nft

8

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

## 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 by 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.

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.

## 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`

. 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

    					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

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

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

`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.

## See Also

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

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

## Authors

nftables was written by Patrick McHardy.

## Copyright

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.