nft

package: nftables

title: nft(8)

text:
# nft(8)

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

## SYNOPSIS

```
nft [ -nNscaeSupyjtT ] [ -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.

-D, --define name=value

Define a variable. You can only combine this option with -f.

-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.

-o, --optimize

Optimize your ruleset. You can combine this option with -c to inspect the proposed optimizations.

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 by 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 the -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
undefine variable
redefine variable = expr
$variable
```

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

Using symbolic variables.

```
define int_if1 = eth0
define int_if2 = eth1
define int_ifs = { $int_if1, $int_if2 }
redefine int_if2 = wlan0
undefine 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 on ingress and egress.

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 and egress 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.

egress

All packets leaving the system are processed by this hook. It is invoked after layer 3 protocol handlers and before tc egress. It can be used for late filtering and policing.

Tunneled packets (such as vxlan) are processed by netdev family hooks both in decapsulated and encapsulated (tunneled) form. So a packet can be filtered on the overlay network as well as on the underlying network.

Note that the order of netfilter and tc is mirrored on ingress versus egress. This ensures symmetry for NAT and other packet mangling.

Ingress packets which are redirected out some other interface are only processed by netfilter on egress if they have passed through netfilter ingress processing before. Thus, ingress packets which are redirected by tc are not subjected to netfilter. But they are if they are redirected by netfilter on ingress. Conceptually, tc and netfilter can be thought of as layers, with netfilter layered above tc: If the packet hasn’t been passed up from the tc layer to the netfilter layer, it’s not subjected to netfilter on egress.

## 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 [ {comment comment ;} { flags 'flags ; }]
{delete | destroy | list | flush} table [family] table
list tables [family]
delete table [family] handle handle
destroy 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 filter; }
# 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.

destroy

Delete the specified table, it does not fail if it does not exist.

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 ;] [comment comment ;] }]
{delete | destroy | list | flush} chain ['family] table chain
list chains [family]
delete chain [family] table handle handle
destroy 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.

destroy

Delete the specified chain, it does not fail if it does not exist. 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, input, output, postrouting

Chains of this type perform 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 one 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 two combinations, namely filter type with ingress hook and filter type with egress hook. Base chains in this family also require the device parameter to be present since they exist per 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 device parameter accepts a network interface name as a string, and is required when adding a base chain that filters traffic on the ingress or egress hooks. Any ingress or egress chains will only filter traffic from the interface specified in the device parameter.

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

With nat type chains, there’s a lower excluding limit of -200 for priority values, because conntrack hooks at this priority and NAT requires it.

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, arp, netdev

all

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 from the standard value.

Base chains also allow one 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 | reset} rule [family] table chain handle handle
destroy rule [family] table chain handle handle
reset rules [family] [table [chain]]
```

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.

destroy

Delete the specified rule, it does not fail if it does not exist.

reset

Reset rule-contained state, e.g. counter and quota statement values.

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 filter; 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 ;] [comment comment ;] [policy 'policy ;] [auto-merge ;] }
{delete | destroy | list | flush | reset } set [family] table set
list sets [family]
delete set [family] table handle handle
{add | delete | destroy } 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 properties of a set.

delete

Delete the specified set.

destroy

Delete the specified set, it does not fail if it does not exist.

list

Display the elements in the specified set.

flush

Remove all elements from the specified set.

reset

Reset state in all contained elements, e.g. counter and quota statement values.

Table 8. Set specifications

Keyword

Description

Type

type

data type of set elements

string: ipv4_addr, ipv6_addr, ether_addr, inet_proto, inet_service, mark

typeof

data type of set element

expression to derive the data type from

flags

set flags

string: constant, dynamic, interval, timeout. Used to describe the sets properties.

timeout

time an element stays in the set, mandatory if set is added to from the packet path (ruleset)

string, decimal followed by unit. Units are: d, h, m, s

gc-interval

garbage collection interval, only available when timeout or flag timeout are active

string, decimal followed by unit. Units are: d, h, m, s

elements

elements contained by the set

set data type

size

maximum number of elements in the set, mandatory if set is added to from the packet path (ruleset)

unsigned integer (64 bit)

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 ;] [comment comment ;] [policy 'policy ;] }

{delete | destroy | list | flush | reset } 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.

destroy

Delete the specified map, it does not fail if it does not exist.

list

Display the elements in the specified map.

flush

Remove all elements from the specified map.

reset

Reset state in all contained elements, e.g. counter and quota statement values.

Table 9. Map specifications

Keyword

Description

Type

type

data type of map elements

string: ipv4_addr, 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 element

expression to derive the data type from

flags

map flags

string, same as set flags

elements

elements contained by the map

map data type

size

maximum number of elements in the map

unsigned integer (64 bit)

policy

map policy

string: performance [default], memory

Users can specifiy the properties/features that the set/map must support. This allows the kernel to pick an optimal internal representation. If a required flag is missing, the ruleset might still work, as nftables will auto-enable features if it can infer this from the ruleset. This may not work for all cases, however, so it is recommended to specify all required features in the set/map definition manually.

