nftables manual

NFT(8)                                                                  NFT(8)

NAME
nft - Administration tool of the nftables framework for packet
filtering and classification

SYNOPSIS
nft [ -nNscaeSupyjt ] [ -I directory ] [ -f filename | -i | cmd ...]
nft -h
nft -v

DESCRIPTION
nft is the command line tool used to set up, maintain and inspect
packet filtering and classification rules in the Linux kernel, in the
nftables framework. The Linux kernel subsystem is known as nf_tables,
and ‘nf’ stands for Netfilter.

OPTIONS
The command accepts several different options which are documented here
in groups for better understanding of their meaning. You can get
information about options by running nft --help.

General options:

-h, --help
Show help message and all options.

-v, --version
Show version.

-V
Show long version information, including compile-time
configuration.

Ruleset input handling options that specify to how to load rulesets:

-f, --file filename
Read input from filename. If filename is -, read from stdin.

-i, --interactive
Read input from an interactive readline CLI. You can use quit to
exit, or use the EOF marker, normally this is CTRL-D.

-I, --includepath directory
Add the directory directory to the list of directories to be
searched for included files. This option may be specified multiple
times.

-c, --check
Check commands validity without actually applying the changes.

Ruleset list output formatting that modify the output of the list
ruleset command:

-a, --handle
Show object handles in output.

-s, --stateless
Omit stateful information of rules and stateful objects.

-t, --terse
Omit contents of sets from output.

-S, --service
Translate ports to service names as defined by /etc/services.

-N, --reversedns
Translate IP address to names via reverse DNS lookup. This may slow
down your listing since it generates network traffic.

-u, --guid
Translate numeric UID/GID to names as defined by /etc/passwd and
/etc/group.

-n, --numeric
Print fully numerical output.

-y, --numeric-priority
Display base chain priority numerically.

-p, --numeric-protocol
Display layer 4 protocol numerically.

-T, --numeric-time
Show time, day and hour values in numeric format.

Command output formatting:

-e, --echo
When inserting items into the ruleset using add, insert or replace
commands, print notifications just like nft monitor.

-j, --json
Format output in JSON. See libnftables-json(5) for a schema
description.

-d, --debug level
Enable debugging output. The debug level can be any of scanner,
parser, eval, netlink, mnl, proto-ctx, segtree, all. You can
combine more than one by separating by the , symbol, for example -d
eval,mnl.

INPUT FILE FORMATS
LEXICAL CONVENTIONS
Input is parsed line-wise. When the last character of a line, just
before the newline character, is a non-quoted backslash (\), the next
line is treated as a continuation. Multiple commands on the same line
can be separated using a semicolon (;).

A hash sign (#) begins a comment. All following characters on the same
line are ignored.

Identifiers begin with an alphabetic character (a-z,A-Z), followed zero
or more alphanumeric characters (a-z,A-Z,0-9) and the characters slash
(/), backslash (\), underscore (_) and dot (.). Identifiers using
different characters or clashing with a keyword need to be enclosed in
double quotes (").

INCLUDE FILES
include filename

Other files can be included by using the include statement. The
directories to be searched for include files can be specified using the
-I/--includepath option. You can override this behaviour either by
prepending ‘./’ to your path to force inclusion of files located in the
current working directory (i.e. relative path) or / for file location
expressed as an absolute path.

If -I/--includepath is not specified, then nft relies on the default
directory that is specified at compile time. You can retrieve this
default directory via -h/--help option.

Include statements support the usual shell wildcard symbols (\*,?,[]).
Having no matches for an include statement is not an error, if wildcard
symbols are used in the include statement. This allows having
potentially empty include directories for statements like include
"/etc/firewall/rules/". The wildcard matches are loaded in alphabetical
order. Files beginning with dot (.) are not matched by include
statements.

SYMBOLIC VARIABLES
define variable = expr
$variable

Symbolic variables can be defined using the define statement. Variable
references are expressions and can be used initialize other variables.
The scope of a definition is the current block and all blocks contained
within.

Using symbolic variables.

define int_if1 = eth0
define int_if2 = eth1
define int_ifs = { $int_if1, $int_if2 }

filter input iif $int_ifs accept

ADDRESS FAMILIES
Address families determine the type of packets which are processed. For
each address family, the kernel contains so called hooks at specific
stages of the packet processing paths, which invoke nftables if rules
for these hooks exist.

ip       IPv4 address family.

ip6      IPv6 address family.

inet     Internet (IPv4/IPv6)
address family.

arp      ARP address family,
handling IPv4 ARP packets.

bridge   Bridge address family,
handling packets which
traverse a bridge device.

netdev   Netdev address family,
handling packets from
ingress.

All nftables objects exist in address family specific namespaces,
therefore all identifiers include an address family. If an identifier
is specified without an address family, the ip family is used by
default.

IPV4/IPV6/INET ADDRESS FAMILIES
The IPv4/IPv6/Inet address families handle IPv4, IPv6 or both types of
packets. They contain five hooks at different packet processing stages
in the network stack.

Table 1. IPv4/IPv6/Inet address family hooks
┌────────────┬────────────────────────────┐
│Hook        │ Description                │
├────────────┼────────────────────────────┤
│            │                            │
│prerouting  │ All packets entering the   │
│            │ system are processed by    │
│            │ the prerouting hook. It is │
│            │ invoked before the routing │
│            │ process and is used for    │
│            │ early filtering or         │
│            │ changing packet attributes │
│            │ that affect routing.       │
├────────────┼────────────────────────────┤
│            │                            │
│input       │ Packets delivered to the   │
│            │ local system are processed │
│            │ by the input hook.         │
├────────────┼────────────────────────────┤
│            │                            │
│forward     │ Packets forwarded to a     │
│            │ different host are         │
│            │ processed by the forward   │
│            │ hook.                      │
├────────────┼────────────────────────────┤
│            │                            │
│output      │ Packets sent by local      │
│            │ processes are processed by │
│            │ the output hook.           │
├────────────┼────────────────────────────┤
│            │                            │
│postrouting │ All packets leaving the    │
│            │ system are processed by    │
│            │ the postrouting hook.      │
├────────────┼────────────────────────────┤
│            │                            │
│ingress     │ All packets entering the   │
│            │ system are processed by    │
│            │ this hook. It is invoked   │
│            │ before layer 3 protocol    │
│            │ handlers, hence before the │
│            │ prerouting hook, and it    │
│            │ can be used for filtering  │
│            │ and policing. Ingress is   │
│            │ only available for Inet    │
│            │ family (since Linux kernel │
│            │ 5.10).                     │
└────────────┴────────────────────────────┘

ARP ADDRESS FAMILY
The ARP address family handles ARP packets received and sent by the
system. It is commonly used to mangle ARP packets for clustering.

Table 2. ARP address family hooks
┌───────┬────────────────────────────┐
│Hook   │ Description                │
├───────┼────────────────────────────┤
│       │                            │
│input  │ Packets delivered to the   │
│       │ local system are processed │
│       │ by the input hook.         │
├───────┼────────────────────────────┤
│       │                            │
│output │ Packets send by the local  │
│       │ system are processed by    │
│       │ the output hook.           │
└───────┴────────────────────────────┘

BRIDGE ADDRESS FAMILY
The bridge address family handles Ethernet packets traversing bridge
devices.

The list of supported hooks is identical to IPv4/IPv6/Inet address
families above.

NETDEV ADDRESS FAMILY
The Netdev address family handles packets from the device ingress path.
This family allows you to filter packets of any ethertype such as ARP,
VLAN 802.1q, VLAN 802.1ad (Q-in-Q) as well as IPv4 and IPv6 packets.

Table 3. Netdev address family hooks
┌────────┬────────────────────────────┐
│Hook    │ Description                │
├────────┼────────────────────────────┤
│        │                            │
│ingress │ All packets entering the   │
│        │ system are processed by    │
│        │ this hook. It is invoked   │
│        │ after the network taps     │
│        │ (ie. tcpdump), right after │
│        │ tc ingress and before      │
│        │ layer 3 protocol handlers, │
│        │ it can be used for early   │
│        │ filtering and policing.    │
└────────┴────────────────────────────┘

RULESET
{list | flush} ruleset [family]

The ruleset keyword is used to identify the whole set of tables,
chains, etc. currently in place in kernel. The following ruleset
commands exist:

list    Print the ruleset in
human-readable format.

flush   Clear the whole ruleset.
Note that, unlike
iptables, this will remove
all tables and whatever
they contain, effectively
leading to an empty
ruleset - no packet
filtering will happen
anymore, so the kernel
accepts any valid packet
it receives.

It is possible to limit list and flush to a specific address family
only. For a list of valid family names, see the section called “ADDRESS
FAMILIES” above.

By design, list ruleset command output may be used as input to nft -f.
Effectively, this is the nft-equivalent of iptables-save and
iptables-restore.

TABLES
{add | create} table [family] table [{ flags flags ; }]
{delete | list | flush} table [family] table
list tables [family]
delete table [family] handle handle

Tables are containers for chains, sets and stateful objects. They are
identified by their address family and their name. The address family
must be one of ip, ip6, inet, arp, bridge, netdev. The inet address
family is a dummy family which is used to create hybrid IPv4/IPv6
tables. The meta expression nfproto keyword can be used to test which
family (ipv4 or ipv6) context the packet is being processed in. When no
address family is specified, ip is used by default. The only difference
between add and create is that the former will not return an error if
the specified table already exists while create will return an error.

Table 4. Table flags
┌────────┬────────────────────────────┐
│Flag    │ Description                │
├────────┼────────────────────────────┤
│        │                            │
│dormant │ table is not evaluated any │
│        │ more (base chains are      │
│        │ unregistered).             │
└────────┴────────────────────────────┘

Add, change, delete a table.

# start nft in interactive mode
nft --interactive

# create a new table.
create table inet mytable

# add a new base chain: get input packets
add chain inet mytable myin { type filter hook input priority 0; }

# add a single counter to the chain
add rule inet mytable myin counter

# disable the table temporarily -- rules are not evaluated anymore
add table inet mytable { flags dormant; }

# make table active again:
add table inet mytable

add      Add a new table for the
given family with the
given name.

delete   Delete the specified
table.

list     List all chains and rules
of the specified table.

flush    Flush all chains and rules
of the specified table.

CHAINS
{add | create} chain [family] table chain [{ type type hook hook [device device] priority priority ; [policy policy ;] }]
{delete | list | flush} chain [family] table chain
list chains [family]
delete chain [family] table handle handle
rename chain [family] table chain newname

Chains are containers for rules. They exist in two kinds, base chains
and regular chains. A base chain is an entry point for packets from the
networking stack, a regular chain may be used as jump target and is
used for better rule organization.

add      Add a new chain in the
specified table. When a
hook and priority value
are specified, the chain
is created as a base chain
and hooked up to the
networking stack.

create   Similar to the add
command, but returns an
error if the chain already
exists.

delete   Delete the specified
chain. The chain must not
contain any rules or be
used as jump target.

rename   Rename the specified
chain.

list     List all rules of the
specified chain.

flush    Flush all rules of the
specified chain.

For base chains, type, hook and priority parameters are mandatory.

Table 5. Supported chain types
┌───────┬───────────────┬────────────────┬──────────────────┐
│Type   │ Families      │ Hooks          │ Description      │
├───────┼───────────────┼────────────────┼──────────────────┤
│       │               │                │                  │
│filter │ all           │ all            │ Standard chain   │
│       │               │                │ type to use in   │
│       │               │                │ doubt.           │
├───────┼───────────────┼────────────────┼──────────────────┤
│       │               │                │                  │
│nat    │ ip, ip6, inet │ prerouting,    │ Chains of this   │
│       │               │ input, output, │ type perform     │
│       │               │ postrouting    │ Native Address   │
│       │               │                │ Translation      │
│       │               │                │ based on         │
│       │               │                │ conntrack        │
│       │               │                │ entries. Only    │
│       │               │                │ the first packet │
│       │               │                │ of a connection  │
│       │               │                │ actually         │
│       │               │                │ traverses this   │
│       │               │                │ chain - its      │
│       │               │                │ rules usually    │
│       │               │                │ define details   │
│       │               │                │ of the created   │
│       │               │                │ conntrack entry  │
│       │               │                │ (NAT statements  │
│       │               │                │ for instance).   │
├───────┼───────────────┼────────────────┼──────────────────┤
│       │               │                │                  │
│route  │ ip, ip6       │ output         │ If a packet has  │
│       │               │                │ traversed a      │
│       │               │                │ chain of this    │
│       │               │                │ type and is      │
│       │               │                │ about to be      │
│       │               │                │ accepted, a new  │
│       │               │                │ route lookup is  │
│       │               │                │ performed if     │
│       │               │                │ relevant parts   │
│       │               │                │ of the IP header │
│       │               │                │ have changed.    │
│       │               │                │ This allows to   │
│       │               │                │ e.g. implement   │
│       │               │                │ policy routing   │
│       │               │                │ selectors in     │
│       │               │                │ nftables.        │
└───────┴───────────────┴────────────────┴──────────────────┘

Apart from the special cases illustrated above (e.g. nat type not
supporting forward hook or route type only supporting output hook),
there are three further quirks worth noticing:

•   The netdev family supports merely a single combination, namely
filter type and ingress hook. Base chains in this family also
require the device parameter to be present since they exist per
incoming interface only.

•   The arp family supports only the input and output hooks, both in
chains of type filter.

•   The inet family also supports the ingress hook (since Linux kernel
5.10), to filter IPv4 and IPv6 packet at the same location as the
netdev ingress hook. This inet hook allows you to share sets and
maps between the usual prerouting, input, forward, output,
postrouting and this ingress hook.

The priority parameter accepts a signed integer value or a standard
priority name which specifies the order in which chains with same hook
value are traversed. The ordering is ascending, i.e. lower priority
values have precedence over higher ones.

Standard priority values can be replaced with easily memorizable names.
Not all names make sense in every family with every hook (see the
compatibility matrices below) but their numerical value can still be
used for prioritizing chains.

These names and values are defined and made available based on what
priorities are used by xtables when registering their default chains.

Most of the families use the same values, but bridge uses different
ones from the others. See the following tables that describe the values
and compatibility.

Table 6. Standard priority names, family and hook compatibility matrix
┌─────────┬───────┬────────────────┬─────────────┐
│Name     │ Value │ Families       │ Hooks       │
├─────────┼───────┼────────────────┼─────────────┤
│         │       │                │             │
│raw      │ -300  │ ip, ip6, inet  │ all         │
├─────────┼───────┼────────────────┼─────────────┤
│         │       │                │             │
│mangle   │ -150  │ ip, ip6, inet  │ all         │
├─────────┼───────┼────────────────┼─────────────┤
│         │       │                │             │
│dstnat   │ -100  │ ip, ip6, inet  │ prerouting  │
├─────────┼───────┼────────────────┼─────────────┤
│         │       │                │             │
│filter   │ 0     │ ip, ip6, inet, │ all         │
│         │       │ arp, netdev    │             │
├─────────┼───────┼────────────────┼─────────────┤
│         │       │                │             │
│security │ 50    │ ip, ip6, inet  │ all         │
├─────────┼───────┼────────────────┼─────────────┤
│         │       │                │             │
│srcnat   │ 100   │ ip, ip6, inet  │ postrouting │
└─────────┴───────┴────────────────┴─────────────┘

Table 7. Standard priority names and hook compatibility for the bridge
family
┌───────┬───────┬─────────────┐
│       │       │             │
│Name   │ Value │ Hooks       │
├───────┼───────┼─────────────┤
│       │       │             │
│dstnat │ -300  │ prerouting  │
├───────┼───────┼─────────────┤
│       │       │             │
│filter │ -200  │ all         │
├───────┼───────┼─────────────┤
│       │       │             │
│out    │ 100   │ output      │
├───────┼───────┼─────────────┤
│       │       │             │
│srcnat │ 300   │ postrouting │
└───────┴───────┴─────────────┘

Basic arithmetic expressions (addition and subtraction) can also be
achieved with these standard names to ease relative prioritizing, e.g.
mangle - 5 stands for -155. Values will also be printed like this until
the value is not further than 10 form the standard value.

Base chains also allow to set the chain’s policy, i.e. what happens to
packets not explicitly accepted or refused in contained rules.
Supported policy values are accept (which is the default) or drop.

RULES
{add | insert} rule [family] table chain [handle handle | index index] statement ... [comment comment]
replace rule [family] table chain handle handle statement ... [comment comment]
delete rule [family] table chain handle handle

Rules are added to chains in the given table. If the family is not
specified, the ip family is used. Rules are constructed from two kinds
of components according to a set of grammatical rules: expressions and
statements.

