path: root/doc/nft.txt

nft(8)
======

NAME
----
nft - Administration tool of the nftables framework for packet filtering and classification


SYNOPSIS
--------
[verse]
*nft* [ *-nNscaeSupyjt* ] [ *-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
-------
The command accepts several different options which are documented here in groups for better
understanding of their meaning. You can get information about options by running *nft --help*.

.General options:

*-h*::
*--help*::
	Show help message and all options.

*-v*::
*--version*::
	Show version.

*-V*::
	Show long version information, including compile-time configuration.

.Ruleset input handling options that specify to how to load rulesets:

*-f*::
*--file 'filename'*::
	Read input from 'filename'. If 'filename' is -, read from stdin.

*-i*::
*--interactive*::
	Read input from an interactive readline CLI. You can use quit to exit, or use the EOF marker,
	normally this is CTRL-D.

*-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.

*-c*::
*--check*::
	Check commands validity without actually applying the changes.

.Ruleset list output formatting that modify the output of the list ruleset command:

*-a*::
*--handle*::
	Show object handles in output.

*-s*::
*--stateless*::
	Omit stateful information of rules and stateful objects.

*-t*::
*--terse*::
	Omit contents of sets from output.

*-S*::
*--service*::
	Translate ports to service names as defined by /etc/services.

*-N*::
*--reversedns*::
	Translate IP address to names via reverse DNS lookup. This may slow down
	your listing since it generates network traffic.

*-u*::
*--guid*::
	Translate numeric UID/GID to names as defined by /etc/passwd and /etc/group.

*-n*::
*--numeric*::
	Print fully numerical output.

*-y*::
*--numeric-priority*::
	Display base chain priority numerically.

*-p*::
*--numeric-protocol*::
	Display layer 4 protocol numerically.

*-T*::
*--numeric-time*::
	Show time, day and hour values in numeric format.

.Command output formatting:

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

*-j*::
*--json*::
	Format output in JSON. See libnftables-json(5) for a schema description.

*-d*::
*--debug* 'level'::
	Enable debugging output. The debug level can be any of *scanner*, *parser*, *eval*,
        *netlink*, *mnl*, *proto-ctx*, *segtree*, *all*. You can combine more than one by
        separating by the ',' symbol, for example '-d eval,mnl'.

INPUT FILE FORMATS
------------------
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 by 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
~~~~~~~~~~~~~
[verse]
*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 (i.e.
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 the *-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
~~~~~~~~~~~~~~~~~~
[verse]
*define* 'variable' *=* 'expr'
*$variable*

Symbolic variables can be defined using the *define* statement. Variable
references are expressions and can be used to 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
----------------
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.

[horizontal]
*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
[options="header"]
|==================
|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.
|ingress |
All packets entering the system are processed by this hook. It is invoked before
layer 3 protocol handlers, hence before the prerouting hook, and it can be used
for filtering and policing. Ingress is only available for Inet family (since
Linux kernel 5.10).
|===================

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
[options="header"]
|=================
|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 the device ingress path. This
family allows you to filter packets of any ethertype such as ARP, VLAN 802.1q,
VLAN 802.1ad (Q-in-Q) as well as IPv4 and IPv6 packets.

.Netdev address family hooks
[options="header"]
|=================
|Hook | Description
|ingress |
All packets entering the system are processed by this hook. It is invoked after
the network taps (ie. *tcpdump*), right after *tc* ingress and before layer 3
protocol handlers, it can be used for early filtering and policing.
|=================

RULESET
-------
[verse]
{*list* | *flush*} *ruleset* ['family']

The *ruleset* keyword is used to identify the whole set of tables, chains, etc.
currently in place in kernel. The following *ruleset* commands exist:

[horizontal]
*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.

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.

By design, *list ruleset* command output may be used as input to *nft -f*.
Effectively, this is the nft-equivalent of *iptables-save* and
*iptables-restore*.

TABLES
------
[verse]
{*add* | *create*} *table* ['family'] 'table' [*{ flags* 'flags' *; }*]
{*delete* | *list* | *flush*} *table* ['family'] 'table'
*list tables* ['family']
*delete table* ['family'] *handle* 'handle'

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. The only difference between add and create is that the former will
not return an error if the specified table already exists while *create* will
return an error.

