Manpages

NAME

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

SYNOPSIS

nft [ -nNscae ] [ -I directory ] [ -f filename | -i | cmd ...]
nft -h
nft -v

DESCRIPTION

nft is the command line tool used to set up, maintain and inspect packet filtering and classification rules in the Linux kernel, in the nftables framework. The Linux kernel subsystem is known as nf_tables, and ’nf’ stands for Netfilter.

OPTIONS

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

Show help message and all options.

−v, −−version

Show version.

−n, −−numeric

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

−N, −−reversedns

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

−s, −−stateless

Omit stateful information of rules and stateful objects.

−c, −−check

Check commands validity without actually applying the changes.

−a, −−handle

Show object handles in output.

−e, −−echo

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

−I, −−includepath directory

Add the directory directory to the list of directories to be searched for included files. This option may be specified multiple times.

−f, −−file filename

Read input from filename. If filename is −, read from stdin.

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

−i, −−interactive

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

INPUT FILE FORMAT

LEXICAL CONVENTIONS
Input is parsed line-wise. When the last character of a line, just before the newline character, is a non-quoted backslash (\), the next line is treated as a continuation. Multiple commands on the same line can be separated using a semicolon (;).

A hash sign (#) begins a comment. All following characters on the same line are ignored.

Identifiers begin with an alphabetic character (a−z,A−Z), followed zero or more alphanumeric characters (a−z,A−Z,0−9) and the characters slash (/), backslash (\), underscore (_) and dot (.). Identifiers using different characters or clashing with a keyword need to be enclosed in double quotes (").

INCLUDE FILES
include
filename

Other files can be included by using the include statement. The directories to be searched for include files can be specified using the −I/−−includepath option. You can override this behaviour either by prepending ./ to your path to force inclusion of files located in the current working directory (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.

IPv4/IPv6/Inet address family hooks

ARP ADDRESS FAMILY
The ARP address family handles ARP packets received and sent by the system. It is commonly used to mangle ARP packets for clustering.

ARP address family hooks

BRIDGE ADDRESS FAMILY
The bridge address family handles Ethernet packets traversing bridge devices.

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

NETDEV ADDRESS FAMILY
The Netdev address family handles packets from ingress.

Netdev address family hooks

RULESET

{list | flush} ruleset [family]
export [ruleset] format

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

list

Print the ruleset in human-readable format.

flush

Clear the whole ruleset. Note that unlike iptables, this will remove all tables and whatever they contain, effectively leading to an empty ruleset - no packet filtering will happen anymore, so the kernel accepts any valid packet it receives.

export

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

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

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

TABLES

{add | create} table [family] table [ { flags flags } ]
{delete | list | flush} table [family] table
delete table [family] handle handle

Tables are containers for chains, sets and stateful objects. They are identified by their address family and their name. The address family must be one of ip, ip6, inet, arp, bridge, netdev. The inet address family is a dummy family which is used to create hybrid IPv4/IPv6 tables. The meta expression nfproto keyword can be used to test which family (IPv4 or IPv6) context the packet is being processed in. When no address family is specified, ip is used by default. The only difference between add and create is that the former will not return an error if the specified table already exists while create will return an error.

Table flags

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
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.

Supported chain types

Apart from the special cases illustrated above (e.g. nat type not supporting forward hook or route type only supporting output hook), there are two further quirks worth noticing:

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

arp family supports only input and output hooks, both in chains of type filter.

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

Base chains also allow to set the chain’s policy, i.e. what happens to packets not explicitly accepted or refused in contained rules. Supported policy values are accept (which is the default) or drop.

RULES

[add | insert] rule [family] table chain [ {handle | position} handle | index index ] statement...
replace rule [family] table chain handle handle statement...
delete rule [family] table chain handle handle

Rules are added to chain 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 of an existing rule or an index (starting at zero). 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.

add

Add a new rule described by the list of statements. The rule is appended to the given chain unless a handle is specified, in which case the rule is appended to the rule given by the handle. The alternative name position is deprecated and should not be used anymore.

insert

Similar to the add command, but the rule is prepended to the beginning of the chain or before the rule with the given handle.

replace

Similar to the add command, 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 auto-merge ;] }
{delete | list | flush} set [family] table set
delete set [family] table handle handle
{add | delete} element [family] table set { element[,...] }

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

add

Add a new set in the specified table.

delete

Delete the specified set.

list

Display the elements in the specified set.

flush

Remove all elements from the specified set.

add element

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

delete element

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

Set specifications

MAPS

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

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

add

Add a new map in the specified table.

delete

Delete the specified map.

list

Display the elements in the specified map.

flush

Remove all elements from the specified map.

add element

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

delete element

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

Map specifications

FLOWTABLES

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

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

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

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

add

Add a new stateful object in the specified table.

delete

Delete the specified object.

list

Display stateful information the object holds.

reset

List-and-reset stateful object.

