Provided by: nftables_0.8.2-1_amd64 
NAME
nft - Administration tool of the nftables framework for packet filtering and classification
SYNOPSIS
nft [ -nNscae ] [ -I directory ] [ -f filename | -i | cmd ...]
nft -h
nft -v
DESCRIPTION
nft is the command line tool used to set up, maintain and inspect packet filtering and classification
rules in the Linux kernel, in the nftables framework. The Linux kernel subsystem is known as nf_tables,
and 'nf' stands for Netfilter.
OPTIONS
For a full summary of options, run nft --help.
-h, --help
Show help message and all options.
-v, --version
Show version.
-n, --numeric
Show data numerically. When used once (the default behaviour), skip lookup of addresses to symbol‐
ic names. Use twice to also show Internet services (port numbers) numerically. Use three times to
also show protocols and UIDs/GIDs numerically.
-N, --reversedns
Translate IP addresses to names. Usually requires network traffic for DNS lookup.
-s, --stateless
Omit stateful information of rules and stateful objects.
-c, --check
Check commands validity without actually applying the changes.
-a, --handle
Show rule handles in output.
-e, --echo
When inserting items into the ruleset using add, insert or replace commands, print notifications
just like nft monitor.
-I, --includepath directory
Add the directory directory to the list of directories to be searched for included files. This op‐
tion may be specified multiple times.
-f, --file filename
Read input from filename.
nft scripts must start #!/usr/sbin/nft -f
-i, --interactive
Read input from an interactive readline CLI.
INPUT FILE FORMAT
LEXICAL CONVENTIONS
Input is parsed line-wise. When the last character of a line, just before the newline character, is a
non-quoted backslash (\), the next line is treated as a continuation. Multiple commands on the same line
can be separated using a semicolon (;).
A hash sign (#) begins a comment. All following characters on the same line are ignored.
Identifiers begin with an alphabetic character (a-z,A-Z), followed zero or more alphanumeric characters
(a-z,A-Z,0-9) and the characters slash (/), backslash (\), underscore (_) and dot (.). Identifiers using
different characters or clashing with a keyword need to be enclosed in double quotes (").
INCLUDE FILES
include filename
Other files can be included by using the include statement. The directories to be searched for include
files can be specified using the -I/--includepath option. You can override this behaviour either by
prepending ./ to your path to force inclusion of files located in the current working directory (ie. rel‐
ative 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 com‐
pile 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 po‐
tentially 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 state‐
ments.
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 de‐
fault.
IPV4/IPV6/INET ADDRESS FAMILIES
The IPv4/IPv6/Inet address families handle IPv4, IPv6 or both types of packets. They contain five hooks
at different packet processing stages in the network stack.
IPv4/IPv6/Inet address family hooks
┌─────────────┬───────────────────────────────────────┐
│ Hook │ Description │
├─────────────┼───────────────────────────────────────┤
│ prerouting │ All packets entering the system are │
│ │ processed by the prerouting hook. It │
│ │ is invoked before the routing process │
│ │ and is used for early filtering or │
│ │ changing packet attributes that af‐ │
│ │ fect routing. │
├─────────────┼───────────────────────────────────────┤
│ input │ Packets delivered to the local system │
│ │ are processed by the input hook. │
├─────────────┼───────────────────────────────────────┤
│ forward │ Packets forwarded to a different host │
│ │ are processed by the forward hook. │
├─────────────┼───────────────────────────────────────┤
│ output │ Packets sent by local processes are │
│ │ processed by the output hook. │
├─────────────┼───────────────────────────────────────┤
│ postrouting │ All packets leaving the system are │
│ │ processed by the postrouting hook. │
└─────────────┴───────────────────────────────────────┘
ARP ADDRESS FAMILY
The ARP address family handles ARP packets received and sent by the system. It is commonly used to mangle
ARP packets for clustering.
ARP address family hooks
┌────────┬───────────────────────────────────────┐
│ Hook │ Description │
├────────┼───────────────────────────────────────┤
│ input │ Packets delivered to the local system │
│ │ are processed by the input hook. │
├────────┼───────────────────────────────────────┤
│ output │ Packets send by the local system are │
│ │ processed by the output hook. │
└────────┴───────────────────────────────────────┘
BRIDGE ADDRESS FAMILY
The bridge address family handles ethernet packets traversing bridge devices.
The list of supported hooks is identical to IPv4/IPv6/Inet address families above.
NETDEV ADDRESS FAMILY
The Netdev address family handles packets from ingress.
Netdev address family hooks
┌─────────┬───────────────────────────────────────┐
│ Hook │ Description │
├─────────┼───────────────────────────────────────┤
│ ingress │ All packets entering the system are │
│ │ processed by this hook. It is invoked │
│ │ before layer 3 protocol handlers and │
│ │ it can be used for early filtering │
│ │ and policing. │
└─────────┴───────────────────────────────────────┘
RULESET
{list | flush} ruleset [family]
{export} [ruleset] {format}
The ruleset keyword is used to identify the whole set of tables, chains, etc. currently in place in ker‐
nel. The following ruleset commands exist:
list Print the ruleset in human-readable format.
flush Clear the whole ruleset. Note that unlike iptables, this will remove all tables and whatever they
contain, effectively leading to an empty ruleset - no packet filtering will happen anymore, so the
kernel accepts any valid packet it receives.
export Print the ruleset in machine readable format. The mandatory format parameter may be either xml or
json.
It is possible to limit list and flush to a specific address family only. For a list of valid family
names, see ADDRESS FAMILIES above.
Note that contrary to what one might assume, the output generated by export is not parseable by nft -f.
Instead, the output of list command serves well for that purpose.
TABLES
{add | delete | list | flush} table [family] table
Tables are containers for chains, sets and stateful objects. They are identified by their address family
and their name. The address family must be one of ip, ip6, inet, arp, bridge, netdev. The inet address
family is a dummy family which is used to create hybrid IPv4/IPv6 tables. The meta expression nfproto
keyword can be used to test which family (ipv4 or ipv6) context the packet is being processed in. When
no address family is specified, ip is used by default.
add Add a new table for the given family with the given name.
delete Delete the specified table.
list List all chains and rules of the specified table.
flush Flush all chains and rules of the specified table.
CHAINS
{add | create} chain [family] table chain [ { type type hook hook [device device] priority priority ;
[policy policy ;] } ]
{delete | list | flush} chain [family] table chain
rename chain [family] table chain newname
Chains are containers for rules. They exist in two kinds, base chains and regular chains. A base chain is
an entry point for packets from the networking stack, a regular chain may be used as jump target and is
used for better rule organization.
add Add a new chain in the specified table. When a hook and priority value are specified, the chain is
created as a base chain and hooked up to the networking stack.
create Similar to the add command, but returns an error if the chain already exists.
delete Delete the specified chain. The chain must not contain any rules or be used as jump target.
rename Rename the specified chain.
list List all rules of the specified chain.
flush Flush all rules of the specified chain.
For base chains, type, hook and priority parameters are mandatory.
Supported chain types
┌────────┬──────────┬────────────────────────┬────────────────────────┐
│ Type │ Families │ Hooks │ Description │
├────────┼──────────┼────────────────────────┼────────────────────────┤
│ filter │ all │ all │ Standard chain type to │
│ │ │ │ use in doubt. │
├────────┼──────────┼────────────────────────┼────────────────────────┤
│ nat │ ip, ip6 │ prerouting, input, │ Chains of this type │
│ │ │ output, postrouting │ perform Network Ad‐ │
│ │ │ │ dress Translation │
│ │ │ │ based on conntrack en‐ │
│ │ │ │ tries. Only the first │
│ │ │ │ packet of a connection │
│ │ │ │ actually traverses │
│ │ │ │ this chain - its rules │
│ │ │ │ usually define details │
│ │ │ │ of the created con‐ │
│ │ │ │ ntrack entry (NAT │
│ │ │ │ statements for in‐ │
│ │ │ │ stance). │
├────────┼──────────┼────────────────────────┼────────────────────────┤
│ route │ ip, ip6 │ output │ If a packet has tra‐ │
│ │ │ │ versed a chain of this │
│ │ │ │ type and is about to │
│ │ │ │ be accepted, a new │
│ │ │ │ route lookup is per‐ │
│ │ │ │ formed if relevant │
│ │ │ │ parts of the IP header │
│ │ │ │ have changed. This al‐ │
│ │ │ │ lows to e.g. imple‐ │
│ │ │ │ ment policy routing │
│ │ │ │ selectors in nftables. │
└────────┴──────────┴────────────────────────┴────────────────────────┘
Apart from the special cases illustrated above (e.g. nat type not supporting forward hook or route type
only supporting output hook), there are two further quirks worth noticing:
• netdev family supports merely a single combination, namely filter type and ingress hook. Base chains in
this family also require the device parameter to be present since they exist per incoming interface on‐
ly.
• arp family supports only input and output hooks, both in chains of type filter.
The priority parameter accepts a signed integer value which specifies the order in which chains with same
hook value are traversed. The ordering is ascending, i.e. lower priority values have precedence over
higher ones.
Base chains also allow to set the chain's policy, i.e. what happens to packets not explicitly accepted or
refused in contained rules. Supported policy values are accept (which is the default) or drop.
RULES
[add | insert] rule [family] table chain [position position] statement...
replace rule [family] table chain handle handle statement...
delete rule [family] table chain handle handle
Rules are constructed from two kinds of components according to a set of grammatical rules: expressions
and statements.
add Add a new rule described by the list of statements. The rule is appended to the given chain unless
a position is specified, in which case the rule is appended to the rule given by the position.
insert Similar to the add command, but the rule is prepended to the beginning of the chain or before the
rule at the given position.
replace
Similar to the add command, but the rule replaces the specified rule.
delete Delete the specified rule.