.Table flags
[options="header"]
|=================
|Flag | Description
|dormant |
table is not evaluated any more (base chains are unregistered).
|=================

.*Add, change, delete a table*
---------------------------------------
# start nft in interactive mode
nft --interactive

# create a new table.
create table inet mytable

# add a new base chain: get input packets
add chain inet mytable myin { type filter hook input priority filter; }

# add a single counter to the chain
add rule inet mytable myin counter

# disable the table temporarily -- rules are not evaluated anymore
add table inet mytable { flags dormant; }

# make table active again:
add table inet mytable
---------------------------------------

[horizontal]
*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
------
[verse]
{*add* | *create*} *chain* ['family'] 'table' 'chain' [*{ type* 'type' *hook* 'hook' [*device* 'device'] *priority* 'priority' *;* [*policy* 'policy' *;*] *}*]
{*delete* | *list* | *flush*} *chain* ['family'] 'table' 'chain'
*list chains* ['family']
*delete chain* ['family'] 'table' *handle* 'handle'
*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.

[horizontal]
*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
[options="header"]
|=================
|Type | Families | Hooks | Description
|filter | all | all |
Standard chain type to use in doubt.
|nat | ip, ip6, inet |
prerouting, input, output, postrouting |
Chains of this type perform Native 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 three
further quirks worth noticing:

* The 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.
* The arp family supports only the *input* and *output* hooks, both in chains of type
  *filter*.
* The inet family also supports the *ingress* hook (since Linux kernel 5.10),
  to filter IPv4 and IPv6 packet at the same location as the netdev *ingress*
  hook. This inet hook allows you to share sets and maps between the usual
  *prerouting*, *input*, *forward*, *output*, *postrouting* and this *ingress*
  hook.

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

Standard priority values can be replaced with easily memorizable names.  Not all
names make sense in every family with every hook (see the compatibility matrices
below) but their numerical value can still be used for prioritizing chains.

These names and values are defined and made available based on what priorities
are used by xtables when registering their default chains.

Most of the families use the same values, but bridge uses different ones from
the others. See the following tables that describe the values and compatibility.

.Standard priority names, family and hook compatibility matrix
[options="header"]
|==================
| Name | Value | Families | Hooks
| raw | -300 | ip, ip6, inet | all
| mangle | -150 | ip, ip6, inet | all
| dstnat | -100 | ip, ip6, inet | prerouting
| filter | 0 | ip, ip6, inet, arp, netdev | all
| security | 50 | ip, ip6, inet | all
| srcnat | 100 | ip, ip6, inet | postrouting
|===================

.Standard priority names and hook compatibility for the bridge family
[option="header"]
|==================
| Name | Value | Hooks
| dstnat | -300 | prerouting
| filter | -200 | all
| out | 100 | output
| srcnat | 300 | postrouting
|==================

Basic arithmetic expressions (addition and subtraction) can also be achieved
with these standard names to ease relative prioritizing, e.g. *mangle - 5* stands
for *-155*.  Values will also be printed like this until the value is not
further than 10 from the standard value.

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
-----
[verse]
{*add* | *insert*} *rule* ['family'] 'table' 'chain' [*handle* 'handle' | *index* 'index'] 'statement' ... [*comment* 'comment']
*replace rule* ['family'] 'table' 'chain' *handle* 'handle' 'statement' ... [*comment* 'comment']
*delete rule* ['family'] 'table' 'chain' *handle* 'handle'

Rules are added to chains in the given table. If the family is not specified, the
ip family is used. Rules are constructed from two kinds of components according
to a set of grammatical rules: expressions and statements.

The add and insert commands support an optional location specifier, which is
either a 'handle' or the 'index' (starting at zero) of an existing rule.
Internally, rule locations are always identified by 'handle' and the translation
from 'index' happens in userspace. This has two potential implications in case a
concurrent ruleset change happens after the translation was done: The effective
rule index might change if a rule was inserted or deleted before the referred
one. If the referred rule was deleted, the command is rejected by the kernel
just as if an invalid 'handle' was given.

A 'comment' is a single word or a double-quoted (") multi-word string which can
be used to make notes regarding the actual rule. *Note:* If you use bash for
adding rules, you have to escape the quotation marks, e.g. \"enable ssh for
servers\".

[horizontal]
*add*:: Add a new rule described by the list of statements. The
rule is appended to the given chain unless a location is specified, in which
case the rule is inserted after the specified rule.
*insert*:: Same as *add* except the rule is inserted at the
beginning of the chain or before the specified rule.
*replace*:: Similar to *add*, but the rule replaces the specified rule.
*delete*:: Delete the specified rule.