The add and insert commands support an optional location specifier,
which is either a handle or the index (starting at zero) of an existing
rule. Internally, rule locations are always identified by handle and
the translation from index happens in userspace. This has two potential
implications in case a concurrent ruleset change happens after the
translation was done: The effective rule index might change if a rule
was inserted or deleted before the referred one. If the referred rule
was deleted, the command is rejected by the kernel just as if an
invalid handle was given.

A comment is a single word or a double-quoted (") multi-word string
which can be used to make notes regarding the actual rule. Note: If you
use bash for adding rules, you have to escape the quotation marks, e.g.
\"enable ssh for servers\".

add       Add a new rule described
by the list of statements.
The rule is appended to
the given chain unless a
location is specified, in
which case the rule is
inserted after the
specified rule.

insert    Same as add except the
rule is inserted at the
beginning of the chain or
before the specified rule.

replace   Similar to add, but the
rule replaces the
specified rule.

delete    Delete the specified rule.

add a rule to ip table output chain.

nft add rule filter output ip daddr 192.168.0.0/24 accept # 'ip filter' is assumed
# same command, slightly more verbose
nft add rule ip filter output ip daddr 192.168.0.0/24 accept

delete rule from inet table.

# nft -a list ruleset
table inet filter {
chain input {
type filter hook input priority 0; policy accept;
ct state established,related accept # handle 4
ip saddr 10.1.1.1 tcp dport ssh accept # handle 5
...
# delete the rule with handle 5
# nft delete rule inet filter input handle 5

SETS
nftables offers two kinds of set concepts. Anonymous sets are sets that
have no specific name. The set members are enclosed in curly braces,
with commas to separate elements when creating the rule the set is used
in. Once that rule is removed, the set is removed as well. They cannot
be updated, i.e. once an anonymous set is declared it cannot be changed
anymore except by removing/altering the rule that uses the anonymous
set.

Using anonymous sets to accept particular subnets and ports.

nft add rule filter input ip saddr { 10.0.0.0/8, 192.168.0.0/16 } tcp dport { 22, 443 } accept

Named sets are sets that need to be defined first before they can be
referenced in rules. Unlike anonymous sets, elements can be added to or
removed from a named set at any time. Sets are referenced from rules
using an @ prefixed to the sets name.

Using named sets to accept addresses and ports.

nft add rule filter input ip saddr @allowed_hosts tcp dport @allowed_ports accept

The sets allowed_hosts and allowed_ports need to be created first. The
next section describes nft set syntax in more detail.

add set [family] table set { type type | typeof expression ; [flags flags ;] [timeout timeout ;] [gc-interval gc-interval ;] [elements = { element[, ...] } ;] [size size ;] [policy policy ;] [auto-merge ;] }
{delete | list | flush} set [family] table set
list sets [family]
delete set [family] table handle handle
{add | delete} element [family] table set { element[, ...] }

Sets are element containers of a user-defined data type, they are
uniquely identified by a user-defined name and attached to tables.
Their behaviour can be tuned with the flags that can be specified at
set creation time.

add      Add a new set in the
specified table. See the
Set specification table
below for more information
about how to specify a
sets properties.

delete   Delete the specified set.

list     Display the elements in
the specified set.

flush    Remove all elements from
the specified set.

Table 8. Set specifications
┌────────────┬──────────────────────┬─────────────────────┐
│Keyword     │ Description          │ Type                │
├────────────┼──────────────────────┼─────────────────────┤
│            │                      │                     │
│type        │ data type of set     │ string: ipv4_addr,  │
│            │ elements             │ ipv6_addr,          │
│            │                      │ ether_addr,         │
│            │                      │ inet_proto,         │
│            │                      │ inet_service, mark  │
├────────────┼──────────────────────┼─────────────────────┤
│            │                      │                     │
│typeof      │ data type of set     │ expression to       │
│            │ element              │ derive the data     │
│            │                      │ type from           │
├────────────┼──────────────────────┼─────────────────────┤
│            │                      │                     │
│flags       │ set flags            │ string: constant,   │
│            │                      │ dynamic, interval,  │
│            │                      │ timeout             │
├────────────┼──────────────────────┼─────────────────────┤
│            │                      │                     │
│timeout     │ time an element      │ string, decimal     │
│            │ stays in the set,    │ followed by unit.   │
│            │ mandatory if set is  │ Units are: d, h, m, │
│            │ added to from the    │ s                   │
│            │ packet path          │                     │
│            │ (ruleset).           │                     │
├────────────┼──────────────────────┼─────────────────────┤
│            │                      │                     │
│gc-interval │ garbage collection   │ string, decimal     │
│            │ interval, only       │ followed by unit.   │
│            │ available when       │ Units are: d, h, m, │
│            │ timeout or flag      │ s                   │
│            │ timeout are active   │                     │
├────────────┼──────────────────────┼─────────────────────┤
│            │                      │                     │
│elements    │ elements contained   │ set data type       │
│            │ by the set           │                     │
├────────────┼──────────────────────┼─────────────────────┤
│            │                      │                     │
│size        │ maximum number of    │ unsigned integer    │
│            │ elements in the      │ (64 bit)            │
│            │ set, mandatory if    │                     │
│            │ set is added to      │                     │
│            │ from the packet      │                     │
│            │ path (ruleset).      │                     │
├────────────┼──────────────────────┼─────────────────────┤
│            │                      │                     │
│policy      │ set policy           │ string: performance │
│            │                      │ [default], memory   │
├────────────┼──────────────────────┼─────────────────────┤
│            │                      │                     │
│auto-merge  │ automatic merge of   │                     │
│            │ adjacent/overlapping │                     │
│            │ set elements (only   │                     │
│            │ for interval sets)   │                     │
└────────────┴──────────────────────┴─────────────────────┘

MAPS
add map [family] table map { type type | typeof expression [flags flags ;] [elements = { element[, ...] } ;] [size size ;] [policy policy ;] }
{delete | list | flush} map [family] table map
list maps [family]

Maps store data based on some specific key used as input. They are
uniquely identified by a user-defined name and attached to tables.

add              Add a new map in the
specified table.

delete           Delete the specified map.

list             Display the elements in
the specified map.

flush            Remove all elements from
the specified map.

add element      Comma-separated list of
elements to add into the
specified map.

delete element   Comma-separated list of
element keys to delete
from the specified map.

Table 9. Map specifications
┌─────────┬─────────────────────┬─────────────────────┐
│Keyword  │ Description         │ Type                │
├─────────┼─────────────────────┼─────────────────────┤
│         │                     │                     │
│type     │ data type of map    │ string: ipv4_addr,  │
│         │ elements            │ ipv6_addr,          │
│         │                     │ ether_addr,         │
│         │                     │ inet_proto,         │
│         │                     │ inet_service, mark, │
│         │                     │ counter, quota.     │
│         │                     │ Counter and quota   │
│         │                     │ can’t be used as    │
│         │                     │ keys                │
├─────────┼─────────────────────┼─────────────────────┤
│         │                     │                     │
│typeof   │ data type of set    │ expression to       │
│         │ element             │ derive the data     │
│         │                     │ type from           │
├─────────┼─────────────────────┼─────────────────────┤
│         │                     │                     │
│flags    │ map flags           │ string: constant,   │
│         │                     │ interval            │
├─────────┼─────────────────────┼─────────────────────┤
│         │                     │                     │
│elements │ elements contained  │ map data type       │
│         │ by the map          │                     │
├─────────┼─────────────────────┼─────────────────────┤
│         │                     │                     │
│size     │ maximum number of   │ unsigned integer    │
│         │ elements in the map │ (64 bit)            │
├─────────┼─────────────────────┼─────────────────────┤
│         │                     │                     │
│policy   │ map policy          │ string: performance │
│         │                     │ [default], memory   │
└─────────┴─────────────────────┴─────────────────────┘

ELEMENTS
{add | create | delete | get } element [family] table set { ELEMENT[, ...] }

ELEMENT := key_expression OPTIONS [: value_expression]
OPTIONS := [timeout TIMESPEC] [expires TIMESPEC] [comment string]
TIMESPEC := [numd][numh][numm][num[s]]

Element-related commands allow to change contents of named sets and
maps. key_expression is typically a value matching the set type.
value_expression is not allowed in sets but mandatory when adding to
maps, where it matches the data part in it’s type definition. When
deleting from maps, it may be specified but is optional as
key_expression uniquely identifies the element.

create command is similar to add with the exception that none of the
listed elements may already exist.

get command is useful to check if an element is contained in a set
which may be non-trivial in very large and/or interval sets. In the
latter case, the containing interval is returned instead of just the
element itself.

Table 10. Element options
┌────────┬───────────────────────────┐
│Option  │ Description               │
├────────┼───────────────────────────┤
│        │                           │
│timeout │ timeout value for         │
│        │ sets/maps with flag       │
│        │ timeout                   │
├────────┼───────────────────────────┤
│        │                           │
│expires │ the time until given      │
│        │ element expires, useful   │
│        │ for ruleset replication   │
│        │ only                      │
├────────┼───────────────────────────┤
│        │                           │
│comment │ per element comment field │
└────────┴───────────────────────────┘

FLOWTABLES
{add | create} flowtable [family] table flowtable { hook hook priority priority ; devices = { device[, ...] } ; }
list flowtables [family]
{delete | list} flowtable [family] table flowtable
delete flowtable [family] table handle handle

Flowtables allow you to accelerate packet forwarding in software.
Flowtables entries are represented through a tuple that is composed of
the input interface, source and destination address, source and
destination port; and layer 3/4 protocols. Each entry also caches the
destination interface and the gateway address - to update the
destination link-layer address - to forward packets. The ttl and
hoplimit fields are also decremented. Hence, flowtables provides an
alternative path that allow packets to bypass the classic forwarding
path. Flowtables reside in the ingress hook that is located before the
prerouting hook. You can select which flows you want to offload through
the flow expression from the forward chain. Flowtables are identified
by their address family and their name. The address family must be one
of ip, ip6, or inet. The inet address family is a dummy family which is
used to create hybrid IPv4/IPv6 tables. When no address family is
specified, ip is used by default.

The priority can be a signed integer or filter which stands for 0.
Addition and subtraction can be used to set relative priority, e.g.
filter + 5 equals to 5.

add      Add a new flowtable for
the given family with the
given name.

delete   Delete the specified
flowtable.

list     List all flowtables.

STATEFUL OBJECTS
{add | delete | list | reset} type [family] table object
delete type [family] table handle handle
list counters [family]
list quotas [family]

Stateful objects are attached to tables and are identified by an unique
name. They group stateful information from rules, to reference them in
rules the keywords "type name" are used e.g. "counter name".

add      Add a new stateful object
in the specified table.

delete   Delete the specified
object.

list     Display stateful
information the object
holds.

reset    List-and-reset stateful
object.

CT HELPER
ct helper helper { type type protocol protocol ; [l3proto family ;] }

Ct helper is used to define connection tracking helpers that can then
be used in combination with the ct helper set statement. type and
protocol are mandatory, l3proto is derived from the table family by
default, i.e. in the inet table the kernel will try to load both the
ipv4 and ipv6 helper backends, if they are supported by the kernel.

Table 11. conntrack helper specifications
┌─────────┬─────────────────────┬─────────────────────┐
│Keyword  │ Description         │ Type                │
├─────────┼─────────────────────┼─────────────────────┤
│         │                     │                     │
│type     │ name of helper type │ quoted string (e.g. │
│         │                     │ "ftp")              │
├─────────┼─────────────────────┼─────────────────────┤
│         │                     │                     │
│protocol │ layer 4 protocol of │ string (e.g. ip)    │
│         │ the helper          │                     │
├─────────┼─────────────────────┼─────────────────────┤
│         │                     │                     │
│l3proto  │ layer 3 protocol of │ address family      │
│         │ the helper          │ (e.g. ip)           │
└─────────┴─────────────────────┴─────────────────────┘

defining and assigning ftp helper.

Unlike iptables, helper assignment needs to be performed after the conntrack
lookup has completed, for example with the default 0 hook priority.

table inet myhelpers {
ct helper ftp-standard {
type "ftp" protocol tcp
}
chain prerouting {
type filter hook prerouting priority 0;
tcp dport 21 ct helper set "ftp-standard"
}
}

CT TIMEOUT
ct timeout name { protocol protocol ; policy = { state: value [, ...] } ; [l3proto family ;] }

Ct timeout is used to update connection tracking timeout values.Timeout
policies are assigned with the ct timeout set statement. protocol and
policy are mandatory, l3proto is derived from the table family by
default.

Table 12. conntrack timeout specifications
┌─────────┬─────────────────────┬──────────────────┐
│Keyword  │ Description         │ Type             │
├─────────┼─────────────────────┼──────────────────┤
│         │                     │                  │
│protocol │ layer 4 protocol of │ string (e.g. ip) │
│         │ the timeout object  │                  │
├─────────┼─────────────────────┼──────────────────┤
│         │                     │                  │
│state    │ connection state    │ string (e.g.     │
│         │ name                │ "established")   │
├─────────┼─────────────────────┼──────────────────┤
│         │                     │                  │
│value    │ timeout value for   │ unsigned integer │
│         │ connection state    │                  │
├─────────┼─────────────────────┼──────────────────┤
│         │                     │                  │
│l3proto  │ layer 3 protocol of │ address family   │
│         │ the timeout object  │ (e.g. ip)        │
└─────────┴─────────────────────┴──────────────────┘

defining and assigning ct timeout policy.

table ip filter {
ct timeout customtimeout {
protocol tcp;
l3proto ip
policy = { established: 120, close: 20 }
}

chain output {
type filter hook output priority filter; policy accept;
ct timeout set "customtimeout"
}
}

testing the updated timeout policy.

% conntrack -E

It should display:

[UPDATE] tcp      6 120 ESTABLISHED src=172.16.19.128 dst=172.16.19.1
sport=22 dport=41360 [UNREPLIED] src=172.16.19.1 dst=172.16.19.128
sport=41360 dport=22

CT EXPECTATION
ct expectation name { protocol protocol ; dport dport ; timeout timeout ; size size ; [*l3proto family ;] }

Ct expectation is used to create connection expectations. Expectations
are assigned with the ct expectation set statement. protocol, dport,
timeout and size are mandatory, l3proto is derived from the table
family by default.

Table 13. conntrack expectation specifications
┌─────────┬─────────────────────┬──────────────────┐
│Keyword  │ Description         │ Type             │
├─────────┼─────────────────────┼──────────────────┤
│         │                     │                  │
│protocol │ layer 4 protocol of │ string (e.g. ip) │
│         │ the expectation     │                  │
│         │ object              │                  │
├─────────┼─────────────────────┼──────────────────┤
│         │                     │                  │
│dport    │ destination port of │ unsigned integer │
│         │ expected connection │                  │
├─────────┼─────────────────────┼──────────────────┤
│         │                     │                  │
│timeout  │ timeout value for   │ unsigned integer │
│         │ expectation         │                  │
├─────────┼─────────────────────┼──────────────────┤
│         │                     │                  │
│size     │ size value for      │ unsigned integer │
│         │ expectation         │                  │
├─────────┼─────────────────────┼──────────────────┤
│         │                     │                  │
│l3proto  │ layer 3 protocol of │ address family   │
│         │ the expectation     │ (e.g. ip)        │
│         │ object              │                  │
└─────────┴─────────────────────┴──────────────────┘

defining and assigning ct expectation policy.

table ip filter {
ct expectation expect {
protocol udp
dport 9876
timeout 2m
size 8
l3proto ip
}

chain input {
type filter hook input priority filter; policy accept;
ct expectation set "expect"
}
}

COUNTER
counter [packets bytes]

Table 14. Counter specifications
┌────────┬──────────────────┬──────────────────┐
│Keyword │ Description      │ Type             │
├────────┼──────────────────┼──────────────────┤
│        │                  │                  │
│packets │ initial count of │ unsigned integer │
│        │ packets          │ (64 bit)         │
├────────┼──────────────────┼──────────────────┤
│        │                  │                  │
│bytes   │ initial count of │ unsigned integer │
│        │ bytes            │ (64 bit)         │
└────────┴──────────────────┴──────────────────┘

QUOTA
quota [over | until] [used]

Table 15. Quota specifications
┌────────┬───────────────────┬────────────────────┐
│Keyword │ Description       │ Type               │
├────────┼───────────────────┼────────────────────┤
│        │                   │                    │
│quota   │ quota limit, used │ Two arguments,     │
│        │ as the quota name │ unsigned integer   │
│        │                   │ (64 bit) and       │
│        │                   │ string: bytes,     │
│        │                   │ kbytes, mbytes.    │
│        │                   │ "over" and "until" │
│        │                   │ go before these    │
│        │                   │ arguments          │
├────────┼───────────────────┼────────────────────┤
│        │                   │                    │
│used    │ initial value of  │ Two arguments,     │
│        │ used quota        │ unsigned integer   │
│        │                   │ (64 bit) and       │
│        │                   │ string: bytes,     │
│        │                   │ kbytes, mbytes     │
└────────┴───────────────────┴────────────────────┘

EXPRESSIONS
Expressions represent values, either constants like network addresses,
port numbers, etc., or data gathered from the packet during ruleset
evaluation. Expressions can be combined using binary, logical,
relational and other types of expressions to form complex or relational
(match) expressions. They are also used as arguments to certain types
of operations, like NAT, packet marking etc.

