Provided by: nftables_0.9.3-2_amd64 
NAME
nft - Administration tool of the nftables framework for packet filtering and classification
SYNOPSIS
nft [ -nNscaeSupyjt ] [ -I directory ] [ -f filename | -i | cmd ...]
nft -h
nft -v
DESCRIPTION
nft is the command line tool used to set up, maintain and inspect packet filtering and classification
rules in the Linux kernel, in the nftables framework. The Linux kernel subsystem is known as nf_tables,
and ‘nf’ stands for Netfilter.
OPTIONS
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.
-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.
-T, --numeric-time
Show time, day and hour values in numeric format.
-t, --terse
Omit contents of sets from output.
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 [family]
delete table [family] handle handle
Tables are containers for chains, sets and stateful objects. They are identified by their address family
and their name. The address family must be one of ip, ip6, inet, arp, bridge, netdev. The inet address
family is a dummy family which is used to create hybrid IPv4/IPv6 tables. The meta expression nfproto
keyword can be used to test which family (ipv4 or ipv6) context the packet is being processed in. When no
address family is specified, ip is used by default. The only difference between add and create is that
the former will not return an error if the specified table already exists while create will return an
error.
Table 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 [family]
delete chain [family] table handle handle
rename chain [family] table chain newname
Chains are containers for rules. They exist in two kinds, base chains and regular chains. A base chain is
an entry point for packets from the networking stack, a regular chain may be used as jump target and is
used for better rule organization.
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, │ Chains of this type │
│ │ │ output, postrouting │ 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, │ all │
│ │ │ netdev │ │
├─────────┼───────┼─────────────────────┼─────────────┤
│ │ │ │ │
│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 [family]
delete set [family] table handle handle
{add | delete} element [family] table set { element[, ...] }
Sets are element containers of a user-defined data type, they are uniquely identified by a user-defined
name and attached to tables. Their behaviour can be tuned with the flags that can be specified at set
creation time.
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 │ string, decimal followed by │
│ │ set, mandatory if set is │ unit. Units are: d, h, m, s │
│ │ added to from the packet │ │
│ │ path (ruleset). │ │
├────────────┼──────────────────────────────┼─────────────────────────────┤
│ │ │ │
│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 │ maximum number of elements │ unsigned integer (64 bit) │
│ │ in the set, mandatory if set │ │
│ │ is added to from the packet │ │
│ │ path (ruleset). │ │
├────────────┼──────────────────────────────┼─────────────────────────────┤
│ │ │ │
│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 [family]
{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 data type │
│ │ map │ │
├─────────┼────────────────────────────┼──────────────────────────────┤
│ │ │ │
│size │ maximum number of elements │ unsigned integer (64 bit) │
│ │ in the map │ │
├─────────┼────────────────────────────┼──────────────────────────────┤
│ │ │ │
│policy │ map policy │ string: performance │
│ │ │ [default], memory │
└─────────┴────────────────────────────┴──────────────────────────────┘
FLOWTABLES
{add | create} flowtable [family] table flowtable { hook hook priority priority ; devices = { device[, ...] } ; }
list flowtables [family]
{delete | list} flowtable [family] table flowtable
delete flowtable [family] table handle handle
Flowtables allow you to accelerate packet forwarding in software. Flowtables entries are represented
through a tuple that is composed of the input interface, source and destination address, source and
destination port; and layer 3/4 protocols. Each entry also caches the destination interface and the
gateway address - to update the destination link-layer address - to forward packets. The ttl and hoplimit
fields are also decremented. Hence, flowtables provides an alternative path that allow packets to bypass
the classic forwarding path. Flowtables reside in the ingress hook that is located before the prerouting
hook. You can select which flows you want to offload through the flow expression from the forward chain.
Flowtables are identified by their address family and their name. The address family must be one of ip,
ip6, or inet. The inet address family is a dummy family which is used to create hybrid IPv4/IPv6 tables.
When no address family is specified, ip is used by default.
