nft man page

nft — Administration tool for packet filtering and classification

Synopsis

nft [-n | --numeric] [-N | --reversedns] [-s | --stateless] [-c | --check] [-a | --handle] [-e | --echo] [ {-I | --includepath} directory ] [ {-f | --file} filename |  {-i | --interactive} |  cmd ...] nft [-h | --help] [-v | --version]

Description

nft is used to set up, maintain and inspect packet filtering and classification rules in the Linux kernel.

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.

-s, --stateless

Omit stateful information of rules and stateful objects.

-c, --check

Check commands validity without actually applying the changes.

-N, --reversedns

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

-a, --handle

Show rule handles in output.

-e, --echo

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

-I, --includepath directory

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

-f, --file filename

Read input from filename.

-i, --interactive

Read input from an interactive readline CLI.

Input File Format

Lexical Conventions

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

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

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

Include Files

include filename

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

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.

ARP address family hooks

Hook Description
input Packets delivered to the local system are processed by the input hook.
output Packets send by the local system are processed by the output hook.

Bridge Address Family

The bridge address family handles ethernet packets traversing bridge devices.

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

Netdev Address Family

The Netdev address family handles packets from ingress.

Netdev address family hooks

Hook Description
ingress All packets entering the system are processed by this hook. It is invoked before layer 3 protocol handlers and it can be used for early filtering and policing.

Ruleset

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

The ruleset keyword is used to identify the whole set of tables, chains, etc. currently in place in 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 | delete | list | flush} table [family] {table}

Tables are containers for chains, sets and stateful objects. They are identified by their address family and their name. The address family must be one of ip, ip6, inet, arp, bridge, netdev. The inet address family is a dummy family which is used to create hybrid IPv4/IPv6 tables. The meta expression nfproto keyword can be used to test which family (ipv4 or ipv6) context the packet is being processed in. When no address family is specified, ip is used by default.

add

Add a new table for the given family with the given name.

delete

Delete the specified table.

list

List all chains and rules of the specified table.

flush

Flush all chains and rules of the specified table.

Chains

{add | create} chain [family] table chain [ { {type} {hook} [device] {priority ;} [policy ;] } ] {delete | list | flush} chain [family] {table} {chain} {rename} chain [family] {table} {chain} {newname}

Chains are containers for rules. They exist in two kinds, base chains and regular chains. A base chain is an entry point for packets from the networking stack, a regular chain may be used as jump target and is used for better rule organization.

add

Add a new chain in the specified table. When a hook and priority value are specified, the chain is created as a base chain and hooked up to the networking stack.

create

Similar to the add command, but returns an error if the chain already exists.

delete

Delete the specified chain. The chain must not contain any rules or be used as jump target.

rename

Rename the specified chain.

list

List all rules of the specified chain.

flush

Flush all rules of the specified chain.

For base chains, type, hook and priority parameters are mandatory.

Supported chain types

Type Families Hooks Description
filter all all Standard chain type to use in doubt.
nat ip, ip6 prerouting, input, output, postrouting Chains of this type perform Native Address Translation based on conntrack entries. Only the first packet of a connection actually traverses this chain - its rules usually define details of the created conntrack entry (NAT statements for instance).
route ip, ip6 output If a packet has traversed a chain of this type and is about to be accepted, a new route lookup is performed if relevant parts of the IP header have changed. This allows to e.g. implement policy routing selectors in nftables.

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

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

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

Rules

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

Rules are constructed from two kinds of components according to a set of grammatical rules: expressions and statements.

add

Add a new rule described by the list of statements. The rule is appended to the given chain unless a position is specified, in which case the rule is appended to the rule given by the position.

insert

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

replace

Similar to the add command, but the rule replaces the specified rule.

delete

Delete the specified rule.

Sets

{add} set [family] {table} {set} { {type} [flags] [timeout] [gc-interval] [elements] [size] [policy] } {delete | list | flush} set [family] {table} {set} {add | delete} element [family] {table} {set} { {elements} }

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

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, interval, timeout
timeout time an element stays in the set string, decimal followed by unit. Units are: d, h, m, s
gc-interval garbage collection interval, only available when timeout or flag timeout are active string, decimal followed by unit. Units are: d, h, m, s
elements elements contained by the set set data type
size maximun number of elements in the set unsigned integer (64 bit)
policy set policy string: performance [default], memory

Maps

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

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

add

Add a new map in the specified table.

delete

Delete the specified map.

list

Display the elements in the specified map.

flush

Remove all elements from the specified map.

add element

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

delete element

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

Map specifications

Keyword Description Type
type data type of map elements string ':' string: ipv4_addr, ipv6_addr, ether_addr, inet_proto, inet_service, mark, counter, quota. Counter and quota can't be used as keys
flags map flags string: constant, interval
elements elements contained by the map map data type
size maximun number of elements in the map unsigned integer (64 bit)
policy map policy string: performance [default], memory