Table 10. Set and Map flags

Flag

Description

constant

Set contents will never change after creation

dynamic

Set must support updates from the packet path with the add, update or delete keywords.

interval

Set must be able to store intervals (ranges)

timeout

Set must support element timeouts (auto-removal of elements once they expire).

## ELEMENTS

```
{add | create | delete | destroy | get | reset } 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 one 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 its 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.

reset command resets state attached to the given element(s), e.g. counter and quota statement values.

Table 11. 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 | destroy | 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.

destroy

Delete the specified flowtable, it does not fail if it does not exist.

list

List all flowtables.

## LISTING

```
list { secmarks | synproxys | flow tables | meters | hooks } [family]
list { secmarks | synproxys | flow tables | meters | hooks } table [family] table
list ct { timeout | expectation | helper | helpers } table [family] table
```

Inspect configured objects. list hooks shows the full hook pipeline, including those registered by kernel modules, such as nf_conntrack.

## STATEFUL OBJECTS

```
{add | delete | destroy | list | reset} counter [family] table object
{add | delete | destroy | list | reset} quota [family] table object
{add | delete | destroy | list} limit [family] table object
delete counter [family] table handle handle
delete quota [family] table handle handle
delete limit [family] table handle handle
destroy counter [family] table handle handle
destroy quota [family] table handle handle
destroy limit [family] table handle handle
list counters [family]
list quotas [family]
list limits [family]
reset counters [family]
reset quotas [family]
reset counters [family] table
reset quotas [family] table
```

Stateful objects are attached to tables and are identified by a 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.

destroy

Delete the specified object, it does not fail if it does not exist.

list

Display stateful information the object holds.

reset

List-and-reset stateful object.

### CT HELPER

```
add ct helper [family] table name { type type protocol protocol ; [l3proto family ;] }
delete ct helper [family] table name
list ct helpers
```

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 12. conntrack helper specifications

Keyword

Description

Type

type

name of helper type

quoted string (e.g. "ftp")

protocol

layer 4 protocol of the helper

string (e.g. ip)

l3proto

layer 3 protocol of the helper

address family (e.g. ip)

comment

per ct helper comment field

string

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 filter;

      tcp dport 21 ct helper set "ftp-standard"

  }
}
```

### CT TIMEOUT

```
add ct timeout  [family] table name { protocol protocol ; policy = { state: value [, ...] } ; [l3proto family ;] }
delete ct timeout [family] table name
list ct timeouts
```

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 13. conntrack timeout specifications

Keyword

Description

Type

protocol

layer 4 protocol of the timeout object

string (e.g. ip)

state

connection state name

string (e.g. "established")

value

timeout value for connection state

unsigned integer

l3proto

layer 3 protocol of the timeout object

address family (e.g. ip)

comment

per ct timeout comment field

string

tcp connection state names that can have a specific timeout value are:

close, close_wait, established, fin_wait, last_ack, retrans, syn_recv, syn_sent, time_wait and unack.

You can use sysctl -a |grep net.netfilter.nf_conntrack_tcp_timeout_ to view and change the system-wide defaults. ct timeout allows for flow-specific settings, without changing the global timeouts.

For example, tcp port 53 could have much lower settings than other traffic.

udp state names that can have a specific timeout value are replied and unreplied.

defining and assigning ct timeout policy.

```
table ip filter {

        ct timeout customtimeout {

                protocol tcp;

                l3proto ip

                policy = { established: 2m, close: 20s }

        }

        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

```
add ct expectation  [family] table name { protocol protocol ; dport dport ; timeout timeout ; size size ; [*l3proto family ;] }
delete ct expectation  [family] table name
list ct expectations
```

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 14. conntrack expectation specifications

Keyword

Description

Type

protocol

layer 4 protocol of the expectation object

string (e.g. ip)

dport

destination port of expected connection

unsigned integer

timeout

timeout value for expectation

unsigned integer

size

size value for expectation

unsigned integer

l3proto

layer 3 protocol of the expectation object

address family (e.g. ip)

comment

per ct expectation comment field

string

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

```
add counter [family] table name [{ [ packets packets bytes bytes ; ] [ comment comment ; }]
delete counter [family] table name
list counters
```

Table 15. Counter specifications

Keyword

Description

Type

packets

initial count of packets

unsigned integer (64 bit)

bytes

initial count of bytes

unsigned integer (64 bit)

comment

per counter comment field

string

Using named counters.

```
nft add counter filter http
nft add rule filter input tcp dport 80 counter name \"http\"
```

Using named counters with maps.