SETS
add set [family] table set { type type ; [flags flags ;] [timeout timeout ;] [gc-interval gc-interval ;]
[elements = { element[,...] } ;] [size size ;] [policy policy ;] [auto-merge auto-merge ;] }
{delete | list | flush} set [family] table set
{add | delete} element [family] table set { element[,...] }
Sets are elements containers of an user-defined data type, they are uniquely identified by an user-de‐
fined name and attached to tables.
add Add a new set in the specified table.
delete Delete the specified set.
list Display the elements in the specified set.
flush Remove all elements from the specified set.
add element
Comma-separated list of elements to add into the specified set.
delete element
Comma-separated list of elements to delete from the specified set.
Set specifications
┌─────────────┬──────────────────────────────┬──────────────────────────────┐
│ Keyword │ Description │ Type │
├─────────────┼──────────────────────────────┼──────────────────────────────┤
│ type │ data type of set elements │ string: ipv4_addr, ipv6_ad‐ │
│ │ │ dr, ether_addr, inet_proto, │
│ │ │ inet_service, mark │
├─────────────┼──────────────────────────────┼──────────────────────────────┤
│ flags │ set flags │ string: constant, interval, │
│ │ │ timeout │
├─────────────┼──────────────────────────────┼──────────────────────────────┤
│ timeout │ time an element stays in the │ string, decimal followed by │
│ │ set │ unit. Units are: d, h, m, s │
├─────────────┼──────────────────────────────┼──────────────────────────────┤
│ gc-interval │ garbage collection interval, │ string, decimal followed by │
│ │ only available when timeout │ unit. Units are: d, h, m, s │
│ │ or flag timeout are active │ │
├─────────────┼──────────────────────────────┼──────────────────────────────┤
│ elements │ elements contained by the │ set data type │
│ │ set │ │
├─────────────┼──────────────────────────────┼──────────────────────────────┤
│ size │ maximun number of elements │ unsigned integer (64 bit) │
│ │ in the set │ │
├─────────────┼──────────────────────────────┼──────────────────────────────┤
│ policy │ set policy │ string: performance [de‐ │
│ │ │ fault], memory │
├─────────────┼──────────────────────────────┼──────────────────────────────┤
│ auto-merge │ automatic merge of adja‐ │ │
│ │ cent/overlapping set ele‐ │ │
│ │ ments (only for interval │ │
│ │ sets) │ │
└─────────────┴──────────────────────────────┴──────────────────────────────┘
MAPS
add map [family] table map { type type [flags flags ;] [elements = { element[,...] } ;] [size size ;]
[policy policy ;] }
{delete | list | flush} map [family] table map
{add | delete} element [family] table map { elements = { element[,...] } ; }
Maps store data based on some specific key used as input, they are uniquely identified by an user-defined
name and attached to tables.
add Add a new map in the specified table.
delete Delete the specified map.
list Display the elements in the specified map.
flush Remove all elements from the specified map.
add element
Comma-separated list of elements to add into the specified map.
delete element
Comma-separated list of element keys to delete from the specified map.
Map specifications
┌──────────┬──────────────────────────────┬──────────────────────────────┐
│ Keyword │ Description │ Type │
├──────────┼──────────────────────────────┼──────────────────────────────┤
│ type │ data type of map elements │ string ':' string: ipv4_ad‐ │
│ │ │ dr, 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 data type │
│ │ map │ │
├──────────┼──────────────────────────────┼──────────────────────────────┤
│ size │ maximun number of elements │ unsigned integer (64 bit) │
│ │ in the map │ │
├──────────┼──────────────────────────────┼──────────────────────────────┤
│ policy │ map policy │ string: performance [de‐ │
│ │ │ fault], memory │
└──────────┴──────────────────────────────┴──────────────────────────────┘
STATEFUL OBJECTS
{add | delete | list | reset} type [family] table object
Stateful objects are attached to tables and are identified by an unique name. They group stateful infor‐
mation from rules, to reference them in rules the keywords "type name" are used e.g. "counter name".
add Add a new stateful object in the specified table.
delete Delete the specified object.
list Display stateful information the object holds.
reset List-and-reset stateful object.
CT
ct helper helper { type type protocol protocol ; [l3proto family ;] }
Ct helper is used to define connection tracking helpers that can then be used in combination with the "ct
helper set" statement. type and protocol are mandatory, l3proto is derived from the table family by de‐
fault, i.e. in the inet table the kernel will try to load both the ipv4 and ipv6 helper backends, if they
are supported by the kernel.
conntrack helper specifications
┌──────────┬──────────────────────────────┬────────────────────────────┐
│ Keyword │ Description │ Type │
├──────────┼──────────────────────────────┼────────────────────────────┤
│ type │ name of helper type │ quoted string (e.g. "ftp") │
├──────────┼──────────────────────────────┼────────────────────────────┤
│ protocol │ layer 4 protocol of the │ string (e.g. tcp) │
│ │ helper │ │
├──────────┼──────────────────────────────┼────────────────────────────┤
│ l3proto │ layer 3 protocol of the │ address family (e.g. ip) │
│ │ helper │ │
└──────────┴──────────────────────────────┴────────────────────────────┘
defining and assigning ftp helper
Unlike iptables, helper assignment needs to be performed after the conntrack lookup has completed, for
example with the default 0 hook priority.
table inet myhelpers {
ct helper ftp-standard {
type "ftp" protocol tcp
}
chain prerouting {
type filter hook prerouting priority 0;
tcp dport 21 ct helper set "ftp-standard"
}
}
COUNTER
counter [packets bytes]
Counter specifications
┌─────────┬──────────────────────────┬───────────────────────────┐
│ Keyword │ Description │ Type │
├─────────┼──────────────────────────┼───────────────────────────┤
│ packets │ initial count of packets │ unsigned integer (64 bit) │
├─────────┼──────────────────────────┼───────────────────────────┤
│ bytes │ initial count of bytes │ unsigned integer (64 bit) │
└─────────┴──────────────────────────┴───────────────────────────┘
QUOTA
quota [over | until] [used]
Quota specifications
┌─────────┬──────────────────────────────┬──────────────────────────────┐
│ Keyword │ Description │ Type │
├─────────┼──────────────────────────────┼──────────────────────────────┤
│ quota │ quota limit, used as the │ Two arguments, unsigned in‐ │
│ │ quota name │ terger (64 bit) and string: │
│ │ │ bytes, kbytes, mbytes. │
│ │ │ "over" and "until" go before │
│ │ │ these arguments │
├─────────┼──────────────────────────────┼──────────────────────────────┤
│ used │ initial value of used quota │ Two arguments, unsigned in‐ │
│ │ │ terger (64 bit) and string: │
│ │ │ bytes, kbytes, mbytes │
└─────────┴──────────────────────────────┴──────────────────────────────┘
EXPRESSIONS
Expressions represent values, either constants like network addresses, port numbers etc. or data gathered
from the packet during ruleset evaluation. Expressions can be combined using binary, logical, relational
and other types of expressions to form complex or relational (match) expressions. They are also used as
arguments to certain types of operations, like NAT, packet marking etc.
Each expression has a data type, which determines the size, parsing and representation of symbolic values
and type compatibility with other expressions.
DESCRIBE COMMAND
describe expression
The describe command shows information about the type of an expression and its data type.
The describe command
$ nft describe tcp flags
payload expression, datatype tcp_flag (TCP flag) (basetype bitmask, integer), 8 bits
pre-defined symbolic constants:
fin 0x01
syn 0x02
rst 0x04
psh 0x08
ack 0x10
urg 0x20
ecn 0x40
cwr 0x80
DATA TYPES
Data types determine the size, parsing and representation of symbolic values and type compatibility of
expressions. A number of global data types exist, in addition some expression types define further data
types specific to the expression type. Most data types have a fixed size, some however may have a dynamic
size, f.i. the string type.
Types may be derived from lower order types, f.i. the IPv4 address type is derived from the integer type,
meaning an IPv4 address can also be specified as an integer value.
In certain contexts (set and map definitions) it is necessary to explicitly specify a data type. Each
type has a name which is used for this.
INTEGER TYPE
┌─────────┬─────────┬──────────┬───────────┐
│ Name │ Keyword │ Size │ Base type │
├─────────┼─────────┼──────────┼───────────┤
│ Integer │ integer │ variable │ - │
└─────────┴─────────┴──────────┴───────────┘
The integer type is used for numeric values. It may be specified as decimal, hexadecimal or octal number.
The integer type doesn't have a fixed size, its size is determined by the expression for which it is
used.
BITMASK TYPE
┌─────────┬─────────┬──────────┬───────────┐
│ Name │ Keyword │ Size │ Base type │
├─────────┼─────────┼──────────┼───────────┤
│ Bitmask │ bitmask │ variable │ integer │
└─────────┴─────────┴──────────┴───────────┘
The bitmask type (bitmask) is used for bitmasks.
STRING TYPE
┌────────┬─────────┬──────────┬───────────┐
│ Name │ Keyword │ Size │ Base type │
├────────┼─────────┼──────────┼───────────┤
│ String │ string │ variable │ - │
└────────┴─────────┴──────────┴───────────┘
The string type is used to for character strings. A string begins with an alphabetic character (a-zA-Z)
followed by zero or more alphanumeric characters or the characters /, -, _ and .. In addition anything
enclosed in double quotes (") is recognized as a string.
String specification
# Interface name
filter input iifname eth0
# Weird interface name
filter input iifname "(eth0)"
LINK LAYER ADDRESS TYPE
┌────────────────────┬─────────┬──────────┬───────────┐
│ Name │ Keyword │ Size │ Base type │
├────────────────────┼─────────┼──────────┼───────────┤
│ Link layer address │ lladdr │ variable │ integer │
└────────────────────┴─────────┴──────────┴───────────┘
The link layer address type is used for link layer addresses. Link layer addresses are specified as a
variable amount of groups of two hexadecimal digits separated using colons (:).
