nft

8

nft

Administration tool of the nftables framework for packet filtering and classification

    			nft

    				 -nNscae

    			 -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

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

`-h, --help`

- Show help message and all options.

`-v, --version`

- Show version.

`-n, --numeric`

- Show data numerically. When used once (the default behaviour), skip lookup of addresses to symbolic names. Use twice to also show Internet services (port numbers) numerically. Use three times to also show protocols and UIDs/GIDs numerically.

`-N, --reversedns`

- Translate IP addresses to names. Usually requires network traffic for DNS lookup.

`-s, --stateless`

- Omit stateful information of rules and stateful objects.

`-c, --check`

- Check commands validity without actually applying the changes.

`-a, --handle`

- Show rule handles in output.

`-e, --echo`

- When inserting items into the ruleset using `add`, `insert` or `replace` commands, print notifications just like `nft monitor`.

`-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.

`-f, --file `filename``

- Read input from `filename`.

  nft scripts must start `#!/usr/sbin/nft -f`

`-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. You can override this behaviour either by prepending ./ to your path to force inclusion of files located in the current working directory (ie. 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 -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

    					$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 IPv4 ARP packets.

`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.

The list of supported hooks is identical to IPv4/IPv6/Inet address families above.

### 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.

## Ruleset

    					list
    					flush

    				ruleset
    				family

    				export
    				ruleset
    				format

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.

`export`

- Print the ruleset in machine readable format. The mandatory `format` parameter may be either `xml` or `json`.

It is possible to limit `list` and `flush` to a specific address family only. For a list of valid family names, see `ADDRESS FAMILIES` above.

Note that contrary to what one might assume, the output generated by `export` is not parseable by `nft -f`. Instead, the output of `list` command serves well for that purpose.

## Tables

    					add
    					delete
    					list
    					flush

    				table
    				family
    				table

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.

`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
    					create

    				chain
    				family
    				table
    				chain

    						type type
    						hook hook
    						device device
    						priority priority ;
    						policy policy ;

    					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`

- 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.

`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.

## Supported chain types

Type | Families | Hooks | Description
filter | all | all | Standard chain type to use in doubt.
nat | ip, ip6 | prerouting, input, output, postrouting | Chains of this type perform Network 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 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 two further quirks worth noticing:

- `netdev` family supports merely a single combination, namely `filter` type and `ingress` hook. Base chains in this family also require the `device` parameter to be present since they exist per incoming interface only.
- `arp` family supports only `input` and `output` hooks, both in chains of type `filter`.

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

Base chains also allow 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
    				position position
    				statement...

    				replace rule
    				family
    				table
    				chain
    				 handle handle
    				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.

`replace`

- Similar to the `add` command, but the rule replaces the specified rule.

`delete`

- Delete the specified rule.

## Sets

    				add
    				 set
    				family
    				table
    				set
    				{ type
    				type ;
    				flags flags ;
    				timeout timeout ;
    				gc-interval gc-interval ;
    				elements = { element[,...] } ;
    				size size ;
    				policy policy ;
    				auto-merge auto-merge ;
    				}

    					delete
    					list
    					flush

    				 set
    				family
    				table
    				set

    					add
    					delete

    				 element
    				family
    				table
    				set
    				{
    				element[,...] }

Sets are elements containers of an user-defined data type, they are uniquely identified by an user-defined name and attached to tables.

`add`

- Add a new set in the specified table.

`delete`

- Delete the specified set.

`list`

- Display the elements in the specified set.

`flush`

- Remove all elements from the specified set.

`add element`

- Comma-separated list of elements to add into the specified set.

`delete element`

- Comma-separated list of elements to delete from the specified set.

## Set specifications

Keyword | Description | Type
type | data type of set elements | string: ipv4_addr, ipv6_addr, ether_addr, inet_proto, inet_service, mark
flags | set flags | string: constant, interval, timeout
timeout | time an element stays in the set | 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 | maximun number of elements in the set | 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
    				flags flags ;
    				elements = { element[,...] } ;
    				size size ;
    				policy policy ;
    				}

    					delete
    					list
    					flush

    				 map
    				family
    				table
    				map

    					add
    					delete

    				 element
    				family
    				table
    				map
    				{
    				elements = { element[,...] } ;
    				}

Maps store data based on some specific key used as input, they are uniquely identified by an user-defined name and attached to tables.

`add`

- Add a new map in the specified table.

`delete`

- Delete the specified map.

`list`

- Display the elements in the specified map.

`flush`

- Remove all elements from the specified map.

`add element`

- Comma-separated list of elements to add into the specified map.

`delete element`

- Comma-separated list of element keys to delete from the specified map.

## Map specifications

Keyword | Description | Type
type | data type of map elements | string ':' string: ipv4_addr, ipv6_addr, ether_addr, inet_proto, inet_service, mark, counter, quota. Counter and quota can't be used as keys
flags | map flags | string: constant, interval
elements | elements contained by the map | map data type
size | maximun number of elements in the map | unsigned integer (64 bit)
policy | map policy | string: performance [default], memory

## Stateful objects

    					add
    					delete
    					list
    					reset

    				 type
    				family
    				table
    				object

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

`list`

- Display stateful information the object holds.