.*add a rule to ip table output chain*
-------------
nft add rule filter output ip daddr 192.168.0.0/24 accept # 'ip filter' is assumed
# same command, slightly more verbose
nft add rule ip filter output ip daddr 192.168.0.0/24 accept
--------------

.*delete rule from inet table*
-----------------------
# nft -a list ruleset
table inet filter {
	chain input {
		type filter hook input priority filter; policy accept;
		ct state established,related accept # handle 4
		ip saddr 10.1.1.1 tcp dport ssh accept # handle 5
	  ...
# delete the rule with handle 5
nft delete rule inet filter input handle 5
-------------------------

SETS
----
nftables offers two kinds of set concepts. Anonymous sets are sets that have no
specific name. The set members are enclosed in curly braces, with commas to
separate elements when creating the rule the set is used in. Once that rule is
removed, the set is removed as well. They cannot be updated, i.e. once an
anonymous set is declared it cannot be changed anymore except by
removing/altering the rule that uses the anonymous set.

.*Using anonymous sets to accept particular subnets and ports*
----------
nft add rule filter input ip saddr { 10.0.0.0/8, 192.168.0.0/16 } tcp dport { 22, 443 } accept
----------

Named sets are sets that need to be defined first before they can be referenced
in rules. Unlike anonymous sets, elements can be added to or removed from a
named set at any time. Sets are referenced from rules using an @ prefixed to the
sets name.

.*Using named sets to accept addresses and ports*
------------------
nft add rule filter input ip saddr @allowed_hosts tcp dport @allowed_ports accept
------------------

The sets allowed_hosts and allowed_ports need to be created first. The next
section describes nft set syntax in more detail.

[verse]
*add set* ['family'] 'table' 'set' *{ type* 'type' | *typeof* 'expression' *;* [*flags* 'flags' *;*] [*timeout* 'timeout' *;*] [*gc-interval* 'gc-interval' *;*] [*elements = {* 'element'[*,* ...] *} ;*] [*size* 'size' *;*] [*policy* 'policy' *;*] [*auto-merge ;*] *}*
{*delete* | *list* | *flush*} *set* ['family'] 'table' 'set'
*list sets* ['family']
*delete set* ['family'] 'table' *handle* 'handle'
{*add* | *delete*} *element* ['family'] 'table' 'set' *{* 'element'[*,* ...] *}*

Sets are element containers of a user-defined data type, they are uniquely
identified by a user-defined name and attached to tables. Their behaviour can
be tuned with the flags that can be specified at set creation time.

[horizontal]
*add*:: Add a new set in the specified table. See the Set specification table below for more information about how to specify properties of a set.
*delete*:: Delete the specified set.
*list*:: Display the elements in the specified set.
*flush*:: Remove all elements from the specified set.

.Set specifications
[options="header"]
|=================
|Keyword | Description | Type
|type |
data type of set elements |
string: ipv4_addr, ipv6_addr, ether_addr, inet_proto, inet_service, mark
|typeof |
data type of set element |
expression to derive the data type from
|flags |
set flags |
string: constant, dynamic, interval, timeout
|timeout |
time an element stays in the set, mandatory if set is added to from the packet path (ruleset)|
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 |
maximum number of elements in the set, mandatory if set is added to from the packet path (ruleset)|
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
-----
[verse]
*add map* ['family'] 'table' 'map' *{ type* 'type' | *typeof* 'expression' [*flags* 'flags' *;*] [*elements = {* 'element'[*,* ...] *} ;*] [*size* 'size' *;*] [*policy* 'policy' *;*] *}*
{*delete* | *list* | *flush*} *map* ['family'] 'table' 'map'
*list maps* ['family']

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

[horizontal]
*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
[options="header"]
|=================
|Keyword | Description | Type
|type |
data type of map elements |
string: ipv4_addr, ipv6_addr, ether_addr, inet_proto, inet_service, mark, counter, quota. Counter and quota can't be used as keys
|typeof |
data type of set element |
expression to derive the data type from
|flags |
map flags |
string: constant, interval
|elements |
elements contained by the map |
map data type
|size |
maximum number of elements in the map |
unsigned integer (64 bit)
| policy |
map policy |
string: performance [default], memory
|=================


ELEMENTS
--------
[verse]
____
{*add* | *create* | *delete* | *get* } *element* ['family'] 'table' 'set' *{* 'ELEMENT'[*,* ...] *}*

'ELEMENT' := 'key_expression' 'OPTIONS' [*:* 'value_expression']
'OPTIONS' := [*timeout* 'TIMESPEC'] [*expires* 'TIMESPEC'] [*comment* 'string']
'TIMESPEC' := ['num'*d*]['num'*h*]['num'*m*]['num'[*s*]]
____
Element-related commands allow to change contents of named sets and maps.
'key_expression' is typically a value matching the set type.
'value_expression' is not allowed in sets but mandatory when adding to maps, where it
matches the data part in its type definition. When deleting from maps, it may
be specified but is optional as 'key_expression' uniquely identifies the
element.