Link layer address specification
# Ethernet destination MAC address
filter input ether daddr 20:c9:d0:43:12:d9
IPV4 ADDRESS TYPE
┌──────────────┬───────────┬────────┬───────────┐
│ Name │ Keyword │ Size │ Base type │
├──────────────┼───────────┼────────┼───────────┤
│ IPv4 address │ ipv4_addr │ 32 bit │ integer │
└──────────────┴───────────┴────────┴───────────┘
The IPv4 address type is used for IPv4 addresses. Addresses are specified in either dotted decimal, dot‐
ted hexadecimal, dotted octal, decimal, hexadecimal, octal notation or as a host name. A host name will
be resolved using the standard system resolver.
IPv4 address specification
# dotted decimal notation
filter output ip daddr 127.0.0.1
# host name
filter output ip daddr localhost
IPV6 ADDRESS TYPE
┌──────────────┬───────────┬─────────┬───────────┐
│ Name │ Keyword │ Size │ Base type │
├──────────────┼───────────┼─────────┼───────────┤
│ IPv6 address │ ipv6_addr │ 128 bit │ integer │
└──────────────┴───────────┴─────────┴───────────┘
The IPv6 address type is used for IPv6 addresses. FIXME
IPv6 address specification
# abbreviated loopback address
filter output ip6 daddr ::1
BOOLEAN TYPE
┌─────────┬─────────┬───────┬───────────┐
│ Name │ Keyword │ Size │ Base type │
├─────────┼─────────┼───────┼───────────┤
│ Boolean │ boolean │ 1 bit │ integer │
└─────────┴─────────┴───────┴───────────┘
The boolean type is a syntactical helper type in user space. It's use is in the right-hand side of a
(typically implicit) relational expression to change the expression on the left-hand side into a boolean
check (usually for existence).
The following keywords will automatically resolve into a boolean type with given value:
┌─────────┬───────┐
│ Keyword │ Value │
├─────────┼───────┤
│ exists │ 1 │
├─────────┼───────┤
│ missing │ 0 │
└─────────┴───────┘
Boolean specification
The following expressions support a boolean comparison:
┌────────────┬───────────────────────────────────────┐
│ Expression │ Behaviour │
├────────────┼───────────────────────────────────────┤
│ fib │ Check route existence. │
├────────────┼───────────────────────────────────────┤
│ exthdr │ Check IPv6 extension header exis‐ │
│ │ tence. │
├────────────┼───────────────────────────────────────┤
│ tcp option │ Check TCP option header existence. │
└────────────┴───────────────────────────────────────┘
# match if route exists
filter input fib daddr . iif oif exists
# match only non-fragmented packets in IPv6 traffic
filter input exthdr frag missing
# match if TCP timestamp option is present
filter input tcp option timestamp exists
ICMP TYPE TYPE
┌───────────┬───────────┬───────┬───────────┐
│ Name │ Keyword │ Size │ Base type │
├───────────┼───────────┼───────┼───────────┤
│ ICMP Type │ icmp_type │ 8 bit │ integer │
└───────────┴───────────┴───────┴───────────┘
The ICMP Type type is used to conveniently specify the ICMP header's type field.
The following keywords may be used when specifying the ICMP type:
┌─────────────────────────┬───────┐
│ Keyword │ Value │
├─────────────────────────┼───────┤
│ echo-reply │ 0 │
├─────────────────────────┼───────┤
│ destination-unreachable │ 3 │
├─────────────────────────┼───────┤
│ source-quench │ 4 │
├─────────────────────────┼───────┤
│ redirect │ 5 │
├─────────────────────────┼───────┤
│ echo-request │ 8 │
├─────────────────────────┼───────┤
│ router-advertisement │ 9 │
├─────────────────────────┼───────┤
│ router-solicitation │ 10 │
├─────────────────────────┼───────┤
│ time-exceeded │ 11 │
├─────────────────────────┼───────┤
│ parameter-problem │ 12 │
├─────────────────────────┼───────┤
│ timestamp-request │ 13 │
├─────────────────────────┼───────┤
│ timestamp-reply │ 14 │
├─────────────────────────┼───────┤
│ info-request │ 15 │
├─────────────────────────┼───────┤
│ info-reply │ 16 │
├─────────────────────────┼───────┤
│ address-mask-request │ 17 │
├─────────────────────────┼───────┤
│ address-mask-reply │ 18 │
└─────────────────────────┴───────┘
ICMP Type specification
# match ping packets
filter output icmp type { echo-request, echo-reply }
ICMP CODE TYPE
┌───────────┬───────────┬───────┬───────────┐
│ Name │ Keyword │ Size │ Base type │
├───────────┼───────────┼───────┼───────────┤
│ ICMP Code │ icmp_code │ 8 bit │ integer │
└───────────┴───────────┴───────┴───────────┘
The ICMP Code type is used to conveniently specify the ICMP header's code field.
The following keywords may be used when specifying the ICMP code:
┌──────────────────┬───────┐
│ Keyword │ Value │
├──────────────────┼───────┤
│ net-unreachable │ 0 │
├──────────────────┼───────┤
│ host-unreachable │ 1 │
├──────────────────┼───────┤
│ prot-unreachable │ 2 │
├──────────────────┼───────┤
│ port-unreachable │ 3 │
├──────────────────┼───────┤
│ net-prohibited │ 9 │
├──────────────────┼───────┤
│ host-prohibited │ 10 │
├──────────────────┼───────┤
│ admin-prohibited │ 13 │
└──────────────────┴───────┘
ICMPV6 TYPE TYPE
┌─────────────┬─────────────┬───────┬───────────┐
│ Name │ Keyword │ Size │ Base type │
├─────────────┼─────────────┼───────┼───────────┤
│ ICMPv6 Type │ icmpv6_type │ 8 bit │ integer │
└─────────────┴─────────────┴───────┴───────────┘
The ICMPv6 Type type is used to conveniently specify the ICMPv6 header's type field.
The following keywords may be used when specifying the ICMPv6 type:
┌─────────────────────────┬───────┐
│ Keyword │ Value │
├─────────────────────────┼───────┤
│ destination-unreachable │ 1 │
├─────────────────────────┼───────┤
│ packet-too-big │ 2 │
├─────────────────────────┼───────┤
│ time-exceeded │ 3 │
├─────────────────────────┼───────┤
│ parameter-problem │ 4 │
├─────────────────────────┼───────┤
│ echo-request │ 128 │
├─────────────────────────┼───────┤
│ echo-reply │ 129 │
├─────────────────────────┼───────┤
│ mld-listener-query │ 130 │
├─────────────────────────┼───────┤
│ mld-listener-report │ 131 │
├─────────────────────────┼───────┤
│ mld-listener-done │ 132 │
├─────────────────────────┼───────┤
│ mld-listener-reduction │ 132 │
├─────────────────────────┼───────┤
│ nd-router-solicit │ 133 │
├─────────────────────────┼───────┤
│ nd-router-advert │ 134 │
├─────────────────────────┼───────┤
│ nd-neighbor-solicit │ 135 │
├─────────────────────────┼───────┤
│ nd-neighbor-advert │ 136 │
├─────────────────────────┼───────┤
│ nd-redirect │ 137 │
├─────────────────────────┼───────┤
│ router-renumbering │ 138 │
├─────────────────────────┼───────┤
│ ind-neighbor-solicit │ 141 │
├─────────────────────────┼───────┤
│ ind-neighbor-advert │ 142 │
├─────────────────────────┼───────┤
│ mld2-listener-report │ 143 │
└─────────────────────────┴───────┘
ICMPv6 Type specification
# match ICMPv6 ping packets
filter output icmpv6 type { echo-request, echo-reply }
ICMPV6 CODE TYPE
┌─────────────┬─────────────┬───────┬───────────┐
│ Name │ Keyword │ Size │ Base type │
├─────────────┼─────────────┼───────┼───────────┤
│ ICMPv6 Code │ icmpv6_code │ 8 bit │ integer │
└─────────────┴─────────────┴───────┴───────────┘
The ICMPv6 Code type is used to conveniently specify the ICMPv6 header's code field.
The following keywords may be used when specifying the ICMPv6 code:
┌──────────────────┬───────┐
│ Keyword │ Value │
├──────────────────┼───────┤
│ no-route │ 0 │
├──────────────────┼───────┤
│ admin-prohibited │ 1 │
├──────────────────┼───────┤
│ addr-unreachable │ 3 │
├──────────────────┼───────┤
│ port-unreachable │ 4 │
├──────────────────┼───────┤
│ policy-fail │ 5 │
├──────────────────┼───────┤
│ reject-route │ 6 │
└──────────────────┴───────┘
ICMPVX CODE TYPE
┌─────────────┬────────────┬───────┬───────────┐
│ Name │ Keyword │ Size │ Base type │
├─────────────┼────────────┼───────┼───────────┤
│ ICMPvX Code │ icmpx_code │ 8 bit │ integer │
└─────────────┴────────────┴───────┴───────────┘
The ICMPvX Code type abstraction is a set of values which overlap between ICMP and ICMPv6 Code types to
be used from the inet family.