Each expression has a data type, which determines the size, parsing and
representation of symbolic values and type compatibility with other
expressions.

DESCRIBE COMMAND
describe expression | data type

The describe command shows information about the type of an expression
and its data type. A data type may also be given, in which nft will
display more information about the type.

The describe command.

$ nft describe tcp flags
payload expression, datatype tcp_flag (TCP flag) (basetype bitmask, integer), 8 bits

predefined symbolic constants:
fin                           0x01
syn                           0x02
rst                           0x04
psh                           0x08
ack                           0x10
urg                           0x20
ecn                           0x40
cwr                           0x80

DATA TYPES
Data types determine the size, parsing and representation of symbolic
values and type compatibility of expressions. A number of global data
types exist, in addition some expression types define further data
types specific to the expression type. Most data types have a fixed
size, some however may have a dynamic size, f.i. the string type. Some
types also have predefined symbolic constants. Those can be listed
using the nft describe command:

$ nft describe ct_state
datatype ct_state (conntrack state) (basetype bitmask, integer), 32 bits

pre-defined symbolic constants (in hexadecimal):
invalid                         0x00000001
new ...

Types may be derived from lower order types, f.i. the IPv4 address type
is derived from the integer type, meaning an IPv4 address can also be
specified as an integer value.

In certain contexts (set and map definitions), it is necessary to
explicitly specify a data type. Each type has a name which is used for
this.

INTEGER TYPE
┌────────┬─────────┬──────────┬───────────┐
│Name    │ Keyword │ Size     │ Base type │
├────────┼─────────┼──────────┼───────────┤
│        │         │          │           │
│Integer │ integer │ variable │ -         │
└────────┴─────────┴──────────┴───────────┘

The integer type is used for numeric values. It may be specified as a
decimal, hexadecimal or octal number. The integer type does not have a
fixed size, its size is determined by the expression for which it is
used.

BITMASK TYPE
┌────────┬─────────┬──────────┬───────────┐
│Name    │ Keyword │ Size     │ Base type │
├────────┼─────────┼──────────┼───────────┤
│        │         │          │           │
│Bitmask │ bitmask │ variable │ integer   │
└────────┴─────────┴──────────┴───────────┘

The bitmask type (bitmask) is used for bitmasks.

STRING TYPE
┌───────┬─────────┬──────────┬───────────┐
│Name   │ Keyword │ Size     │ Base type │
├───────┼─────────┼──────────┼───────────┤
│       │         │          │           │
│String │ string  │ variable │ -         │
└───────┴─────────┴──────────┴───────────┘

The string type is used for character strings. A string begins with an
alphabetic character (a-zA-Z) followed by zero or more alphanumeric
characters or the characters /, -, _ and .. In addition, anything
enclosed in double quotes (") is recognized as a string.

String specification.

# Interface name
filter input iifname eth0

# Weird interface name
filter input iifname "(eth0)"

LINK LAYER ADDRESS TYPE
┌───────────┬─────────┬──────────┬───────────┐
│Name       │ Keyword │ Size     │ Base type │
├───────────┼─────────┼──────────┼───────────┤
│           │         │          │           │
│Link layer │ lladdr  │ variable │ integer   │
│address    │         │          │           │
└───────────┴─────────┴──────────┴───────────┘

The link layer address type is used for link layer addresses. Link
layer addresses are specified as a variable amount of groups of two
hexadecimal digits separated using colons (:).

Link layer address specification.

# Ethernet destination MAC address
filter input ether daddr 20:c9:d0:43:12:d9

IPV4 ADDRESS TYPE
┌─────────────┬───────────┬────────┬───────────┐
│Name         │ Keyword   │ Size   │ Base type │
├─────────────┼───────────┼────────┼───────────┤
│             │           │        │           │
│IPV4 address │ ipv4_addr │ 32 bit │ integer   │
└─────────────┴───────────┴────────┴───────────┘

The IPv4 address type is used for IPv4 addresses. Addresses are
specified in either dotted decimal, dotted hexadecimal, dotted octal,
decimal, hexadecimal, octal notation or as a host name. A host name
will be resolved using the standard system resolver.

IPv4 address specification.

# dotted decimal notation
filter output ip daddr 127.0.0.1

# host name
filter output ip daddr localhost

IPV6 ADDRESS TYPE
┌─────────────┬───────────┬─────────┬───────────┐
│Name         │ Keyword   │ Size    │ Base type │
├─────────────┼───────────┼─────────┼───────────┤
│             │           │         │           │
│IPv6 address │ ipv6_addr │ 128 bit │ integer   │
└─────────────┴───────────┴─────────┴───────────┘

The IPv6 address type is used for IPv6 addresses. Addresses are
specified as a host name or as hexadecimal halfwords separated by
colons. Addresses might be enclosed in square brackets ("[]") to
differentiate them from port numbers.

IPv6 address specification.

# abbreviated loopback address
filter output ip6 daddr ::1

IPv6 address specification with bracket notation.

# without [] the port number (22) would be parsed as part of the
# ipv6 address
ip6 nat prerouting tcp dport 2222 dnat to [1ce::d0]:22

BOOLEAN TYPE
┌────────┬─────────┬───────┬───────────┐
│Name    │ Keyword │ Size  │ Base type │
├────────┼─────────┼───────┼───────────┤
│        │         │       │           │
│Boolean │ boolean │ 1 bit │ integer   │
└────────┴─────────┴───────┴───────────┘

The boolean type is a syntactical helper type in userspace. Its use is
in the right-hand side of a (typically implicit) relational expression
to change the expression on the left-hand side into a boolean check
(usually for existence).

Table 16. The following keywords will automatically resolve into a
boolean type with given value
┌────────┬───────┐
│Keyword │ Value │
├────────┼───────┤
│        │       │
│exists  │ 1     │
├────────┼───────┤
│        │       │
│missing │ 0     │
└────────┴───────┘

Table 17. expressions support a boolean comparison
┌───────────┬─────────────────────────┐
│Expression │ Behaviour               │
├───────────┼─────────────────────────┤
│           │                         │
│fib        │ Check route existence.  │
├───────────┼─────────────────────────┤
│           │                         │
│exthdr     │ Check IPv6 extension    │
│           │ header existence.       │
├───────────┼─────────────────────────┤
│           │                         │
│tcp option │ Check TCP option header │
│           │ existence.              │
└───────────┴─────────────────────────┘

Boolean specification.

# match if route exists
filter input fib daddr . iif oif exists

# match only non-fragmented packets in IPv6 traffic
filter input exthdr frag missing

# match if TCP timestamp option is present
filter input tcp option timestamp exists

ICMP TYPE TYPE
┌──────────┬───────────┬───────┬───────────┐
│Name      │ Keyword   │ Size  │ Base type │
├──────────┼───────────┼───────┼───────────┤
│          │           │       │           │
│ICMP Type │ icmp_type │ 8 bit │ integer   │
└──────────┴───────────┴───────┴───────────┘

The ICMP Type type is used to conveniently specify the ICMP header’s
type field.

Table 18. Keywords may be used when specifying the ICMP type
┌────────────────────────┬───────┐
│Keyword                 │ Value │
├────────────────────────┼───────┤
│                        │       │
│echo-reply              │ 0     │
├────────────────────────┼───────┤
│                        │       │
│destination-unreachable │ 3     │
├────────────────────────┼───────┤
│                        │       │
│source-quench           │ 4     │
├────────────────────────┼───────┤
│                        │       │
│redirect                │ 5     │
├────────────────────────┼───────┤
│                        │       │
│echo-request            │ 8     │
├────────────────────────┼───────┤
│                        │       │
│router-advertisement    │ 9     │
├────────────────────────┼───────┤
│                        │       │
│router-solicitation     │ 10    │
├────────────────────────┼───────┤
│                        │       │
│time-exceeded           │ 11    │
├────────────────────────┼───────┤
│                        │       │
│parameter-problem       │ 12    │
├────────────────────────┼───────┤
│                        │       │
│timestamp-request       │ 13    │
├────────────────────────┼───────┤
│                        │       │
│timestamp-reply         │ 14    │
├────────────────────────┼───────┤
│                        │       │
│info-request            │ 15    │
├────────────────────────┼───────┤
│                        │       │
│info-reply              │ 16    │
├────────────────────────┼───────┤
│                        │       │
│address-mask-request    │ 17    │
├────────────────────────┼───────┤
│                        │       │
│address-mask-reply      │ 18    │
└────────────────────────┴───────┘

ICMP Type specification.

# match ping packets
filter output icmp type { echo-request, echo-reply }

ICMP CODE TYPE
┌──────────┬───────────┬───────┬───────────┐
│Name      │ Keyword   │ Size  │ Base type │
├──────────┼───────────┼───────┼───────────┤
│          │           │       │           │
│ICMP Code │ icmp_code │ 8 bit │ integer   │
└──────────┴───────────┴───────┴───────────┘

The ICMP Code type is used to conveniently specify the ICMP header’s
code field.

Table 19. Keywords may be used when specifying the ICMP code
┌─────────────────┬───────┐
│Keyword          │ Value │
├─────────────────┼───────┤
│                 │       │
│net-unreachable  │ 0     │
├─────────────────┼───────┤
│                 │       │
│host-unreachable │ 1     │
├─────────────────┼───────┤
│                 │       │
│prot-unreachable │ 2     │
├─────────────────┼───────┤
│                 │       │
│port-unreachable │ 3     │
├─────────────────┼───────┤
│                 │       │
│frag-needed      │ 4     │
├─────────────────┼───────┤
│                 │       │
│net-prohibited   │ 9     │
├─────────────────┼───────┤
│                 │       │
│host-prohibited  │ 10    │
├─────────────────┼───────┤
│                 │       │
│admin-prohibited │ 13    │
└─────────────────┴───────┘

ICMPV6 TYPE TYPE
┌────────────┬────────────┬───────┬───────────┐
│Name        │ Keyword    │ Size  │ Base type │
├────────────┼────────────┼───────┼───────────┤
│            │            │       │           │
│ICMPv6 Type │ icmpx_code │ 8 bit │ integer   │
└────────────┴────────────┴───────┴───────────┘

The ICMPv6 Type type is used to conveniently specify the ICMPv6
header’s type field.

Table 20. keywords may be used when specifying the ICMPv6 type:
┌────────────────────────┬───────┐
│Keyword                 │ Value │
├────────────────────────┼───────┤
│                        │       │
│destination-unreachable │ 1     │
├────────────────────────┼───────┤
│                        │       │
│packet-too-big          │ 2     │
├────────────────────────┼───────┤
│                        │       │
│time-exceeded           │ 3     │
├────────────────────────┼───────┤
│                        │       │
│parameter-problem       │ 4     │
├────────────────────────┼───────┤
│                        │       │
│echo-request            │ 128   │
├────────────────────────┼───────┤
│                        │       │
│echo-reply              │ 129   │
├────────────────────────┼───────┤
│                        │       │
│mld-listener-query      │ 130   │
├────────────────────────┼───────┤
│                        │       │
│mld-listener-report     │ 131   │
├────────────────────────┼───────┤
│                        │       │
│mld-listener-done       │ 132   │
├────────────────────────┼───────┤
│                        │       │
│mld-listener-reduction  │ 132   │
├────────────────────────┼───────┤
│                        │       │
│nd-router-solicit       │ 133   │
├────────────────────────┼───────┤
│                        │       │
│nd-router-advert        │ 134   │
├────────────────────────┼───────┤
│                        │       │
│nd-neighbor-solicit     │ 135   │
├────────────────────────┼───────┤
│                        │       │
│nd-neighbor-advert      │ 136   │
├────────────────────────┼───────┤
│                        │       │
│nd-redirect             │ 137   │
├────────────────────────┼───────┤
│                        │       │
│router-renumbering      │ 138   │
├────────────────────────┼───────┤
│                        │       │
│ind-neighbor-solicit    │ 141   │
├────────────────────────┼───────┤
│                        │       │
│ind-neighbor-advert     │ 142   │
├────────────────────────┼───────┤
│                        │       │
│mld2-listener-report    │ 143   │
└────────────────────────┴───────┘

ICMPv6 Type specification.

# match ICMPv6 ping packets
filter output icmpv6 type { echo-request, echo-reply }

ICMPV6 CODE TYPE
┌────────────┬─────────────┬───────┬───────────┐
│Name        │ Keyword     │ Size  │ Base type │
├────────────┼─────────────┼───────┼───────────┤
│            │             │       │           │
│ICMPv6 Code │ icmpv6_code │ 8 bit │ integer   │
└────────────┴─────────────┴───────┴───────────┘

The ICMPv6 Code type is used to conveniently specify the ICMPv6
header’s code field.

Table 21. keywords may be used when specifying the ICMPv6 code
┌─────────────────┬───────┐
│Keyword          │ Value │
├─────────────────┼───────┤
│                 │       │
│no-route         │ 0     │
├─────────────────┼───────┤
│                 │       │
│admin-prohibited │ 1     │
├─────────────────┼───────┤
│                 │       │
│addr-unreachable │ 3     │
├─────────────────┼───────┤
│                 │       │
│port-unreachable │ 4     │
├─────────────────┼───────┤
│                 │       │
│policy-fail      │ 5     │
├─────────────────┼───────┤
│                 │       │
│reject-route     │ 6     │
└─────────────────┴───────┘

ICMPVX CODE TYPE
┌────────────┬─────────────┬───────┬───────────┐
│Name        │ Keyword     │ Size  │ Base type │
├────────────┼─────────────┼───────┼───────────┤
│            │             │       │           │
│ICMPvX Code │ icmpv6_type │ 8 bit │ integer   │
└────────────┴─────────────┴───────┴───────────┘

The ICMPvX Code type abstraction is a set of values which overlap
between ICMP and ICMPv6 Code types to be used from the inet family.

Table 22. keywords may be used when specifying the ICMPvX code
┌─────────────────┬───────┐
│Keyword          │ Value │
├─────────────────┼───────┤
│                 │       │
│no-route         │ 0     │
├─────────────────┼───────┤
│                 │       │
│port-unreachable │ 1     │
├─────────────────┼───────┤
│                 │       │
│host-unreachable │ 2     │
├─────────────────┼───────┤
│                 │       │
│admin-prohibited │ 3     │
└─────────────────┴───────┘

CONNTRACK TYPES
Table 23. overview of types used in ct expression and statement
┌─────────────────┬───────────┬─────────┬───────────┐
│Name             │ Keyword   │ Size    │ Base type │
├─────────────────┼───────────┼─────────┼───────────┤
│                 │           │         │           │
│conntrack state  │ ct_state  │ 4 byte  │ bitmask   │
├─────────────────┼───────────┼─────────┼───────────┤
│                 │           │         │           │
│conntrack        │ ct_dir    │ 8 bit   │ integer   │
│direction        │           │         │           │
├─────────────────┼───────────┼─────────┼───────────┤
│                 │           │         │           │
│conntrack status │ ct_status │ 4 byte  │ bitmask   │
├─────────────────┼───────────┼─────────┼───────────┤
│                 │           │         │           │
│conntrack event  │ ct_event  │ 4 byte  │ bitmask   │
│bits             │           │         │           │
├─────────────────┼───────────┼─────────┼───────────┤
│                 │           │         │           │
│conntrack label  │ ct_label  │ 128 bit │ bitmask   │
└─────────────────┴───────────┴─────────┴───────────┘

For each of the types above, keywords are available for convenience:

Table 24. conntrack state (ct_state)
┌────────────┬───────┐
│Keyword     │ Value │
├────────────┼───────┤
│            │       │
│invalid     │ 1     │
├────────────┼───────┤
│            │       │
│established │ 2     │
├────────────┼───────┤
│            │       │
│related     │ 4     │
├────────────┼───────┤
│            │       │
│new         │ 8     │
├────────────┼───────┤
│            │       │
│untracked   │ 64    │
└────────────┴───────┘

Table 25. conntrack direction (ct_dir)
┌─────────┬───────┐
│Keyword  │ Value │
├─────────┼───────┤
│         │       │
│original │ 0     │
├─────────┼───────┤
│         │       │
│reply    │ 1     │
└─────────┴───────┘

Table 26. conntrack status (ct_status)
┌───────────┬───────┐
│Keyword    │ Value │
├───────────┼───────┤
│           │       │
│expected   │ 1     │
├───────────┼───────┤
│           │       │
│seen-reply │ 2     │
├───────────┼───────┤
│           │       │
│assured    │ 4     │
├───────────┼───────┤
│           │       │
│confirmed  │ 8     │
├───────────┼───────┤
│           │       │
│snat       │ 16    │
├───────────┼───────┤
│           │       │
│dnat       │ 32    │
├───────────┼───────┤
│           │       │
│dying      │ 512   │
└───────────┴───────┘

Table 27. conntrack event bits (ct_event)
┌──────────┬───────┐
│Keyword   │ Value │
├──────────┼───────┤
│          │       │
│new       │ 1     │
├──────────┼───────┤
│          │       │
│related   │ 2     │
├──────────┼───────┤
│          │       │
│destroy   │ 4     │
├──────────┼───────┤
│          │       │
│reply     │ 8     │
├──────────┼───────┤
│          │       │
│assured   │ 16    │
├──────────┼───────┤
│          │       │
│protoinfo │ 32    │
├──────────┼───────┤
│          │       │
│helper    │ 64    │
├──────────┼───────┤
│          │       │
│mark      │ 128   │
├──────────┼───────┤
│          │       │
│seqadj    │ 256   │
├──────────┼───────┤
│          │       │
│secmark   │ 512   │
├──────────┼───────┤
│          │       │
│label     │ 1024  │
└──────────┴───────┘

Possible keywords for conntrack label type (ct_label) are read at
runtime from /etc/connlabel.conf.