```
nft add counter filter http
nft add counter filter https
nft add rule filter input counter name tcp dport map { 80 : \"http\", 443 : \"https\" }
```

### QUOTA

```
add quota [family] table name { [over|until] bytes BYTE_UNIT [ used bytes BYTE_UNIT ] ; [ comment comment ; ] }
BYTE_UNIT := bytes | kbytes | mbytes
delete quota [family] table name
list quotas
```

Table 16. Quota specifications

Keyword

Description

Type

quota

quota limit, used as the quota name

Two arguments, unsigned integer (64 bit) and string: bytes, kbytes, mbytes. "over" and "until" go before these arguments

used

initial value of used quota

Two arguments, unsigned integer (64 bit) and string: bytes, kbytes, mbytes

comment

per quota comment field

string

Using named quotas.

```
nft add quota filter user123 { over 20 mbytes }
nft add rule filter input ip saddr 192.168.10.123 quota name \"user123\"
```

Using named quotas with maps.

```
nft add quota filter user123 { over 20 mbytes }
nft add quota filter user124 { over 20 mbytes }
nft add rule filter input quota name ip saddr map { 192.168.10.123 : \"user123\", 192.168.10.124 : \"user124\" }
```

## 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 address

lladdr

variable

integer

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 17. The following keywords will automatically resolve into a boolean type with given value

Keyword

Value

exists

1

missing

0

Table 18. 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 19. 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 20. 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 21. 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 22. 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 23. 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 24. overview of types used in ct expression and statement

Name

Keyword

Size

Base type

conntrack state

ct_state

4 byte

bitmask

conntrack direction

ct_dir

8 bit

integer

conntrack status

ct_status

4 byte

bitmask

conntrack event bits

ct_event

4 byte

bitmask

conntrack label

ct_label

128 bit

bitmask

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

Table 25. conntrack state (ct_state)

Keyword

Value

invalid

1

established

2

related

4

new

8

untracked

64

Table 26. conntrack direction (ct_dir)

Keyword

Value

original

0

reply

1

Table 27. conntrack status (ct_status)

Keyword

Value

expected

1

seen-reply

2

assured

4

confirmed

8

snat

16

dnat

32

dying

512

Table 28. 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 29. 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 30. Meta expression types

Keyword

Description

Type

length

Length of the packet in bytes

integer (32-bit)

nfproto

real hook protocol family, useful only in inet table

integer (32 bit)

l4proto

layer 4 protocol, skips ipv6 extension headers

integer (8 bit)

protocol

EtherType protocol value

ether_type

priority

TC packet priority

tc_handle

mark

Packet mark

mark

iif

Input interface index

iface_index

iifname

Input interface name

ifname

iiftype

Input interface type

iface_type

oif

Output interface index

iface_index

oifname

Output interface name

ifname

oiftype

Output interface hardware type

iface_type

sdif

Slave device input interface index

iface_index

sdifname

Slave device interface name

ifname

skuid

UID associated with originating socket

uid

skgid

GID associated with originating socket

gid

rtclassid

Routing realm

realm

ibrname

Input bridge interface name

ifname

obrname

Output bridge interface name

ifname

pkttype

packet type

pkt_type

cpu

cpu number processing the packet

integer (32 bit)

iifgroup

incoming device group

devgroup

oifgroup

outgoing device group

devgroup

cgroup

control group id

integer (32 bit)

random

pseudo-random number

integer (32 bit)

ipsec

true if packet was ipsec encrypted

boolean (1 bit)

iifkind

Input interface kind

oifkind

Output interface kind

time

Absolute time of packet reception

Integer (32 bit) or string

day

Day of week

Integer (8 bit) or string

hour

Hour of day

String

Table 31. 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 cgroupv2 level NUM
```

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). You can also use it to match on the socket cgroupv2 at a given ancestor level, e.g. if the socket belongs to cgroupv2 a/b, ancestor level 1 checks for a matching on cgroup a and ancestor level 2 checks for a matching on cgroup b.

Table 32. Available socket attributes

Name

Description

Type

transparent

Value of the IP_TRANSPARENT socket option in the found socket. It can be 0 or 1.

boolean (1 bit)

mark

Value of the socket mark (SOL_SOCKET, SO_MARK).

mark

wildcard

Indicates whether the socket is wildcard-bound (e.g. 0.0.0.0 or ::0).

boolean (1 bit)

cgroupv2

cgroup version 2 for this socket (path from /sys/fs/cgroup)

cgroupv2

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 mangle; 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 mangle; policy accept;

        socket mark 0x0000000f nftrace set 1

    }
}
# Set packet mark to socket mark
table inet x {

    chain y {

        type filter hook prerouting priority mangle; policy accept;

        tcp dport 8080 mark set socket mark

    }
}
# Count packets for cgroupv2 "user.slice" at level 1
table inet x {

    chain y {

        type filter hook input priority filter; policy accept;

