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nft

Administration tool for packet filtering and classification


nft
[ -n/--numeric ] [ -I/--includepath directory ] [ -f/--file filename | -i/--interactive | cmd ...]

nft
[ -h/--help ] [ -v/--version ]

nft is used to set up, maintain and inspect packet filtering and classification rules in the Linux kernel.

For a full summary of options, run nft --help.

Show help message and all options.
Show version.
Numeric output: Addresses and other information that might need network traffic to resolve to symbolic names are shown numerically (default behaviour). When used twice, internet services are translated. When used twice, internet services and UIDs/GIDs are also shown numerically. When used three times, protocol numbers are also shown numerically.
Translate IP addresses to DNS names.
Show rule handles in output.
Add the directory directory to the list of directories to by searched for included files.
Read input from filename.
Read input from an interactive readline CLI.

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

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

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


include
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.


define
variable expr

$variable

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

Using symbolic variables

define int_if1 = eth0
define int_if2 = eth1
define int_ifs = { $int_if1, $int_if2 }
filter input iif $int_ifs accept

Address families 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.

IPv4 address family.
IPv6 address family.
Internet (IPv4/IPv6) address family.
ARP address family, handling packets vi
Bridge address family, handling packets which traverse a bridge device.
Netdev address family, handling packets from ingress.

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

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.

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.

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

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.

The bridge address family handles ethernet packets traversing bridge devices.

The Netdev address family handles packets from ingress.

Netdev address family hooks

Hook Description
ingress All packets entering the system are processed by this hook. It is invoked before layer 3 protocol handlers and it can be used for early filtering and policing.


{add | delete | list | flush} table [family] {table}

Tables are containers for chains and sets. 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. When no address family is specified, ip is used by default.

Add a new table for the given family with the given name.
Delete the specified table.
List all chains and rules of the specified table.
Flush all chains and rules of the specified table.


{add} chain [family] {table} {chain} {hook} {priority} {policy} {device}
{add | create | delete | list | flush} chain [family] {table} {chain}
{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 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.
Simlar to the add command, but returns an error if the chain already exists.
Delete the specified chain. The chain must not contain any rules or be used as jump target.
Rename the specified chain.
List all rules of the specified chain.
Flush all rules of the specified chain.


[add | insert] rule [family] {table} {chain} [position position] {statement}...
{delete} rule [family] {table} {chain} {handle handle}

Rules are constructed from two kinds of components according to a set of grammatical rules: expressions and statements.

Add a new rule described by the list of statements. The rule is appended to the given chain unless a position is specified, in which case the rule is appended to the rule given by the position.
Similar to the add command, but the rule is prepended to the beginning of the chain or before the rule at the given position.
Delete the specified rule.

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
{expression}

The describe command shows information about the type of an expression and its data type.

The describe command

$ nft describe tcp flags
payload expression, datatype tcp_flag (TCP flag) (basetype bitmask, integer), 8 bits
pre-defined symbolic constants:
fin                           	0x01
syn                           	0x02
rst                           	0x04
psh                           	0x08
ack                           	0x10
urg                           	0x20
ecn                           	0x40
cwr                           	0x80

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.

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.

Name Keyword Size Base type
Integer integer variable -

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

Name Keyword Size Base type
Bitmask bitmask variable integer

The bitmask type (bitmask) is used for bitmasks.

Name Keyword Size Base type
String string variable -

The string type is used to 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)"
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

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

Name Keyword Size Base type
IPv6 address ipv6_addr 128 bit integer

The IPv6 address type is used for IPv6 addresses. FIXME

IPv6 address specification

# abbreviated loopback address
filter output ip6 daddr ::1

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
{length | nfproto | l4proto | protocol | priority}

[meta] {mark | iif | iifname | iiftype | oif | oifname | oiftype | skuid | skgid | nftrace | rtclassid}

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 expression types

Keyword Description Type
length Length of the packet in bytes integer (32 bit)
protocol Ethertype protocol value ether_type
priority TC packet priority integer (32 bit)
mark Packet mark packetmark
iif Input interface index iface_index
iifname Input interface name string
iiftype Input interface type iface_type
oif Output interface index iface_index
oifname Output interface name string
oiftype Output interface hardware type iface_type
skuid UID associated with originating socket uid
skgid GID associated with originating socket gid
rtclassid Routing realm realm

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.