The following keywords may be used when specifying the ICMPvX code:
┌──────────────────┬───────┐
│ Keyword │ Value │
├──────────────────┼───────┤
│ no-route │ 0 │
├──────────────────┼───────┤
│ port-unreachable │ 1 │
├──────────────────┼───────┤
│ host-unreachable │ 2 │
├──────────────────┼───────┤
│ admin-prohibited │ 3 │
└──────────────────┴───────┘
CONNTRACK TYPES
This is an overview of types used in ct expression and statement:
┌──────────────────────┬───────────┬─────────┬───────────┐
│ Name │ Keyword │ Size │ Base type │
├──────────────────────┼───────────┼─────────┼───────────┤
│ conntrack state │ ct_state │ 4 byte │ bitmask │
├──────────────────────┼───────────┼─────────┼───────────┤
│ conntrack direction │ ct_dir │ 8 bit │ integer │
├──────────────────────┼───────────┼─────────┼───────────┤
│ conntrack status │ ct_status │ 4 byte │ bitmask │
├──────────────────────┼───────────┼─────────┼───────────┤
│ conntrack event bits │ ct_event │ 4 byte │ bitmask │
├──────────────────────┼───────────┼─────────┼───────────┤
│ conntrack label │ ct_label │ 128 bit │ bitmask │
└──────────────────────┴───────────┴─────────┴───────────┘
For each of the types above, keywords are available for convenience:
conntrack state (ct_state)
┌─────────────┬───────┐
│ Keyword │ Value │
├─────────────┼───────┤
│ invalid │ 1 │
├─────────────┼───────┤
│ established │ 2 │
├─────────────┼───────┤
│ related │ 4 │
├─────────────┼───────┤
│ new │ 8 │
├─────────────┼───────┤
│ untracked │ 64 │
└─────────────┴───────┘
conntrack direction (ct_dir)
┌──────────┬───────┐
│ Keyword │ Value │
├──────────┼───────┤
│ original │ 0 │
├──────────┼───────┤
│ reply │ 1 │
└──────────┴───────┘
conntrack status (ct_status)
┌────────────┬───────┐
│ Keyword │ Value │
├────────────┼───────┤
│ expected │ 1 │
├────────────┼───────┤
│ seen-reply │ 2 │
├────────────┼───────┤
│ assured │ 4 │
├────────────┼───────┤
│ confirmed │ 8 │
├────────────┼───────┤
│ snat │ 16 │
├────────────┼───────┤
│ dnat │ 32 │
├────────────┼───────┤
│ dying │ 512 │
└────────────┴───────┘
conntrack event bits (ct_event)
┌───────────┬───────┐
│ Keyword │ Value │
├───────────┼───────┤
│ new │ 1 │
├───────────┼───────┤
│ related │ 2 │
├───────────┼───────┤
│ destroy │ 4 │
├───────────┼───────┤
│ reply │ 8 │
├───────────┼───────┤
│ assured │ 16 │
├───────────┼───────┤
│ protoinfo │ 32 │
├───────────┼───────┤
│ helper │ 64 │
├───────────┼───────┤
│ mark │ 128 │
├───────────┼───────┤
│ seqadj │ 256 │
├───────────┼───────┤
│ secmark │ 512 │
├───────────┼───────┤
│ label │ 1024 │
└───────────┴───────┘
Possible keywords for conntrack label type (ct_label) are read at runtime from /etc/connlabel.conf.
PRIMARY EXPRESSIONS
The lowest order expression is a primary expression, representing either a constant or a single datum
from a packet's payload, meta data or a stateful module.
META EXPRESSIONS
meta {length | nfproto | l4proto | protocol | priority}
[meta] {mark | iif | iifname | iiftype | oif | oifname | oiftype | skuid | skgid | nftrace | rtclassid |
ibriport | obriport | pkttype | cpu | iifgroup | oifgroup | cgroup | random | secpath}
A meta expression refers to meta data associated with a packet.
There are two types of meta expressions: unqualified and qualified meta expressions. Qualified meta ex‐
pressions require the meta keyword before the meta key, unqualified meta expressions can be specified by
using the meta key directly or as qualified meta expressions.
Meta expression types
┌───────────┬──────────────────────────────┬───────────────────┐
│ Keyword │ Description │ Type │
├───────────┼──────────────────────────────┼───────────────────┤
│ length │ Length of the packet in │ integer (32 bit) │
│ │ bytes │ │
├───────────┼──────────────────────────────┼───────────────────┤
│ nfproto │ real hook protocol family, │ integer (32 bit) │
│ │ useful only in inet table │ │
├───────────┼──────────────────────────────┼───────────────────┤
│ protocol │ Ethertype protocol value │ ether_type │
├───────────┼──────────────────────────────┼───────────────────┤
│ priority │ TC packet priority │ tc_handle │
├───────────┼──────────────────────────────┼───────────────────┤
│ mark │ Packet mark │ mark │
├───────────┼──────────────────────────────┼───────────────────┤
│ iif │ Input interface index │ iface_index │
├───────────┼──────────────────────────────┼───────────────────┤
│ iifname │ Input interface name │ string │
├───────────┼──────────────────────────────┼───────────────────┤
│ iiftype │ Input interface type │ iface_type │
├───────────┼──────────────────────────────┼───────────────────┤
│ oif │ Output interface index │ iface_index │
├───────────┼──────────────────────────────┼───────────────────┤
│ oifname │ Output interface name │ string │
├───────────┼──────────────────────────────┼───────────────────┤
│ oiftype │ Output interface hardware │ iface_type │
│ │ type │ │
├───────────┼──────────────────────────────┼───────────────────┤
│ skuid │ UID associated with origi‐ │ uid │
│ │ nating socket │ │
├───────────┼──────────────────────────────┼───────────────────┤
│ skgid │ GID associated with origi‐ │ gid │
│ │ nating socket │ │
├───────────┼──────────────────────────────┼───────────────────┤
│ rtclassid │ Routing realm │ realm │
├───────────┼──────────────────────────────┼───────────────────┤
│ ibriport │ Input bridge interface name │ string │
├───────────┼──────────────────────────────┼───────────────────┤
│ obriport │ Output bridge interface name │ string │
├───────────┼──────────────────────────────┼───────────────────┤
│ pkttype │ packet type │ pkt_type │
├───────────┼──────────────────────────────┼───────────────────┤
│ cpu │ cpu number processing the │ integer (32 bits) │
│ │ packet │ │
├───────────┼──────────────────────────────┼───────────────────┤
│ iifgroup │ incoming device group │ devgroup │
├───────────┼──────────────────────────────┼───────────────────┤
│ oifgroup │ outgoing device group │ devgroup │
├───────────┼──────────────────────────────┼───────────────────┤
│ cgroup │ control group id │ integer (32 bits) │
├───────────┼──────────────────────────────┼───────────────────┤
│ random │ pseudo-random number │ integer (32 bits) │
├───────────┼──────────────────────────────┼───────────────────┤
│ secpath │ boolean │ boolean (1 bit) │
└───────────┴──────────────────────────────┴───────────────────┘
Meta expression specific types
┌───────────────┬───────────────────────────────────────┐
│ Type │ Description │
├───────────────┼───────────────────────────────────────┤
│ iface_index │ Interface index (32 bit number). Can │
│ │ be specified numerically or as name │
│ │ of an existing interface. │
├───────────────┼───────────────────────────────────────┤
│ ifname │ Interface name (16 byte string). Does │
│ │ not have to exist. │
├───────────────┼───────────────────────────────────────┤
│ iface_type │ Interface type (16 bit number). │
├───────────────┼───────────────────────────────────────┤
│ uid │ User ID (32 bit number). Can be spec‐ │
│ │ ified numerically or as user name. │
├───────────────┼───────────────────────────────────────┤
│ gid │ Group ID (32 bit number). Can be │
│ │ specified numerically or as group │
│ │ name. │
├───────────────┼───────────────────────────────────────┤
│ realm │ Routing Realm (32 bit number). Can be │
│ │ specified numerically or as symbolic │
│ │ name defined in │
│ │ /etc/iproute2/rt_realms. │
├───────────────┼───────────────────────────────────────┤
│ devgroup_type │ Device group (32 bit number). Can be │
│ │ specified numerically or as symbolic │
│ │ name defined in /etc/iproute2/group. │
├───────────────┼───────────────────────────────────────┤
│ pkt_type │ Packet type: Unicast (addressed to │
│ │ local host), Broadcast (to all), Mul‐ │
│ │ ticast (to group). │
└───────────────┴───────────────────────────────────────┘
Using meta expressions
# qualified meta expression
filter output meta oif eth0
# unqualified meta expression
filter output oif eth0
# packed was subject to ipsec processing
raw prerouting meta secpath exists accept
FIB EXPRESSIONS
fib {saddr | daddr | {mark | iif | oif}} {oif | oifname | type}
A fib expression queries the fib (forwarding information base) to obtain information such as the output
interface index a particular address would use. The input is a tuple of elements that is used as input to
the fib lookup functions.
fib expression specific types
┌─────────┬────────────────────────┬──────────────────┐
│ Keyword │ Description │ Type │
├─────────┼────────────────────────┼──────────────────┤
│ oif │ Output interface index │ integer (32 bit) │
├─────────┼────────────────────────┼──────────────────┤
│ oifname │ Output interface name │ string │
├─────────┼────────────────────────┼──────────────────┤
│ type │ Address type │ fib_addrtype │
└─────────┴────────────────────────┴──────────────────┘
Using fib expressions
# drop packets without a reverse path
filter prerouting fib saddr . iif oif missing drop
# drop packets to address not configured on ininterface
filter prerouting fib daddr . iif type != { local, broadcast, multicast } drop
# perform lookup in a specific 'blackhole' table (0xdead, needs ip appropriate ip rule)
filter prerouting meta mark set 0xdead fib daddr . mark type vmap { blackhole : drop, prohibit : jump prohibited, unreachable : drop }
ROUTING EXPRESSIONS
rt {classid | nexthop}
A routing expression refers to routing data associated with a packet.
Routing expression types
┌─────────┬──────────────────────────────┬─────────────────────┐
│ Keyword │ Description │ Type │
├─────────┼──────────────────────────────┼─────────────────────┤
│ classid │ Routing realm │ realm │
├─────────┼──────────────────────────────┼─────────────────────┤
│ nexthop │ Routing nexthop │ ipv4_addr/ipv6_addr │
├─────────┼──────────────────────────────┼─────────────────────┤
│ mtu │ TCP maximum segment size of │ integer (16 bit) │
│ │ route │ │
└─────────┴──────────────────────────────┴─────────────────────┘
Routing expression specific types
┌───────┬───────────────────────────────────────┐
│ Type │ Description │
├───────┼───────────────────────────────────────┤
│ realm │ Routing Realm (32 bit number). Can be │
│ │ specified numerically or as symbolic │
│ │ name defined in │
│ │ /etc/iproute2/rt_realms. │
└───────┴───────────────────────────────────────┘
Using routing expressions
# IP family independent rt expression
filter output rt classid 10
# IP family dependent rt expressions
ip filter output rt nexthop 192.168.0.1
ip6 filter output rt nexthop fd00::1
inet filter output rt ip nexthop 192.168.0.1
inet filter output rt ip6 nexthop fd00::1
PAYLOAD EXPRESSIONS
Payload expressions refer to data from the packet's payload.