CT
ct
helper helper { type type protocol protocol ; [l3proto family ;] }

Ct helper is used to define connection tracking helpers that can then be used in combination with the "ct helper set" statement. type and protocol are mandatory, l3proto is derived from the table family by default, i.e. in the inet table the kernel will try to load both the IPv4 and IPv6 helper backends, if they are supported by the kernel.

conntrack helper specifications

defining and assigning ftp helper

Unlike iptables, helper assignment needs to be performed after the conntrack lookup has completed, for example with the default 0 hook priority.

table inet myhelpers {
ct helper ftp−standard {
type "ftp" protocol tcp
}
chain prerouting {
type filter hook prerouting priority 0;
tcp dport 21 ct helper set "ftp−standard"
}
}

COUNTER
counter
[packets bytes]

Counter specifications

QUOTA
quota
[over | until] [used]

Quota specifications

EXPRESSIONS

Expressions represent values, either constants like network addresses, port numbers etc. or data gathered from the packet during ruleset evaluation. Expressions can be combined using binary, logical, relational and other types of expressions to form complex or relational (match) expressions. They are also used as arguments to certain types of operations, like NAT, packet marking etc.

Each expression has a data type, which determines the size, parsing and representation of symbolic values and type compatibility with other expressions.

DESCRIBE COMMAND
describe
expression

The describe command shows information about the type of an expression and its data type.

The describe command

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

predefined symbolic constants:
fin 0x01
syn 0x02
rst 0x04
psh 0x08
ack 0x10
urg 0x20
ecn 0x40
cwr 0x80

DATA TYPES

Data types determine the size, parsing and representation of symbolic values and type compatibility of expressions. A number of global data types exist, in addition some expression types define further data types specific to the expression type. Most data types have a fixed size, some however may have a dynamic size, f.i. the string type.

Types may be derived from lower order types, f.i. the IPv4 address type is derived from the integer type, meaning an IPv4 address can also be specified as an integer value.

In certain contexts (set and map definitions) it is necessary to explicitly specify a data type. Each type has a name which is used for this.

INTEGER TYPE
The integer type is used for numeric values. It may be specified as decimal, hexadecimal or octal number. The integer type doesn’t have a fixed size, its size is determined by the expression for which it is used.

BITMASK TYPE
The bitmask type (bitmask) is used for bitmasks.

STRING TYPE
The string type is used to for character strings. A string begins with an alphabetic character (a-zA-Z) followed by zero or more alphanumeric characters or the characters /, −, _ and .. In addition anything enclosed in double quotes (") is recognized as a string.

String specification

# Interface name
filter input iifname eth0

# Weird interface name
filter input iifname "(eth0)"

LINK LAYER ADDRESS TYPE
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
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
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 specificationIPv6 address specification with bracket notation

# abbreviated loopback address
filter output ip6 daddr ::1

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

BOOLEAN TYPE
The boolean type is a syntactical helper type in user space. It’s use is in the right-hand side of a (typically implicit) relational expression to change the expression on the left-hand side into a boolean check (usually for existence).

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

Boolean specification

The following expressions support a boolean comparison:

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

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

ICMP Type specification

# match ping packets
filter output icmp type { echo−request, echo−reply }

ICMP CODE TYPE
The ICMP Code type is used to conveniently specify the ICMP header’s code field.

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

ICMPV6 TYPE TYPE
The ICMPv6 Type type is used to conveniently specify the ICMPv6 header’s type field.

The following keywords may be used when specifying the ICMPv6 type:

ICMPv6 Type specification

# match ICMPv6 ping packets
filter output icmpv6 type { echo−request, echo−reply }

ICMPV6 CODE TYPE
The ICMPv6 Code type is used to conveniently specify the ICMPv6 header’s code field.