DCCP PKTTYPE TYPE
┌─────────────────┬──────────────┬───────┬───────────┐
│Name             │ Keyword      │ Size  │ Base type │
├─────────────────┼──────────────┼───────┼───────────┤
│                 │              │       │           │
│DCCP packet type │ dccp_pkttype │ 4 bit │ integer   │
└─────────────────┴──────────────┴───────┴───────────┘

The DCCP packet type abstracts the different legal values of the
respective four bit field in the DCCP header, as stated by RFC4340.
Note that possible values 10-15 are considered reserved and therefore
not allowed to be used. In iptables' dccp match, these values are
aliased INVALID. With nftables, one may simply match on the numeric
value range, i.e. 10-15.

Table 28. keywords may be used when specifying the DCCP packet type
┌─────────┬───────┐
│Keyword  │ Value │
├─────────┼───────┤
│         │       │
│request  │ 0     │
├─────────┼───────┤
│         │       │
│response │ 1     │
├─────────┼───────┤
│         │       │
│data     │ 2     │
├─────────┼───────┤
│         │       │
│ack      │ 3     │
├─────────┼───────┤
│         │       │
│dataack  │ 4     │
├─────────┼───────┤
│         │       │
│closereq │ 5     │
├─────────┼───────┤
│         │       │
│close    │ 6     │
├─────────┼───────┤
│         │       │
│reset    │ 7     │
├─────────┼───────┤
│         │       │
│sync     │ 8     │
├─────────┼───────┤
│         │       │
│syncack  │ 9     │
└─────────┴───────┘

PRIMARY EXPRESSIONS
The lowest order expression is a primary expression, representing
either a constant or a single datum from a packet’s payload, meta data
or a stateful module.

META EXPRESSIONS
meta {length | nfproto | l4proto | protocol | priority}
[meta] {mark | iif | iifname | iiftype | oif | oifname | oiftype | skuid | skgid | nftrace | rtclassid | ibrname | obrname | pkttype | cpu | iifgroup | oifgroup | cgroup | random | ipsec | iifkind | oifkind | time | hour | day }

A meta expression refers to meta data associated with a packet.

There are two types of meta expressions: unqualified and qualified meta
expressions. Qualified meta expressions require the meta keyword before
the meta key, unqualified meta expressions can be specified by using
the meta key directly or as qualified meta expressions. Meta l4proto is
useful to match a particular transport protocol that is part of either
an IPv4 or IPv6 packet. It will also skip any IPv6 extension headers
present in an IPv6 packet.

meta iif, oif, iifname and oifname are used to match the interface a
packet arrived on or is about to be sent out on.

iif and oif are used to match on the interface index, whereas iifname
and oifname are used to match on the interface name. This is not the
same — assuming the rule

filter input meta iif "foo"

Then this rule can only be added if the interface "foo" exists. Also,
the rule will continue to match even if the interface "foo" is renamed
to "bar".

This is because internally the interface index is used. In case of
dynamically created interfaces, such as tun/tap or dialup interfaces
(ppp for example), it might be better to use iifname or oifname
instead.

In these cases, the name is used so the interface doesn’t have to exist
to add such a rule, it will stop matching if the interface gets renamed
and it will match again in case interface gets deleted and later a new
interface with the same name is created.

Like with iptables, wildcard matching on interface name prefixes is
available for iifname and oifname matches by appending an asterisk (*)
character. Note however that unlike iptables, nftables does not accept
interface names consisting of the wildcard character only - users are
supposed to just skip those always matching expressions. In order to
match on literal asterisk character, one may escape it using backslash
(\).

Table 29. Meta expression types
┌──────────┬─────────────────────┬─────────────────────┐
│Keyword   │ Description         │ Type                │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│length    │ Length of the       │ integer (32-bit)    │
│          │ packet in bytes     │                     │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│nfproto   │ real hook protocol  │ integer (32 bit)    │
│          │ family, useful only │                     │
│          │ in inet table       │                     │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│l4proto   │ layer 4 protocol,   │ integer (8 bit)     │
│          │ skips ipv6          │                     │
│          │ extension headers   │                     │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│protocol  │ EtherType protocol  │ ether_type          │
│          │ value               │                     │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│priority  │ TC packet priority  │ tc_handle           │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│mark      │ Packet mark         │ mark                │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│iif       │ Input interface     │ iface_index         │
│          │ index               │                     │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│iifname   │ Input interface     │ ifname              │
│          │ name                │                     │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│iiftype   │ Input interface     │ iface_type          │
│          │ type                │                     │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│oif       │ Output interface    │ iface_index         │
│          │ index               │                     │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│oifname   │ Output interface    │ ifname              │
│          │ name                │                     │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│oiftype   │ Output interface    │ iface_type          │
│          │ hardware type       │                     │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│sdif      │ Slave device input  │ iface_index         │
│          │ interface index     │                     │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│sdifname  │ Slave device        │ ifname              │
│          │ interface name      │                     │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│skuid     │ UID associated with │ uid                 │
│          │ originating socket  │                     │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│skgid     │ GID associated with │ gid                 │
│          │ originating socket  │                     │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│rtclassid │ Routing realm       │ realm               │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│ibrname   │ Input bridge        │ ifname              │
│          │ interface name      │                     │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│obrname   │ Output bridge       │ ifname              │
│          │ interface name      │                     │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│pkttype   │ packet type         │ pkt_type            │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│cpu       │ cpu number          │ integer (32 bit)    │
│          │ processing the      │                     │
│          │ packet              │                     │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│iifgroup  │ incoming device     │ devgroup            │
│          │ group               │                     │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│oifgroup  │ outgoing device     │ devgroup            │
│          │ group               │                     │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│cgroup    │ control group id    │ integer (32 bit)    │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│random    │ pseudo-random       │ integer (32 bit)    │
│          │ number              │                     │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│ipsec     │ true if packet was  │ boolean (1 bit)     │
│          │ ipsec encrypted     │                     │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│iifkind   │ Input interface     │                     │
│          │ kind                │                     │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│oifkind   │ Output interface    │                     │
│          │ kind                │                     │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│time      │ Absolute time of    │ Integer (32 bit) or │
│          │ packet reception    │ string              │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│day       │ Day of week         │ Integer (8 bit) or  │
│          │                     │ string              │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│hour      │ Hour of day         │ String              │
└──────────┴─────────────────────┴─────────────────────┘

Table 30. Meta expression specific types
┌──────────────┬────────────────────────────┐
│Type          │ Description                │
├──────────────┼────────────────────────────┤
│              │                            │
│iface_index   │ Interface index (32 bit    │
│              │ number). Can be specified  │
│              │ numerically or as name of  │
│              │ an existing interface.     │
├──────────────┼────────────────────────────┤
│              │                            │
│ifname        │ Interface name (16 byte    │
│              │ string). Does not have to  │
│              │ exist.                     │
├──────────────┼────────────────────────────┤
│              │                            │
│iface_type    │ Interface type (16 bit     │
│              │ number).                   │
├──────────────┼────────────────────────────┤
│              │                            │
│uid           │ User ID (32 bit number).   │
│              │ Can be specified           │
│              │ numerically or as user     │
│              │ name.                      │
├──────────────┼────────────────────────────┤
│              │                            │
│gid           │ Group ID (32 bit number).  │
│              │ Can be specified           │
│              │ numerically or as group    │
│              │ name.                      │
├──────────────┼────────────────────────────┤
│              │                            │
│realm         │ Routing Realm (32 bit      │
│              │ number). Can be specified  │
│              │ numerically or as symbolic │
│              │ name defined in            │
│              │ /etc/iproute2/rt_realms.   │
├──────────────┼────────────────────────────┤
│              │                            │
│devgroup_type │ Device group (32 bit       │
│              │ number). Can be specified  │
│              │ numerically or as symbolic │
│              │ name defined in            │
│              │ /etc/iproute2/group.       │
├──────────────┼────────────────────────────┤
│              │                            │
│pkt_type      │ Packet type: host          │
│              │ (addressed to local host), │
│              │ broadcast (to all),        │
│              │ multicast (to group),      │
│              │ other (addressed to        │
│              │ another host).             │
├──────────────┼────────────────────────────┤
│              │                            │
│ifkind        │ Interface kind (16 byte    │
│              │ string). See TYPES in      │
│              │ ip-link(8) for a list.     │
├──────────────┼────────────────────────────┤
│              │                            │
│time          │ Either an integer or a     │
│              │ date in ISO format. For    │
│              │ example: "2019-06-06       │
│              │ 17:00". Hour and seconds   │
│              │ are optional and can be    │
│              │ omitted if desired. If     │
│              │ omitted, midnight will be  │
│              │ assumed. The following     │
│              │ three would be equivalent: │
│              │ "2019-06-06", "2019-06-06  │
│              │ 00:00" and "2019-06-06     │
│              │ 00:00:00". When an integer │
│              │ is given, it is assumed to │
│              │ be a UNIX timestamp.       │
├──────────────┼────────────────────────────┤
│              │                            │
│day           │ Either a day of week       │
│              │ ("Monday", "Tuesday",      │
│              │ etc.), or an integer       │
│              │ between 0 and 6. Strings   │
│              │ are matched                │
│              │ case-insensitively, and a  │
│              │ full match is not expected │
│              │ (e.g. "Mon" would match    │
│              │ "Monday"). When an integer │
│              │ is given, 0 is Sunday and  │
│              │ 6 is Saturday.             │
├──────────────┼────────────────────────────┤
│              │                            │
│hour          │ A string representing an   │
│              │ hour in 24-hour format.    │
│              │ Seconds can optionally be  │
│              │ specified. For example,    │
│              │ 17:00 and 17:00:00 would   │
│              │ be equivalent.             │
└──────────────┴────────────────────────────┘

Using meta expressions.

# qualified meta expression
filter output meta oif eth0
filter forward meta iifkind { "tun", "veth" }

# unqualified meta expression
filter output oif eth0

# incoming packet was subject to ipsec processing
raw prerouting meta ipsec exists accept

SOCKET EXPRESSION
socket {transparent | mark | wildcard}

Socket expression can be used to search for an existing open TCP/UDP
socket and its attributes that can be associated with a packet. It
looks for an established or non-zero bound listening socket (possibly
with a non-local address).

Table 31. Available socket attributes
┌────────────┬─────────────────────┬─────────────────┐
│Name        │ Description         │ Type            │
├────────────┼─────────────────────┼─────────────────┤
│            │                     │                 │
│transparent │ Value of the        │ boolean (1 bit) │
│            │ IP_TRANSPARENT      │                 │
│            │ socket option in    │                 │
│            │ the found socket.   │                 │
│            │ It can be 0 or 1.   │                 │
├────────────┼─────────────────────┼─────────────────┤
│            │                     │                 │
│mark        │ Value of the socket │ mark            │
│            │ mark (SOL_SOCKET,   │                 │
│            │ SO_MARK).           │                 │
├────────────┼─────────────────────┼─────────────────┤
│            │                     │                 │
│wildcard    │ Indicates whether   │ boolean (1 bit) │
│            │ the socket is       │                 │
│            │ wildcard-bound      │                 │
│            │ (e.g. 0.0.0.0 or    │                 │
│            │ ::0).               │                 │
└────────────┴─────────────────────┴─────────────────┘

Using socket expression.

# Mark packets that correspond to a transparent socket. "socket wildcard 0"
# means that zero-bound listener sockets are NOT matched (which is usually
# exactly what you want).
table inet x {
chain y {
type filter hook prerouting priority -150; policy accept;
socket transparent 1 socket wildcard 0 mark set 0x00000001 accept
}
}

# Trace packets that corresponds to a socket with a mark value of 15
table inet x {
chain y {
type filter hook prerouting priority -150; policy accept;
socket mark 0x0000000f nftrace set 1
}
}

# Set packet mark to socket mark
table inet x {
chain y {
type filter hook prerouting priority -150; policy accept;
tcp dport 8080 mark set socket mark
}
}

OSF EXPRESSION
osf [ttl {loose | skip}] {name | version}

The osf expression does passive operating system fingerprinting. This
expression compares some data (Window Size, MSS, options and their
order, DF, and others) from packets with the SYN bit set.

Table 32. Available osf attributes
┌────────┬─────────────────────┬────────┐
│Name    │ Description         │ Type   │
├────────┼─────────────────────┼────────┤
│        │                     │        │
│ttl     │ Do TTL checks on    │ string │
│        │ the packet to       │        │
│        │ determine the       │        │
│        │ operating system.   │        │
├────────┼─────────────────────┼────────┤
│        │                     │        │
│version │ Do OS version       │        │
│        │ checks on the       │        │
│        │ packet.             │        │
├────────┼─────────────────────┼────────┤
│        │                     │        │
│name    │ Name of the OS      │ string │
│        │ signature to match. │        │
│        │ All signatures can  │        │
│        │ be found at pf.os   │        │
│        │ file. Use "unknown" │        │
│        │ for OS signatures   │        │
│        │ that the expression │        │
│        │ could not detect.   │        │
└────────┴─────────────────────┴────────┘

Available ttl values.

If no TTL attribute is passed, make a true IP header and fingerprint TTL true comparison. This generally works for LANs.

* loose: Check if the IP header's TTL is less than the fingerprint one. Works for globally-routable addresses.
* skip: Do not compare the TTL at all.

Using osf expression.

# Accept packets that match the "Linux" OS genre signature without comparing TTL.
table inet x {
chain y {
type filter hook input priority 0; policy accept;
osf ttl skip name "Linux"
}
}

FIB EXPRESSIONS
fib {saddr | daddr | mark | iif | oif} [. ...] {oif | oifname | type}

A fib expression queries the fib (forwarding information base) to
obtain information such as the output interface index a particular
address would use. The input is a tuple of elements that is used as
input to the fib lookup functions.

Table 33. fib expression specific types
┌────────┬──────────────────┬──────────────────┐
│Keyword │ Description      │ Type             │
├────────┼──────────────────┼──────────────────┤
│        │                  │                  │
│oif     │ Output interface │ integer (32 bit) │
│        │ index            │                  │
├────────┼──────────────────┼──────────────────┤
│        │                  │                  │
│oifname │ Output interface │ string           │
│        │ name             │                  │
├────────┼──────────────────┼──────────────────┤
│        │                  │                  │
│type    │ Address type     │ fib_addrtype     │
└────────┴──────────────────┴──────────────────┘

Use nft describe fib_addrtype to get a list of all address types.

Using fib expressions.

# drop packets without a reverse path
filter prerouting fib saddr . iif oif missing drop

In this example, 'saddr . iif' looks up routing information based on the source address and the input interface.
oif picks the output interface index from the routing information.
If no route was found for the source address/input interface combination, the output interface index is zero.
In case the input interface is specified as part of the input key, the output interface index is always the same as the input interface index or zero.
If only 'saddr oif' is given, then oif can be any interface index or zero.

# drop packets to address not configured on incoming interface
filter prerouting fib daddr . iif type != { local, broadcast, multicast } drop

# perform lookup in a specific 'blackhole' table (0xdead, needs ip appropriate ip rule)
filter prerouting meta mark set 0xdead fib daddr . mark type vmap { blackhole : drop, prohibit : jump prohibited, unreachable : drop }

ROUTING EXPRESSIONS
rt [ip | ip6] {classid | nexthop | mtu | ipsec}

A routing expression refers to routing data associated with a packet.

