Scroll to navigation

NFT(8)   NFT(8)

NAME

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

SYNOPSIS

nft [ -nNscaeSupyj ] [ -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

Print fully numerical output.

-s, --stateless

Omit stateful information of rules and stateful objects.

-N, --reversedns

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

-S, --service

Translate ports to service names as defined by /etc/services.

-u, --guid

Translate numeric UID/GID to names as defined by /etc/passwd and /etc/group.

-p, --numeric-protocol

Display layer 4 protocol numerically.

-y, --numeric-priority

Display base chain priority numerically.

-c, --check

Check commands validity without actually applying the changes.

-a, --handle

Show object handles in output.

-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.

-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. If filename is -, read from stdin.

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

-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.

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 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 (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 -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.

Table 1. 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.

Table 2. 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.

Table 3. 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]

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.

It is possible to limit list and flush to a specific address family only. For a list of valid family names, see the section called “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

{add | create} table [family] table [{ flags flags ; }]
{delete | list | flush} table [family] table
list tables
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 4. Table flags

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 0; }
# 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

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
list chains
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.

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.

Table 5. Supported chain types

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 two 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 priority parameter accepts a signed integer value or a standard priority name 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.

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.

Table 6. Standard priority names, family and hook compatibility matrix

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

Table 7. Standard priority names and hook compatibility for the bridge family

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 form 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

{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\".

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 input 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 0; 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.

add set [family] table set { type type ; [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
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.

add Add a new set in the specified table. See the Set specification table below for more information about how to specify a sets properties.
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.

Table 8. 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, 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

add map [family] table map { type type [flags flags ;] [elements = { element[, ...] } ;] [size size ;] [policy policy ;] }

{delete | list | flush} map [family] table map list maps {add | delete} element [family] table map { elements = { element[, ...] } ; }

Maps store data based on some specific key used as input. They are uniquely identified by a 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.

Table 9. 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 maximum number of elements in the map unsigned integer (64 bit)
policy map policy string: performance [default], memory

FLOWTABLES

{add | create} flowtable [family] table flowtable { hook hook priority priority ; devices = { device[, ...] } ; }

{delete | list} flowtable [family] table flowtable

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.

add Add a new flowtable for the given family with the given name.
delete Delete the specified flowtable.
list List all flowtables.

STATEFUL OBJECTS

{add | delete | list | reset} type [family] table object
delete type [family] table handle handle
list counters
list quotas

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 HELPER

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.

Table 10. 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. ip)
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"
  }
}

CT TIMEOUT

ct timeout name { protocol protocol ; policy = { state: value [, ...] } ; [l3proto family ;] }

Ct timeout is used to update connection tracking timeout values.Timeout policies are assigned with the ct timeout set statement. protocol and policy are mandatory, l3proto is derived from the table family by default.

Table 11. conntrack timeout specifications

Keyword Description Type
protocol layer 4 protocol of the timeout object string (e.g. ip)
state connection state name string (e.g. "established")
value timeout value for connection state unsigned integer
l3proto layer 3 protocol of the timeout object address family (e.g. ip)

defining and assigning ct timeout policy.

table ip filter {
        ct timeout customtimeout {
                protocol tcp;
                l3proto ip
                policy = { established: 120, close: 20 }
        }
        chain output {
                type filter hook output priority filter; policy accept;
                ct timeout set "customtimeout"
        }
}

testing the updated timeout policy.

% conntrack -E
It should display:
[UPDATE] tcp      6 120 ESTABLISHED src=172.16.19.128 dst=172.16.19.1
sport=22 dport=41360 [UNREPLIED] src=172.16.19.1 dst=172.16.19.128
sport=41360 dport=22

COUNTER

counter [packets bytes]

Table 12. 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]

Table 13. Quota specifications

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

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 a decimal, hexadecimal or octal number. The integer type does not 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 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)"

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. Addresses are specified as a host name or as hexadecimal halfwords separated by colons. Addresses might be enclosed in square brackets ("[]") to differentiate them from port numbers.

IPv6 address specification.

# abbreviated loopback address
filter output ip6 daddr ::1

IPv6 address specification with bracket notation.