The following keywords may be used when specifying the ICMPv6 code:

ICMPVX CODE TYPE
The ICMPvX Code type abstraction is a set of values which overlap between ICMP and ICMPv6 Code types to be used from the inet family.

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

CONNTRACK TYPES
This is an overview of types used in ct expression and statement:

For each of the types above, keywords are available for convenience:

conntrack state (ct_state)

conntrack direction (ct_dir)

conntrack status (ct_status)

conntrack event bits (ct_event)

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 | secpath}

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

There are two types of meta expressions: unqualified and qualified meta expressions. Qualified meta expressions require the meta keyword before the meta key, unqualified meta expressions can be specified by using the meta key directly or as qualified meta expressions. Meta 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 expression types

Meta expression specific types

Using meta expressions

# qualified meta expression
filter output meta oif eth0

# unqualified meta expression
filter output oif eth0

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

FIB EXPRESSIONS
fib
{saddr | daddr | {mark | iif | oif}} {oif | oifname | type}

A fib expression queries the fib (forwarding information base) to obtain information such as the output interface index a particular address would use. The input is a tuple of elements that is used as input to the fib lookup functions.

fib expression specific types

Using fib expressions

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

# drop packets to address not configured on ininterface
filter prerouting fib daddr . iif type != { local, broadcast, multicast } drop

# perform lookup in a specific ’blackhole’ table (0xdead, needs ip appropriate ip rule)
filter prerouting meta mark set 0xdead fib daddr . mark type vmap { blackhole : drop, prohibit : jump prohibited, unreachable : drop }

ROUTING EXPRESSIONS
rt
{classid | nexthop}

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

Routing expression types

Routing expression specific types

Using routing expressions

# IP family independent rt expression
filter output rt classid 10

# IP family dependent rt expressions
ip filter output rt nexthop 192.168.0.1
ip6 filter output rt nexthop fd00::1
inet filter output rt ip nexthop 192.168.0.1
inet filter output rt ip6 nexthop fd00::1

PAYLOAD EXPRESSIONS

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

ETHERNET HEADER EXPRESSION
ether
[Ethernet header field]

Ethernet header expression types

VLAN HEADER EXPRESSION
vlan
[VLAN header field]

VLAN header expression

ARP HEADER EXPRESSION
arp
[ARP header field]

ARP header expression

IPV4 HEADER EXPRESSION
ip
[IPv4 header field]

IPv4 header expression

ICMP HEADER EXPRESSION
icmp
[ICMP header field]

ICMP header expression

IPV6 HEADER EXPRESSION
ip6
[IPv6 header field]

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.

IPv6 header expression

matching if first extension header indicates a fragment

ip6 nexthdr ipv6−frag counter

ICMPV6 HEADER EXPRESSION
icmpv6
[ICMPv6 header field]

ICMPv6 header expression

TCP HEADER EXPRESSION
tcp
[TCP header field]

TCP header expression

UDP HEADER EXPRESSION
udp
[UDP header field]

UDP header expression

UDP-LITE HEADER EXPRESSION
udplite
[UDP-Lite header field]

UDP-Lite header expression

SCTP HEADER EXPRESSION
sctp
[SCTP header field]

SCTP header expression

DCCP HEADER EXPRESSION
dccp
[DCCP header field]

DCCP header expression

AUTHENTICATION HEADER EXPRESSION
ah
[AH header field]

AH header expression

ENCRYPTED SECURITY PAYLOAD HEADER EXPRESSION
esp
[ESP header field]

ESP header expression

IPCOMP HEADER EXPRESSION
comp
[IPComp header field]

IPComp header expression

RAW PAYLOAD EXPRESSION
@
[base,offset,length]

The raw payload expression instructs to load lengthbits starting at offsetbits. Bit 0 refers the 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 my using meta l4proto 5 before the raw expression.