Table 34. Routing expression types
┌────────┬─────────────────────┬─────────────────────┐
│Keyword │ Description         │ Type                │
├────────┼─────────────────────┼─────────────────────┤
│        │                     │                     │
│classid │ Routing realm       │ realm               │
├────────┼─────────────────────┼─────────────────────┤
│        │                     │                     │
│nexthop │ Routing nexthop     │ ipv4_addr/ipv6_addr │
├────────┼─────────────────────┼─────────────────────┤
│        │                     │                     │
│mtu     │ TCP maximum segment │ integer (16 bit)    │
│        │ size of route       │                     │
├────────┼─────────────────────┼─────────────────────┤
│        │                     │                     │
│ipsec   │ route via ipsec     │ boolean             │
│        │ tunnel or transport │                     │
└────────┴─────────────────────┴─────────────────────┘

Table 35. Routing expression specific types
┌──────┬────────────────────────────┐
│Type  │ Description                │
├──────┼────────────────────────────┤
│      │                            │
│realm │ Routing Realm (32 bit      │
│      │ number). Can be specified  │
│      │ numerically or as symbolic │
│      │ name defined in            │
│      │ /etc/iproute2/rt_realms.   │
└──────┴────────────────────────────┘

Using routing expressions.

# IP family independent rt expression
filter output rt classid 10

# IP family dependent rt expressions
ip filter output rt nexthop 192.168.0.1
ip6 filter output rt nexthop fd00::1
inet filter output rt ip nexthop 192.168.0.1
inet filter output rt ip6 nexthop fd00::1

# outgoing packet will be encapsulated/encrypted by ipsec
filter output rt ipsec exists

IPSEC EXPRESSIONS
ipsec {in | out} [ spnum NUM ]  {reqid | spi}
ipsec {in | out} [ spnum NUM ]  {ip | ip6} {saddr | daddr}

An ipsec expression refers to ipsec data associated with a packet.

The in or out keyword needs to be used to specify if the expression
should examine inbound or outbound policies. The in keyword can be used
in the prerouting, input and forward hooks. The out keyword applies to
forward, output and postrouting hooks. The optional keyword spnum can
be used to match a specific state in a chain, it defaults to 0.

Table 36. Ipsec expression types
┌────────┬─────────────────────┬─────────────────────┐
│Keyword │ Description         │ Type                │
├────────┼─────────────────────┼─────────────────────┤
│        │                     │                     │
│reqid   │ Request ID          │ integer (32 bit)    │
├────────┼─────────────────────┼─────────────────────┤
│        │                     │                     │
│spi     │ Security Parameter  │ integer (32 bit)    │
│        │ Index               │                     │
├────────┼─────────────────────┼─────────────────────┤
│        │                     │                     │
│saddr   │ Source address of   │ ipv4_addr/ipv6_addr │
│        │ the tunnel          │                     │
├────────┼─────────────────────┼─────────────────────┤
│        │                     │                     │
│daddr   │ Destination address │ ipv4_addr/ipv6_addr │
│        │ of the tunnel       │                     │
└────────┴─────────────────────┴─────────────────────┘

NUMGEN EXPRESSION
numgen {inc | random} mod NUM [ offset NUM ]

Create a number generator. The inc or random keywords control its
operation mode: In inc mode, the last returned value is simply
incremented. In random mode, a new random number is returned. The value
after mod keyword specifies an upper boundary (read: modulus) which is
not reached by returned numbers. The optional offset allows to
increment the returned value by a fixed offset.

A typical use-case for numgen is load-balancing:

Using numgen expression.

# round-robin between 192.168.10.100 and 192.168.20.200:
add rule nat prerouting dnat to numgen inc mod 2 map \
{ 0 : 192.168.10.100, 1 : 192.168.20.200 }

# probability-based with odd bias using intervals:
add rule nat prerouting dnat to numgen random mod 10 map \
{ 0-2 : 192.168.10.100, 3-9 : 192.168.20.200 }

HASH EXPRESSIONS
jhash {ip saddr | ip6 daddr | tcp dport | udp sport | ether saddr} [. ...] mod NUM [ seed NUM ] [ offset NUM ]
symhash mod NUM [ offset NUM ]

Use a hashing function to generate a number. The functions available
are jhash, known as Jenkins Hash, and symhash, for Symmetric Hash. The
jhash requires an expression to determine the parameters of the packet
header to apply the hashing, concatenations are possible as well. The
value after mod keyword specifies an upper boundary (read: modulus)
which is not reached by returned numbers. The optional seed is used to
specify an init value used as seed in the hashing function. The
optional offset allows to increment the returned value by a fixed
offset.

A typical use-case for jhash and symhash is load-balancing:

Using hash expressions.

# load balance based on source ip between 2 ip addresses:
add rule nat prerouting dnat to jhash ip saddr mod 2 map \
{ 0 : 192.168.10.100, 1 : 192.168.20.200 }

# symmetric load balancing between 2 ip addresses:
add rule nat prerouting dnat to symhash mod 2 map \
{ 0 : 192.168.10.100, 1 : 192.168.20.200 }

PAYLOAD EXPRESSIONS
Payload expressions refer to data from the packet’s payload.

ETHERNET HEADER EXPRESSION
ether {daddr | saddr | type}

Table 37. Ethernet header expression types
┌────────┬────────────────────┬────────────┐
│Keyword │ Description        │ Type       │
├────────┼────────────────────┼────────────┤
│        │                    │            │
│daddr   │ Destination MAC    │ ether_addr │
│        │ address            │            │
├────────┼────────────────────┼────────────┤
│        │                    │            │
│saddr   │ Source MAC address │ ether_addr │
├────────┼────────────────────┼────────────┤
│        │                    │            │
│type    │ EtherType          │ ether_type │
└────────┴────────────────────┴────────────┘

VLAN HEADER EXPRESSION
vlan {id | cfi | pcp | type}

Table 38. VLAN header expression
┌────────┬─────────────────────┬──────────────────┐
│Keyword │ Description         │ Type             │
├────────┼─────────────────────┼──────────────────┤
│        │                     │                  │
│id      │ VLAN ID (VID)       │ integer (12 bit) │
├────────┼─────────────────────┼──────────────────┤
│        │                     │                  │
│cfi     │ Canonical Format    │ integer (1 bit)  │
│        │ Indicator           │                  │
├────────┼─────────────────────┼──────────────────┤
│        │                     │                  │
│pcp     │ Priority code point │ integer (3 bit)  │
├────────┼─────────────────────┼──────────────────┤
│        │                     │                  │
│type    │ EtherType           │ ether_type       │
└────────┴─────────────────────┴──────────────────┘

ARP HEADER EXPRESSION
arp {htype | ptype | hlen | plen | operation | saddr { ip | ether } | daddr { ip | ether }

Table 39. ARP header expression
┌────────────┬─────────────────────┬──────────────────┐
│Keyword     │ Description         │ Type             │
├────────────┼─────────────────────┼──────────────────┤
│            │                     │                  │
│htype       │ ARP hardware type   │ integer (16 bit) │
├────────────┼─────────────────────┼──────────────────┤
│            │                     │                  │
│ptype       │ EtherType           │ ether_type       │
├────────────┼─────────────────────┼──────────────────┤
│            │                     │                  │
│hlen        │ Hardware address    │ integer (8 bit)  │
│            │ len                 │                  │
├────────────┼─────────────────────┼──────────────────┤
│            │                     │                  │
│plen        │ Protocol address    │ integer (8 bit)  │
│            │ len                 │                  │
├────────────┼─────────────────────┼──────────────────┤
│            │                     │                  │
│operation   │ Operation           │ arp_op           │
├────────────┼─────────────────────┼──────────────────┤
│            │                     │                  │
│saddr ether │ Ethernet sender     │ ether_addr       │
│            │ address             │                  │
├────────────┼─────────────────────┼──────────────────┤
│            │                     │                  │
│daddr ether │ Ethernet target     │ ether_addr       │
│            │ address             │                  │
├────────────┼─────────────────────┼──────────────────┤
│            │                     │                  │
│saddr ip    │ IPv4 sender address │ ipv4_addr        │
├────────────┼─────────────────────┼──────────────────┤
│            │                     │                  │
│daddr ip    │ IPv4 target address │ ipv4_addr        │
└────────────┴─────────────────────┴──────────────────┘

IPV4 HEADER EXPRESSION
ip {version | hdrlength | dscp | ecn | length | id | frag-off | ttl | protocol | checksum | saddr | daddr }

Table 40. IPv4 header expression
┌──────────┬─────────────────────┬──────────────────┐
│Keyword   │ Description         │ Type             │
├──────────┼─────────────────────┼──────────────────┤
│          │                     │                  │
│version   │ IP header version   │ integer (4 bit)  │
│          │ (4)                 │                  │
├──────────┼─────────────────────┼──────────────────┤
│          │                     │                  │
│hdrlength │ IP header length    │ integer (4 bit)  │
│          │ including options   │ FIXME scaling    │
├──────────┼─────────────────────┼──────────────────┤
│          │                     │                  │
│dscp      │ Differentiated      │ dscp             │
│          │ Services Code Point │                  │
├──────────┼─────────────────────┼──────────────────┤
│          │                     │                  │
│ecn       │ Explicit Congestion │ ecn              │
│          │ Notification        │                  │
├──────────┼─────────────────────┼──────────────────┤
│          │                     │                  │
│length    │ Total packet length │ integer (16 bit) │
├──────────┼─────────────────────┼──────────────────┤
│          │                     │                  │
│id        │ IP ID               │ integer (16 bit) │
├──────────┼─────────────────────┼──────────────────┤
│          │                     │                  │
│frag-off  │ Fragment offset     │ integer (16 bit) │
├──────────┼─────────────────────┼──────────────────┤
│          │                     │                  │
│ttl       │ Time to live        │ integer (8 bit)  │
├──────────┼─────────────────────┼──────────────────┤
│          │                     │                  │
│protocol  │ Upper layer         │ inet_proto       │
│          │ protocol            │                  │
├──────────┼─────────────────────┼──────────────────┤
│          │                     │                  │
│checksum  │ IP header checksum  │ integer (16 bit) │
├──────────┼─────────────────────┼──────────────────┤
│          │                     │                  │
│saddr     │ Source address      │ ipv4_addr        │
├──────────┼─────────────────────┼──────────────────┤
│          │                     │                  │
│daddr     │ Destination address │ ipv4_addr        │
└──────────┴─────────────────────┴──────────────────┘

ICMP HEADER EXPRESSION
icmp {type | code | checksum | id | sequence | gateway | mtu}

This expression refers to ICMP header fields. When using it in inet,
bridge or netdev families, it will cause an implicit dependency on IPv4
to be created. To match on unusual cases like ICMP over IPv6, one has
to add an explicit meta protocol ip6 match to the rule.

Table 41. ICMP header expression
┌─────────┬─────────────────────┬──────────────────┐
│Keyword  │ Description         │ Type             │
├─────────┼─────────────────────┼──────────────────┤
│         │                     │                  │
│type     │ ICMP type field     │ icmp_type        │
├─────────┼─────────────────────┼──────────────────┤
│         │                     │                  │
│code     │ ICMP code field     │ integer (8 bit)  │
├─────────┼─────────────────────┼──────────────────┤
│         │                     │                  │
│checksum │ ICMP checksum field │ integer (16 bit) │
├─────────┼─────────────────────┼──────────────────┤
│         │                     │                  │
│id       │ ID of echo          │ integer (16 bit) │
│         │ request/response    │                  │
├─────────┼─────────────────────┼──────────────────┤
│         │                     │                  │
│sequence │ sequence number of  │ integer (16 bit) │
│         │ echo                │                  │
│         │ request/response    │                  │
├─────────┼─────────────────────┼──────────────────┤
│         │                     │                  │
│gateway  │ gateway of          │ integer (32 bit) │
│         │ redirects           │                  │
├─────────┼─────────────────────┼──────────────────┤
│         │                     │                  │
│mtu      │ MTU of path MTU     │ integer (16 bit) │
│         │ discovery           │                  │
└─────────┴─────────────────────┴──────────────────┘

IGMP HEADER EXPRESSION
igmp {type | mrt | checksum | group}

This expression refers to IGMP header fields. When using it in inet,
bridge or netdev families, it will cause an implicit dependency on IPv4
to be created. To match on unusual cases like IGMP over IPv6, one has
to add an explicit meta protocol ip6 match to the rule.

Table 42. IGMP header expression
┌─────────┬─────────────────────┬──────────────────┐
│Keyword  │ Description         │ Type             │
├─────────┼─────────────────────┼──────────────────┤
│         │                     │                  │
│type     │ IGMP type field     │ igmp_type        │
├─────────┼─────────────────────┼──────────────────┤
│         │                     │                  │
│mrt      │ IGMP maximum        │ integer (8 bit)  │
│         │ response time field │                  │
├─────────┼─────────────────────┼──────────────────┤
│         │                     │                  │
│checksum │ IGMP checksum field │ integer (16 bit) │
├─────────┼─────────────────────┼──────────────────┤
│         │                     │                  │
│group    │ Group address       │ integer (32 bit) │
└─────────┴─────────────────────┴──────────────────┘

IPV6 HEADER EXPRESSION
ip6 {version | dscp | ecn | flowlabel | length | nexthdr | hoplimit | saddr | daddr}

This expression refers to the ipv6 header fields. Caution when using
ip6 nexthdr, the value only refers to the next header, i.e. ip6 nexthdr
tcp will only match if the ipv6 packet does not contain any extension
headers. Packets that are fragmented or e.g. contain a routing
extension headers will not be matched. Please use meta l4proto if you
wish to match the real transport header and ignore any additional
extension headers instead.

Table 43. IPv6 header expression
┌──────────┬─────────────────────┬──────────────────┐
│Keyword   │ Description         │ Type             │
├──────────┼─────────────────────┼──────────────────┤
│          │                     │                  │
│version   │ IP header version   │ integer (4 bit)  │
│          │ (6)                 │                  │
├──────────┼─────────────────────┼──────────────────┤
│          │                     │                  │
│dscp      │ Differentiated      │ dscp             │
│          │ Services Code Point │                  │
├──────────┼─────────────────────┼──────────────────┤
│          │                     │                  │
│ecn       │ Explicit Congestion │ ecn              │
│          │ Notification        │                  │
├──────────┼─────────────────────┼──────────────────┤
│          │                     │                  │
│flowlabel │ Flow label          │ integer (20 bit) │
├──────────┼─────────────────────┼──────────────────┤
│          │                     │                  │
│length    │ Payload length      │ integer (16 bit) │
├──────────┼─────────────────────┼──────────────────┤
│          │                     │                  │
│nexthdr   │ Nexthdr protocol    │ inet_proto       │
├──────────┼─────────────────────┼──────────────────┤
│          │                     │                  │
│hoplimit  │ Hop limit           │ integer (8 bit)  │
├──────────┼─────────────────────┼──────────────────┤
│          │                     │                  │
│saddr     │ Source address      │ ipv6_addr        │
├──────────┼─────────────────────┼──────────────────┤
│          │                     │                  │
│daddr     │ Destination address │ ipv6_addr        │
└──────────┴─────────────────────┴──────────────────┘

Using ip6 header expressions.

# matching if first extension header indicates a fragment
ip6 nexthdr ipv6-frag

ICMPV6 HEADER EXPRESSION
icmpv6 {type | code | checksum | parameter-problem | packet-too-big | id | sequence | max-delay}

This expression refers to ICMPv6 header fields. When using it in inet,
bridge or netdev families, it will cause an implicit dependency on IPv6
to be created. To match on unusual cases like ICMPv6 over IPv4, one has
to add an explicit meta protocol ip match to the rule.