`reset`

- List-and-reset stateful object.

### Ct

    					ct
    					helper
    					helper
    					 { type
    					type
    					 protocol
    					protocol
    					 ;
    					l3proto family ;
    					 }

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.

### 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. tcp)
l3proto | layer 3 protocol of the helper | address family (e.g. ip)

#### 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 0;
          tcp dport 21 ct helper set "ftp-standard"
      }
    }

### Counter

    					counter
    					packets bytes

### Counter specifications

Keyword | Description | Type
packets | initial count of packets | unsigned integer (64 bit)
bytes | initial count of bytes | unsigned integer (64 bit)

### Quota

    					quota

    						over
    						until

    					used

### Quota specifications

Keyword | Description | Type
quota | quota limit, used as the quota name | Two arguments, unsigned interger (64 bit) and string: bytes, kbytes, mbytes. "over" and "until" go before these arguments
used | initial value of used quota | Two arguments, unsigned interger (64 bit) and string: bytes, kbytes, mbytes

## 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

### Boolean type

Name | Keyword | Size | Base type
Boolean | boolean | 1 bit | integer

The boolean type is a syntactical helper type in user space. It's 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).

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

Keyword | Value
exists | 1
missing | 0

#### Boolean specification

The following expressions support a boolean comparison:

Expression | Behaviour
fib | Check route existence.
exthdr | Check IPv6 extension header existence.
tcp option | Check TCP option header existence.

    # 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.

The following 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.

The following keywords may be used when specifying the ICMP code:

Keyword | Value
net-unreachable | 0
host-unreachable | 1
prot-unreachable | 2
port-unreachable | 3
net-prohibited | 9
host-prohibited | 10
admin-prohibited | 13

### ICMPv6 Type type

Name | Keyword | Size | Base type
ICMPv6 Type | icmpv6_type | 8 bit | integer

The ICMPv6 Type type is used to conveniently specify the ICMPv6 header's type field.

The following 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.

The following 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 | icmpx_code | 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.

The following 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

This is an 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:

### conntrack state (ct_state)

Keyword | Value
invalid | 1
established | 2
related | 4
new | 8
untracked | 64

### conntrack direction (ct_dir)

Keyword | Value
original | 0
reply | 1

### conntrack status (ct_status)

Keyword | Value
expected | 1
seen-reply | 2
assured | 4
confirmed | 8
snat | 16
dnat | 32
dying | 512

### 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`.

## 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
    						random
    						secpath

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)
nfproto | real hook protocol family, useful only in inet table | integer (32 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 | 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
oifgroup | outgoing device group | devgroup
cgroup | control group id | integer (32 bits)
random | pseudo-random number | integer (32 bits)
secpath | boolean | boolean (1 bit)

### 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

    # packed was subject to ipsec processing
    raw prerouting meta secpath exists accept

### 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.

### fib expression specific types

Keyword | Description | Type
oif | Output interface index | integer (32 bit)
oifname | Output interface name | string
type | Address type | fib_addrtype

#### Using fib expressions

    # drop packets without a reverse path
    filter prerouting fib saddr . iif oif missing drop

    # drop packets to address not configured on ininterface
    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

    						classid
    						nexthop

A routing expression refers to routing data associated with a packet.

### 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)

### 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

## 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 | integer (1 bit)
pcp | Priority code point | integer (3 bit)
type | EtherType | ether_type

### ARP header expression

    					arp
    					ARP header field

### 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

### 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 | 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

### ICMP header expression

    					icmp
    					ICMP header field

### 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)

### IPv6 header expression

    					ip6
    					IPv6 header field

### 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

### ICMPv6 header expression

    					icmpv6
    					ICMPv6 header field

### 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)

### 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_flag
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
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_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
    					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_proto
flags | Flags | bitmask
cpi | Compression Parameter Index | integer (16 bit)

### Extension header expressions

Extension header expressions refer to data from variable-sized protocol headers, such as IPv6 extension headers and TCPs options.

nftables currently supports matching (finding) a given ipv6 extension header or TCP 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

    				tcp option

    					eol
    					noop
    					maxseg
    					window
    					sack-permitted
    					sack
    					sack0
    					sack1
    					sack2
    					sack3
    					timestamp

    				tcp_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
    					noop
    					maxseg
    					window
    					sack-permitted
    					sack
    					sack0
    					sack1
    					sack2
    					sack3
    					timestamp

### IPv6 extension headers

Keyword | Description
hbh | Hop by Hop
rt | Routing Header
frag | Fragmentation header
dst | dst options
mh | Mobility Header