# without [] the port number (22) would be parsed as part of the
# ipv6 address
ip6 nat prerouting tcp dport 2222 dnat to [1ce::d0]:22

BOOLEAN TYPE

Name Keyword Size Base type
Boolean boolean 1 bit integer

The boolean type is a syntactical helper type in userspace. Its 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).

Table 14. The following keywords will automatically resolve into a boolean type with given value

Keyword Value
exists 1
missing 0

Table 15. expressions support a boolean comparison

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

Boolean specification.

# 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.

Table 16. 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.

Table 17. 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 icmpx_code 8 bit integer

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

Table 18. 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.

Table 19. 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 icmpv6_type 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.

Table 20. 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

Table 21. 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:

Table 22. conntrack state (ct_state)

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

Table 23. conntrack direction (ct_dir)

Keyword Value
original 0
reply 1

Table 24. conntrack status (ct_status)

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

Table 25. 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 | ibrname | obrname | pkttype | cpu | iifgroup | oifgroup | cgroup | random | ipsec | iifkind | oifkind}

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 l4proto is useful to match a particular transport protocol that is part of either an IPv4 or IPv6 packet. It will also skip any IPv6 extension headers present in an IPv6 packet.

meta iif, oif, iifname and oifname are used to match the interface a packet arrived on or is about to be sent out on.

iif and oif are used to match on the interface index, whereas iifname and oifname are used to match on the interface name. This is not the same — assuming the rule

filter input meta iif "foo"

Then this rule can only be added if the interface "foo" exists. Also, the rule will continue to match even if the interface "foo" is renamed to "bar".

This is because internally the interface index is used. In case of dynamically created interfaces, such as tun/tap or dialup interfaces (ppp for example), it might be better to use iifname or oifname instead.

In these cases, the name is used so the interface doesn’t have to exist to add such a rule, it will stop matching if the interface gets renamed and it will match again in case interface gets deleted and later a new interface with the same name is created.

Table 26. 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)
l4proto layer 4 protocol, skips ipv6 extension headers integer (8 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 ifname
iiftype Input interface type iface_type
oif Output interface index iface_index
oifname Output interface name ifname
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
ibrname Input bridge interface name ifname
obrname Output bridge interface name ifname
pkttype packet type pkt_type
cpu cpu number processing the packet integer (32 bit)
iifgroup incoming device group devgroup
oifgroup outgoing device group devgroup
cgroup control group id integer (32 bit)
random pseudo-random number integer (32 bit)
ipsec boolean boolean (1 bit)
iifkind Input interface kind

Table 27. 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: host (addressed to local host), broadcast (to all), multicast (to group), other (addressed to another host).
ifkind Interface kind (16 byte string). Does not have to exist.

Using meta expressions.

# qualified meta expression
filter output meta oif eth0
# unqualified meta expression
filter output oif eth0
# packet was subject to ipsec processing
raw prerouting meta ipsec exists accept

SOCKET EXPRESSION

socket {transparent | mark}

Socket expression can be used to search for an existing open TCP/UDP socket and its attributes that can be associated with a packet. It looks for an established or non-zero bound listening socket (possibly with a non-local address).

Table 28. Available socket attributes

Name Description Type
transparent Value of the IP_TRANSPARENT socket option in the found socket. It can be 0 or 1. boolean (1 bit)
mark Value of the socket mark (SOL_SOCKET, SO_MARK). mark

Using socket expression.

# Mark packets that correspond to a transparent socket
table inet x {
    chain y {
        type filter hook prerouting priority -150; policy accept;
        socket transparent 1 mark set 0x00000001 accept
    }
}
# Trace packets that corresponds to a socket with a mark value of 15
table inet x {
    chain y {
        type filter hook prerouting priority -150; policy accept;
        socket mark 0x0000000f nftrace set 1
    }
}
# Set packet mark to socket mark
table inet x {
    chain y {
        type filter hook prerouting priority -150; policy accept;
        tcp dport 8080 mark set socket mark
    }
}

OSF EXPRESSION

osf [ttl {loose | skip}] {name | version}

The osf expression does passive operating system fingerprinting. This expression compares some data (Window Size, MSS, options and their order, DF, and others) from packets with the SYN bit set.