The priority can be a signed integer or filter which stands for 0. Addition and subtraction can be used
to set relative priority, e.g. filter + 5 equals to 5.
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 [family]
list quotas [family]
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 │ string (e.g. ip) │
│ │ 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"
}
}
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 │ string (e.g. ip) │
│ │ timeout object │ │
├─────────┼──────────────────────────────┼─────────────────────────────┤
│ │ │ │
│state │ connection state name │ string (e.g. "established") │
├─────────┼──────────────────────────────┼─────────────────────────────┤
│ │ │ │
│value │ timeout value for connection │ unsigned integer │
│ │ state │ │
├─────────┼──────────────────────────────┼─────────────────────────────┤
│ │ │ │
│l3proto │ layer 3 protocol of the │ address family (e.g. ip) │
│ │ timeout object │ │
└─────────┴──────────────────────────────┴─────────────────────────────┘
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
CT EXPECTATION
ct expectation name { protocol protocol ; dport dport ; timeout timeout ; size size ; [*l3proto family ;] }
Ct expectation is used to create connection expectations. Expectations are assigned with the ct
expectation set statement. protocol, dport, timeout and size are mandatory, l3proto is derived from the
table family by default.
Table 12. conntrack expectation specifications
┌─────────┬──────────────────────────────┬──────────────────────────┐
│Keyword │ Description │ Type │
├─────────┼──────────────────────────────┼──────────────────────────┤
│ │ │ │
│protocol │ layer 4 protocol of the │ string (e.g. ip) │
│ │ expectation object │ │
├─────────┼──────────────────────────────┼──────────────────────────┤
│ │ │ │
│dport │ destination port of expected │ unsigned integer │
│ │ connection │ │
├─────────┼──────────────────────────────┼──────────────────────────┤
│ │ │ │
│timeout │ timeout value for │ unsigned integer │
│ │ expectation │ │
├─────────┼──────────────────────────────┼──────────────────────────┤
│ │ │ │
│size │ size value for expectation │ unsigned integer │
├─────────┼──────────────────────────────┼──────────────────────────┤
│ │ │ │
│l3proto │ layer 3 protocol of the │ address family (e.g. ip) │
│ │ expectation object │ │
└─────────┴──────────────────────────────┴──────────────────────────┘
defining and assigning ct expectation policy.
table ip filter {
ct expectation expect {
protocol udp
dport 9876
timeout 2m
size 8
l3proto ip
}
chain input {
type filter hook input priority filter; policy accept;
ct expectation set "expect"
}
}
COUNTER
counter [packets bytes]
Table 13. 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 14. Quota specifications
┌────────┬─────────────────────────────┬──────────────────────────────┐
│Keyword │ Description │ Type │
├────────┼─────────────────────────────┼──────────────────────────────┤
│ │ │ │
│quota │ quota limit, used as the │ Two arguments, unsigned │
│ │ quota name │ 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 | data type
The describe command shows information about the type of an expression and its data type. A data type may
also be given, in which nft will display more information about the type.
The describe command.
$ nft describe tcp flags
payload expression, datatype tcp_flag (TCP flag) (basetype bitmask, integer), 8 bits
predefined symbolic constants:
fin 0x01
syn 0x02
rst 0x04
psh 0x08
ack 0x10
urg 0x20
ecn 0x40
cwr 0x80
DATA TYPES
Data types determine the size, parsing and representation of symbolic values and type compatibility of
expressions. A number of global data types exist, in addition some expression types define further data
types specific to the expression type. Most data types have a fixed size, some however may have a dynamic
size, f.i. the string type. Some types also have predefined symbolic constants. Those can be listed using
the nft describe command:
$ nft describe ct_state
datatype ct_state (conntrack state) (basetype bitmask, integer), 32 bits
pre-defined symbolic constants (in hexadecimal):
invalid 0x00000001
new ...
Types may be derived from lower order types, f.i. the IPv4 address type is derived from the integer type,
meaning an IPv4 address can also be specified as an integer value.
