nft(8)

NAME
nft - Administration tool for packet filtering and classification

SYNOPSIS
nft
[ -n/--numeric ] [ -I/--includepath directory ] [ -f/--file filename | -i/--interactive | cmd ...]
nft
[ -h/--help ] [ -v/--version ]

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

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

\FC-h/--help\F[]
    Show help message and all options.

\FC-v/--version\F[]
    Show version.

\FC-n/--numeric\F[]
    Numeric output: Addresses and other information that might need network traffic to resolve to symbolic names are shown numerically. When used twice, internet services and UIDs/GIDs are also shown numerically. When used thrice, protocol numbers are also shown numerically.

\FC-a/--handle\F[]
    Show rule handles in output.

\FC-I/--includepath \F[]directory
    Add the directory directory to the list of directories to by searched for included files.

\FC-f/--file \F[]filename
    Read input from filename.

\FC-i/--interactive\F[]
    Read input from an interactive readline CLI.

INPUT FILE FORMAT

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

A hash sign (\FC#\F[]) begins a comment. All following characters on the same line are ignored.

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

  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 \FC-I/--includepath\F[] option.

  SYMBOLIC VARIABLES
define
variable expr
$variable

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

\FC
define int_if1 = eth0
define int_if2 = eth1
define int_ifs = { $int_if1, $int_if2 }
filter input iif $int_ifs accept
					\F[]

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.

\FCip\F[]
    IPv4 address family.

\FCip6\F[]
    IPv6 address family.

\FCinet\F[]
    Internet (IPv4/IPv6) address family.

\FCarp\F[]
    ARP address family, handling packets vi

\FCbridge\F[]
    Bridge address family, handling packets which traverse a bridge device.

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 \FCip\F[] 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.

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.

  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.

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.

TABLES
{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 \FCip\F[], \FCip6\F[], \FCinet\F[], \FCarp\F[], \FCbridge\F[]. The \FCinet\F[] address family is a dummy family which is used to create hybrid IPv4/IPv6 tables. When no address family is specified, \FCip\F[] is used by default.

\FCadd\F[]
    Add a new table for the given family with the given name.

\FCdelete\F[]
    Delete the specified table.

\FClist\F[]
    List all chains and rules of the specified table.

\FCflush\F[]
    Flush all chains and rules of the specified table.

CHAINS
{add} chain [family] {table} {chain} {hook} {priority}
{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.

\FCadd\F[]
    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.

\FCcreate\F[]
    Simlar to the add command, but returns an error if the chain already exists.

\FCdelete\F[]
    Delete the specified chain. The chain must not contain any rules or be used as jump target.

\FCflush\F[]
    Rename the specified chain.

\FClist\F[]
    List all rules of the specified chain.

\FCflush\F[]
    Flush all rules of the specified chain.

RULES
[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.

\FCadd\F[]
    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.

\FCinsert\F[]
    Similar to the add command, but the rule is prepended to the beginning of the chain or before the rule at the given position.

\FCdelete\F[]
    Delete the specified rule.

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}

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

The describe command

\FC
$ 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
				\F[]

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.

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 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.

  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 to for character strings. A string begins with an alphabetic character (a-zA-Z) followed by zero or more alphanumeric characters or the characters \FC/\F[], \FC-\F[], \FC_\F[] and \FC.\F[]. In addition anything enclosed in double quotes (\FC"\F[]) is recognized as a string.

String specification

\FC
# Interface name
filter input iifname eth0
# Weird interface name
filter input iifname "(eth0)"
				\F[]

  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 (\FC:\F[]).

Link layer address specification

\FC
# Ethernet destination MAC address
filter input ether daddr 20:c9:d0:43:12:d9
				\F[]

  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

\FC
# dotted decimal notation
filter output ip daddr 127.0.0.1
# host name
filter output ip daddr localhost
				\F[]

  IPV6 ADDRESS TYPE
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

\FC
# abbreviated loopback address
filter output ip6 daddr ::1
				\F[]

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}

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

\FC
# qualified meta expression
filter output meta oif eth0
# unqualified meta expression
filter output oif eth0
					\F[]

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

  ETHERNET HEADER EXPRESSION
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 HEADER EXPRESSION
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 HEADER EXPRESSION
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

  IPV4 HEADER EXPRESSION
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

  IPV6 HEADER EXPRESSION
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 HEADER EXPRESSION
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 HEADER EXPRESSION
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)

  UDP-LITE HEADER EXPRESSION
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 HEADER EXPRESSION
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 HEADER EXPRESSION
dccp
[DCCP header field]

DCCP header expression
Keyword | Description | Type
sport | Source port | inet_service
dport | Destination port | inet_service

  AUTHENTICATION HEADER EXPRESSION
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)

  ENCRYPTED SECURITY PAYLOAD HEADER EXPRESSION
esp
[ESP header field]

ESP header expression
Keyword | Description | Type
spi | Security Parameter Index | integer (32 bit)
sequence | Sequence number | integer (32 bit)

  IPCOMP HEADER EXPRESSION
ipcomp
[IPComp header field]

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

BLA

  IPV6 EXTENSION HEADER EXPRESSIONS
IPv6 extension header expressions refer to data from an IPv6 packet's extension headers.

  CONNTRACK EXPRESSIONS
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
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}

\FCaccept\F[]
    Terminate ruleset evaluation and accept the packet.

\FCdrop\F[]
    Terminate ruleset evaluation and drop the packet.

\FCqueue\F[]
    Terminate ruleset evaluation and queue the packet to userspace.

\FCcontinue\F[]
    Continue ruleset evaluation with the next rule. FIXME

\FCreturn\F[]
    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.

\FCjump \F[]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 of a return verdict is issued.

\FCgoto \F[]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.

Verdict statements

\FC
# 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
					\F[]

  LOG STATEMENT

  REJECT STATEMENT

  COUNTER STATEMENT

  META STATEMENT

  LIMIT STATEMENT

  NAT STATEMENT

  QUEUE STATEMENT

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 carrets (\FC^\F[]). If the error results from the combination of two expressions or statements, the part imposing the constraints which are violated is marked using tildes (\FC~\F[]).

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

\FC
<cmdline>:1:19-22: Error: Interface does not exist
filter output oif eth0
^^^
			\F[]

Error caused by invalid combination of two expressions

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

Error returned by the kernel

\FC
<cmdline>:0:0-23: Error: Could not process rule: Operation not permitted
filter output oif wlan0
^^^^^^^^^^^^^^^^^^^^^^^
			\F[]

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.

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

AUTHORS
nftables was written by Patrick McHardy.

COPYRIGHT
Copyright 2008-2014 Patrick McHardy <\FCkaber@trash.net\F[]>

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, .