Supported payload protocol bases

Matching destination port of both UDP and TCP

inet filter input meta l4proto {tcp, udp} @th,16,16 { dns, http }

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

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

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

nftables currently supports matching (finding) a given ipv6 extension header or TCP option.
hbh
{nexthdr | hdrlength}
frag
{nexthdr | frag-off | more-fragments | id}
rt
{nexthdr | hdrlength | type | seg-left}
dst
{nexthdr | hdrlength}
mh
{nexthdr | hdrlength | checksum | type}
srh
{flags | tag | sid | seg-left}
tcp option
{eol | noop | maxseg | window | sack-permitted | sack | sack0 | sack1 | sack2 | sack3 | timestamp} tcp_option_field

The following syntaxes are valid only in a relational expression with boolean type on right-hand side for checking header existence only:
exthdr
{hbh | frag | rt | dst | mh}
tcp option
{eol | noop | maxseg | window | sack-permitted | sack | sack0 | sack1 | sack2 | sack3 | timestamp}

IPv6 extension headers

TCP Options

finding TCP options

filter input tcp option sack−permitted kind 1 counter

matching IPv6 exthdr

ip6 filter input frag more−fragments 1 counter

CONNTRACK EXPRESSIONS
Conntrack expressions refer to meta data of the connection tracking entry associated with a packet.

There are three types of conntrack expressions. Some conntrack expressions require the flow direction before the conntrack key, others must be used directly because they are direction agnostic. The packets, bytes and avgpkt keywords can be used with or without a direction. If the direction is omitted, the sum of the original and the reply direction is returned. The same is true for the zone, if a direction is given, the zone is only matched if the zone id is tied to the given direction.

ct {state | direction | status | mark | expiration | helper | label | l3proto | protocol | bytes | packets | avgpkt | zone}
ct
{original | reply} {l3proto | protocol | proto-src | proto-dst | bytes | packets | avgpkt | zone}
ct
{original | reply} {ip | ip6} {saddr | daddr}

Conntrack expressions

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

STATEMENTS

Statements represent actions to be performed. They can alter control flow (return, jump to a different chain, accept or drop the packet) or can perform actions, such as logging, rejecting a packet, etc.

Statements exist in two kinds. Terminal statements unconditionally terminate evaluation of the current rule, non-terminal statements either only conditionally or never terminate evaluation of the current rule, in other words, they are passive from the ruleset evaluation perspective. There can be an arbitrary amount of non-terminal statements in a rule, but only a single terminal statement as the final statement.

VERDICT STATEMENT
The verdict statement alters control flow in the ruleset and issues policy decisions for packets.

{accept | drop | queue | continue | return}
{jump | goto} chain

accept

Terminate ruleset evaluation and accept the packet.

drop

Terminate ruleset evaluation and drop the packet.

queue

Terminate ruleset evaluation and queue the packet to userspace.

continue

Continue ruleset evaluation with the next rule. FIXME

return

Return from the current chain and continue evaluation at the next rule in the last chain. If issued in a base chain, it is equivalent to accept.

jump chain

Continue evaluation at the first rule in chain. The current position in the ruleset is pushed to a call stack and evaluation will continue there when the new chain is entirely evaluated of a return verdict is issued.

goto chain

Similar to jump, but the current position is not pushed to the call stack, meaning that after the new chain evaluation will continue at the last chain instead of the one containing the goto statement.

Verdict statements

# process packets from eth0 and the internal network in from_lan
# chain, drop all packets from eth0 with different source addresses.

filter input iif eth0 ip saddr 192.168.0.0/24 jump from_lan
filter input iif eth0 drop

PAYLOAD STATEMENT
The payload statement alters packet content. It can be used for example to set ip DSCP (differv) header field or ipv6 flow labels.

route some packets instead of bridging

# redirect tcp:http from 192.160.0.0/16 to local machine for routing instead of bridging
# assumes 00:11:22:33:44:55 is local MAC address.
bridge input meta iif eth0 ip saddr 192.168.0.0/16 tcp dport 80 meta pkttype set unicast ether daddr set 00:11:22:33:44:55

Set IPv4 DSCP header field

ip forward ip dscp set 42

EXTENSION HEADER STATEMENT
The extension header statement alters packet content in variable-sized headers. This can currently be used to alter the TCP Maximum segment size of packets, similar to TCPMSS.

change tcp mss

tcp flags syn tcp option maxseg size set 1360
# set a size based on route information:
tcp flags syn tcp option maxseg size set rt mtu

LOG STATEMENT
log
[prefix quoted_string] [level syslog-level] [flags log-flags]
log
group nflog_group [prefix quoted_string] [queue-threshold value] [snaplen size]