In certain contexts (set and map definitions), it is necessary to explicitly specify a data type. Each
type has a name which is used for this.
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)"
LINK LAYER ADDRESS TYPE
┌───────────────────┬─────────┬──────────┬───────────┐
│Name │ Keyword │ Size │ Base type │
├───────────────────┼─────────┼──────────┼───────────┤
│ │ │ │ │
│Link layer address │ lladdr │ variable │ integer │
└───────────────────┴─────────┴──────────┴───────────┘
The link layer address type is used for link layer addresses. Link layer addresses are specified as a
variable amount of groups of two hexadecimal digits separated using colons (:).
Link layer address specification.
# Ethernet destination MAC address
filter input ether daddr 20:c9:d0:43:12:d9
IPV4 ADDRESS TYPE
┌─────────────┬───────────┬────────┬───────────┐
│Name │ Keyword │ Size │ Base type │
├─────────────┼───────────┼────────┼───────────┤
│ │ │ │ │
│IPV4 address │ ipv4_addr │ 32 bit │ integer │
└─────────────┴───────────┴────────┴───────────┘
The IPv4 address type is used for IPv4 addresses. Addresses are specified in either dotted decimal,
dotted hexadecimal, dotted octal, decimal, hexadecimal, octal notation or as a host name. A host name
will be resolved using the standard system resolver.
IPv4 address specification.
# dotted decimal notation
filter output ip daddr 127.0.0.1
# host name
filter output ip daddr localhost
IPV6 ADDRESS TYPE
┌─────────────┬───────────┬─────────┬───────────┐
│Name │ Keyword │ Size │ Base type │
├─────────────┼───────────┼─────────┼───────────┤
│ │ │ │ │
│IPv6 address │ ipv6_addr │ 128 bit │ integer │
└─────────────┴───────────┴─────────┴───────────┘
The IPv6 address type is used for IPv6 addresses. 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 15. The following keywords will automatically resolve into a boolean type with given value
┌────────┬───────┐
│Keyword │ Value │
├────────┼───────┤
│ │ │
│exists │ 1 │
├────────┼───────┤
│ │ │
│missing │ 0 │
└────────┴───────┘
Table 16. 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 17. 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 18. 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 19. 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 20. 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 21. 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 22. 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 23. conntrack state (ct_state)
┌────────────┬───────┐
│Keyword │ Value │
├────────────┼───────┤
│ │ │
│invalid │ 1 │
├────────────┼───────┤
│ │ │
│established │ 2 │
├────────────┼───────┤
│ │ │
│related │ 4 │
├────────────┼───────┤
│ │ │
│new │ 8 │
├────────────┼───────┤
│ │ │
│untracked │ 64 │
└────────────┴───────┘
Table 24. conntrack direction (ct_dir)
┌─────────┬───────┐
│Keyword │ Value │
├─────────┼───────┤
│ │ │
│original │ 0 │
├─────────┼───────┤
│ │ │
│reply │ 1 │
└─────────┴───────┘
Table 25. conntrack status (ct_status)
┌───────────┬───────┐
│Keyword │ Value │
├───────────┼───────┤
│ │ │
│expected │ 1 │
├───────────┼───────┤
│ │ │
│seen-reply │ 2 │
├───────────┼───────┤
│ │ │
│assured │ 4 │
├───────────┼───────┤
│ │ │
│confirmed │ 8 │
├───────────┼───────┤
│ │ │
│snat │ 16 │
├───────────┼───────┤
│ │ │
│dnat │ 32 │
├───────────┼───────┤
│ │ │
│dying │ 512 │
└───────────┴───────┘
Table 26. 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 | time | hour | day }
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 27. 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 │ │
├──────────┼──────────────────────────────┼────────────────────────────┤
│ │ │ │
│l4proto │ layer 4 protocol, skips ipv6 │ integer (8 bit) │
│ │ extension headers │ │
├──────────┼──────────────────────────────┼────────────────────────────┤
│ │ │ │
│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 │ iface_type │
│ │ type │ │
├──────────┼──────────────────────────────┼────────────────────────────┤
│ │ │ │
│skuid │ UID associated with │ uid │
│ │ originating socket │ │
├──────────┼──────────────────────────────┼────────────────────────────┤