ETHERNET HEADER EXPRESSION
ether [ethernet header field]
Ethernet header expression types
┌─────────┬─────────────────────────┬────────────┐
│ Keyword │ Description │ Type │
├─────────┼─────────────────────────┼────────────┤
│ daddr │ Destination MAC address │ ether_addr │
├─────────┼─────────────────────────┼────────────┤
│ saddr │ Source MAC address │ ether_addr │
├─────────┼─────────────────────────┼────────────┤
│ type │ EtherType │ ether_type │
└─────────┴─────────────────────────┴────────────┘
VLAN HEADER EXPRESSION
vlan [VLAN header field]
VLAN header expression
┌─────────┬────────────────────────────┬──────────────────┐
│ Keyword │ Description │ Type │
├─────────┼────────────────────────────┼──────────────────┤
│ id │ VLAN ID (VID) │ integer (12 bit) │
├─────────┼────────────────────────────┼──────────────────┤
│ cfi │ Canonical Format Indicator │ integer (1 bit) │
├─────────┼────────────────────────────┼──────────────────┤
│ pcp │ Priority code point │ integer (3 bit) │
├─────────┼────────────────────────────┼──────────────────┤
│ type │ EtherType │ ether_type │
└─────────┴────────────────────────────┴──────────────────┘
ARP HEADER EXPRESSION
arp [ARP header field]
ARP header expression
┌───────────┬──────────────────────┬──────────────────┐
│ Keyword │ Description │ Type │
├───────────┼──────────────────────┼──────────────────┤
│ htype │ ARP hardware type │ integer (16 bit) │
├───────────┼──────────────────────┼──────────────────┤
│ ptype │ EtherType │ ether_type │
├───────────┼──────────────────────┼──────────────────┤
│ hlen │ Hardware address len │ integer (8 bit) │
├───────────┼──────────────────────┼──────────────────┤
│ plen │ Protocol address len │ integer (8 bit) │
├───────────┼──────────────────────┼──────────────────┤
│ operation │ Operation │ arp_op │
└───────────┴──────────────────────┴──────────────────┘
IPV4 HEADER EXPRESSION
ip [IPv4 header field]
IPv4 header expression
┌───────────┬──────────────────────────────┬──────────────────────────────┐
│ Keyword │ Description │ Type │
├───────────┼──────────────────────────────┼──────────────────────────────┤
│ version │ IP header version (4) │ integer (4 bit) │
├───────────┼──────────────────────────────┼──────────────────────────────┤
│ hdrlength │ IP header length including │ integer (4 bit) FIXME scal‐ │
│ │ options │ ing │
├───────────┼──────────────────────────────┼──────────────────────────────┤
│ dscp │ Differentiated Services Code │ dscp │
│ │ Point │ │
├───────────┼──────────────────────────────┼──────────────────────────────┤
│ ecn │ Explicit Congestion Notifi‐ │ ecn │
│ │ cation │ │
├───────────┼──────────────────────────────┼──────────────────────────────┤
│ length │ Total packet length │ integer (16 bit) │
├───────────┼──────────────────────────────┼──────────────────────────────┤
│ id │ IP ID │ integer (16 bit) │
├───────────┼──────────────────────────────┼──────────────────────────────┤
│ frag-off │ Fragment offset │ integer (16 bit) │
├───────────┼──────────────────────────────┼──────────────────────────────┤
│ ttl │ Time to live │ integer (8 bit) │
├───────────┼──────────────────────────────┼──────────────────────────────┤
│ protocol │ Upper layer protocol │ inet_proto │
├───────────┼──────────────────────────────┼──────────────────────────────┤
│ checksum │ IP header checksum │ integer (16 bit) │
├───────────┼──────────────────────────────┼──────────────────────────────┤
│ saddr │ Source address │ ipv4_addr │
├───────────┼──────────────────────────────┼──────────────────────────────┤
│ daddr │ Destination address │ ipv4_addr │
└───────────┴──────────────────────────────┴──────────────────────────────┘
ICMP HEADER EXPRESSION
icmp [ICMP header field]
ICMP header expression
┌──────────┬──────────────────────────────┬──────────────────┐
│ Keyword │ Description │ Type │
├──────────┼──────────────────────────────┼──────────────────┤
│ type │ ICMP type field │ icmp_type │
├──────────┼──────────────────────────────┼──────────────────┤
│ code │ ICMP code field │ integer (8 bit) │
├──────────┼──────────────────────────────┼──────────────────┤
│ checksum │ ICMP checksum field │ integer (16 bit) │
├──────────┼──────────────────────────────┼──────────────────┤
│ id │ ID of echo request/response │ integer (16 bit) │
├──────────┼──────────────────────────────┼──────────────────┤
│ sequence │ sequence number of echo re‐ │ integer (16 bit) │
│ │ quest/response │ │
├──────────┼──────────────────────────────┼──────────────────┤
│ gateway │ gateway of redirects │ integer (32 bit) │
├──────────┼──────────────────────────────┼──────────────────┤
│ mtu │ MTU of path MTU discovery │ integer (16 bit) │
└──────────┴──────────────────────────────┴──────────────────┘
IPV6 HEADER EXPRESSION
ip6 [IPv6 header field]
IPv6 header expression
┌───────────┬──────────────────────────────┬──────────────────┐
│ Keyword │ Description │ Type │
├───────────┼──────────────────────────────┼──────────────────┤
│ version │ IP header version (6) │ integer (4 bit) │
├───────────┼──────────────────────────────┼──────────────────┤
│ dscp │ Differentiated Services Code │ dscp │
│ │ Point │ │
├───────────┼──────────────────────────────┼──────────────────┤
│ ecn │ Explicit Congestion Notifi‐ │ ecn │
│ │ cation │ │
├───────────┼──────────────────────────────┼──────────────────┤
│ flowlabel │ Flow label │ integer (20 bit) │
├───────────┼──────────────────────────────┼──────────────────┤
│ length │ Payload length │ integer (16 bit) │
├───────────┼──────────────────────────────┼──────────────────┤
│ nexthdr │ Nexthdr protocol │ inet_proto │
├───────────┼──────────────────────────────┼──────────────────┤
│ hoplimit │ Hop limit │ integer (8 bit) │
├───────────┼──────────────────────────────┼──────────────────┤
│ saddr │ Source address │ ipv6_addr │
├───────────┼──────────────────────────────┼──────────────────┤
│ daddr │ Destination address │ ipv6_addr │
└───────────┴──────────────────────────────┴──────────────────┘
ICMPV6 HEADER EXPRESSION
icmpv6 [ICMPv6 header field]
ICMPv6 header expression
┌───────────────────┬──────────────────────────────┬──────────────────┐
│ Keyword │ Description │ Type │
├───────────────────┼──────────────────────────────┼──────────────────┤
│ type │ ICMPv6 type field │ icmpv6_type │
├───────────────────┼──────────────────────────────┼──────────────────┤
│ code │ ICMPv6 code field │ integer (8 bit) │
├───────────────────┼──────────────────────────────┼──────────────────┤
│ checksum │ ICMPv6 checksum field │ integer (16 bit) │
├───────────────────┼──────────────────────────────┼──────────────────┤
│ parameter-problem │ pointer to problem │ integer (32 bit) │
├───────────────────┼──────────────────────────────┼──────────────────┤
│ packet-too-big │ oversized MTU │ integer (32 bit) │
├───────────────────┼──────────────────────────────┼──────────────────┤
│ id │ ID of echo request/response │ integer (16 bit) │
├───────────────────┼──────────────────────────────┼──────────────────┤
│ sequence │ sequence number of echo re‐ │ integer (16 bit) │
│ │ quest/response │ │
├───────────────────┼──────────────────────────────┼──────────────────┤
│ max-delay │ maximum response delay of │ integer (16 bit) │
│ │ MLD queries │ │
└───────────────────┴──────────────────────────────┴──────────────────┘
TCP HEADER EXPRESSION
tcp [TCP header field]
TCP header expression
┌──────────┬────────────────────────┬──────────────────────────────┐
│ Keyword │ Description │ Type │
├──────────┼────────────────────────┼──────────────────────────────┤
│ sport │ Source port │ inet_service │
├──────────┼────────────────────────┼──────────────────────────────┤
│ dport │ Destination port │ inet_service │
├──────────┼────────────────────────┼──────────────────────────────┤
│ sequence │ Sequence number │ integer (32 bit) │
├──────────┼────────────────────────┼──────────────────────────────┤
│ ackseq │ Acknowledgement number │ integer (32 bit) │
├──────────┼────────────────────────┼──────────────────────────────┤
│ doff │ Data offset │ integer (4 bit) FIXME scal‐ │
│ │ │ ing │
├──────────┼────────────────────────┼──────────────────────────────┤
│ reserved │ Reserved area │ integer (4 bit) │
├──────────┼────────────────────────┼──────────────────────────────┤
│ flags │ TCP flags │ tcp_flag │
├──────────┼────────────────────────┼──────────────────────────────┤
│ window │ Window │ integer (16 bit) │
├──────────┼────────────────────────┼──────────────────────────────┤
│ checksum │ Checksum │ integer (16 bit) │
├──────────┼────────────────────────┼──────────────────────────────┤
│ urgptr │ Urgent pointer │ integer (16 bit) │
└──────────┴────────────────────────┴──────────────────────────────┘
UDP HEADER EXPRESSION
udp [UDP header field]
UDP header expression
┌──────────┬─────────────────────┬──────────────────┐
│ Keyword │ Description │ Type │
├──────────┼─────────────────────┼──────────────────┤
│ sport │ Source port │ inet_service │
├──────────┼─────────────────────┼──────────────────┤
│ dport │ Destination port │ inet_service │
├──────────┼─────────────────────┼──────────────────┤
│ length │ Total packet length │ integer (16 bit) │
├──────────┼─────────────────────┼──────────────────┤
│ checksum │ Checksum │ integer (16 bit) │
└──────────┴─────────────────────┴──────────────────┘
UDP-LITE HEADER EXPRESSION
udplite [UDP-Lite header field]
UDP-Lite header expression
┌──────────┬──────────────────┬──────────────────┐
│ Keyword │ Description │ Type │
├──────────┼──────────────────┼──────────────────┤
│ sport │ Source port │ inet_service │