        socket cgroupv2 level 1 "user.slice" counter

    }
}
```

### 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 33. Available osf attributes

Name

Description

Type

ttl

Do TTL checks on the packet to determine the operating system.

string

version

Do OS version checks on the packet.

name

Name of the OS signature to match. All signatures can be found at pf.os file. Use "unknown" for OS signatures that the expression could not detect.

string

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 filter; 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 34. fib expression specific types

Keyword

Description

Type

oif

Output interface index

integer (32 bit)

oifname

Output interface name

string

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 35. Routing expression types

Keyword

Description

Type

classid

Routing realm

realm

nexthop

Routing nexthop

ipv4_addr/ipv6_addr

mtu

TCP maximum segment size of route

integer (16 bit)

ipsec

route via ipsec tunnel or transport

boolean

Table 36. 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 37. Ipsec expression types

Keyword

Description

Type

reqid

Request ID

integer (32 bit)

spi

Security Parameter Index

integer (32 bit)

saddr

Source address of the tunnel

ipv4_addr/ipv6_addr

daddr

Destination address of the tunnel

ipv4_addr/ipv6_addr

Note: When using xfrm_interface, this expression is not useable in output hook as the plain packet does not traverse it with IPsec info attached - use a chain in postrouting hook instead.

### 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 one 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 one 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 38. Ethernet header expression types

Keyword

Description

Type

daddr

Destination MAC address

ether_addr

saddr

Source MAC address

ether_addr

type

EtherType

ether_type

### VLAN HEADER EXPRESSION

```
vlan {id | dei | pcp | type}
```

The vlan expression is used to match on the vlan header fields. This expression will not work in the ip, ip6 and inet families, unless the vlan interface is configured with the reorder_hdr off setting. The default is reorder_hdr on which will automatically remove the vlan tag from the packet. See ip-link(8) for more information. For these families its easier to match the vlan interface name instead, using the meta iif or meta iifname expression.

Table 39. VLAN header expression

Keyword

Description

Type

id

VLAN ID (VID)

integer (12 bit)

dei

Drop Eligible Indicator

integer (1 bit)

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 40. ARP header expression

Keyword

Description

Type

htype

ARP hardware type

integer (16 bit)

ptype

EtherType

ether_type

hlen

Hardware address len

integer (8 bit)

plen

Protocol address len

integer (8 bit)

operation

Operation

arp_op

saddr ether

Ethernet sender address

ether_addr

daddr ether

Ethernet target address

ether_addr

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 41. IPv4 header expression

Keyword

Description

Type

version

IP header version (4)

integer (4 bit)

hdrlength

IP header length including options

integer (4 bit) FIXME scaling

dscp

Differentiated Services Code Point

dscp

ecn

Explicit Congestion Notification

ecn

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 protocol

inet_proto

checksum

IP header checksum

integer (16 bit)

saddr

Source address

ipv4_addr

daddr

Destination address

ipv4_addr

Careful with matching on ip length: If GRO/GSO is enabled, then the Linux kernel might aggregate several packets into one big packet that is larger than MTU. Moreover, if GRO/GSO maximum size is larger than 65535 (see man ip-link(8), specifically gro_ipv6_max_size and gso_ipv6_max_size), then ip length might be 0 for such jumbo packets. meta length allows you to match on the packet length including the IP header size. If you want to perform heuristics on the ip length field, then disable GRO/GSO.

### 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 42. 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 request/response

integer (16 bit)

sequence

sequence number of echo request/response

integer (16 bit)

gateway

gateway of redirects

integer (32 bit)

mtu

MTU of path MTU discovery

integer (16 bit)

### 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 43. IGMP header expression

Keyword

Description

Type

type

IGMP type field

igmp_type

mrt

IGMP maximum response time field

integer (8 bit)

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 44. IPv6 header expression

Keyword

Description

Type

version

IP header version (6)

integer (4 bit)

dscp

Differentiated Services Code Point

dscp

ecn

Explicit Congestion Notification

ecn

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

Careful with matching on ip6 length: If GRO/GSO is enabled, then the Linux kernel might aggregate several packets into one big packet that is larger than MTU. Moreover, if GRO/GSO maximum size is larger than 65535 (see man ip-link(8), specifically gro_ipv6_max_size and gso_ipv6_max_size), then ip6 length might be 0 for such jumbo packets. meta length allows you to match on the packet length including the IP header size. If you want to perform heuristics on the ip6 length field, then disable GRO/GSO.

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 | taddr | daddr}
```

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 45. ICMPv6 header expression

Keyword

Description

Type

type

ICMPv6 type field

icmpv6_type

code

ICMPv6 code field

integer (8 bit)

checksum

ICMPv6 checksum field

integer (16 bit)

parameter-problem

pointer to problem

integer (32 bit)

packet-too-big

oversized MTU

integer (32 bit)

id

ID of echo request/response

integer (16 bit)

sequence

sequence number of echo request/response

integer (16 bit)

max-delay

maximum response delay of MLD queries

integer (16 bit)

taddr

target address of neighbor solicit/advert, redirect or MLD

ipv6_addr

daddr

destination address of redirect

ipv6_addr

### TCP HEADER EXPRESSION

```
tcp {sport | dport | sequence | ackseq | doff | reserved | flags | window | checksum | urgptr}
```