Table 29. Available osf attributes

Name Description Type
ttl Do TTL checks on the packet to determine the operating system. string
version Do OS version checks on the packet.
name Name of the OS signature to match. All signatures can be found at pf.os file. Use "unknown" for OS signatures that the expression could not detect. string

Available ttl values.

If no TTL attribute is passed, make a true IP header and fingerprint TTL true comparison. This generally works for LANs.
* loose: Check if the IP header's TTL is less than the fingerprint one. Works for globally-routable addresses.
* skip: Do not compare the TTL at all.

Using osf expression.

# Accept packets that match the "Linux" OS genre signature without comparing TTL.
table inet x {
    chain y {
        type filter hook input priority 0; policy accept;
        osf ttl skip name "Linux"
    }
}

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.

Table 30. 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 [ip | ip6] {classid | nexthop | mtu | ipsec}

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

Table 31. 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)
ipsec route via ipsec tunnel or transport boolean

Table 32. 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
filter output rt ipsec missing
# 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

IPSEC EXPRESSIONS

ipsec {in | out} [ spnum NUM ]  {reqid | spi}
ipsec {in | out} [ spnum NUM ]  {ip | ip6} {saddr | daddr}

An ipsec expression refers to ipsec data associated with a packet.

The in or out keyword needs to be used to specify if the expression should examine inbound or outbound policies. The in keyword can be used in the prerouting, input and forward hooks. The out keyword applies to forward, output and postrouting hooks. The optional keyword spnum can be used to match a specific state in a chain, it defaults to 0.

Table 33. Ipsec expression types

Keyword Description Type
reqid Request ID integer (32 bit)
spi Security Parameter Index integer (32 bit)
saddr Source address of the tunnel ipv4_addr/ipv6_addr
daddr Destination address of the tunnel ipv4_addr/ipv6_addr

PAYLOAD EXPRESSIONS

Payload expressions refer to data from the packet’s payload.

ETHERNET HEADER EXPRESSION

ether {daddr | saddr | type}

Table 34. 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 {id | cfi | pcp | type}

Table 35. 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 {htype | ptype | hlen | plen | operation | saddr { ip | ether } | daddr { ip | ether }

Table 36. 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
saddr ether Ethernet sender address ether_addr
daddr ether Ethernet target address ether_addr
saddr ip IPv4 sender address ipv4_addr
daddr ip IPv4 target address ipv4_addr

IPV4 HEADER EXPRESSION

ip {version | hdrlength | dscp | ecn | length | id | frag-off | ttl | protocol | checksum | saddr | daddr }

Table 37. 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 {type | code | checksum | id | sequence | gateway | mtu}

This expression refers to ICMP header fields. When using it in inet, bridge or netdev families, it will cause an implicit dependency on IPv4 to be created. To match on unusual cases like ICMP over IPv6, one has to add an explicit meta protocol ip6 match to the rule.

Table 38. 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)

IGMP HEADER EXPRESSION

igmp {type | mrt | checksum | group}

This expression refers to IGMP header fields. When using it in inet, bridge or netdev families, it will cause an implicit dependency on IPv4 to be created. To match on unusual cases like IGMP over IPv6, one has to add an explicit meta protocol ip6 match to the rule.

Table 39. IGMP header expression

Keyword Description Type
type IGMP type field igmp_type
mrt IGMP maximum response time field integer (8 bit)
checksum ICMP checksum field integer (16 bit)
group Group address integer (32 bit)

IPV6 HEADER EXPRESSION

ip6 {version | dscp | ecn | flowlabel | length | nexthdr | hoplimit | saddr | daddr}

This expression refers to the ipv6 header fields. Caution when using ip6 nexthdr, the value only refers to the next header, i.e. ip6 nexthdr tcp will only match if the ipv6 packet does not contain any extension headers. Packets that are fragmented or e.g. contain a routing extension headers will not be matched. Please use meta l4proto if you wish to match the real transport header and ignore any additional extension headers instead.

Table 40. 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

Using ip6 header expressions.