Table 44. ICMPv6 header expression
┌──────────────────┬────────────────────┬──────────────────┐
│Keyword           │ Description        │ Type             │
├──────────────────┼────────────────────┼──────────────────┤
│                  │                    │                  │
│type              │ ICMPv6 type field  │ icmpv6_type      │
├──────────────────┼────────────────────┼──────────────────┤
│                  │                    │                  │
│code              │ ICMPv6 code field  │ integer (8 bit)  │
├──────────────────┼────────────────────┼──────────────────┤
│                  │                    │                  │
│checksum          │ ICMPv6 checksum    │ integer (16 bit) │
│                  │ field              │                  │
├──────────────────┼────────────────────┼──────────────────┤
│                  │                    │                  │
│parameter-problem │ pointer to problem │ integer (32 bit) │
├──────────────────┼────────────────────┼──────────────────┤
│                  │                    │                  │
│packet-too-big    │ oversized MTU      │ integer (32 bit) │
├──────────────────┼────────────────────┼──────────────────┤
│                  │                    │                  │
│id                │ ID of echo         │ integer (16 bit) │
│                  │ request/response   │                  │
├──────────────────┼────────────────────┼──────────────────┤
│                  │                    │                  │
│sequence          │ sequence number of │ integer (16 bit) │
│                  │ echo               │                  │
│                  │ request/response   │                  │
├──────────────────┼────────────────────┼──────────────────┤
│                  │                    │                  │
│max-delay         │ maximum response   │ integer (16 bit) │
│                  │ delay of MLD       │                  │
│                  │ queries            │                  │
└──────────────────┴────────────────────┴──────────────────┘

TCP HEADER EXPRESSION
tcp {sport | dport | sequence | ackseq | doff | reserved | flags | window | checksum | urgptr}

Table 45. TCP header expression
┌─────────┬──────────────────┬──────────────────┐
│Keyword  │ Description      │ Type             │
├─────────┼──────────────────┼──────────────────┤
│         │                  │                  │
│sport    │ Source port      │ inet_service     │
├─────────┼──────────────────┼──────────────────┤
│         │                  │                  │
│dport    │ Destination port │ inet_service     │
├─────────┼──────────────────┼──────────────────┤
│         │                  │                  │
│sequence │ Sequence number  │ integer (32 bit) │
├─────────┼──────────────────┼──────────────────┤
│         │                  │                  │
│ackseq   │ Acknowledgement  │ integer (32 bit) │
│         │ number           │                  │
├─────────┼──────────────────┼──────────────────┤
│         │                  │                  │
│doff     │ Data offset      │ integer (4 bit)  │
│         │                  │ FIXME scaling    │
├─────────┼──────────────────┼──────────────────┤
│         │                  │                  │
│reserved │ Reserved area    │ integer (4 bit)  │
├─────────┼──────────────────┼──────────────────┤
│         │                  │                  │
│flags    │ TCP flags        │ tcp_flag         │
├─────────┼──────────────────┼──────────────────┤
│         │                  │                  │
│window   │ Window           │ integer (16 bit) │
├─────────┼──────────────────┼──────────────────┤
│         │                  │                  │
│checksum │ Checksum         │ integer (16 bit) │
├─────────┼──────────────────┼──────────────────┤
│         │                  │                  │
│urgptr   │ Urgent pointer   │ integer (16 bit) │
└─────────┴──────────────────┴──────────────────┘

UDP HEADER EXPRESSION
udp {sport | dport | length | checksum}

Table 46. UDP header expression
┌─────────┬─────────────────────┬──────────────────┐
│Keyword  │ Description         │ Type             │
├─────────┼─────────────────────┼──────────────────┤
│         │                     │                  │
│sport    │ Source port         │ inet_service     │
├─────────┼─────────────────────┼──────────────────┤
│         │                     │                  │
│dport    │ Destination port    │ inet_service     │
├─────────┼─────────────────────┼──────────────────┤
│         │                     │                  │
│length   │ Total packet length │ integer (16 bit) │
├─────────┼─────────────────────┼──────────────────┤
│         │                     │                  │
│checksum │ Checksum            │ integer (16 bit) │
└─────────┴─────────────────────┴──────────────────┘

UDP-LITE HEADER EXPRESSION
udplite {sport | dport | checksum}

Table 47. UDP-Lite header expression
┌─────────┬──────────────────┬──────────────────┐
│Keyword  │ Description      │ Type             │
├─────────┼──────────────────┼──────────────────┤
│         │                  │                  │
│sport    │ Source port      │ inet_service     │
├─────────┼──────────────────┼──────────────────┤
│         │                  │                  │
│dport    │ Destination port │ inet_service     │
├─────────┼──────────────────┼──────────────────┤
│         │                  │                  │
│checksum │ Checksum         │ integer (16 bit) │
└─────────┴──────────────────┴──────────────────┘

SCTP HEADER EXPRESSION
sctp {sport | dport | vtag | checksum}

Table 48. SCTP header expression
┌─────────┬──────────────────┬──────────────────┐
│Keyword  │ Description      │ Type             │
├─────────┼──────────────────┼──────────────────┤
│         │                  │                  │
│sport    │ Source port      │ inet_service     │
├─────────┼──────────────────┼──────────────────┤
│         │                  │                  │
│dport    │ Destination port │ inet_service     │
├─────────┼──────────────────┼──────────────────┤
│         │                  │                  │
│vtag     │ Verification Tag │ integer (32 bit) │
├─────────┼──────────────────┼──────────────────┤
│         │                  │                  │
│checksum │ Checksum         │ integer (32 bit) │
└─────────┴──────────────────┴──────────────────┘

DCCP HEADER EXPRESSION
dccp {sport | dport | type}

Table 49. DCCP header expression
┌────────┬──────────────────┬──────────────┐
│Keyword │ Description      │ Type         │
├────────┼──────────────────┼──────────────┤
│        │                  │              │
│sport   │ Source port      │ inet_service │
├────────┼──────────────────┼──────────────┤
│        │                  │              │
│dport   │ Destination port │ inet_service │
├────────┼──────────────────┼──────────────┤
│        │                  │              │
│type    │ Packet type      │ dccp_pkttype │
└────────┴──────────────────┴──────────────┘

AUTHENTICATION HEADER EXPRESSION
ah {nexthdr | hdrlength | reserved | spi | sequence}

Table 50. AH header expression
┌──────────┬────────────────────┬──────────────────┐
│Keyword   │ Description        │ Type             │
├──────────┼────────────────────┼──────────────────┤
│          │                    │                  │
│nexthdr   │ Next header        │ inet_proto       │
│          │ protocol           │                  │
├──────────┼────────────────────┼──────────────────┤
│          │                    │                  │
│hdrlength │ AH Header length   │ integer (8 bit)  │
├──────────┼────────────────────┼──────────────────┤
│          │                    │                  │
│reserved  │ Reserved area      │ integer (16 bit) │
├──────────┼────────────────────┼──────────────────┤
│          │                    │                  │
│spi       │ Security Parameter │ integer (32 bit) │
│          │ Index              │                  │
├──────────┼────────────────────┼──────────────────┤
│          │                    │                  │
│sequence  │ Sequence number    │ integer (32 bit) │
└──────────┴────────────────────┴──────────────────┘

ENCRYPTED SECURITY PAYLOAD HEADER EXPRESSION
esp {spi | sequence}

Table 51. ESP header expression
┌─────────┬────────────────────┬──────────────────┐
│Keyword  │ Description        │ Type             │
├─────────┼────────────────────┼──────────────────┤
│         │                    │                  │
│spi      │ Security Parameter │ integer (32 bit) │
│         │ Index              │                  │
├─────────┼────────────────────┼──────────────────┤
│         │                    │                  │
│sequence │ Sequence number    │ integer (32 bit) │
└─────────┴────────────────────┴──────────────────┘

IPCOMP HEADER EXPRESSION
comp {nexthdr | flags | cpi}

Table 52. IPComp header expression
┌────────┬─────────────────┬──────────────────┐
│Keyword │ Description     │ Type             │
├────────┼─────────────────┼──────────────────┤
│        │                 │                  │
│nexthdr │ Next header     │ inet_proto       │
│        │ protocol        │                  │
├────────┼─────────────────┼──────────────────┤
│        │                 │                  │
│flags   │ Flags           │ bitmask          │
├────────┼─────────────────┼──────────────────┤
│        │                 │                  │
│cpi     │ compression     │ integer (16 bit) │
│        │ Parameter Index │                  │
└────────┴─────────────────┴──────────────────┘

RAW PAYLOAD EXPRESSION
@base,offset,length

The raw payload expression instructs to load length bits starting at
offset bits. Bit 0 refers to the very first bit — in the C programming
language, this corresponds to the topmost bit, i.e. 0x80 in case of an
octet. They are useful to match headers that do not have a
human-readable template expression yet. Note that nft will not add
dependencies for Raw payload expressions. If you e.g. want to match
protocol fields of a transport header with protocol number 5, you need
to manually exclude packets that have a different transport header, for
instance by using meta l4proto 5 before the raw expression.

Table 53. Supported payload protocol bases
┌─────┬─────────────────────────┐
│Base │ Description             │
├─────┼─────────────────────────┤
│     │                         │
│ll   │ Link layer, for example │
│     │ the Ethernet header     │
├─────┼─────────────────────────┤
│     │                         │
│nh   │ Network header, for     │
│     │ example IPv4 or IPv6    │
├─────┼─────────────────────────┤
│     │                         │
│th   │ Transport Header, for   │
│     │ example TCP             │
└─────┴─────────────────────────┘

Matching destination port of both UDP and TCP.

inet filter input meta l4proto {tcp, udp} @th,16,16 { 53, 80 }

The above can also be written as

inet filter input meta l4proto {tcp, udp} th dport { 53, 80 }

it is more convenient, but like the raw expression notation no
dependencies are created or checked. It is the users responsibility to
restrict matching to those header types that have a notion of ports.
Otherwise, rules using raw expressions will errnously match unrelated
packets, e.g. mis-interpreting ESP packets SPI field as a port.

Rewrite arp packet target hardware address if target protocol address
matches a given address.

input meta iifname enp2s0 arp ptype 0x0800 arp htype 1 arp hlen 6 arp plen 4 @nh,192,32 0xc0a88f10 @nh,144,48 set 0x112233445566 accept

EXTENSION HEADER EXPRESSIONS
Extension header expressions refer to data from variable-sized protocol
headers, such as IPv6 extension headers, TCP options and IPv4 options.

nftables currently supports matching (finding) a given ipv6 extension
header, TCP option or IPv4 option.

hbh {nexthdr | hdrlength}
frag {nexthdr | frag-off | more-fragments | id}
rt {nexthdr | hdrlength | type | seg-left}
dst {nexthdr | hdrlength}
mh {nexthdr | hdrlength | checksum | type}
srh {flags | tag | sid | seg-left}
tcp option {eol | nop | maxseg | window | sack-perm | sack | sack0 | sack1 | sack2 | sack3 | timestamp} tcp_option_field
ip option { lsrr | ra | rr | ssrr } ip_option_field

The following syntaxes are valid only in a relational expression with
boolean type on right-hand side for checking header existence only:

exthdr {hbh | frag | rt | dst | mh}
tcp option {eol | nop | maxseg | window | sack-perm | sack | sack0 | sack1 | sack2 | sack3 | timestamp}
ip option { lsrr | ra | rr | ssrr }

Table 54. IPv6 extension headers
┌────────┬────────────────────────┐
│Keyword │ Description            │
├────────┼────────────────────────┤
│        │                        │
│hbh     │ Hop by Hop             │
├────────┼────────────────────────┤
│        │                        │
│rt      │ Routing Header         │
├────────┼────────────────────────┤
│        │                        │
│frag    │ Fragmentation header   │
├────────┼────────────────────────┤
│        │                        │
│dst     │ dst options            │
├────────┼────────────────────────┤
│        │                        │
│mh      │ Mobility Header        │
├────────┼────────────────────────┤
│        │                        │
│srh     │ Segment Routing Header │
└────────┴────────────────────────┘

Table 55. TCP Options
┌──────────┬─────────────────────┬─────────────────────┐
│Keyword   │ Description         │ TCP option fields   │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│eol       │ End if option list  │ kind                │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│nop       │ 1 Byte TCP Nop      │ kind                │
│          │ padding option      │                     │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│maxseg    │ TCP Maximum Segment │ kind, length, size  │
│          │ Size                │                     │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│window    │ TCP Window Scaling  │ kind, length, count │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│sack-perm │ TCP SACK permitted  │ kind, length        │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│sack      │ TCP Selective       │ kind, length, left, │
│          │ Acknowledgement     │ right               │
│          │ (alias of block 0)  │                     │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│sack0     │ TCP Selective       │ kind, length, left, │
│          │ Acknowledgement     │ right               │
│          │ (block 0)           │                     │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│sack1     │ TCP Selective       │ kind, length, left, │
│          │ Acknowledgement     │ right               │
│          │ (block 1)           │                     │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│sack2     │ TCP Selective       │ kind, length, left, │
│          │ Acknowledgement     │ right               │
│          │ (block 2)           │                     │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│sack3     │ TCP Selective       │ kind, length, left, │
│          │ Acknowledgement     │ right               │
│          │ (block 3)           │                     │
├──────────┼─────────────────────┼─────────────────────┤
│          │                     │                     │
│timestamp │ TCP Timestamps      │ kind, length,       │
│          │                     │ tsval, tsecr        │
└──────────┴─────────────────────┴─────────────────────┘

TCP option matching also supports raw expression syntax to access
arbitrary options:

tcp option

tcp option @number,offset,length

Table 56. IP Options
┌────────┬─────────────────────┬─────────────────────┐
│Keyword │ Description         │ IP option fields    │
├────────┼─────────────────────┼─────────────────────┤
│        │                     │                     │
│lsrr    │ Loose Source Route  │ type, length, ptr,  │
│        │                     │ addr                │
├────────┼─────────────────────┼─────────────────────┤
│        │                     │                     │
│ra      │ Router Alert        │ type, length, value │
├────────┼─────────────────────┼─────────────────────┤
│        │                     │                     │
│rr      │ Record Route        │ type, length, ptr,  │
│        │                     │ addr                │
├────────┼─────────────────────┼─────────────────────┤
│        │                     │                     │
│ssrr    │ Strict Source Route │ type, length, ptr,  │
│        │                     │ addr                │
└────────┴─────────────────────┴─────────────────────┘

finding TCP options.

filter input tcp option sack-perm kind 1 counter

matching IPv6 exthdr.

ip6 filter input frag more-fragments 1 counter

finding IP option.

filter input ip option lsrr exists counter

CONNTRACK EXPRESSIONS
Conntrack expressions refer to meta data of the connection tracking
entry associated with a packet.

There are three types of conntrack expressions. Some conntrack
expressions require the flow direction before the conntrack key, others
must be used directly because they are direction agnostic. The packets,
bytes and avgpkt keywords can be used with or without a direction. If
the direction is omitted, the sum of the original and the reply
direction is returned. The same is true for the zone, if a direction is
given, the zone is only matched if the zone id is tied to the given
direction.

ct {state | direction | status | mark | expiration | helper | label}
ct [original | reply] {l3proto | protocol | bytes | packets | avgpkt | zone | id}
ct {original | reply} {proto-src | proto-dst}
ct {original | reply} {ip | ip6} {saddr | daddr}

The conntrack-specific types in this table are described in the
sub-section CONNTRACK TYPES above.

Table 57. Conntrack expressions
┌───────────┬─────────────────────┬─────────────────────┐
│Keyword    │ Description         │ Type                │
├───────────┼─────────────────────┼─────────────────────┤
│           │                     │                     │
│state      │ State of the        │ ct_state            │
│           │ connection          │                     │
├───────────┼─────────────────────┼─────────────────────┤
│           │                     │                     │
│direction  │ Direction of the    │ ct_dir              │
│           │ packet relative to  │                     │
│           │ the connection      │                     │
├───────────┼─────────────────────┼─────────────────────┤
│           │                     │                     │
│status     │ Status of the       │ ct_status           │
│           │ connection          │                     │
├───────────┼─────────────────────┼─────────────────────┤
│           │                     │                     │
│mark       │ Connection mark     │ mark                │
├───────────┼─────────────────────┼─────────────────────┤
│           │                     │                     │
│expiration │ Connection          │ time                │
│           │ expiration time     │                     │
├───────────┼─────────────────────┼─────────────────────┤
│           │                     │                     │
│helper     │ Helper associated   │ string              │
│           │ with the connection │                     │
├───────────┼─────────────────────┼─────────────────────┤
│           │                     │                     │
│label      │ Connection tracking │ ct_label            │
│           │ label bit or        │                     │
│           │ symbolic name       │                     │
│           │ defined in          │                     │
│           │ connlabel.conf in   │                     │
│           │ the nftables        │                     │
│           │ include path        │                     │
├───────────┼─────────────────────┼─────────────────────┤
│           │                     │                     │
│l3proto    │ Layer 3 protocol of │ nf_proto            │
│           │ the connection      │                     │
├───────────┼─────────────────────┼─────────────────────┤
│           │                     │                     │
│saddr      │ Source address of   │ ipv4_addr/ipv6_addr │
│           │ the connection for  │                     │
│           │ the given direction │                     │
├───────────┼─────────────────────┼─────────────────────┤
│           │                     │                     │
│daddr      │ Destination address │ ipv4_addr/ipv6_addr │
│           │ of the connection   │                     │
│           │ for the given       │                     │
│           │ direction           │                     │
├───────────┼─────────────────────┼─────────────────────┤
│           │                     │                     │
│protocol   │ Layer 4 protocol of │ inet_proto          │
│           │ the connection for  │                     │
│           │ the given direction │                     │
├───────────┼─────────────────────┼─────────────────────┤
│           │                     │                     │
│proto-src  │ Layer 4 protocol    │ integer (16 bit)    │
│           │ source for the      │                     │
│           │ given direction     │                     │
├───────────┼─────────────────────┼─────────────────────┤
│           │                     │                     │
│proto-dst  │ Layer 4 protocol    │ integer (16 bit)    │
│           │ destination for the │                     │
│           │ given direction     │                     │
├───────────┼─────────────────────┼─────────────────────┤
│           │                     │                     │
│packets    │ packet count seen   │ integer (64 bit)    │
│           │ in the given        │                     │
│           │ direction or sum of │                     │
│           │ original and reply  │                     │
├───────────┼─────────────────────┼─────────────────────┤
│           │                     │                     │
│bytes      │ byte count seen,    │ integer (64 bit)    │
│           │ see description for │                     │
│           │ packets keyword     │                     │
├───────────┼─────────────────────┼─────────────────────┤
│           │                     │                     │
│avgpkt     │ average bytes per   │ integer (64 bit)    │
│           │ packet, see         │                     │
│           │ description for     │                     │
│           │ packets keyword     │                     │
├───────────┼─────────────────────┼─────────────────────┤
│           │                     │                     │
│zone       │ conntrack zone      │ integer (16 bit)    │
├───────────┼─────────────────────┼─────────────────────┤
│           │                     │                     │
│count      │ number of current   │ integer (32 bit)    │
│           │ connections         │                     │
├───────────┼─────────────────────┼─────────────────────┤
│           │                     │                     │
│id         │ Connection id       │ ct_id               │
└───────────┴─────────────────────┴─────────────────────┘

restrict the number of parallel connections to a server.

nft add set filter ssh_flood '{ type ipv4_addr; flags dynamic; }'
nft add rule filter input tcp dport 22 add @ssh_flood '{ ip saddr ct count over 2 }' reject

STATEMENTS
Statements represent actions to be performed. They can alter control
flow (return, jump to a different chain, accept or drop the packet) or
can perform actions, such as logging, rejecting a packet, etc.