├──────────┼──────────────────┼──────────────────┤
│ dport │ Destination port │ inet_service │
├──────────┼──────────────────┼──────────────────┤
│ checksum │ Checksum │ integer (16 bit) │
└──────────┴──────────────────┴──────────────────┘
SCTP HEADER EXPRESSION
sctp [SCTP header field]
SCTP header expression
┌──────────┬──────────────────┬──────────────────┐
│ Keyword │ Description │ Type │
├──────────┼──────────────────┼──────────────────┤
│ sport │ Source port │ inet_service │
├──────────┼──────────────────┼──────────────────┤
│ dport │ Destination port │ inet_service │
├──────────┼──────────────────┼──────────────────┤
│ vtag │ Verfication Tag │ integer (32 bit) │
├──────────┼──────────────────┼──────────────────┤
│ checksum │ Checksum │ integer (32 bit) │
└──────────┴──────────────────┴──────────────────┘
DCCP HEADER EXPRESSION
dccp [DCCP header field]
DCCP header expression
┌─────────┬──────────────────┬──────────────┐
│ Keyword │ Description │ Type │
├─────────┼──────────────────┼──────────────┤
│ sport │ Source port │ inet_service │
├─────────┼──────────────────┼──────────────┤
│ dport │ Destination port │ inet_service │
└─────────┴──────────────────┴──────────────┘
AUTHENTICATION HEADER EXPRESSION
ah [AH header field]
AH header expression
┌───────────┬──────────────────────────┬──────────────────┐
│ Keyword │ Description │ Type │
├───────────┼──────────────────────────┼──────────────────┤
│ nexthdr │ Next header protocol │ inet_proto │
├───────────┼──────────────────────────┼──────────────────┤
│ hdrlength │ AH Header length │ integer (8 bit) │
├───────────┼──────────────────────────┼──────────────────┤
│ reserved │ Reserved area │ integer (16 bit) │
├───────────┼──────────────────────────┼──────────────────┤
│ spi │ Security Parameter Index │ integer (32 bit) │
├───────────┼──────────────────────────┼──────────────────┤
│ sequence │ Sequence number │ integer (32 bit) │
└───────────┴──────────────────────────┴──────────────────┘
ENCRYPTED SECURITY PAYLOAD HEADER EXPRESSION
esp [ESP header field]
ESP header expression
┌──────────┬──────────────────────────┬──────────────────┐
│ Keyword │ Description │ Type │
├──────────┼──────────────────────────┼──────────────────┤
│ spi │ Security Parameter Index │ integer (32 bit) │
├──────────┼──────────────────────────┼──────────────────┤
│ sequence │ Sequence number │ integer (32 bit) │
└──────────┴──────────────────────────┴──────────────────┘
IPCOMP HEADER EXPRESSION
comp [IPComp header field]
IPComp header expression
┌─────────┬─────────────────────────────┬──────────────────┐
│ Keyword │ Description │ Type │
├─────────┼─────────────────────────────┼──────────────────┤
│ nexthdr │ Next header protocol │ inet_proto │
├─────────┼─────────────────────────────┼──────────────────┤
│ flags │ Flags │ bitmask │
├─────────┼─────────────────────────────┼──────────────────┤
│ cpi │ Compression Parameter Index │ integer (16 bit) │
└─────────┴─────────────────────────────┴──────────────────┘
EXTENSION HEADER EXPRESSIONS
Extension header expressions refer to data from variable-sized protocol headers, such as IPv6 extension
headers and TCPs options.
nftables currently supports matching (finding) a given ipv6 extension header or TCP option.
hbh {nexthdr | hdrlength}
frag {nexthdr | frag-off | more-fragments | id}
rt {nexthdr | hdrlength | type | seg-left}
dst {nexthdr | hdrlength}
mh {nexthdr | hdrlength | checksum | type}
tcp option {eol | noop | maxseg | window | sack-permitted | sack | sack0 | sack1 | sack2 | sack3 |
timestamp} tcp_option_field
The following syntaxes are valid only in a relational expression with boolean type on right-hand side for
checking header existence only:
exthdr {hbh | frag | rt | dst | mh}
tcp option {eol | noop | maxseg | window | sack-permitted | sack | sack0 | sack1 | sack2 | sack3 |
timestamp}
IPv6 extension headers
┌─────────┬──────────────────────┐
│ Keyword │ Description │
├─────────┼──────────────────────┤
│ hbh │ Hop by Hop │
├─────────┼──────────────────────┤
│ rt │ Routing Header │
├─────────┼──────────────────────┤
│ frag │ Fragmentation header │
├─────────┼──────────────────────┤
│ dst │ dst options │
├─────────┼──────────────────────┤
│ mh │ Mobility Header │
└─────────┴──────────────────────┘
TCP Options
┌────────────────┬──────────────────────────────┬────────────────────────────┐
│ Keyword │ Description │ TCP option fields │
├────────────────┼──────────────────────────────┼────────────────────────────┤
│ eol │ End of option list │ kind │
├────────────────┼──────────────────────────────┼────────────────────────────┤
│ noop │ 1 Byte TCP No-op options │ kind │
├────────────────┼──────────────────────────────┼────────────────────────────┤
│ maxseg │ TCP Maximum Segment Size │ kind, length, size │
├────────────────┼──────────────────────────────┼────────────────────────────┤
│ window │ TCP Window Scaling │ kind, length, count │
├────────────────┼──────────────────────────────┼────────────────────────────┤
│ sack-permitted │ TCP SACK permitted │ kind, length │
├────────────────┼──────────────────────────────┼────────────────────────────┤
│ sack │ TCP Selective Acknowledge‐ │ kind, length, left, right │
│ │ ment (alias of block 0) │ │
├────────────────┼──────────────────────────────┼────────────────────────────┤
│ sack0 │ TCP Selective Acknowledge‐ │ kind, length, left, right │
│ │ ment (block 0) │ │
├────────────────┼──────────────────────────────┼────────────────────────────┤
│ sack1 │ TCP Selective Acknowledge‐ │ kind, length, left, right │
│ │ ment (block 1) │ │
├────────────────┼──────────────────────────────┼────────────────────────────┤
│ sack2 │ TCP Selective Acknowledge‐ │ kind, length, left, right │
│ │ ment (block 2) │ │
├────────────────┼──────────────────────────────┼────────────────────────────┤
│ sack3 │ TCP Selective Acknowledge‐ │ kind, length, left, right │
│ │ ment (block 3) │ │
├────────────────┼──────────────────────────────┼────────────────────────────┤
│ 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 be‐
fore the conntrack key, others must be used directly because they are direction agnostic. The packets,
bytes and avgpkt keywords can be used with or without a direction. If the direction is omitted, the sum
of the original and the reply direction is returned. The same is true for the zone, if a direction is
given, the zone is only matched if the zone id is tied to the given direction.
ct {state | direction | status | mark | expiration | helper | label | l3proto | protocol | bytes |
packets | avgpkt | zone}
ct {original | reply} {l3proto | protocol | proto-src | proto-dst | bytes | packets | avgpkt | zone}
ct {original | reply} {ip | ip6} {saddr | daddr}
Conntrack expressions
┌────────────┬──────────────────────────────┬─────────────────────┐
│ Keyword │ Description │ Type │
├────────────┼──────────────────────────────┼─────────────────────┤
│ state │ State of the connection │ ct_state │
├────────────┼──────────────────────────────┼─────────────────────┤
│ direction │ Direction of the packet rel‐ │ ct_dir │
│ │ ative to the connection │ │
├────────────┼──────────────────────────────┼─────────────────────┤
│ status │ Status of the connection │ ct_status │
├────────────┼──────────────────────────────┼─────────────────────┤
│ mark │ Connection mark │ mark │
├────────────┼──────────────────────────────┼─────────────────────┤
│ expiration │ Connection expiration time │ time │
├────────────┼──────────────────────────────┼─────────────────────┤
│ helper │ Helper associated with the │ string │
│ │ connection │ │
├────────────┼──────────────────────────────┼─────────────────────┤
│ label │ Connection tracking label │ ct_label │
│ │ bit or symbolic name defined │ │
│ │ in connlabel.conf in the │ │
│ │ nftables include path │ │
├────────────┼──────────────────────────────┼─────────────────────┤
│ l3proto │ Layer 3 protocol of the con‐ │ nf_proto │
│ │ nection │ │
├────────────┼──────────────────────────────┼─────────────────────┤
│ saddr │ Source address of the con‐ │ ipv4_addr/ipv6_addr │
│ │ nection for the given direc‐ │ │
│ │ tion │ │
├────────────┼──────────────────────────────┼─────────────────────┤
│ daddr │ Destination address of the │ ipv4_addr/ipv6_addr │
│ │ connection for the given di‐ │ │
│ │ rection │ │
├────────────┼──────────────────────────────┼─────────────────────┤
│ protocol │ Layer 4 protocol of the con‐ │ inet_proto │
│ │ nection for the given direc‐ │ │
│ │ tion │ │
├────────────┼──────────────────────────────┼─────────────────────┤
│ proto-src │ Layer 4 protocol source for │ integer (16 bit) │
│ │ the given direction │ │
├────────────┼──────────────────────────────┼─────────────────────┤
│ proto-dst │ Layer 4 protocol destination │ integer (16 bit) │
│ │ for the given direction │ │
├────────────┼──────────────────────────────┼─────────────────────┤
│ packets │ packet count seen in the │ integer (64 bit) │
│ │ given direction or sum of │ │
│ │ original and reply │ │
├────────────┼──────────────────────────────┼─────────────────────┤
│ bytes │ bytecount seen, see descrip‐ │ integer (64 bit) │
│ │ tion for packets keyword │ │
├────────────┼──────────────────────────────┼─────────────────────┤
│ avgpkt │ average bytes per packet, │ integer (64 bit) │
│ │ see description for packets │ │
│ │ keyword │ │
├────────────┼──────────────────────────────┼─────────────────────┤
│ zone │ conntrack zone │ integer (16 bit) │
└────────────┴──────────────────────────────┴─────────────────────┘
A description of conntrack-specific types listed above can be found sub-section CONNTRACK TYPES above.