# matching if first extension header indicates a fragment
ip6 nexthdr ipv6-frag

ICMPV6 HEADER EXPRESSION

icmpv6 {type | code | checksum | parameter-problem | packet-too-big | id | sequence | max-delay}

This expression refers to ICMPv6 header fields. When using it in inet, bridge or netdev families, it will cause an implicit dependency on IPv6 to be created. To match on unusual cases like ICMPv6 over IPv4, one has to add an explicit meta protocol ip match to the rule.

Table 41. 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 {sport | dport | sequence | ackseq | doff | reserved | flags | window | checksum | urgptr}

Table 42. 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 {sport | dport | length | checksum}

Table 43. 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 {sport | dport | checksum}

Table 44. 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 {sport | dport | vtag | checksum}

Table 45. SCTP header expression

Keyword Description Type
sport Source port inet_service
dport Destination port inet_service
vtag Verification Tag integer (32 bit)
checksum Checksum integer (32 bit)

DCCP HEADER EXPRESSION

dccp {sport | dport}

Table 46. DCCP header expression

Keyword Description Type
sport Source port inet_service
dport Destination port inet_service

AUTHENTICATION HEADER EXPRESSION

ah {nexthdr | hdrlength | reserved | spi | sequence}

Table 47. 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 {spi | sequence}

Table 48. ESP header expression

Keyword Description Type
spi Security Parameter Index integer (32 bit)
sequence Sequence number integer (32 bit)

IPCOMP HEADER EXPRESSION

comp {nexthdr | flags | cpi}

Table 49. IPComp header expression

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

RAW PAYLOAD EXPRESSION

@base,offset,length

The raw payload expression instructs to load length bits starting at offset bits. Bit 0 refers to the very first bit — in the C programming language, this corresponds to the topmost bit, i.e. 0x80 in case of an octet. They are useful to match headers that do not have a human-readable template expression yet. Note that nft will not add dependencies for Raw payload expressions. If you e.g. want to match protocol fields of a transport header with protocol number 5, you need to manually exclude packets that have a different transport header, for instance by using meta l4proto 5 before the raw expression.

Table 50. Supported payload protocol bases

Base Description
ll Link layer, for example the Ethernet header
nh Network header, for example IPv4 or IPv6
th Transport Header, for example TCP

Matching destination port of both UDP and TCP.

inet filter input meta l4proto {tcp, udp} @th,16,16 { 53, 80 }

Rewrite arp packet target hardware address if target protocol address matches a given address.

input meta iifname enp2s0 arp ptype 0x0800 arp htype 1 arp hlen 6 arp plen 4 @nh,192,32 0xc0a88f10 @nh,144,48 set 0x112233445566 accept

EXTENSION HEADER EXPRESSIONS

Extension header expressions refer to data from variable-sized protocol headers, such as IPv6 extension headers and TCP 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}
srh {flags | tag | sid | seg-left}
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}

Table 51. IPv6 extension headers

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

Table 52. TCP Options

Keyword Description TCP option fields
eol End if 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}
ct [original | reply] {l3proto | protocol | bytes | packets | avgpkt | zone}
ct {original | reply} {proto-src | proto-dst}
ct {original | reply} {ip | ip6} {saddr | daddr}

Table 53. 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 byte count 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.

restrict the number of parallel connections to a server.

filter input tcp dport 22 meter test { ip saddr ct count over 2 } reject

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 and drop are absolute verdicts — they terminate ruleset evaluation immediately.

accept Terminate ruleset evaluation and accept the packet. The packet can still be dropped later by another hook, for instance accept in the forward hook still allows to drop the packet later in the postrouting hook, or another forward base chain that has a higher priority number and is evaluated afterwards in the processing pipeline.
drop Terminate ruleset evaluation and drop the packet. The drop occurs instantly, no further chains or hooks are evaluated. It is not possible to accept the packet in a later chain again, as those are not evaluated anymore for the packet.
queue Terminate ruleset evaluation and queue the packet to userspace. Userspace must provide a drop or accept verdict. In case of accept, processing resumes with the next base chain hook, not the rule following the queue verdict.
continue Continue ruleset evaluation with the next rule. This is the default behaviour in case a rule issues no verdict.
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 the base chain policy.
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 or a return verdict is issued. In case an absolute verdict is issued by a rule in the chain, ruleset evaluation terminates immediately and the specific action is taken.
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.