Table 46. TCP header expression

Keyword

Description

Type

sport

Source port

inet_service

dport

Destination port

inet_service

sequence

Sequence number

integer (32 bit)

ackseq

Acknowledgement number

integer (32 bit)

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 47. 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 48. 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}

sctp chunk CHUNK [ FIELD ]
CHUNK := data | init | init-ack | sack | heartbeat |

           heartbeat-ack | abort | shutdown | shutdown-ack | error |

           cookie-echo | cookie-ack | ecne | cwr | shutdown-complete

           | asconf-ack | forward-tsn | asconf
FIELD := COMMON_FIELD | DATA_FIELD | INIT_FIELD | INIT_ACK_FIELD |

           SACK_FIELD | SHUTDOWN_FIELD | ECNE_FIELD | CWR_FIELD |

           ASCONF_ACK_FIELD | FORWARD_TSN_FIELD | ASCONF_FIELD
COMMON_FIELD := type | flags | length
DATA_FIELD := tsn | stream | ssn | ppid
INIT_FIELD := init-tag | a-rwnd | num-outbound-streams |

                num-inbound-streams | initial-tsn
INIT_ACK_FIELD := INIT_FIELD
SACK_FIELD := cum-tsn-ack | a-rwnd | num-gap-ack-blocks |

                num-dup-tsns
SHUTDOWN_FIELD := cum-tsn-ack
ECNE_FIELD := lowest-tsn
CWR_FIELD := lowest-tsn
ASCONF_ACK_FIELD := seqno
FORWARD_TSN_FIELD := new-cum-tsn
ASCONF_FIELD := seqno
```

Table 49. 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)

chunk

Search chunk in packet

without FIELD, boolean indicating existence

Table 50. SCTP chunk fields

Name

Width in bits

Chunk

Notes

type

8

all

not useful, defined by chunk type

flags

8

all

semantics defined on per-chunk basis

length

16

all

length of this chunk in bytes excluding padding

tsn

32

data

transmission sequence number

stream

16

data

stream identifier

ssn

16

data

stream sequence number

ppid

32

data

payload protocol identifier

init-tag

32

init, init-ack

initiate tag

a-rwnd

32

init, init-ack, sack

advertised receiver window credit

num-outbound-streams

16

init, init-ack

number of outbound streams

num-inbound-streams

16

init, init-ack

number of inbound streams

initial-tsn

32

init, init-ack

initial transmit sequence number

cum-tsn-ack

32

sack, shutdown

cumulative transmission sequence number acknowledged

num-gap-ack-blocks

16

sack

number of Gap Ack Blocks included

num-dup-tsns

16

sack

number of duplicate transmission sequence numbers received

lowest-tsn

32

ecne, cwr

lowest transmission sequence number

seqno

32

asconf-ack, asconf

sequence number

new-cum-tsn

32

forward-tsn

new cumulative transmission sequence number

### DCCP HEADER EXPRESSION

```
dccp {sport | dport | type}
```

Table 51. 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 52. AH header expression

Keyword

Description

Type

nexthdr

Next header protocol

inet_proto

hdrlength

AH Header length

integer (8 bit)

reserved

Reserved area

integer (16 bit)

spi

Security Parameter Index

integer (32 bit)

sequence

Sequence number

integer (32 bit)

### ENCRYPTED SECURITY PAYLOAD HEADER EXPRESSION

```
esp {spi | sequence}
```

Table 53. ESP header expression

Keyword

Description

Type

spi

Security Parameter Index

integer (32 bit)

sequence

Sequence number

integer (32 bit)

### IPCOMP HEADER EXPRESSION
comp {nexthdr | flags | cpi}

Table 54. IPComp header expression

Keyword

Description

Type

nexthdr

Next header protocol

inet_proto

flags

Flags

bitmask

cpi

compression Parameter Index

integer (16 bit)

### GRE HEADER EXPRESSION

```
gre {flags | version | protocol}

gre ip {version | hdrlength | dscp | ecn | length | id | frag-off | ttl | protocol | checksum | saddr | daddr }
gre ip6 {version | dscp | ecn | flowlabel | length | nexthdr | hoplimit | saddr | daddr}
```

The gre expression is used to match on the gre header fields. This expression also allows to match on the IPv4 or IPv6 packet within the gre header.

Table 55. GRE header expression

Keyword

Description

Type

flags

checksum, routing, key, sequence and strict source route flags

integer (5 bit)

version

gre version field, 0 for GRE and 1 for PPTP

integer (3 bit)

protocol

EtherType of encapsulated packet

integer (16 bit)

Matching inner IPv4 destination address encapsulated in gre.