Statements exist in two kinds. Terminal statements unconditionally
terminate evaluation of the current rule, non-terminal statements
either only conditionally or never terminate evaluation of the current
rule, in other words, they are passive from the ruleset evaluation
perspective. There can be an arbitrary amount of non-terminal
statements in a rule, but only a single terminal statement as the final
statement.

VERDICT STATEMENT
The verdict statement alters control flow in the ruleset and issues
policy decisions for packets.

{accept | drop | queue | continue | return}
{jump | goto} chain

accept and drop are absolute verdicts — they terminate ruleset
evaluation immediately.

accept       Terminate ruleset
evaluation and accept the
packet. The packet can
still be dropped later by
another hook, for instance
accept in the forward hook
still allows to drop the
packet later in the
postrouting hook, or
another forward base chain
that has a higher priority
number and is evaluated
afterwards in the
processing pipeline.

drop         Terminate ruleset
evaluation and drop the
packet. The drop occurs
instantly, no further
chains or hooks are
evaluated. It is not
possible to accept the
packet in a later chain
again, as those are not
evaluated anymore for the
packet.

queue        Terminate ruleset
evaluation and queue the
packet to userspace.
Userspace must provide a
drop or accept verdict. In
case of accept, processing
resumes with the next base
chain hook, not the rule
following the queue
verdict.

continue     Continue ruleset
evaluation with the next
rule. This is the default
behaviour in case a rule
issues no verdict.

return       Return from the current
chain and continue
evaluation at the next
rule in the last chain. If
issued in a base chain, it
is equivalent to the base
chain policy.

jump chain   Continue evaluation at the
first rule in chain. The
current position in the
ruleset is pushed to a
call stack and evaluation
will continue there when
the new chain is entirely
evaluated or a return
verdict is issued. In case
an absolute verdict is
issued by a rule in the
chain, ruleset evaluation
terminates immediately and
the specific action is
taken.

goto chain   Similar to jump, but the
current position is not
pushed to the call stack,
meaning that after the new
chain evaluation will
continue at the last chain
instead of the one
containing the goto
statement.

Using verdict statements.

# process packets from eth0 and the internal network in from_lan
# chain, drop all packets from eth0 with different source addresses.

filter input iif eth0 ip saddr 192.168.0.0/24 jump from_lan
filter input iif eth0 drop

PAYLOAD STATEMENT
payload_expression set value

The payload statement alters packet content. It can be used for example
to set ip DSCP (diffserv) header field or ipv6 flow labels.

route some packets instead of bridging.

# redirect tcp:http from 192.160.0.0/16 to local machine for routing instead of bridging
# assumes 00:11:22:33:44:55 is local MAC address.
bridge input meta iif eth0 ip saddr 192.168.0.0/16 tcp dport 80 meta pkttype set unicast ether daddr set 00:11:22:33:44:55

Set IPv4 DSCP header field.

ip forward ip dscp set 42

EXTENSION HEADER STATEMENT
extension_header_expression set value

The extension header statement alters packet content in variable-sized
headers. This can currently be used to alter the TCP Maximum segment
size of packets, similar to TCPMSS.

change tcp mss.

tcp flags syn tcp option maxseg size set 1360
# set a size based on route information:
tcp flags syn tcp option maxseg size set rt mtu

LOG STATEMENT
log [prefix quoted_string] [level syslog-level] [flags log-flags]
log group nflog_group [prefix quoted_string] [queue-threshold value] [snaplen size]
log level audit

The log statement enables logging of matching packets. When this
statement is used from a rule, the Linux kernel will print some
information on all matching packets, such as header fields, via the
kernel log (where it can be read with dmesg(1) or read in the syslog).

In the second form of invocation (if nflog_group is specified), the
Linux kernel will pass the packet to nfnetlink_log which will multicast
the packet through a netlink socket to the specified multicast group.
One or more userspace processes may subscribe to the group to receive
the packets, see libnetfilter_queue documentation for details.

In the third form of invocation (if level audit is specified), the
Linux kernel writes a message into the audit buffer suitably formatted
for reading with auditd. Therefore no further formatting options (such
as prefix or flags) are allowed in this mode.

This is a non-terminating statement, so the rule evaluation continues
after the packet is logged.

Table 58. log statement options
┌────────────────┬─────────────────────┬───────────────────┐
│Keyword         │ Description         │ Type              │
├────────────────┼─────────────────────┼───────────────────┤
│                │                     │                   │
│prefix          │ Log message prefix  │ quoted string     │
├────────────────┼─────────────────────┼───────────────────┤
│                │                     │                   │
│level           │ Syslog level of     │ string: emerg,    │
│                │ logging             │ alert, crit, err, │
│                │                     │ warn [default],   │
│                │                     │ notice, info,     │
│                │                     │ debug, audit      │
├────────────────┼─────────────────────┼───────────────────┤
│                │                     │                   │
│group           │ NFLOG group to send │ unsigned integer  │
│                │ messages to         │ (16 bit)          │
├────────────────┼─────────────────────┼───────────────────┤
│                │                     │                   │
│snaplen         │ Length of packet    │ unsigned integer  │
│                │ payload to include  │ (32 bit)          │
│                │ in netlink message  │                   │
├────────────────┼─────────────────────┼───────────────────┤
│                │                     │                   │
│queue-threshold │ Number of packets   │ unsigned integer  │
│                │ to queue inside the │ (32 bit)          │
│                │ kernel before       │                   │
│                │ sending them to     │                   │
│                │ userspace           │                   │
└────────────────┴─────────────────────┴───────────────────┘

Table 59. log-flags
┌─────────────┬───────────────────────────┐
│Flag         │ Description               │
├─────────────┼───────────────────────────┤
│             │                           │
│tcp sequence │ Log TCP sequence numbers. │
├─────────────┼───────────────────────────┤
│             │                           │
│tcp options  │ Log options from the TCP  │
│             │ packet header.            │
├─────────────┼───────────────────────────┤
│             │                           │
│ip options   │ Log options from the      │
│             │ IP/IPv6 packet header.    │
├─────────────┼───────────────────────────┤
│             │                           │
│skuid        │ Log the userid of the     │
│             │ process which generated   │
│             │ the packet.               │
├─────────────┼───────────────────────────┤
│             │                           │
│ether        │ Decode MAC addresses and  │
│             │ protocol.                 │
├─────────────┼───────────────────────────┤
│             │                           │
│all          │ Enable all log flags      │
│             │ listed above.             │
└─────────────┴───────────────────────────┘

Using log statement.

# log the UID which generated the packet and ip options
ip filter output log flags skuid flags ip options

# log the tcp sequence numbers and tcp options from the TCP packet
ip filter output log flags tcp sequence,options

# enable all supported log flags
ip6 filter output log flags all

REJECT STATEMENT
reject [ with REJECT_WITH ]

REJECT_WITH := icmp type icmp_code |
icmpv6 type icmpv6_code |
icmpx type icmpx_code |
tcp reset

A reject statement is used to send back an error packet in response to
the matched packet otherwise it is equivalent to drop so it is a
terminating statement, ending rule traversal. This statement is only
valid in base chains using the input, forward or output hooks, and
user-defined chains which are only called from those chains.

Table 60. different ICMP reject variants are meant for use in different
table families
┌────────┬────────┬─────────────┐
│Variant │ Family │ Type        │
├────────┼────────┼─────────────┤
│        │        │             │
│icmp    │ ip     │ icmp_code   │
├────────┼────────┼─────────────┤
│        │        │             │
│icmpv6  │ ip6    │ icmpv6_code │
├────────┼────────┼─────────────┤
│        │        │             │
│icmpx   │ inet   │ icmpx_code  │
└────────┴────────┴─────────────┘

For a description of the different types and a list of supported
keywords refer to DATA TYPES section above. The common default reject
value is port-unreachable.

Note that in bridge family, reject statement is only allowed in base
chains which hook into input or prerouting.

COUNTER STATEMENT
A counter statement sets the hit count of packets along with the number
of bytes.

counter packets number bytes number
counter { packets number | bytes number }

CONNTRACK STATEMENT
The conntrack statement can be used to set the conntrack mark and
conntrack labels.

ct {mark | event | label | zone} set value

The ct statement sets meta data associated with a connection. The zone
id has to be assigned before a conntrack lookup takes place, i.e. this
has to be done in prerouting and possibly output (if locally generated
packets need to be placed in a distinct zone), with a hook priority of
-300.

Unlike iptables, where the helper assignment happens in the raw table,
the helper needs to be assigned after a conntrack entry has been found,
i.e. it will not work when used with hook priorities equal or before
-200.

Table 61. Conntrack statement types
┌────────┬─────────────────────┬──────────────────┐
│Keyword │ Description         │ Value            │
├────────┼─────────────────────┼──────────────────┤
│        │                     │                  │
│event   │ conntrack event     │ bitmask, integer │
│        │ bits                │ (32 bit)         │
├────────┼─────────────────────┼──────────────────┤
│        │                     │                  │
│helper  │ name of ct helper   │ quoted string    │
│        │ object to assign to │                  │
│        │ the connection      │                  │
├────────┼─────────────────────┼──────────────────┤
│        │                     │                  │
│mark    │ Connection tracking │ mark             │
│        │ mark                │                  │
├────────┼─────────────────────┼──────────────────┤
│        │                     │                  │
│label   │ Connection tracking │ label            │
│        │ label               │                  │
├────────┼─────────────────────┼──────────────────┤
│        │                     │                  │
│zone    │ conntrack zone      │ integer (16 bit) │
└────────┴─────────────────────┴──────────────────┘

save packet nfmark in conntrack.

ct mark set meta mark

set zone mapped via interface.

table inet raw {
chain prerouting {
type filter hook prerouting priority -300;
ct zone set iif map { "eth1" : 1, "veth1" : 2 }
}
chain output {
type filter hook output priority -300;
ct zone set oif map { "eth1" : 1, "veth1" : 2 }
}
}

restrict events reported by ctnetlink.

ct event set new,related,destroy

NOTRACK STATEMENT
The notrack statement allows to disable connection tracking for certain
packets.

notrack

Note that for this statement to be effective, it has to be applied to
packets before a conntrack lookup happens. Therefore, it needs to sit
in a chain with either prerouting or output hook and a hook priority of
-300 or less.

See SYNPROXY STATEMENT for an example usage.

META STATEMENT
A meta statement sets the value of a meta expression. The existing meta
fields are: priority, mark, pkttype, nftrace.

meta {mark | priority | pkttype | nftrace} set value

A meta statement sets meta data associated with a packet.

Table 62. Meta statement types
┌─────────┬─────────────────────┬───────────┐
│Keyword  │ Description         │ Value     │
├─────────┼─────────────────────┼───────────┤
│         │                     │           │
│priority │ TC packet priority  │ tc_handle │
├─────────┼─────────────────────┼───────────┤
│         │                     │           │
│mark     │ Packet mark         │ mark      │
├─────────┼─────────────────────┼───────────┤
│         │                     │           │
│pkttype  │ packet type         │ pkt_type  │
├─────────┼─────────────────────┼───────────┤
│         │                     │           │
│nftrace  │ ruleset packet      │ 0, 1      │
│         │ tracing on/off. Use │           │
│         │ monitor trace       │           │
│         │ command to watch    │           │
│         │ traces              │           │
└─────────┴─────────────────────┴───────────┘

LIMIT STATEMENT
limit rate [over] packet_number / TIME_UNIT [burst packet_number packets]
limit rate [over] byte_number BYTE_UNIT / TIME_UNIT [burst byte_number BYTE_UNIT]

TIME_UNIT := second | minute | hour | day
BYTE_UNIT := bytes | kbytes | mbytes

A limit statement matches at a limited rate using a token bucket
filter. A rule using this statement will match until this limit is
reached. It can be used in combination with the log statement to give
limited logging. The optional over keyword makes it match over the
specified rate. Default burst is 5. if you specify burst, it must be
non-zero value.

Table 63. limit statement values
┌──────────────┬───────────────────┬──────────────────┐
│Value         │ Description       │ Type             │
├──────────────┼───────────────────┼──────────────────┤
│              │                   │                  │
│packet_number │ Number of packets │ unsigned integer │
│              │                   │ (32 bit)         │
├──────────────┼───────────────────┼──────────────────┤
│              │                   │                  │
│byte_number   │ Number of bytes   │ unsigned integer │
│              │                   │ (32 bit)         │
└──────────────┴───────────────────┴──────────────────┘

NAT STATEMENTS
snat to address [:port] [PRF_FLAGS]
snat to address - address [:port - port] [PRF_FLAGS]
snat { ip | ip6 } to address - address [:port - port] [PR_FLAGS]
dnat to address [:port] [PRF_FLAGS]
dnat to address [:port - port] [PR_FLAGS]
dnat { ip | ip6 } to address [:port - port] [PR_FLAGS]
masquerade to [:port] [PRF_FLAGS]
masquerade to [:port - port] [PRF_FLAGS]
redirect to [:port] [PRF_FLAGS]
redirect to [:port - port] [PRF_FLAGS]

PRF_FLAGS := PRF_FLAG [, PRF_FLAGS]
PR_FLAGS  := PR_FLAG [, PR_FLAGS]
PRF_FLAG  := PR_FLAG | fully-random
PR_FLAG   := persistent | random

The nat statements are only valid from nat chain types.

The snat and masquerade statements specify that the source address of
the packet should be modified. While snat is only valid in the
postrouting and input chains, masquerade makes sense only in
postrouting. The dnat and redirect statements are only valid in the
prerouting and output chains, they specify that the destination address
of the packet should be modified. You can use non-base chains which are
called from base chains of nat chain type too. All future packets in
this connection will also be mangled, and rules should cease being
examined.

The masquerade statement is a special form of snat which always uses
the outgoing interface’s IP address to translate to. It is particularly
useful on gateways with dynamic (public) IP addresses.

The redirect statement is a special form of dnat which always
translates the destination address to the local host’s one. It comes in
handy if one only wants to alter the destination port of incoming
traffic on different interfaces.

When used in the inet family (available with kernel 5.2), the dnat and
snat statements require the use of the ip and ip6 keyword in case an
address is provided, see the examples below.

Before kernel 4.18 nat statements require both prerouting and
postrouting base chains to be present since otherwise packets on the
return path won’t be seen by netfilter and therefore no reverse
translation will take place.