Using 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

payload_expression set value

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

extension_header_expression set value

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]
log level audit

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).

In the second form of invocation (if nflog_group 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.

In the third form of invocation (if level audit is specified), the Linux kernel writes a message into the audit buffer suitably formatted for reading with auditd. Therefore no further formatting options (such as prefix or flags) are allowed in this mode.

This is a non-terminating statement, so the rule evaluation continues after the packet is logged.

Table 54. 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, audit
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)

Table 55. 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 REJECT_WITH ]
REJECT_WITH := icmp type icmp_code |
                 icmpv6 type icmpv6_code |
                 icmpx type icmpx_code |
                 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.

Table 56. different ICMP reject variants are meant for use in different table families

Variant Family Type
icmp ip icmp_code
icmpv6 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.

Note that in bridge family, reject statement is only allowed in base chains which hook into input or prerouting.

COUNTER STATEMENT

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

counter packets number bytes number
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.

Table 57. 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.

Table 58. 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 / TIME_UNIT [burst packet_number packets]

limit rate [over] byte_number BYTE_UNIT / TIME_UNIT [burst byte_number BYTE_UNIT] TIME_UNIT := second | minute | hour | day BYTE_UNIT := bytes | kbytes | mbytes

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 optional over keyword makes it match over the specified rate.

Table 59. 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] [PRF_FLAGS]

snat to address - address [:port - port] [PRF_FLAGS] snat to { ip | ip6 } address - address [:port - port] [PR_FLAGS] dnat to address [:port] [PRF_FLAGS] dnat to address [:port - port] [PR_FLAGS] dnat to { ip | ip6 } address [:port - port] [PR_FLAGS] masquerade to [:port] [PRF_FLAGS] masquerade to [:port - port] [PRF_FLAGS] redirect to [:port] [PRF_FLAGS] redirect to [:port - port] [PRF_FLAGS] PRF_FLAGS := PRF_FLAG [, PRF_FLAGS] PR_FLAGS := PR_FLAG [, PR_FLAGS] PRF_FLAG := PR_FLAG | fully-random PR_FLAG := persistent | 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.

When used in the inet family (available with kernel 5.2), the dnat and snat statements require the use of the ip and ip6 keyword in case an address is provided, see the examples below.

Before kernel 4.18 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.

Table 60. 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, e.g. abcd::1234, or you can use a mapping, e.g. 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 bit)

Table 61. NAT statement flags

Flag Description
persistent Gives a client the same source-/destination-address for each connection.
random In kernel 5.0 and newer this is the same as fully-random. In earlier kernels the port mapping will be randomized using a 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
# inet family:
# handle ip dnat:
add rule inet nat prerouting dnat ip to 10.0.2.99
# handle ip6 dnat:
add rule inet nat prerouting dnat ip6 to fe80::dead
# this masquerades both ipv4 and ipv6:
add rule inet nat postrouting meta oif ppp0 masquerade

TPROXY STATEMENT

Tproxy redirects the packet to a local socket without changing the packet header in any way. If any of the arguments is missing the data of the incoming packet is used as parameter. Tproxy matching requires another rule that ensures the presence of transport protocol header is specified.

tproxy to address:port
tproxy to {address | :port}

This syntax can be used in ip/ip6 tables where network layer protocol is obvious. Either IP address or port can be specified, but at least one of them is necessary.

tproxy {ip | ip6} to address[:port]
tproxy to :port

This syntax can be used in inet tables. The ip/ip6 parameter defines the family the rule will match. The address parameter must be of this family. When only port is defined, the address family should not be specified. In this case the rule will match for both families.

Table 62. tproxy attributes

Name Description
address IP address the listening socket with IP_TRANSPARENT option is bound to.
port Port the listening socket with IP_TRANSPARENT option is bound to.