STATEMENTS
Statements represent actions to be performed. They can alter control flow (return, jump to a different
chain, accept or drop the packet) or can perform actions, such as logging, rejecting a packet, etc.
Statements exist in two kinds. Terminal statements unconditionally terminate evaluation of the current
rule, non-terminal statements either only conditionally or never terminate evaluation of the current
rule, in other words, they are passive from the ruleset evaluation perspective. There can be an arbitrary
amount of non-terminal statements in a rule, but only a single terminal statement as the final statement.
VERDICT STATEMENT
The verdict statement alters control flow in the ruleset and issues policy decisions for packets.
{accept | drop | queue | continue | return}
{jump | goto} chain
accept Terminate ruleset evaluation and accept the packet.
drop Terminate ruleset evaluation and drop the packet.
queue Terminate ruleset evaluation and queue the packet to userspace.
continue
Continue ruleset evaluation with the next rule. FIXME
return Return from the current chain and continue evaluation at the next rule in the last chain. If is‐
sued in a base chain, it is equivalent to accept.
jump chain
Continue evaluation at the first rule in chain. The current position in the ruleset is pushed to
a call stack and evaluation will continue there when the new chain is entirely evaluated of a re‐
turn verdict is issued.
goto chain
Similar to jump, but the current position is not pushed to the call stack, meaning that after the
new chain evaluation will continue at the last chain instead of the one containing the goto state‐
ment.
Verdict statements
# process packets from eth0 and the internal network in from_lan
# chain, drop all packets from eth0 with different source addresses.
filter input iif eth0 ip saddr 192.168.0.0/24 jump from_lan
filter input iif eth0 drop
PAYLOAD STATEMENT
The payload statement alters packet content. It can be used for example to set ip DSCP (differv) header
field or ipv6 flow labels.
route some packets instead of bridging
# redirect tcp:http from 192.160.0.0/16 to local machine for routing instead of bridging
# assumes 00:11:22:33:44:55 is local MAC address.
bridge input meta iif eth0 ip saddr 192.168.0.0/16 tcp dport 80 meta pkttype set unicast ether daddr set 00:11:22:33:44:55
Set IPv4 DSCP header field
ip forward ip dscp set 42
EXTENSION HEADER STATEMENT
The extension header statement alters packet content in variable-sized headers. This can currently be
used to alter the TCP Maximum segment size of packets, similar to TCPMSS.
change tcp mss
tcp flags syn tcp option maxseg size set 1360
# set a size based on route information:
tcp flags syn tcp option maxseg size set rt mtu
LOG STATEMENT
log [prefix quoted_string] [level syslog-level] [flags log-flags]
log group nflog_group [prefix quoted_string] [queue-threshold value] [snaplen size]
The log statement enables logging of matching packets. When this statement is used from a rule, the Linux
kernel will print some information on all matching packets, such as header fields, via the kernel log
(where it can be read with dmesg(1) or read in the syslog). If the group number is specified, the Linux
kernel will pass the packet to nfnetlink_log which will multicast the packet through a netlink socket to
the specified multicast group. One or more userspace processes may subscribe to the group to receive the
packets, see libnetfilter_queue documentation for details. This is a non-terminating statement, so the
rule evaluation continues after the packet is logged.
log statement options
┌─────────────────┬──────────────────────────────┬──────────────────────────────┐
│ Keyword │ Description │ Type │
├─────────────────┼──────────────────────────────┼──────────────────────────────┤
│ prefix │ Log message prefix │ quoted string │
├─────────────────┼──────────────────────────────┼──────────────────────────────┤
│ level │ Syslog level of logging │ string: emerg, alert, crit, │
│ │ │ err, warn [default], notice, │
│ │ │ info, debug │
├─────────────────┼──────────────────────────────┼──────────────────────────────┤
│ group │ NFLOG group to send messages │ unsigned integer (16 bit) │
│ │ to │ │
├─────────────────┼──────────────────────────────┼──────────────────────────────┤
│ snaplen │ Length of packet payload to │ unsigned integer (32 bit) │
│ │ include in netlink message │ │
├─────────────────┼──────────────────────────────┼──────────────────────────────┤
│ queue-threshold │ Number of packets to queue │ unsigned integer (32 bit) │
│ │ inside the kernel before │ │
│ │ sending them to userspace │ │
└─────────────────┴──────────────────────────────┴──────────────────────────────┘
log-flags
┌──────────────┬───────────────────────────────────────┐
│ Flag │ Description │
├──────────────┼───────────────────────────────────────┤
│ tcp sequence │ Log TCP sequence numbers. │
├──────────────┼───────────────────────────────────────┤
│ tcp options │ Log options from the TCP packet head‐ │
│ │ er. │
├──────────────┼───────────────────────────────────────┤
│ ip options │ Log options from the IP/IPv6 packet │
│ │ header. │
├──────────────┼───────────────────────────────────────┤
│ skuid │ Log the userid of the process which │
│ │ generated the packet. │
├──────────────┼───────────────────────────────────────┤
│ ether │ Decode MAC addresses and protocol. │
├──────────────┼───────────────────────────────────────┤
│ all │ Enable all log flags listed above. │
└──────────────┴───────────────────────────────────────┘
Using log statement
# log the UID which generated the packet and ip options
ip filter output log flags skuid flags ip options
# log the tcp sequence numbers and tcp options from the TCP packet
ip filter output log flags tcp sequence,options
# enable all supported log flags
ip6 filter output log flags all
REJECT STATEMENT
reject [ with {icmp | icmp6 | icmpx} type {icmp_type | icmp6_type | icmpx_type} ]
reject [ with tcp reset ]
A reject statement is used to send back an error packet in response to the matched packet otherwise it is
equivalent to drop so it is a terminating statement, ending rule traversal. This statement is only valid
in the input, forward and output chains, and user-defined chains which are only called from those chains.
The different ICMP reject variants are meant for use in different table families:
┌─────────┬────────┬─────────────┐
│ Variant │ Family │ Type │
├─────────┼────────┼─────────────┤
│ icmp │ ip │ icmp_code │
├─────────┼────────┼─────────────┤
│ icmp6 │ ip6 │ icmpv6_code │
├─────────┼────────┼─────────────┤
│ icmpx │ inet │ icmpx_code │
└─────────┴────────┴─────────────┘
For a description of the different types and a list of supported keywords refer to DATA TYPES section
above. The common default reject value is port-unreachable.
COUNTER STATEMENT
A counter statement sets the hit count of packets along with the number of bytes.
counter [ packets number bytes number ]
CONNTRACK STATEMENT
The conntrack statement can be used to set the conntrack mark and conntrack labels.
ct {mark | event | label | zone} set value
The ct statement sets meta data associated with a connection. The zone id has to be assigned before a
conntrack lookup takes place, i.e. this has to be done in prerouting and possibly output (if locally gen‐
erated packets need to be placed in a distinct zone), with a hook priority of -300.
Conntrack statement types
┌─────────┬──────────────────────────────┬───────────────────────────┐
│ Keyword │ Description │ Value │
├─────────┼──────────────────────────────┼───────────────────────────┤
│ event │ conntrack event bits │ bitmask, integer (32 bit) │
├─────────┼──────────────────────────────┼───────────────────────────┤
│ helper │ name of ct helper object to │ quoted string │
│ │ assign to the connection │ │
├─────────┼──────────────────────────────┼───────────────────────────┤
│ 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, pkt‐
type, nftrace.
meta {mark | priority | pkttype | nftrace} set value
A meta statement sets meta data associated with a packet.
Meta statement types
┌──────────┬──────────────────────────────┬───────────┐
│ Keyword │ Description │ Value │
├──────────┼──────────────────────────────┼───────────┤
│ priority │ TC packet priority │ tc_handle │
├──────────┼──────────────────────────────┼───────────┤
│ mark │ Packet mark │ mark │
├──────────┼──────────────────────────────┼───────────┤
│ pkttype │ packet type │ pkt_type │
├──────────┼──────────────────────────────┼───────────┤
│ nftrace │ ruleset packet tracing │ 0, 1 │
│ │ on/off. Use monitor trace │ │
│ │ command to watch traces │ │
└──────────┴──────────────────────────────┴───────────┘
LIMIT STATEMENT
limit rate [over] packet_number / {second | minute | hour | day} [burst packet_number packets]
limit rate [over] byte_number {bytes | kbytes | mbytes} / {second | minute | hour | day | week} [burst
byte_number bytes]
A limit statement matches at a limited rate using a token bucket filter. A rule using this statement will
match until this limit is reached. It can be used in combination with the log statement to give limited
logging. The over keyword, that is optional, makes it match over the specified rate.
limit statement values
┌───────────────┬───────────────────┬───────────────────────────┐
│ Value │ Description │ Type │
├───────────────┼───────────────────┼───────────────────────────┤
│ packet_number │ Number of packets │ unsigned integer (32 bit) │
├───────────────┼───────────────────┼───────────────────────────┤
│ byte_number │ Number of bytes │ unsigned integer (32 bit) │
└───────────────┴───────────────────┴───────────────────────────┘
NAT STATEMENTS
snat to address [:port] [persistent, random, fully-random]
snat to address - address [:port - port] [persistent, random, fully-random]
dnat to address [:port] [persistent, random, fully-random]
dnat to address [:port - port] [persistent, random, fully-random]
masquerade to [:port] [persistent, random, fully-random]
masquerade to [:port - port] [persistent, random, fully-random]
redirect to [:port] [persistent, random, fully-random]
redirect to [:port - port] [persistent, random, fully-random]
The nat statements are only valid from nat chain types.