Table 64. NAT statement values
┌───────────┬─────────────────────┬─────────────────────┐
│Expression │ Description         │ Type                │
├───────────┼─────────────────────┼─────────────────────┤
│           │                     │                     │
│address    │ Specifies that the  │ ipv4_addr,          │
│           │ source/destination  │ ipv6_addr, e.g.     │
│           │ address of the      │ abcd::1234, or you  │
│           │ packet should be    │ can use a mapping,  │
│           │ modified. You may   │ e.g. meta mark map  │
│           │ specify a mapping   │ { 10 : 192.168.1.2, │
│           │ to relate a list of │ 20 : 192.168.1.3 }  │
│           │ tuples composed of  │                     │
│           │ arbitrary           │                     │
│           │ expression key with │                     │
│           │ address value.      │                     │
├───────────┼─────────────────────┼─────────────────────┤
│           │                     │                     │
│port       │ Specifies that the  │ port number (16     │
│           │ source/destination  │ bit)                │
│           │ address of the      │                     │
│           │ packet should be    │                     │
│           │ modified.           │                     │
└───────────┴─────────────────────┴─────────────────────┘

Table 65. NAT statement flags
┌─────────────┬─────────────────────────────┐
│Flag         │ Description                 │
├─────────────┼─────────────────────────────┤
│             │                             │
│persistent   │ Gives a client the same     │
│             │ source-/destination-address │
│             │ for each connection.        │
├─────────────┼─────────────────────────────┤
│             │                             │
│random       │ In kernel 5.0 and newer     │
│             │ this is the same as         │
│             │ fully-random. In earlier    │
│             │ kernels the port mapping    │
│             │ will be randomized using a  │
│             │ seeded MD5 hash mix using   │
│             │ source and destination      │
│             │ address and destination     │
│             │ port.                       │
├─────────────┼─────────────────────────────┤
│             │                             │
│fully-random │ If used then port mapping   │
│             │ is generated based on a     │
│             │ 32-bit pseudo-random        │
│             │ algorithm.                  │
└─────────────┴─────────────────────────────┘

Using NAT statements.

# create a suitable table/chain setup for all further examples
add table nat
add chain nat prerouting { type nat hook prerouting priority 0; }
add chain nat postrouting { type nat hook postrouting priority 100; }

# translate source addresses of all packets leaving via eth0 to address 1.2.3.4
add rule nat postrouting oif eth0 snat to 1.2.3.4

# redirect all traffic entering via eth0 to destination address 192.168.1.120
add rule nat prerouting iif eth0 dnat to 192.168.1.120

# translate source addresses of all packets leaving via eth0 to whatever
# locally generated packets would use as source to reach the same destination
add rule nat postrouting oif eth0 masquerade

# redirect incoming TCP traffic for port 22 to port 2222
add rule nat prerouting tcp dport 22 redirect to :2222

# inet family:
# handle ip dnat:
add rule inet nat prerouting dnat ip to 10.0.2.99
# handle ip6 dnat:
add rule inet nat prerouting dnat ip6 to fe80::dead
# this masquerades both ipv4 and ipv6:
add rule inet nat postrouting meta oif ppp0 masquerade

TPROXY STATEMENT
Tproxy redirects the packet to a local socket without changing the
packet header in any way. If any of the arguments is missing the data
of the incoming packet is used as parameter. Tproxy matching requires
another rule that ensures the presence of transport protocol header is
specified.

tproxy to address:port
tproxy to {address | :port}

This syntax can be used in ip/ip6 tables where network layer protocol
is obvious. Either IP address or port can be specified, but at least
one of them is necessary.

tproxy {ip | ip6} to address[:port]
tproxy to :port

This syntax can be used in inet tables. The ip/ip6 parameter defines
the family the rule will match. The address parameter must be of this
family. When only port is defined, the address family should not be
specified. In this case the rule will match for both families.

Table 66. tproxy attributes
┌────────┬────────────────────────────┐
│Name    │ Description                │
├────────┼────────────────────────────┤
│        │                            │
│address │ IP address the listening   │
│        │ socket with IP_TRANSPARENT │
│        │ option is bound to.        │
├────────┼────────────────────────────┤
│        │                            │
│port    │ Port the listening socket  │
│        │ with IP_TRANSPARENT option │
│        │ is bound to.               │
└────────┴────────────────────────────┘

Example ruleset for tproxy statement.

table ip x {
chain y {
type filter hook prerouting priority -150; policy accept;
tcp dport ntp tproxy to 1.1.1.1
udp dport ssh tproxy to :2222
}
}
table ip6 x {
chain y {
type filter hook prerouting priority -150; policy accept;
tcp dport ntp tproxy to [dead::beef]
udp dport ssh tproxy to :2222
}
}
table inet x {
chain y {
type filter hook prerouting priority -150; policy accept;
tcp dport 321 tproxy to :ssh
tcp dport 99 tproxy ip to 1.1.1.1:999
udp dport 155 tproxy ip6 to [dead::beef]:smux
}
}

SYNPROXY STATEMENT
This statement will process TCP three-way-handshake parallel in
netfilter context to protect either local or backend system. This
statement requires connection tracking because sequence numbers need to
be translated.

synproxy [mss mss_value] [wscale wscale_value] [SYNPROXY_FLAGS]

Table 67. synproxy statement attributes
┌───────┬────────────────────────────┐
│Name   │ Description                │
├───────┼────────────────────────────┤
│       │                            │
│mss    │ Maximum segment size       │
│       │ announced to clients. This │
│       │ must match the backend.    │
├───────┼────────────────────────────┤
│       │                            │
│wscale │ Window scale announced to  │
│       │ clients. This must match   │
│       │ the backend.               │
└───────┴────────────────────────────┘

Table 68. synproxy statement flags
┌──────────┬────────────────────────────┐
│Flag      │ Description                │
├──────────┼────────────────────────────┤
│          │                            │
│sack-perm │ Pass client selective      │
│          │ acknowledgement option to  │
│          │ backend (will be disabled  │
│          │ if not present).           │
├──────────┼────────────────────────────┤
│          │                            │
│timestamp │ Pass client timestamp      │
│          │ option to backend (will be │
│          │ disabled if not present,   │
│          │ also needed for selective  │
│          │ acknowledgement and window │
│          │ scaling).                  │
└──────────┴────────────────────────────┘

Example ruleset for synproxy statement.

Determine tcp options used by backend, from an external system

tcpdump -pni eth0 -c 1 'tcp[tcpflags] == (tcp-syn|tcp-ack)'
port 80 &
telnet 192.0.2.42 80
18:57:24.693307 IP 192.0.2.42.80 > 192.0.2.43.48757:
Flags [S.], seq 360414582, ack 788841994, win 14480,
options [mss 1460,sackOK,
TS val 1409056151 ecr 9690221,
nop,wscale 9],
length 0

Switch tcp_loose mode off, so conntrack will mark out-of-flow packets as state INVALID.

echo 0 > /proc/sys/net/netfilter/nf_conntrack_tcp_loose

Make SYN packets untracked.

table ip x {
chain y {
type filter hook prerouting priority raw; policy accept;
tcp flags syn notrack
}
}

Catch UNTRACKED (SYN  packets) and INVALID (3WHS ACK packets) states and send
them to SYNPROXY. This rule will respond to SYN packets with SYN+ACK
syncookies, create ESTABLISHED for valid client response (3WHS ACK packets) and
drop incorrect cookies. Flags combinations not expected during  3WHS will not
match and continue (e.g. SYN+FIN, SYN+ACK). Finally, drop invalid packets, this
will be out-of-flow packets that were not matched by SYNPROXY.

table ip foo {
chain z {
type filter hook input priority filter; policy accept;
ct state { invalid, untracked } synproxy mss 1460 wscale 9 timestamp sack-perm
ct state invalid drop
}
}

The outcome ruleset of the steps above should be similar to the one below.

table ip x {
chain y {
type filter hook prerouting priority raw; policy accept;
tcp flags syn notrack
}

chain z {
type filter hook input priority filter; policy accept;
ct state { invalid, untracked } synproxy mss 1460 wscale 9 timestamp sack-perm
ct state invalid drop
}
}

FLOW STATEMENT
A flow statement allows us to select what flows you want to accelerate
forwarding through layer 3 network stack bypass. You have to specify
the flowtable name where you want to offload this flow.

flow add @flowtable

QUEUE STATEMENT
This statement passes the packet to userspace using the nfnetlink_queue
handler. The packet is put into the queue identified by its 16-bit
queue number. Userspace can inspect and modify the packet if desired.
Userspace must then drop or re-inject the packet into the kernel. See
libnetfilter_queue documentation for details.

queue [num queue_number] [bypass]
queue [num queue_number_from - queue_number_to] [QUEUE_FLAGS]

QUEUE_FLAGS := QUEUE_FLAG [, QUEUE_FLAGS]
QUEUE_FLAG  := bypass | fanout

Table 69. queue statement values
┌──────────────────┬────────────────────┬──────────────────┐
│Value             │ Description        │ Type             │
├──────────────────┼────────────────────┼──────────────────┤
│                  │                    │                  │
│queue_number      │ Sets queue number, │ unsigned integer │
│                  │ default is 0.      │ (16 bit)         │
├──────────────────┼────────────────────┼──────────────────┤
│                  │                    │                  │
│queue_number_from │ Sets initial queue │ unsigned integer │
│                  │ in the range, if   │ (16 bit)         │
│                  │ fanout is used.    │                  │
├──────────────────┼────────────────────┼──────────────────┤
│                  │                    │                  │
│queue_number_to   │ Sets closing queue │ unsigned integer │
│                  │ in the range, if   │ (16 bit)         │
│                  │ fanout is used.    │                  │
└──────────────────┴────────────────────┴──────────────────┘

Table 70. queue statement flags
┌───────┬────────────────────────────┐
│Flag   │ Description                │
├───────┼────────────────────────────┤
│       │                            │
│bypass │ Let packets go through if  │
│       │ userspace application      │
│       │ cannot back off. Before    │
│       │ using this flag, read      │
│       │ libnetfilter_queue         │
│       │ documentation for          │
│       │ performance tuning         │
│       │ recommendations.           │
├───────┼────────────────────────────┤
│       │                            │
│fanout │ Distribute packets between │
│       │ several queues.            │
└───────┴────────────────────────────┘

DUP STATEMENT
The dup statement is used to duplicate a packet and send the copy to a
different destination.

dup to device
dup to address device device

Table 71. Dup statement values
┌───────────┬─────────────────────┬─────────────────────┐
│Expression │ Description         │ Type                │
├───────────┼─────────────────────┼─────────────────────┤
│           │                     │                     │
│address    │ Specifies that the  │ ipv4_addr,          │
│           │ copy of the packet  │ ipv6_addr, e.g.     │
│           │ should be sent to a │ abcd::1234, or you  │
│           │ new gateway.        │ can use a mapping,  │
│           │                     │ e.g. ip saddr map { │
│           │                     │ 192.168.1.2 :       │
│           │                     │ 10.1.1.1 }          │
├───────────┼─────────────────────┼─────────────────────┤
│           │                     │                     │
│device     │ Specifies that the  │ string              │
│           │ copy should be      │                     │
│           │ transmitted via     │                     │
│           │ device.             │                     │
└───────────┴─────────────────────┴─────────────────────┘

Using the dup statement.

# send to machine with ip address 10.2.3.4 on eth0
ip filter forward dup to 10.2.3.4 device "eth0"

# copy raw frame to another interface
netdetv ingress dup to "eth0"
dup to "eth0"

# combine with map dst addr to gateways
dup to ip daddr map { 192.168.7.1 : "eth0", 192.168.7.2 : "eth1" }

FWD STATEMENT
The fwd statement is used to redirect a raw packet to another
interface. It is only available in the netdev family ingress hook. It
is similar to the dup statement except that no copy is made.

fwd to device

SET STATEMENT
The set statement is used to dynamically add or update elements in a
set from the packet path. The set setname must already exist in the
given table and must have been created with one or both of the dynamic
and the timeout flags. The dynamic flag is required if the set
statement expression includes a stateful object. The timeout flag is
implied if the set is created with a timeout, and is required if the
set statement updates elements, rather than adding them. Furthermore,
these sets should specify both a maximum set size (to prevent memory
exhaustion), and their elements should have a timeout (so their number
will not grow indefinitely) either from the set definition or from the
statement that adds or updates them. The set statement can be used to
e.g. create dynamic blacklists.

{add | update} @setname { expression [timeout timeout] [comment string] }

Example for simple blacklist.

# declare a set, bound to table "filter", in family "ip".
# Timeout and size are mandatory because we will add elements from packet path.
# Entries will timeout after one minute, after which they might be
# re-added if limit condition persists.
nft add set ip filter blackhole \
"{ type ipv4_addr; flags dynamic; timeout 1m; size 65536; }"

# declare a set to store the limit per saddr.
# This must be separate from blackhole since the timeout is different
nft add set ip filter flood \
"{ type ipv4_addr; flags dynamic; timeout 10s; size 128000; }"

# whitelist internal interface.
nft add rule ip filter input meta iifname "internal" accept

# drop packets coming from blacklisted ip addresses.
nft add rule ip filter input ip saddr @blackhole counter drop

# add source ip addresses to the blacklist if more than 10 tcp connection
# requests occurred per second and ip address.
nft add rule ip filter input tcp flags syn tcp dport ssh \
add @flood { ip saddr limit rate over 10/second } \
add @blackhole { ip saddr } drop

# inspect state of the sets.
nft list set ip filter flood
nft list set ip filter blackhole

# manually add two addresses to the blackhole.
nft add element filter blackhole { 10.2.3.4, 10.23.1.42 }

MAP STATEMENT
The map statement is used to lookup data based on some specific input
key.

expression map { MAP_ELEMENTS }

MAP_ELEMENTS := MAP_ELEMENT [, MAP_ELEMENTS]
MAP_ELEMENT  := key : value

The key is a value returned by expression.

Using the map statement.

# select DNAT target based on TCP dport:
# connections to port 80 are redirected to 192.168.1.100,
# connections to port 8888 are redirected to 192.168.1.101
nft add rule ip nat prerouting dnat tcp dport map { 80 : 192.168.1.100, 8888 : 192.168.1.101 }

# source address based SNAT:
# packets from net 192.168.1.0/24 will appear as originating from 10.0.0.1,
# packets from net 192.168.2.0/24 will appear as originating from 10.0.0.2
nft add rule ip nat postrouting snat to ip saddr map { 192.168.1.0/24 : 10.0.0.1, 192.168.2.0/24 : 10.0.0.2 }

VMAP STATEMENT
The verdict map (vmap) statement works analogous to the map statement,
but contains verdicts as values.

expression vmap { VMAP_ELEMENTS }

VMAP_ELEMENTS := VMAP_ELEMENT [, VMAP_ELEMENTS]
VMAP_ELEMENT  := key : verdict

Using the vmap statement.

# jump to different chains depending on layer 4 protocol type:
nft add rule ip filter input ip protocol vmap { tcp : jump tcp-chain, udp : jump udp-chain , icmp : jump icmp-chain }

ADDITIONAL COMMANDS
These are some additional commands included in nft.

MONITOR
The monitor command allows you to listen to Netlink events produced by
the nf_tables subsystem, related to creation and deletion of objects.
When they occur, nft will print to stdout the monitored events in
either JSON or native nft format.

To filter events related to a concrete object, use one of the keywords
tables, chains, sets, rules, elements, ruleset.

To filter events related to a concrete action, use keyword new or
destroy.

Hit ^C to finish the monitor operation.

Listen to all events, report in native nft format.

% nft monitor

Listen to deleted rules, report in JSON format.

% nft -j monitor destroy rules

Listen to both new and destroyed chains, in native nft format.

% nft monitor chains

Listen to ruleset events such as table, chain, rule, set, counters and
quotas, in native nft format.

% nft monitor ruleset

ERROR REPORTING
When an error is detected, nft shows the line(s) containing the error,
the position of the erroneous parts in the input stream and marks up
the erroneous parts using carets (^). If the error results from the
combination of two expressions or statements, the part imposing the
constraints which are violated is marked using tildes (~).

For errors returned by the kernel, nft cannot detect which parts of the
input caused the error and the entire command is marked.

Error caused by single incorrect expression.

<cmdline>:1:19-22: Error: Interface does not exist
filter output oif eth0
^^^^

Error caused by invalid combination of two expressions.

<cmdline>:1:28-36: Error: Right hand side of relational expression (==) must be constant
filter output tcp dport == tcp dport
~~ ^^^^^^^^^

Error returned by the kernel.

<cmdline>:0:0-23: Error: Could not process rule: Operation not permitted
filter output oif wlan0
^^^^^^^^^^^^^^^^^^^^^^^

EXIT STATUS
On success, nft exits with a status of 0. Unspecified errors cause it
to exit with a status of 1, memory allocation errors with a status of
2, unable to open Netlink socket with 3.

SEE ALSO
libnftables(3), libnftables-json(5), iptables(8), ip6tables(8), arptables(8), ebtables(8), ip(8), tc(8)

There is an official wiki at: https://wiki.nftables.org

AUTHORS
nftables was written by Patrick McHardy and Pablo Neira Ayuso, among
many other contributors from the Netfilter community.

COPYRIGHT
Copyright © 2008-2014 Patrick McHardy <kaber@trash.net> Copyright ©
2013-2018 Pablo Neira Ayuso <pablo@netfilter.org>

nftables is free software; you can redistribute it and/or modify it
under the terms of the GNU General Public License version 2 as
published by the Free Software Foundation.

                          NFTABLES firewall

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