Example ruleset for tproxy statement.

table ip x {
    chain y {
        type filter hook prerouting priority -150; policy accept;
        tcp dport ntp tproxy to 1.1.1.1
        udp dport ssh tproxy to :2222
    }
}
table ip6 x {
    chain y {
       type filter hook prerouting priority -150; policy accept;
       tcp dport ntp tproxy to [dead::beef]
       udp dport ssh tproxy to :2222
    }
}
table inet x {
    chain y {
        type filter hook prerouting priority -150; policy accept;
        tcp dport 321 tproxy to :ssh
        tcp dport 99 tproxy ip to 1.1.1.1:999
        udp dport 155 tproxy ip6 to [dead::beef]:smux
    }
}

FLOW STATEMENT

A flow statement allows us to select what flows you want to accelerate forwarding through layer 3 network stack bypass. You have to specify the flowtable name where you want to offload this flow.

flow add @flowtable

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 re-inject the packet into the kernel. See libnetfilter_queue documentation for details.

queue [num queue_number] [bypass]
queue [num queue_number_from - queue_number_to] [QUEUE_FLAGS]
QUEUE_FLAGS := QUEUE_FLAG [, QUEUE_FLAGS]
QUEUE_FLAG  := bypass | fanout

Table 63. 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)

Table 64. 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 recommendations.
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

Table 65. 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, e.g. abcd::1234, or you can use a mapping, e.g. 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. It 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

SET STATEMENT

The set statement is used to dynamically add or update elements in a set from the packet path. The set setname must already exist in the given table and must have been created with the dynamic flag. Furthermore, these sets must specify both a maximum set size (to prevent memory exhaustion) and a timeout (so that number of entries in set will not grow indefinitely). The set statement can be used to e.g. create dynamic blacklists.

{add | update} @setname { expression [timeout timeout] [comment string] }

Example for simple blacklist.

# declare a set, bound to table "filter", in family "ip". Timeout and size are mandatory because we will add elements from packet path.
nft add set ip filter blackhole "{ type ipv4_addr; flags timeout; size 65536; }"
# whitelist internal interface.
nft add rule ip filter input meta iifname "internal" accept
# drop packets coming from blacklisted ip addresses.
nft add rule ip filter input ip saddr @blackhole counter drop
# add source ip addresses to the blacklist if more than 10 tcp connection requests occurred per second and ip address.
# entries will timeout after one minute, after which they might be re-added if limit condition persists.
nft add rule ip filter input tcp flags syn tcp dport ssh meter flood size 128000 { ip saddr timeout 10s limit rate over 10/second} add @blackhole { ip saddr timeout 1m } drop
# inspect state of the rate limit meter:
nft list meter ip filter flood
# inspect content of blackhole:
nft list set ip filter blackhole
# manually add two addresses to the set:
nft add element filter blackhole { 10.2.3.4, 10.23.1.42 }

MAP STATEMENT

The map statement is used to lookup data based on some specific input key.

expression map { MAP_ELEMENTS }
MAP_ELEMENTS := MAP_ELEMENT [, MAP_ELEMENTS]
MAP_ELEMENT  := key : value

The key is a value returned by expression.

Using the map statement.

# select DNAT target based on TCP dport:
# connections to port 80 are redirected to 192.168.1.100,
# connections to port 8888 are redirected to 192.168.1.101
nft add rule ip nat prerouting dnat tcp dport map { 80 : 192.168.1.100, 8888 : 192.168.1.101 }
# source address based SNAT:
# packets from net 192.168.1.0/24 will appear as originating from 10.0.0.1,
# packets from net 192.168.2.0/24 will appear as originating from 10.0.0.2
nft add rule ip nat postrouting snat to ip saddr map { 192.168.1.0/24 : 10.0.0.1, 192.168.2.0/24 : 10.0.0.2 }

VMAP STATEMENT

The verdict map (vmap) statement works analogous to the map statement, but contains verdicts as values.

expression vmap { VMAP_ELEMENTS }
VMAP_ELEMENTS := VMAP_ELEMENT [, VMAP_ELEMENTS]
VMAP_ELEMENT  := key : verdict

Using the vmap statement.

# jump to different chains depending on layer 4 protocol type:
nft add rule ip filter input ip protocol vmap { tcp : jump tcp-chain, udp : jump udp-chain , icmp : jump icmp-chain }

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 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 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

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

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/.

08/15/2019