```
netdev filter ingress gre ip daddr 9.9.9.9 counter
```

### GENEVE HEADER EXPRESSION

```
geneve {vni | flags}
geneve ether {daddr | saddr | type}
geneve vlan {id | dei | pcp | type}
geneve ip {version | hdrlength | dscp | ecn | length | id | frag-off | ttl | protocol | checksum | saddr | daddr }
geneve ip6 {version | dscp | ecn | flowlabel | length | nexthdr | hoplimit | saddr | daddr}
geneve tcp {sport | dport | sequence | ackseq | doff | reserved | flags | window | checksum | urgptr}
geneve udp {sport | dport | length | checksum}
```

The geneve expression is used to match on the geneve header fields. The geneve header encapsulates a ethernet frame within a udp packet. This expression requires that you restrict the matching to udp packets (usually at port 6081 according to IANA-assigned ports).

Table 56. GENEVE header expression

Keyword

Description

Type

protocol

EtherType of encapsulated packet

integer (16 bit)

vni

Virtual Network ID (VNI)

integer (24 bit)

Matching inner TCP destination port encapsulated in geneve.

```
netdev filter ingress udp dport 4789 geneve tcp dport 80 counter
```

### GRETAP HEADER EXPRESSION

```
gretap {vni | flags}
gretap ether {daddr | saddr | type}
gretap vlan {id | dei | pcp | type}
gretap ip {version | hdrlength | dscp | ecn | length | id | frag-off | ttl | protocol | checksum | saddr | daddr }
gretap ip6 {version | dscp | ecn | flowlabel | length | nexthdr | hoplimit | saddr | daddr}
gretap tcp {sport | dport | sequence | ackseq | doff | reserved | flags | window | checksum | urgptr}
gretap udp {sport | dport | length | checksum}
```

The gretap expression is used to match on the encapsulated ethernet frame within the gre header. Use the gre expression to match on the gre header fields.

Matching inner TCP destination port encapsulated in gretap.

```
netdev filter ingress gretap tcp dport 80 counter
```

### VXLAN HEADER EXPRESSION

```
vxlan {vni | flags}
vxlan ether {daddr | saddr | type}
vxlan vlan {id | dei | pcp | type}
vxlan ip {version | hdrlength | dscp | ecn | length | id | frag-off | ttl | protocol | checksum | saddr | daddr }
vxlan ip6 {version | dscp | ecn | flowlabel | length | nexthdr | hoplimit | saddr | daddr}
vxlan tcp {sport | dport | sequence | ackseq | doff | reserved | flags | window | checksum | urgptr}
vxlan udp {sport | dport | length | checksum}
```

The vxlan expression is used to match on the vxlan header fields. The vxlan header encapsulates a ethernet frame within a udp packet. This expression requires that you restrict the matching to udp packets (usually at port 4789 according to IANA-assigned ports).

Table 57. VXLAN header expression

Keyword

Description

Type

flags

vxlan flags

integer (8 bit)

vni

Virtual Network ID (VNI)

integer (24 bit)

Matching inner TCP destination port encapsulated in vxlan.

```
netdev filter ingress udp dport 4789 vxlan tcp dport 80 counter
```

### ARP HEADER EXPRESSION

```
arp {htype | ptype | hlen | plen | operation | saddr { ip | ether } | daddr { ip | ether }
```

Table 58. ARP header expression

Keyword

Description

Type

htype

ARP hardware type

integer (16 bit)

ptype

EtherType

ether_type

hlen

Hardware address len

integer (8 bit)

plen

Protocol address len

integer (8 bit)

operation

Operation

arp_op

saddr ether

Ethernet sender address

ether_addr

daddr ether

Ethernet target address

ether_addr

saddr ip

IPv4 sender address

ipv4_addr

daddr ip

IPv4 target address

ipv4_addr

### 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 59. 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

ih

Inner Header / Payload, i.e. after the L4 transport level header

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 }
dccp option dccp_option_type
```

Table 60. 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 61. TCP Options

Keyword

Description

TCP option fields

eol

End if option list

-

nop

1 Byte TCP Nop padding option

-

maxseg

TCP Maximum Segment Size

length, size

window

TCP Window Scaling

length, count

sack-perm

TCP SACK permitted

length

sack

TCP Selective Acknowledgement (alias of block 0)

length, left, right

sack0

TCP Selective Acknowledgement (block 0)

length, left, right

sack1

TCP Selective Acknowledgement (block 1)

length, left, right

sack2

TCP Selective Acknowledgement (block 2)

length, left, right

sack3

TCP Selective Acknowledgement (block 3)

length, left, right

timestamp

TCP Timestamps

length, tsval, tsecr

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

```
tcp option
```

```
tcp option @number,offset,length
```

Table 62. 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 exists counter
```

matching TCP options.

```
filter input tcp option maxseg size lt 536
```

matching IPv6 exthdr.

```
ip6 filter input frag more-fragments 1 counter
```

finding IP option.

```
filter input ip option lsrr exists counter
```

finding DCCP option.