The snat and masquerade statements specify that the source address of the packet should be modified.
While snat is only valid in the postrouting and input chains, masquerade makes sense only in postrouting.
The dnat and redirect statements are only valid in the prerouting and output chains, they specify that
the destination address of the packet should be modified. You can use non-base chains which are called
from base chains of nat chain type too. All future packets in this connection will also be mangled, and
rules should cease being examined.
The masquerade statement is a special form of snat which always uses the outgoing interface's IP address
to translate to. It is particularly useful on gateways with dynamic (public) IP addresses.
The redirect statement is a special form of dnat which always translates the destination address to the
local host's one. It comes in handy if one only wants to alter the destination port of incoming traffic
on different interfaces.
Note that all nat statements require both prerouting and postrouting base chains to be present since oth‐
erwise packets on the return path won't be seen by netfilter and therefore no reverse translation will
take place.
NAT statement values
┌────────────┬──────────────────────────────┬──────────────────────────────┐
│ Expression │ Description │ Type │
├────────────┼──────────────────────────────┼──────────────────────────────┤
│ address │ Specifies that the │ ipv4_addr, ipv6_addr, eg. │
│ │ source/destination address │ abcd::1234, or you can use a │
│ │ of the packet should be mod‐ │ mapping, eg. meta mark map { │
│ │ ified. You may specify a │ 10 : 192.168.1.2, 20 : │
│ │ mapping to relate a list of │ 192.168.1.3 } │
│ │ tuples composed of arbitrary │ │
│ │ expression key with address │ │
│ │ value. │ │
├────────────┼──────────────────────────────┼──────────────────────────────┤
│ port │ Specifies that the │ port number (16 bits) │
│ │ source/destination address │ │
│ │ of the packet should be mod‐ │ │
│ │ ified. │ │
└────────────┴──────────────────────────────┴──────────────────────────────┘
NAT statement flags
┌──────────────┬───────────────────────────────────────┐
│ Flag │ Description │
├──────────────┼───────────────────────────────────────┤
│ persistent │ Gives a client the same source-/des‐ │
│ │ tination-address for each connection. │
├──────────────┼───────────────────────────────────────┤
│ random │ If used then port mapping will be │
│ │ randomized using a random seeded MD5 │
│ │ hash mix using source and destination │
│ │ address and destination port. │
├──────────────┼───────────────────────────────────────┤
│ fully-random │ If used then port mapping is generat‐ │
│ │ ed based on a 32-bit pseudo-random │
│ │ algorithm. │
└──────────────┴───────────────────────────────────────┘
Using NAT statements
# create a suitable table/chain setup for all further examples
add table nat
add chain nat prerouting { type nat hook prerouting priority 0; }
add chain nat postrouting { type nat hook postrouting priority 100; }
# translate source addresses of all packets leaving via eth0 to address 1.2.3.4
add rule nat postrouting oif eth0 snat to 1.2.3.4
# redirect all traffic entering via eth0 to destination address 192.168.1.120
add rule nat prerouting iif eth0 dnat to 192.168.1.120
# translate source addresses of all packets leaving via eth0 to whatever
# locally generated packets would use as source to reach the same destination
add rule nat postrouting oif eth0 masquerade
# redirect incoming TCP traffic for port 22 to port 2222
add rule nat prerouting tcp dport 22 redirect to :2222
QUEUE STATEMENT
This statement passes the packet to userspace using the nfnetlink_queue handler. The packet is put into
the queue identified by its 16-bit queue number. Userspace can inspect and modify the packet if desired.
Userspace must then drop or reinject the packet into the kernel. See libnetfilter_queue documentation for
details.
queue [num queue_number] [bypass]
queue [num queue_number_from - queue_number_to] [bypass,fanout]
queue statement values
┌───────────────────┬──────────────────────────────┬───────────────────────────┐
│ Value │ Description │ Type │
├───────────────────┼──────────────────────────────┼───────────────────────────┤
│ queue_number │ Sets queue number, default │ unsigned integer (16 bit) │
│ │ is 0. │ │
├───────────────────┼──────────────────────────────┼───────────────────────────┤
│ queue_number_from │ Sets initial queue in the │ unsigned integer (16 bit) │
│ │ range, if fanout is used. │ │
├───────────────────┼──────────────────────────────┼───────────────────────────┤
│ queue_number_to │ Sets closing queue in the │ unsigned integer (16 bit) │
│ │ range, if fanout is used. │ │
└───────────────────┴──────────────────────────────┴───────────────────────────┘
queue statement flags
┌────────┬───────────────────────────────────────┐
│ Flag │ Description │
├────────┼───────────────────────────────────────┤
│ bypass │ Let packets go through if userspace │
│ │ application cannot back off. Before │
│ │ using this flag, read libnetfil‐ │
│ │ ter_queue documentation for perfor‐ │
│ │ mance tuning recomendations. │
├────────┼───────────────────────────────────────┤
│ fanout │ Distribute packets between several │
│ │ queues. │
└────────┴───────────────────────────────────────┘
DUP STATEMENT
The dup statement is used to duplicate a packet and send the copy to a different destination.
dup to device
dup to address device device
Dup statement values
┌────────────┬──────────────────────────────┬──────────────────────────────┐
│ Expression │ Description │ Type │
├────────────┼──────────────────────────────┼──────────────────────────────┤
│ address │ Specifies that the copy of │ ipv4_addr, ipv6_addr, eg. │
│ │ the packet should be sent to │ abcd::1234, or you can use a │
│ │ a new gateway. │ mapping, eg. ip saddr map { │
│ │ │ 192.168.1.2 : 10.1.1.1 } │
├────────────┼──────────────────────────────┼──────────────────────────────┤
│ device │ Specifies that the copy │ string │
│ │ should be transmitted via │ │
│ │ device. │ │
└────────────┴──────────────────────────────┴──────────────────────────────┘
Using the dup statement
# send to machine with ip address 10.2.3.4 on eth0
ip filter forward dup to 10.2.3.4 device "eth0"
# copy raw frame to another interface
netdetv ingress dup to "eth0"
dup to "eth0"
# combine with map dst addr to gateways
dup to ip daddr map { 192.168.7.1 : "eth0", 192.168.7.2 : "eth1" }
FWD STATEMENT
The fwd statement is used to redirect a raw packet to another interface. Its is only available in the
netdev family ingress hook. It is similar to the dup statement except that no copy is made.
fwd to device
ADDITIONAL COMMANDS
These are some additional commands included in nft.
MONITOR
The monitor command allows you to listen to Netlink events produced by the nf_tables subsystem, related
to creation and deletion of objects. When they occur, nft will print to stdout the monitored events in
either XML, JSON or native nft format.
To filter events related to a concrete object, use one of the keywords 'tables', 'chains', 'sets',
'rules', 'elements' , 'ruleset'.
To filter events related to a concrete action, use keyword 'new' or 'destroy'.
Hit ^C to finish the monitor operation.
Listen to all events, report in native nft format
% nft monitor
Listen to added tables, report in XML format
% nft monitor new tables xml
Listen to deleted rules, report in JSON format
% nft monitor destroy rules json
Listen to both new and destroyed chains, in native nft format
% nft monitor chains
Listen to ruleset events such as table, chain, rule, set, counters and quotas, in native nft format
% nft monitor ruleset
ERROR REPORTING
When an error is detected, nft shows the line(s) containing the error, the position of the erroneous
parts in the input stream and marks up the erroneous parts using carrets (^). If the error results from
the combination of two expressions or statements, the part imposing the constraints which are violated is
marked using tildes (~).
For errors returned by the kernel, nft can't detect which parts of the input caused the error and the en‐
tire command is marked.
Error caused by single incorrect expression
<cmdline>:1:19-22: Error: Interface does not exist
filter output oif eth0
^^^^
Error caused by invalid combination of two expressions
<cmdline>:1:28-36: Error: Right hand side of relational expression (==) must be constant
filter output tcp dport == tcp dport
~~ ^^^^^^^^^
Error returned by the kernel
<cmdline>:0:0-23: Error: Could not process rule: Operation not permitted
filter output oif wlan0
^^^^^^^^^^^^^^^^^^^^^^^
EXIT STATUS
On success, nft exits with a status of 0. Unspecified errors cause it to exit with a status of 1, memory
allocation errors with a status of 2, unable to open Netlink socket with 3.
SEE ALSO
iptables(8), ip6tables(8), arptables(8), ebtables(8), ip(8), tc(8)
There is an official wiki at: https://wiki.nftables.org
AUTHORS
nftables was written by Patrick McHardy and Pablo Neira Ayuso, among many other contributors from the
Netfilter community.
COPYRIGHT
Copyright 2008-2014 Patrick McHardy <kaber@trash.net>
Copyright 2013-2016 Pablo Neira Ayuso <pablo@netfilter.org>
nftables is free software; you can redistribute it and/or modify it under the terms of the GNU General
Public License version 2 as published by the Free Software Foundation.
This documentation is licenced under the terms of the Creative Commons Attribution-ShareAlike 4.0 li‐
cense, CC BY-SA 4.0 ⟨http://creativecommons.org/licenses/by-sa/4.0/⟩ .
02 February 2018 nft(8)