The log statement enables logging of matching packets. When this statement is used from a rule, the Linux kernel will print some information on all matching packets, such as header fields, via the kernel log (where it can be read with dmesg(1) or read in the syslog). If the group number is specified, the Linux kernel will pass the packet to nfnetlink_log which will multicast the packet through a netlink socket to the specified multicast group. One or more userspace processes may subscribe to the group to receive the packets, see libnetfilter_queue documentation for details. This is a non-terminating statement, so the rule evaluation continues after the packet is logged.

log statement options

log-flags

Using log statement

# log the UID which generated the packet and ip options
ip filter output log flags skuid flags ip options

# log the tcp sequence numbers and tcp options from the TCP packet
ip filter output log flags tcp sequence,options

# enable all supported log flags
ip6 filter output log flags all

REJECT STATEMENT
reject
[ with {icmp | icmpv6 | icmpx} type {icmp_code | icmpv6_code | icmpx_code} ]
reject
[ with tcp reset ]

A reject statement is used to send back an error packet in response to the matched packet otherwise it is equivalent to drop so it is a terminating statement, ending rule traversal. This statement is only valid in the input, forward and output chains, and user-defined chains which are only called from those chains.

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

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 ]

CONNTRACK STATEMENT
The conntrack statement can be used to set the conntrack mark and conntrack labels.

ct {mark | event | label | zone} set value

The ct statement sets meta data associated with a connection. The zone id has to be assigned before a conntrack lookup takes place, i.e. this has to be done in prerouting and possibly output (if locally generated packets need to be placed in a distinct zone), with a hook priority of -300.

Conntrack statement types

save packet nfmark in conntrack

ct mark set meta mark

set zone mapped via interface

table inet raw {
chain prerouting {
type filter hook prerouting priority −300;
ct zone set iif map { "eth1" : 1, "veth1" : 2 }
}
chain output {
type filter hook output priority −300;
ct zone set oif map { "eth1" : 1, "veth1" : 2 }
}
}

restrict events reported by ctnetlink

ct event set new,related,destroy

META STATEMENT
A meta statement sets the value of a meta expression. The existing meta fields are: priority, mark, pkttype, nftrace.

meta {mark | priority | pkttype | nftrace} set value

A meta statement sets meta data associated with a packet.

Meta statement types

LIMIT STATEMENT
limit
rate [over] packet_number / {second | minute | hour | day} [burst packet_number packets]
limit
rate [over] byte_number {bytes | kbytes | mbytes} / {second | minute | hour | day | week} [burst byte_number bytes]

A limit statement matches at a limited rate using a token bucket filter. A rule using this statement will match until this limit is reached. It can be used in combination with the log statement to give limited logging. The over keyword, that is optional, makes it match over the specified rate.

limit statement values

NAT STATEMENTS
snat
to address [:port] [persistent, random, fully-random]
snat
to address - address [:port - port] [persistent, random, fully-random]
dnat
to address [:port] [persistent, random, fully-random]
dnat
to address [:port - port] [persistent, random, fully-random]
masquerade
to [:port] [persistent, random, fully-random]
masquerade
to [:port - port] [persistent, random, fully-random]
redirect
to [:port] [persistent, random, fully-random]
redirect
to [:port - port] [persistent, random, fully-random]

The nat statements are only valid from nat chain types.

The snat and masquerade statements specify that the source address of the packet should be modified. While snat is only valid in the postrouting and input chains, masquerade makes sense only in postrouting. The dnat and redirect statements are only valid in the prerouting and output chains, they specify that the destination address of the packet should be modified. You can use non-base chains which are called from base chains of nat chain type too. All future packets in this connection will also be mangled, and rules should cease being examined.

The masquerade statement is a special form of snat which always uses the outgoing interface’s IP address to translate to. It is particularly useful on gateways with dynamic (public) IP addresses.

The redirect statement is a special form of dnat which always translates the destination address to the local host’s one. It comes in handy if one only wants to alter the destination port of incoming traffic on different interfaces.

Note that all nat statements require both prerouting and postrouting base chains to be present since otherwise packets on the return path won’t be seen by netfilter and therefore no reverse translation will take place.