*create* command is similar to *add* with the exception that none of the
listed elements may already exist.

*get* command is useful to check if an element is contained in a set which may
be non-trivial in very large and/or interval sets. In the latter case, the
containing interval is returned instead of just the element itself.

.Element options
[options="header"]
|=================
|Option | Description
|timeout |
timeout value for sets/maps with flag *timeout*
|expires |
the time until given element expires, useful for ruleset replication only
|comment |
per element comment field
|=================


FLOWTABLES
-----------
[verse]
{*add* | *create*} *flowtable* ['family'] 'table' 'flowtable' *{ hook* 'hook' *priority* 'priority' *; devices = {* 'device'[*,* ...] *} ; }*
*list flowtables* ['family']
{*delete* | *list*} *flowtable* ['family'] 'table' 'flowtable'
*delete* *flowtable* ['family'] 'table' *handle* 'handle'

Flowtables allow you to accelerate packet forwarding in software. Flowtables
entries are represented through a tuple that is composed of the input interface,
source and destination address, source and destination port; and layer 3/4
protocols. Each entry also caches the destination interface and the gateway
address - to update the destination link-layer address - to forward packets.
The ttl and hoplimit fields are also decremented. Hence, flowtables provides an
alternative path that allow packets to bypass the classic forwarding path.
Flowtables reside in the ingress hook that is located before the prerouting
hook. You can select which flows you want to offload through the flow
expression from the forward chain. Flowtables are identified by their address
family and their name. The address family must be one of ip, ip6, or inet. 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.

The *priority* can be a signed integer or *filter* which stands for 0. Addition
and subtraction can be used to set relative priority, e.g. filter + 5 equals to
5.

[horizontal]
*add*:: Add a new flowtable for the given family with the given name.
*delete*:: Delete the specified flowtable.
*list*:: List all flowtables.


STATEFUL OBJECTS
----------------
[verse]
{*add* | *delete* | *list* | *reset*} 'type' ['family'] 'table' 'object'
*delete* 'type' ['family'] 'table' *handle* 'handle'
*list counters* ['family']
*list quotas* ['family']

Stateful objects are attached to tables and are identified by a unique name.
They group stateful information from rules, to reference them in rules the
keywords "type name" are used e.g. "counter name".

[horizontal]
*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.

include::stateful-objects.txt[]

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
~~~~~~~~~~~~~~~~
[verse]
*describe* 'expression' | 'data type'

The *describe* command shows information about the type of an expression and its data type.
A data type may also be given, in which nft will display more information
about the type.

.The describe command
---------------------
$ nft describe tcp flags
payload expression, datatype tcp_flag (TCP flag) (basetype bitmask, integer), 8 bits

predefined 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. +
Some types also have predefined symbolic constants.  Those can be listed
using the nft *describe* command:

---------------------
$ nft describe ct_state
datatype ct_state (conntrack state) (basetype bitmask, integer), 32 bits

pre-defined symbolic constants (in hexadecimal):
invalid                         0x00000001
new ...
---------------------

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.

include::data-types.txt[]

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.

include::primary-expression.txt[]

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

include::payload-expression.txt[]

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.

include::statements.txt[]

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. These are either related to creation and deletion of
objects or to packets for which *meta nftrace* was enabled. When they
occur, nft will print to stdout the monitored events in either JSON or
native nft format. +

[verse]
____
*monitor* [*new* | *destroy*] 'MONITOR_OBJECT'
*monitor* *trace*

'MONITOR_OBJECT' := *tables* | *chains* | *sets* | *rules* | *elements* | *ruleset*
____

To filter events related to a concrete object, use one of the keywords in
'MONITOR_OBJECT'.

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

The second form of invocation takes no further options and exclusively prints
events generated for packets with *nftrace* enabled.

Hit ^C to finish the monitor operation.

.Listen to all events, report in native nft format
--------------------------------------------------
% nft monitor
--------------------------------------------------

.Listen to deleted rules, report in JSON format
-----------------------------------------------
% nft -j monitor destroy rules
-----------------------------------------------

.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
---------------------

.Trace incoming packets from host 10.0.0.1
------------------------------------------
% nft add rule filter input ip saddr 10.0.0.1 meta nftrace set 1
% nft monitor trace
------------------------------------------

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 carets (^). 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 cannot 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
--------
[verse]
libnftables(3), libnftables-json(5), 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 © 2013-2018 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 licensed 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/.