### TCP Options

Keyword | Description | TCP option fields
eol | End of option list | kind
noop | 1 Byte TCP No-op options | kind
maxseg | TCP Maximum Segment Size | kind, length, size
window | TCP Window Scaling | kind, length, count
sack-permitted | TCP SACK permitted | kind, length
sack | TCP Selective Acknowledgement (alias of block 0) | kind, length, left, right
sack0 | TCP Selective Acknowledgement (block 0) | kind, length, left, right
sack1 | TCP Selective Acknowledgement (block 1) | kind, length, left, right
sack2 | TCP Selective Acknowledgement (block 2) | kind, length, left, right
sack3 | TCP Selective Acknowledgement (block 3) | kind, length, left, right
timestamp | TCP Timestamps | kind, length, tsval, tsecr

#### finding TCP options

    filter input tcp option sack-permitted kind 1 counter

#### matching IPv6 exthdr

    ip6 filter input frag more-fragments 1 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
    						l3proto
    						protocol
    						bytes
    						packets
    						avgpkt
    						zone

    					ct

    						original
    						reply

    						l3proto
    						protocol
    						proto-src
    						proto-dst
    						bytes
    						packets
    						avgpkt
    						zone

    					ct

    						original
    						reply

    						ip
    						ip6

    						saddr
    						daddr

### 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 | bytecount 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)

A description of conntrack-specific types listed above can be found sub-section `CONNTRACK TYPES` above.

## 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

### Payload statement

The payload statement alters packet content. It can be used for example to set ip DSCP (differv) 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

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 TCPMSS.

#### 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

### Log statement

    					log
    					prefix
    					quoted_string
    					level
    					syslog-level
    					flags
    					log-flags

    					log
    					group
    					nflog_group
    					prefix
    					quoted_string
    					queue-threshold
    					value
    					snaplen
    					size

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). If the group number is specified, the Linux kernel will pass the packet to nfnetlink_log which will multicast the packet through a netlink socket to the specified multicast group. One or more userspace processes may subscribe to the group to receive the packets, see libnetfilter_queue documentation for details. This is a non-terminating statement, so the rule evaluation continues after the packet is logged.

### 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
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)

### 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

    							icmp
    							icmp6
    							icmpx

    						type

    							icmp_type
    							icmp6_type
    							icmpx_type

    					reject

    						 with 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 the input, forward and output chains, and user-defined chains which are only called from those chains.

The different ICMP reject variants are meant for use in different table families:

Variant | Family | Type
icmp | ip | icmp_code
icmp6 | 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`.

### Counter statement

A counter statement sets the hit count of packets along with the number of bytes.

    					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 -300.

### 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 -300;
          ct zone set iif map { "eth1" : 1, "veth1" : 2 }
      }
      chain output {
          type filter hook output priority -300;
          ct zone set oif map { "eth1" : 1, "veth1" : 2 }
      }
    }

#### restrict events reported by ctnetlink

    ct event set new,related,destroy

### 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

    					set
    					value

A meta statement sets meta data associated with a packet.

### 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

### Limit statement

    					limit
    					rate
    					over
    					packet_number
    					/

    						second
    						minute
    						hour
    						day

    					burst packet_number packets

    					limit
    					rate
    					over
    					byte_number

    						bytes
    						kbytes
    						mbytes

    					/

    						second
    						minute
    						hour
    						day
    						week

    					burst byte_number bytes

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 `over` keyword, that is optional, makes it match over the specified rate.

### 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
    						to
    						address
    						:port
    						persistent, random, fully-random

    					        snat
    						to
    						address - address
    						:port - port
    						persistent, random, fully-random

    						dnat
    						to
    						address
    						:port
    						persistent, random, fully-random

    						dnat
    						to
    						address
    						:port - port
    						persistent, random, fully-random

    						masquerade
    						to
    						:port
    						persistent, random, fully-random

    						masquerade
    						to
    						:port - port
    						persistent, random, fully-random

    						redirect
    						to
    						:port
    						persistent, random, fully-random

    						redirect
    						to
    						:port - port
    						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.

Note that all 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.

### 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, eg. abcd::1234, or you can use a mapping, eg. meta mark map { 10 : 192.168.1.2, 20 : 192.168.1.3 }
port | Specifies that the source/destination address of the packet should be modified. | port number (16 bits)

### NAT statement flags

Flag | Description
persistent | Gives a client the same source-/destination-address for each connection.
random | If used then port mapping will be randomized using a random 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 0; }
    add chain nat postrouting { type nat hook postrouting priority 100; }

    # 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

### 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 reinject the packet into the kernel. See libnetfilter_queue documentation for details.

                                            queue
                                            num
                                            queue_number
    					bypass

                                            queue
                                            num
                                            queue_number_from - queue_number_to
    					bypass,fanout

### 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)

### 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 recomendations.
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

### 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, eg. abcd::1234, or you can use a mapping, eg. 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
    netdetv 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. Its is only available in the netdev family ingress hook. It is similar to the dup statement except that no copy is made.

    						fwd
    						to
    						device

## 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, related to creation and deletion of objects. When they occur, 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' , 'ruleset'.

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

#### Listen to ruleset events such as table, chain, rule, set, counters and quotas, in native nft format

    % nft monitor ruleset

## 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: 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 © 2013-2016 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 licenced under the terms of the Creative Commons Attribution-ShareAlike 4.0 license, CC BY-SA 4.0.