│ │ │ │
│skgid │ GID associated with │ gid │
│ │ originating socket │ │
├──────────┼──────────────────────────────┼────────────────────────────┤
│ │ │ │
│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 │ integer (32 bit) │
│ │ packet │ │
├──────────┼──────────────────────────────┼────────────────────────────┤
│ │ │ │
│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 │ │
├──────────┼──────────────────────────────┼────────────────────────────┤
│ │ │ │
│oifkind │ Output interface kind │ │
├──────────┼──────────────────────────────┼────────────────────────────┤
│ │ │ │
│time │ Absolute time of packet │ Integer (32 bit) or string │
│ │ reception │ │
├──────────┼──────────────────────────────┼────────────────────────────┤
│ │ │ │
│day │ Day of week │ Integer (8 bit) or string │
├──────────┼──────────────────────────────┼────────────────────────────┤
│ │ │ │
│hour │ Hour of day │ String │
└──────────┴──────────────────────────────┴────────────────────────────┘
Table 28. 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. │
├──────────────┼───────────────────────────────────────┤
│ │ │
│time │ Either an integer or a date in ISO │
│ │ format. For example: "2019-06-06 │
│ │ 17:00". Hour and seconds are optional │
│ │ and can be omitted if desired. If │
│ │ omitted, midnight will be assumed. │
│ │ The following three would be │
│ │ equivalent: "2019-06-06", "2019-06-06 │
│ │ 00:00" and "2019-06-06 00:00:00". │
│ │ When an integer is given, it is │
│ │ assumed to be a UNIX timestamp. │
├──────────────┼───────────────────────────────────────┤
│ │ │
│day │ Either a day of week ("Monday", │
│ │ "Tuesday", etc.), or an integer │
│ │ between 0 and 6. Strings are matched │
│ │ case-insensitively, and a full match │
│ │ is not expected (e.g. "Mon" would │
│ │ match "Monday"). When an integer is │
│ │ given, 0 is Sunday and 6 is Saturday. │
├──────────────┼───────────────────────────────────────┤
│ │ │
│hour │ A string representing an hour in │
│ │ 24-hour format. Seconds can │
│ │ optionally be specified. For example, │
│ │ 17:00 and 17:00:00 would be │
│ │ equivalent. │
└──────────────┴───────────────────────────────────────┘
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 29. Available socket attributes
┌────────────┬─────────────────────────────┬─────────────────┐
│Name │ Description │ Type │
├────────────┼─────────────────────────────┼─────────────────┤
│ │ │ │
│transparent │ Value of the IP_TRANSPARENT │ boolean (1 bit) │
│ │ socket option in the found │ │
│ │ socket. It can be 0 or 1. │ │
├────────────┼─────────────────────────────┼─────────────────┤
│ │ │ │
│mark │ Value of the socket mark │ mark │
│ │ (SOL_SOCKET, SO_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 30. Available osf attributes
┌────────┬──────────────────────────────┬────────┐
│Name │ Description │ Type │
├────────┼──────────────────────────────┼────────┤
│ │ │ │
│ttl │ Do TTL checks on the packet │ string │
│ │ to determine the operating │ │
│ │ system. │ │
├────────┼──────────────────────────────┼────────┤
│ │ │ │
│version │ Do OS version checks on the │ │
│ │ packet. │ │
├────────┼──────────────────────────────┼────────┤
│ │ │ │
│name │ Name of the OS signature to │ string │
│ │ match. All signatures can be │ │
│ │ found at pf.os file. Use │ │
│ │ "unknown" for OS signatures │ │
│ │ that the expression could │ │
│ │ not detect. │ │
└────────┴──────────────────────────────┴────────┘
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 31. fib expression specific types
┌────────┬────────────────────────┬──────────────────┐
│Keyword │ Description │ Type │
├────────┼────────────────────────┼──────────────────┤
│ │ │ │
│oif │ Output interface index │ integer (32 bit) │
├────────┼────────────────────────┼──────────────────┤
│ │ │ │
│oifname │ Output interface name │ string │
├────────┼────────────────────────┼──────────────────┤
│ │ │ │
│type │ Address type │ fib_addrtype │
└────────┴────────────────────────┴──────────────────┘
Use nft describe fib_addrtype to get a list of all address types.