```
filter input dccp option 40 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 | count | id}
ct [original | reply] {l3proto | protocol | bytes | packets | avgpkt | zone}
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 63. Conntrack expressions

Keyword

Description

Type

state

State of the connection

ct_state

direction

Direction of the packet relative to the connection

ct_dir

status

Status of the connection

ct_status

mark

Connection mark

mark

expiration

Connection expiration time

time

helper

Helper associated with the connection

string

label

Connection tracking label bit or symbolic name defined in connlabel.conf in the nftables include path

ct_label

l3proto

Layer 3 protocol of the connection

nf_proto

saddr

Source address of the connection for the given direction

ipv4_addr/ipv6_addr

daddr

Destination address of the connection for the given direction

ipv4_addr/ipv6_addr

protocol

Layer 4 protocol of the connection for the given direction

inet_proto

proto-src

Layer 4 protocol source for the given direction

integer (16 bit)

proto-dst

Layer 4 protocol destination for the given direction

integer (16 bit)

packets

packet count seen in the given direction or sum of original and reply

integer (64 bit)

bytes

byte count seen, see description for packets keyword

integer (64 bit)

avgpkt

average bytes per packet, see description for packets keyword

integer (64 bit)

zone

conntrack zone

integer (16 bit)

count

number of current connections

integer (32 bit)

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 one 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 the TCPMSS target in iptables.

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
```

You can also remove tcp options via reset keyword.

remove tcp option.

```
tcp flags syn reset tcp option sack-perm
```

### 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 send the log through a netlink socket to the specified group. One userspace process may subscribe to the group to receive the logs, see man(8) ulogd for the Netfilter userspace log daemon and libnetfilter_log documentation for details in case you would like to develop a custom program to digest your logs.

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 64. log statement options

Keyword

Description

Type

prefix

Log message prefix

quoted string

level

Syslog level of logging

string: emerg, alert, crit, err, warn [default], notice, info, debug, audit

group

NFLOG group to send messages to

unsigned integer (16 bit)

snaplen

Length of packet payload to include in netlink message

unsigned integer (32 bit)

queue-threshold

Number of packets to queue inside the kernel before sending them to userspace

unsigned integer (32 bit)

Table 65. 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 icmp_code |

                 icmpv6 icmpv6_code |

                 icmpx 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 66. 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 raw (-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 67. Conntrack statement types

Keyword

Description

Value

event

conntrack event bits

bitmask, integer (32 bit)

helper

name of ct helper object to assign to the connection

quoted string

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 raw;

      ct zone set iif map { "eth1" : 1, "veth1" : 2 }

  }

  chain output {

      type filter hook output priority raw;

      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 one 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 (raw) 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 | broute} set value
```

A meta statement sets meta data associated with a packet.

Table 68. Meta statement types

Keyword

Description

Value

priority

TC packet priority

tc_handle

mark

Packet mark

mark

pkttype

packet type

pkt_type

nftrace

ruleset packet tracing on/off. Use monitor trace command to watch traces

0, 1

broute

broute on/off. packets are routed instead of being bridged

0, 1

### 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.

The burst value influences the bucket size, i.e. jitter tolerance. With packet-based limit, the bucket holds exactly burst packets, by default five. If you specify packet burst, it must be a non-zero value. With byte-based limit, the bucket’s minimum size is the given rate’s byte value and the burst value adds to that, by default zero bytes.

Table 69. 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 [[ip | ip6] [ prefix ] to] ADDR_SPEC [:PORT_SPEC] [FLAGS]

dnat [[ip | ip6] [ prefix ] to] ADDR_SPEC [:PORT_SPEC] [FLAGS]
masquerade [to :PORT_SPEC] [FLAGS]
redirect [to :PORT_SPEC] [FLAGS]
ADDR_SPEC := address | address - address
PORT_SPEC := port | port - port
FLAGS  := FLAG [, FLAGS]
FLAG  := persistent | random | fully-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.

The optional prefix keyword allows to map to map n source addresses to n destination addresses. See Advanced NAT examples below.

Table 70. NAT statement values

Expression

Description

Type

address

Specifies that the source/destination address of the packet should be modified. You may specify a mapping to relate a list of tuples composed of arbitrary expression key with address value.

ipv4_addr, ipv6_addr, e.g. abcd::1234, or you can use a mapping, e.g. meta mark map { 10 : 192.168.1.2, 20 : 192.168.1.3 }

port

Specifies that the source/destination port of the packet should be modified.

port number (16 bit)

Table 71. 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 dstnat; }
add chain nat postrouting { type nat hook postrouting priority srcnat; }
# 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
```

Advanced NAT examples.