NAT statement values

NAT statement flags

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

FLOW OFFLOAD STATEMENT
A flow offload 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 offload @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] [bypass,fanout]

queue statement values

queue statement flags

DUP STATEMENT
The dup statement is used to duplicate a packet and send the copy to a different destination.

dup to device
dup
to address device device

Dup statement values

Using the dup statement

# send to machine with ip address 10.2.3.4 on eth0
ip filter forward dup to 10.2.3.4 device "eth0"

# copy raw frame to another interface
netdetv ingress dup to "eth0"
dup to "eth0"

# combine with map dst addr to gateways
dup to ip daddr map { 192.168.7.1 : "eth0", 192.168.7.2 : "eth1" }

FWD STATEMENT
The fwd statement is used to redirect a raw packet to another interface. Its is only available in the netdev family ingress hook. It is similar to the dup statement except that no copy is made.

fwd to device

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. Furthermore, any set that will be dynamically updated from the nftables ruleset must specify both a maximum set size (to prevent memory exhaustion) and a timeout (so that number of entries in set will not grow indefinitely). The set statement can be used to e.g. create dynamic blacklists.

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

Example for simple blacklist

# declare a set, bound to table "filter", in family "ip". Timeout and size are mandatory because we will add elements from packet path.
nft add set ip filter blackhole "{ type ipv4_addr; flags timeout; size 65536; }"

# whitelist internal interface.
nft add rule ip filter input meta iifname "internal" accept

# drop packets coming from blacklisted ip addresses.
nft add rule ip filter input ip saddr @blackhole counter drop

# add source ip addresses to the blacklist if more than 10 tcp connection requests occurred per second and ip address.
# entries will timeout after one minute, after which they might be re−added if limit condition persists.
nft add rule ip filter input tcp flags syn tcp dport ssh meter flood size 128000 { ip saddr timeout 10s limit rate over 10/second} add @blackhole { ip saddr timeout 1m } drop

# inspect state of the rate limit meter:
nft list meter ip filter flood

# inspect content of blackhole:
nft list set ip filter blackhole

# manually add two addresses to the set:
nft add element filter blackhole { 10.2.3.4, 10.23.1.42 }

ADDITIONAL COMMANDS

These are some additional commands included in nft.

MONITOR
The monitor command allows you to listen to Netlink events produced by the nf_tables subsystem, related to creation and deletion of objects. When they occur, nft will print to stdout the monitored events in either XML, JSON or native nft format.

To filter events related to a concrete object, use one of the keywords ’tables’, ’chains’, ’sets’, ’rules’, ’elements’, ’ruleset’.

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

Hit ^C to finish the monitor operation.

Listen to all events, report in native nft format

% nft monitor

Listen to added tables, report in XML format

% nft monitor new tables xml

Listen to deleted rules, report in JSON format

% nft monitor destroy rules json

Listen to both new and destroyed chains, in native nft format

% nft monitor chains

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

% nft monitor ruleset

ERROR REPORTING

When an error is detected, nft shows the line(s) containing the error, the position of the erroneous parts in the input stream and marks up the erroneous parts using 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 can’t detect which parts of the input caused the error and the entire command is marked.

Error caused by single incorrect expression

<cmdline>:1:19−22: Error: Interface does not exist
filter output oif eth0
^^^^

Error caused by invalid combination of two expressions

<cmdline>:1:28−36: Error: Right hand side of relational expression (==) must be constant
filter output tcp dport == tcp dport
~~ ^^^^^^^^^

Error returned by the kernel

<cmdline>:0:0−23: Error: Could not process rule: Operation not permitted
filter output oif wlan0
^^^^^^^^^^^^^^^^^^^^^^^

EXIT STATUS

On success, nft exits with a status of 0. Unspecified errors cause it to exit with a status of 1, memory allocation errors with a status of 2, unable to open Netlink socket with 3.

SEE ALSO

iptables(8), ip6tables(8), arptables(8), ebtables(8), ip(8), tc(8)

There is an official wiki at: https://wiki.nftables.org

AUTHORS

nftables was written by Patrick McHardy and Pablo Neira Ayuso, among many other contributors from the Netfilter community.

COPYRIGHT

Copyright 2008−2014 Patrick McHardy <kaber [AT] trash.net>
Copyright 2013−2016 Pablo Neira Ayuso <pablo [AT] 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/&rang; .

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