Using fib expressions.
# drop packets without a reverse path
filter prerouting fib saddr . iif oif missing drop
In this example, 'saddr . iif' looks up routing information based on the source address and the input interface.
oif picks the output interface index from the routing information.
If no route was found for the source address/input interface combination, the output interface index is zero.
In case the input interface is specified as part of the input key, the output interface index is always the same as the input interface index or zero.
If only 'saddr oif' is given, then oif can be any interface index or zero.
In this example, 'saddr . iif' lookups up routing information based on the source address and the input interface.
oif picks the output interface index from the routing information.
If no route was found for the source address/input interface combination, the output interface index is zero.
In case the input interface is specified as part of the input key, the output interface index is always the same as the input interface index or zero.
If only 'saddr oif' is given, then oif can be any interface index or zero.
# 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 32. 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 │ │
├────────┼─────────────────────────────┼─────────────────────┤
│ │ │ │
│ipsec │ route via ipsec tunnel or │ boolean │
│ │ transport │ │
└────────┴─────────────────────────────┴─────────────────────┘
Table 33. 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 34. 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 │ ipv4_addr/ipv6_addr │
│ │ tunnel │ │
└────────┴──────────────────────────────┴─────────────────────┘
NUMGEN EXPRESSION
numgen {inc | random} mod NUM [ offset NUM ]
Create a number generator. The inc or random keywords control its operation mode: In inc mode, the last
returned value is simply incremented. In random mode, a new random number is returned. The value after
mod keyword specifies an upper boundary (read: modulus) which is not reached by returned numbers. The
optional offset allows to increment the returned value by a fixed offset.
A typical use-case for numgen is load-balancing:
Using numgen expression.
# round-robin between 192.168.10.100 and 192.168.20.200:
add rule nat prerouting dnat to numgen inc mod 2 map \
{ 0 : 192.168.10.100, 1 : 192.168.20.200 }
# probability-based with odd bias using intervals:
add rule nat prerouting dnat to numgen random mod 10 map \
{ 0-2 : 192.168.10.100, 3-9 : 192.168.20.200 }
PAYLOAD EXPRESSIONS
Payload expressions refer to data from the packet’s payload.
ETHERNET HEADER EXPRESSION
ether {daddr | saddr | type}
Table 35. 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 36. 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 37. 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 38. IPv4 header expression
┌──────────┬──────────────────────────────┬───────────────────────┐
│Keyword │ Description │ Type │
├──────────┼──────────────────────────────┼───────────────────────┤
│ │ │ │
│version │ IP header version (4) │ integer (4 bit) │
├──────────┼──────────────────────────────┼───────────────────────┤
│ │ │ │
│hdrlength │ IP header length including │ integer (4 bit) FIXME │
│ │ options │ scaling │
├──────────┼──────────────────────────────┼───────────────────────┤
│ │ │ │
│dscp │ Differentiated Services Code │ dscp │
│ │ Point │ │
├──────────┼──────────────────────────────┼───────────────────────┤
│ │ │ │
│ecn │ Explicit Congestion │ ecn │
│ │ Notification │ │
├──────────┼──────────────────────────────┼───────────────────────┤
│ │ │ │
│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 39. 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 │ integer (16 bit) │
│ │ request/response │ │
├─────────┼─────────────────────────────┼──────────────────┤
│ │ │ │
│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 40. IGMP header expression
┌─────────┬────────────────────────────┬──────────────────┐
│Keyword │ Description │ Type │
├─────────┼────────────────────────────┼──────────────────┤
│ │ │ │
│type │ IGMP type field │ igmp_type │
├─────────┼────────────────────────────┼──────────────────┤
│ │ │ │
│mrt │ IGMP maximum response time │ integer (8 bit) │
│ │ field │ │