```
# map prefixes in one network to that of another, e.g. 10.141.11.4 is mangled to 192.168.2.4,
# 10.141.11.5 is mangled to 192.168.2.5 and so on.
add rule nat postrouting snat ip prefix to ip saddr map { 10.141.11.0/24 : 192.168.2.0/24 }
# map a source address, source port combination to a pool of destination addresses and ports:
add rule nat postrouting dnat to ip saddr . tcp dport map { 192.168.1.2 . 80 : 10.141.10.2-10.141.10.5 . 8888-8999 }
# The above example generates the following NAT expression:
#
# [ nat dnat ip addr_min reg 1 addr_max reg 10 proto_min reg 9 proto_max reg 11 ]
#
# which expects to obtain the following tuple:
# IP address (min), source port (min), IP address (max), source port (max)
# to be obtained from the map. The given addresses and ports are inclusive.
# This also works with named maps and in combination with both concatenations and ranges:
table ip nat {

        map ipportmap {

                typeof ip saddr : interval ip daddr . tcp dport

                flags interval

                elements = { 192.168.1.2 : 10.141.10.1-10.141.10.3 . 8888-8999, 192.168.2.0/24 : 10.141.11.5-10.141.11.20 . 8888-8999 }

        }

        chain prerouting {

                type nat hook prerouting priority dstnat; policy accept;

                ip protocol tcp dnat ip to ip saddr map @ipportmap

        }
}
@ipportmap maps network prefixes to a range of hosts and ports.
The new destination is taken from the range provided by the map element.
Same for the destination port.
Note the use of the "interval" keyword in the typeof description.
This is required so nftables knows that it has to ask for twice the
amount of storage for each key-value pair in the map.
": ipv4_addr . inet_service" would allow associating one address and one port
with each key.  But for this case, for each key, two addresses and two ports
(The minimum and maximum values for both) have to be stored.
```

### 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 72. 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 mangle; 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 mangle; 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 mangle; 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 73. 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 74. 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 x {

            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 [flags QUEUE_FLAGS] [to queue_number]
queue [flags QUEUE_FLAGS] [to queue_number_from - queue_number_to]
queue [flags QUEUE_FLAGS] [to QUEUE_EXPRESSION ]
QUEUE_FLAGS := QUEUE_FLAG [, QUEUE_FLAGS]
QUEUE_FLAG  := bypass | fanout
QUEUE_EXPRESSION := numgen | hash | symhash | MAP STATEMENT
```

QUEUE_EXPRESSION can be used to compute a queue number at run-time with the hash or numgen expressions. It also allows one to use the map statement to assign fixed queue numbers based on external inputs such as the source ip address or interface names.

Table 75. queue statement values

Value

Description

Type

queue_number

Sets queue number, default is 0.

unsigned integer (16 bit)

queue_number_from

Sets initial queue in the range, if fanout is used.

unsigned integer (16 bit)

queue_number_to

Sets closing queue in the range, if fanout is used.

unsigned integer (16 bit)

Table 76. 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 77. Dup statement values

Expression

Description

Type

address

Specifies that the copy of the packet should be sent to a new gateway.

ipv4_addr, ipv6_addr, e.g. abcd::1234, or you can use a mapping, e.g. ip saddr map { 192.168.1.2 : 10.1.1.1 }

device

Specifies that the copy should be transmitted via device.

string

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
netdev 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 and egress hooks. It is similar to the dup statement except that no copy is made.

You can also specify the address of the next hop and the device to forward the packet to. This updates the source and destination MAC address of the packet by transmitting it through the neighboring layer. This also decrements the ttl field of the IP packet. This provides a way to effectively bypass the classical forwarding path, thus skipping the fib (forwarding information base) lookup.

```
fwd to device
fwd [ip | ip6] to address device device
```

Using the fwd statement.

```
# redirect raw packet to device
netdev ingress fwd to "eth0"
# forward packet to next hop 192.168.200.1 via eth0 device
netdev ingress ether saddr set fwd ip to 192.168.200.1 device "eth0"
```

### 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.

Dynamic updates are also supported with maps. In this case, the add or update rule needs to provide both the key and the data element (value), separated via :.

```
{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 }
```

### XT STATEMENT
This represents an xt statement from xtables compat interface. It is a fallback if translation is not available or not complete.

```
xt TYPE NAME
TYPE := match | target | watcher
```

Seeing this means the ruleset (or parts of it) were created by iptables-nft and one should use that to manage it.

BEWARE: nftables won’t restore these statements.

## 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. These are either related to creation and deletion of objects or to packets for which meta nftrace was enabled. When they occur, nft will print to stdout the monitored events in either JSON or native nft format.

```
monitor [new | destroy] MONITOR_OBJECT
monitor trace
MONITOR_OBJECT := tables | chains | sets | rules | elements | ruleset
```

To filter events related to a concrete object, use one of the keywords in MONITOR_OBJECT.

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

The second form of invocation takes no further options and exclusively prints events generated for packets with nftrace enabled.

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
```

Trace incoming packets from host 10.0.0.1.

```
% nft add rule filter input ip saddr 10.0.0.1 meta nftrace set 1
% nft monitor trace
```

## 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.

This documentation is licensed under the terms of the Creative Commons Attribution-ShareAlike 4.0 license, CC BY-SA 4.0 http://creativecommons.org/licenses/by-sa/4.0/.