man nft (8): Administration tool for packet filtering and classification

SYNOPSIS

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.

-h, --help: Show help message and all options.
-v, --version: Show version.
-n, --numeric: 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.
-N: Translate IP addresses to DNS names.
-a, --handle: Show rule handles in output.
-I, --includepath directory: Add the directory directory to the list of directories to be searched for included files.
-f, --file filename: Read input from filename.
-i, --interactive: 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 (\), the next line is treated as a continuation. Multiple commands on the same line can be separated using a semicolon (;).

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

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

INCLUDE FILES

include filename

Other files can be included by using the include statement. The directories to be searched for include files can be specified using the -I/--includepath option.

SYMBOLIC VARIABLES

define variable expr $variable

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

Using symbolic variables

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

ADDRESS FAMILIES

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

ip: IPv4 address family.
ip6: IPv6 address family.
inet: Internet (IPv4/IPv6) address family.
arp: ARP address family, handling packets vi
bridge: Bridge address family, handling packets which traverse a bridge device.
netdev: Netdev address family, handling packets from ingress.

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

IPV4/IPV6/INET ADDRESS FAMILIES

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

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.

NETDEV ADDRESS FAMILY

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.

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

CHAINS

{add} 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: 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: Simlar to the add command, but returns an error if the chain already exists.
delete: Delete the specified chain. The chain must not contain any rules or be used as jump target.
rename: Rename the specified chain.
list: List all rules of the specified chain.
flush: Flush all rules of the specified chain.

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.

add: 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.
insert: 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: 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

$ 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

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 /, -, _ 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. FIXME

IPv6 address specification

# abbreviated loopback address
filter output ip6 daddr ::1

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 | ibriport | obriport | pkttype | cpu | iifgroup | oifgroup | cgroup}

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
ibriport	Input bridge interface name	string
obriport	Output bridge interface name	string
pkttype	packet type	pkt_type
cpu	cpu number processing the packet	integer (32 bits)
iifgroup	incoming device group	devgroup_type
oifgroup	outgoing device group	devgroup_type
cgroup	control group id	integer (32 bits)

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: Unicast (addressed to local host), Broadcast (to all), Multicast (to group).

Using meta expressions

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

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	integer (16 bit)
ptype	EtherType	ethertype
hlen	Hardware address len	integer (8 bit)
plen	Protocol address len	integer (8 bit)
operation	Operation	arp_op

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
dscp	Differentiated Services Code Point	integer (6 bit)
ecn	Explicit Congestion Notification	integer (2 bit)
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
dscp	Differentiated Services Code Point	integer (6 bit)
ecn	Explicit Congestion Notification	integer (2 bit)
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

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	integer (4 bit)
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	integer (4 bit)
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

comp [IPComp header field]

IPComp header expression

Keyword	Description	Type
nexthdr	Next header protocol	inet_service
flags	Flags	bitmask
cpi	Compression Parameter Index	integer (16 bit)

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.

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 and bytes 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.

ct {state | direction | status | mark | expiration | helper | label | bytes | packets} {original | reply | {l3proto | protocol | saddr | daddr | proto-src | proto-dst | bytes | packets}}

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
label	Connection tracking label	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	bytecount seen, see description for packets keyword	integer (64 bit)

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

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

LOG STATEMENT

REJECT STATEMENT

COUNTER STATEMENT

META STATEMENT

LIMIT STATEMENT

NAT STATEMENT

QUEUE STATEMENT

ADDITIONAL COMMANDS

These are some additional commands included in nft.

EXPORT

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

Examples:

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

MONITOR

The monitor command allows you to listen to Netlink events produced by the nf_tables subsystem, related to creation and deletion of objects. When they 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

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 (^). 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
^^^^^^^^^^^^^^^^^^^^^^^

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.

AUTHORS

nftables was written by Patrick McHardy.

COPYRIGHT

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/> .