├─────────┼────────────────────────────┼──────────────────┤
│ │ │ │
│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 41. IPv6 header expression
┌──────────┬──────────────────────────────┬──────────────────┐
│Keyword │ Description │ Type │
├──────────┼──────────────────────────────┼──────────────────┤
│ │ │ │
│version │ IP header version (6) │ integer (4 bit) │
├──────────┼──────────────────────────────┼──────────────────┤
│ │ │ │
│dscp │ Differentiated Services Code │ dscp │
│ │ Point │ │
├──────────┼──────────────────────────────┼──────────────────┤
│ │ │ │
│ecn │ Explicit Congestion │ ecn │
│ │ Notification │ │
├──────────┼──────────────────────────────┼──────────────────┤
│ │ │ │
│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 42. 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 │ integer (16 bit) │
│ │ request/response │ │
├──────────────────┼─────────────────────────────┼──────────────────┤
│ │ │ │
│max-delay │ maximum response delay of │ integer (16 bit) │
│ │ MLD queries │ │
└──────────────────┴─────────────────────────────┴──────────────────┘
TCP HEADER EXPRESSION
tcp {sport | dport | sequence | ackseq | doff | reserved | flags | window | checksum | urgptr}
Table 43. 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 44. 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 45. 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 46. 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 47. 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 48. 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 49. 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 50. 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 51. 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 }
The above can also be written as
inet filter input meta l4proto {tcp, udp} th dport { 53, 80 }
it is more convenient, but like the raw expression notation no dependencies are created or checked. It is
the users responsibility to restrict matching to those header types that have a notion of ports.
Otherwise, rules using raw expressions will errnously match unrelated packets, e.g. mis-interpreting ESP
packets SPI field as a port.
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, TCP options and IPv4 options.
nftables currently supports matching (finding) a given ipv6 extension header, TCP option or IPv4 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
ip option { lsrr | ra | rr | ssrr } ip_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}
ip option { lsrr | ra | rr | ssrr }
Table 52. 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 53. 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 │ kind, length, left, right │
│ │ Acknowledgement (alias of │ │
│ │ block 0) │ │
├───────────────┼───────────────────────────┼────────────────────────────┤
│ │ │ │
│sack0 │ TCP Selective │ kind, length, left, right │
│ │ Acknowledgement (block 0) │ │
├───────────────┼───────────────────────────┼────────────────────────────┤
│ │ │ │
│sack1 │ TCP Selective │ kind, length, left, right │
│ │ Acknowledgement (block 1) │ │
├───────────────┼───────────────────────────┼────────────────────────────┤
│ │ │ │
│sack2 │ TCP Selective │ kind, length, left, right │
│ │ Acknowledgement (block 2) │ │
├───────────────┼───────────────────────────┼────────────────────────────┤
│ │ │ │
│sack3 │ TCP Selective │ kind, length, left, right │
│ │ Acknowledgement (block 3) │ │
├───────────────┼───────────────────────────┼────────────────────────────┤
│ │ │ │
│timestamp │ TCP Timestamps │ kind, length, tsval, tsecr │
└───────────────┴───────────────────────────┴────────────────────────────┘
Table 54. IP Options
┌────────┬─────────────────────┬─────────────────────────┐
│Keyword │ Description │ IP option fields │
├────────┼─────────────────────┼─────────────────────────┤
│ │ │ │
│lsrr │ Loose Source Route │ type, length, ptr, addr │
├────────┼─────────────────────┼─────────────────────────┤
│ │ │ │
│ra │ Router Alert │ type, length, value │
├────────┼─────────────────────┼─────────────────────────┤
│ │ │ │
│rr │ Record Route │ type, length, ptr, addr │
├────────┼─────────────────────┼─────────────────────────┤
│ │ │ │
│ssrr │ Strict Source Route │ type, length, ptr, addr │
└────────┴─────────────────────┴─────────────────────────┘
finding TCP options.