Using meta expressions

# qualified meta expression
filter output meta oif eth0
# unqualified meta expression
filter output oif eth0

Payload expressions refer to data from the packet's payload.


ether
[ethernet header field]

Ethernet header expression types

Keyword Description Type
daddr Destination MAC address ether_addr
saddr Source MAC address ether_addr
type EtherType ether_type


vlan
[VLAN header field]

VLAN header expression

Keyword Description Type
id VLAN ID (VID) integer (12 bit)
cfi Canonical Format Indicator flag
pcp Priority code point integer (3 bit)
type EtherType ethertype


arp
[ARP header field]

ARP header expression

Keyword Description Type
htype ARP hardware type FIXME
ptype EtherType ethertype
hlen Hardware address len integer (8 bit)
plen Protocol address len integer (8 bit)
op Operation FIXME


ip
[IPv4 header field]

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
tos Type Of Service FIXME
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


ip6
[IPv6 header field]

IPv6 header expression

Keyword Description Type
version IP header version (6) integer (4 bit)
priority
flowlabel Flow label
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


tcp
[TCP header field]

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 FIXME
flags TCP flags tcp_flags
window Window integer (16 bit)
checksum Checksum integer (16 bit)
urgptr Urgent pointer integer (16 bit)


udp
[UDP header field]

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)


udplite
[UDP-Lite header field]

UDP-Lite header expression

Keyword Description Type
sport Source port inet_service
dport Destination port inet_service
cscov Checksum coverage integer (16 bit)
checksum Checksum integer (16 bit)


sctp
[SCTP header field]

SCTP header expression

Keyword Description Type
sport Source port inet_service
dport Destination port inet_service
vtag Verfication Tag integer (32 bit)
checksum Checksum integer (32 bit)


dccp
[DCCP header field]

DCCP header expression

Keyword Description Type
sport Source port inet_service
dport Destination port inet_service


ah
[AH header field]

AH header expression

Keyword Description Type
nexthdr Next header protocol inet_service
hdrlength AH Header length integer (8 bit)
reserved Reserved area FIXME
spi Security Parameter Index integer (32 bit)
sequence Sequence number integer (32 bit)


esp
[ESP header field]

ESP header expression

Keyword Description Type
spi Security Parameter Index integer (32 bit)
sequence Sequence number integer (32 bit)


ipcomp
[IPComp header field]

IPComp header expression

Keyword Description Type
nexthdr Next header protocol inet_service
flags Flags FIXME
cfi Compression Parameter Index FIXME

IPv6 extension header expressions refer to data from an IPv6 packet's extension headers.

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


ct
{state | direction | status | mark | expiration | helper | l3proto | saddr | daddr | protocol | proto-src | proto-dst}

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 packetmark
expiration Connection expiration time time
helper Helper associated with the connection string
l3proto Layer 3 protocol of the connection nf_proto FIXME
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 FIXME
proto-dst Layer 4 protocol destination for the given direction FIXME

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.

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


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

Terminate ruleset evaluation and accept the packet.
Terminate ruleset evaluation and drop the packet.
Terminate ruleset evaluation and queue the packet to userspace.
Continue ruleset evaluation with the next rule. FIXME
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 accept.
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 of a return verdict is issued.
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.

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

These are some additional commands included in nft.

Export your current ruleset in XML or JSON format to stdout.

Examples:

% nft export xml
[...]
% nft export json
[...]

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

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

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

Hit ^C to finish the monitor operation.

Listen to all events, report in native nft format

% nft monitor

Listen to added tables, report in XML format

% nft monitor new tables xml

Listen to deleted rules, report in JSON format

% nft monitor destroy rules json

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

% nft monitor chains

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 carrets (^). 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 can't 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
^^^^^^^^^^^^^^^^^^^^^^^

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.

iptables(8), ip6tables(8), arptables(8), ebtables(8), ip(8), tc(8)

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

nftables was written by Patrick McHardy.

Copyright 2008-2014 Patrick McHardy <kaber@trash.net>

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 licenced 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/⟩ .