filter input tcp option sack-permitted kind 1 counter
matching IPv6 exthdr.
ip6 filter input frag more-fragments 1 counter
finding IP option.
filter input ip option lsrr exists 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 55. Conntrack expressions
┌───────────┬──────────────────────────────┬─────────────────────┐
│Keyword │ Description │ Type │
├───────────┼──────────────────────────────┼─────────────────────┤
│ │ │ │
│state │ State of the connection │ ct_state │
├───────────┼──────────────────────────────┼─────────────────────┤
│ │ │ │
│direction │ Direction of the packet │ ct_dir │
│ │ relative 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 │ nf_proto │
│ │ connection │ │
├───────────┼──────────────────────────────┼─────────────────────┤
│ │ │ │
│saddr │ Source address of the │ ipv4_addr/ipv6_addr │
│ │ connection for the given │ │
│ │ direction │ │
├───────────┼──────────────────────────────┼─────────────────────┤
│ │ │ │
│daddr │ Destination address of the │ ipv4_addr/ipv6_addr │
│ │ connection for the given │ │
│ │ direction │ │
├───────────┼──────────────────────────────┼─────────────────────┤
│ │ │ │
│protocol │ Layer 4 protocol of the │ inet_proto │
│ │ connection for the given │ │
│ │ direction │ │
├───────────┼──────────────────────────────┼─────────────────────┤
│ │ │ │
│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 │ byte count seen, see │ integer (64 bit) │
│ │ description 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.
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 56. 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 │ 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 │ │
└────────────────┴──────────────────────────────┴──────────────────────────────┘
Table 57. 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 58. 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 59. 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,
pkttype, nftrace.
meta {mark | priority | pkttype | nftrace} set value
A meta statement sets meta data associated with a packet.
Table 60. 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 / 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 61. 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 { ip | ip6 } to address - address [:port - port] [PR_FLAGS]
dnat to address [:port] [PRF_FLAGS]
dnat to address [:port - port] [PR_FLAGS]
dnat { ip | ip6 } to 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 62. NAT statement values
┌───────────┬──────────────────────────────┬──────────────────────────────┐
│Expression │ Description │ Type │
├───────────┼──────────────────────────────┼──────────────────────────────┤
│ │ │ │
│address │ Specifies that the │ ipv4_addr, ipv6_addr, e.g. │
│ │ source/destination address │ abcd::1234, or you can use a │
│ │ of the packet should be │ mapping, e.g. meta mark map │
│ │ modified. 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 bit) │
│ │ source/destination address │ │
│ │ of the packet should be │ │
│ │ modified. │ │
└───────────┴──────────────────────────────┴──────────────────────────────┘
Table 63. 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 64. 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
}
}
SYNPROXY STATEMENT
This statement will process TCP three-way-handshake parallel in netfilter context to protect either local
or backend system. This statement requires connection tracking because sequence numbers need to be
translated.
synproxy [mss mss_value] [wscale wscale_value] [SYNPROXY_FLAGS]
Table 65. synproxy statement attributes
┌───────┬───────────────────────────────────────┐
│Name │ Description │
├───────┼───────────────────────────────────────┤
│ │ │
│mss │ Maximum segment size announced to │
│ │ clients. This must match the backend. │
├───────┼───────────────────────────────────────┤
│ │ │
│wscale │ Window scale announced to clients. │
│ │ This must match the backend. │
└───────┴───────────────────────────────────────┘
Table 66. synproxy statement flags
┌──────────┬───────────────────────────────────────┐
│Flag │ Description │
├──────────┼───────────────────────────────────────┤
│ │ │
│sack-perm │ Pass client selective acknowledgement │
│ │ option to backend (will be disabled │
│ │ if not present). │
├──────────┼───────────────────────────────────────┤
│ │ │
│timestamp │ Pass client timestamp option to │
│ │ backend (will be disabled if not │
│ │ present, also needed for selective │
│ │ acknowledgement and window scaling). │
└──────────┴───────────────────────────────────────┘
Example ruleset for synproxy statement.
Determine tcp options used by backend, from an external system
tcpdump -pni eth0 -c 1 'tcp[tcpflags] == (tcp-syn|tcp-ack)'
port 80 &
telnet 192.0.2.42 80
18:57:24.693307 IP 192.0.2.42.80 > 192.0.2.43.48757:
Flags [S.], seq 360414582, ack 788841994, win 14480,
options [mss 1460,sackOK,
TS val 1409056151 ecr 9690221,
nop,wscale 9],
length 0
Switch tcp_loose mode off, so conntrack will mark out-of-flow packets as state INVALID.
echo 0 > /proc/sys/net/netfilter/nf_conntrack_tcp_loose
Make SYN packets untracked.
table ip x {
chain y {
type filter hook prerouting priority raw; policy accept;
tcp flags syn notrack
}
}
Catch UNTRACKED (SYN packets) and INVALID (3WHS ACK packets) states and send
them to SYNPROXY. This rule will respond to SYN packets with SYN+ACK
syncookies, create ESTABLISHED for valid client response (3WHS ACK packets) and
drop incorrect cookies. Flags combinations not expected during 3WHS will not
match and continue (e.g. SYN+FIN, SYN+ACK). Finally, drop invalid packets, this
will be out-of-flow packets that were not matched by SYNPROXY.
table ip foo {
chain z {
type filter hook input priority filter; policy accept;
ct state { invalid, untracked } synproxy mss 1460 wscale 9 timestamp sack-perm
ct state invalid drop
}
}
The outcome ruleset of the steps above should be similar to the one below.
table ip x {
chain y {
type filter hook prerouting priority raw; policy accept;
tcp flags syn notrack
}
chain z {
type filter hook input priority filter; policy accept;
ct state { invalid, untracked } synproxy mss 1460 wscale 9 timestamp sack-perm
ct state invalid drop
}
}
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 67. 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. │ │
└──────────────────┴────────────────────────────┴───────────────────────────┘
Table 68. 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 69. Dup statement values
┌───────────┬──────────────────────────────┬──────────────────────────────┐
│Expression │ Description │ Type │
├───────────┼──────────────────────────────┼──────────────────────────────┤
│ │ │ │
│address │ Specifies that the copy of │ ipv4_addr, ipv6_addr, e.g. │
│ │ the packet should be sent to │ abcd::1234, or you can use a │
│ │ a new gateway. │ mapping, e.g. 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. 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 one or both of the dynamic
and the timeout flags. The dynamic flag is required if the set statement expression includes a stateful
object. The timeout flag is implied if the set is created with a timeout, and is required if the set
statement updates elements, rather than adding them. Furthermore, these sets should specify both a
maximum set size (to prevent memory exhaustion), and their elements should have a timeout (so their
number will not grow indefinitely) either from the set definition or from the statement that adds or
updates them. 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
libnftables(3), libnftables-json(5), iptables(8), ip6tables(8), arptables(8), ebtables(8), ip(8), tc(8) There is an official wiki at: https://wiki.nftables.org
AUTHORS
nftables was written by Patrick McHardy and Pablo Neira Ayuso, among many other contributors from the
Netfilter community.
COPYRIGHT
Copyright © 2008-2014 Patrick McHardy <kaber@trash.net> Copyright © 2013-2018 Pablo Neira Ayuso <pablo@netfilter.org> nftables is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License version 2 as published by the Free Software Foundation. This documentation is licensed under the terms of the Creative Commons Attribution-ShareAlike 4.0 license, CC BY-SA 4.0 http://creativecommons.org/licenses/by-sa/4.0/. 12/17/2019 NFT(8)