Stateful Objects

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

Stateful objects are attached to tables and are identified by an unique name. They group stateful 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} {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

Keyword Description Type
type name of helper type quoted string (e.g. "ftp")
protocol layer 4 protocol of the helper string (e.g. tcp)
l3proto layer 3 protocol of the helper address family (e.g. ip)

defining and assigning ftp helper

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

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

Counter

counter [packets bytes]

Counter specifications

Keyword Description Type
packets initial count of packets unsigned integer (64 bit)
bytes initial count of bytes unsigned integer (64 bit)

Quota

quota [over | until] [used]

Quota specifications

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

Expressions

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

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

Describe Command

describe {expression}

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

The describe command

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

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

Data Types

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

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

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

Integer Type

Name Keyword Size Base type
Integer integer variable -

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

Bitmask Type

Name Keyword Size Base type
Bitmask bitmask variable integer

The bitmask type (bitmask) is used for bitmasks.

String Type

Name Keyword Size Base type
String string variable -

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

String specification

# Interface name
filter input iifname eth0

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

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. FIXME

IPv6 address specification

# abbreviated loopback address
filter output ip6 daddr ::1

Boolean Type

Name Keyword Size Base type
Boolean boolean 1 bit integer

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

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

Keyword Value
exists 1
missing 0

Boolean specification

The following expressions support a boolean comparison:

Expression Behaviour
fib Check route existence.
exthdr Check IPv6 extension header existence.
tcp option Check TCP option header existence.
# match if route exists
filter input fib daddr . iif oif exists

# match only non-fragmented packets in IPv6 traffic
filter input exthdr frag missing

# match if TCP timestamp option is present
filter input tcp option timestamp exists

ICMP Type Type

Name Keyword Size Base type
ICMP Type icmp_type 8 bit integer

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

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

Keyword Value
echo-reply 0
destination-unreachable 3
source-quench 4
redirect 5
echo-request 8
router-advertisement 9
router-solicitation 10
time-exceeded 11
parameter-problem 12
timestamp-request 13
timestamp-reply 14
info-request 15
info-reply 16
address-mask-request 17
address-mask-reply 18

ICMP Type specification

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

ICMP Code Type

Name Keyword Size Base type
ICMP Code icmp_code 8 bit integer

The ICMP Code type is used to conveniently specify the ICMP header's code field.

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

Keyword Value
net-unreachable 0
host-unreachable 1
prot-unreachable 2
port-unreachable 3
net-prohibited 9
host-prohibited 10
admin-prohibited 13

ICMPv6 Type Type

Name Keyword Size Base type
ICMPv6 Type icmpv6_type 8 bit integer

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

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

Keyword Value
destination-unreachable 1
packet-too-big 2
time-exceeded 3
parameter-problem 4
echo-request 128
echo-reply 129
mld-listener-query 130
mld-listener-report 131
mld-listener-done 132
mld-listener-reduction 132
nd-router-solicit 133
nd-router-advert 134
nd-neighbor-solicit 135
nd-neighbor-advert 136
nd-redirect 137
router-renumbering 138
ind-neighbor-solicit 141
ind-neighbor-advert 142
mld2-listener-report 143

ICMPv6 Type specification

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

ICMPv6 Code Type

Name Keyword Size Base type
ICMPv6 Code icmpv6_code 8 bit integer

The ICMPv6 Code type is used to conveniently specify the ICMPv6 header's code field.

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

Keyword Value
no-route 0
admin-prohibited 1
addr-unreachable 3
port-unreachable 4
policy-fail 5
reject-route 6

Icmpvx Code Type

Name Keyword Size Base type
ICMPvX Code icmpx_code 8 bit integer

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

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

Keyword Value
no-route 0
port-unreachable 1
host-unreachable 2
admin-prohibited 3

Conntrack Types

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

Name Keyword Size Base type
conntrack state ct_state 4 byte bitmask
conntrack direction ct_dir 8 bit integer
conntrack status ct_status 4 byte bitmask
conntrack event bits ct_event 4 byte bitmask
conntrack label ct_label 128 bit bitmask

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

conntrack state (ct_state)

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

conntrack direction (ct_dir)

Keyword Value
original 0
reply 1

conntrack status (ct_status)

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

conntrack event bits (ct_event)

Keyword Value
new 1
related 2
destroy 4
reply 8
assured 16
protoinfo 32
helper 64
mark 128
seqadj 256
secmark 512
label 1024

Possible keywords for conntrack label type (ct_label) are read at runtime from /etc/connlabel.conf.

Primary Expressions

The lowest order expression is a primary expression, representing either a constant or a single datum from a packet's payload, meta data or a stateful module.

Meta Expressions

meta {length | nfproto | l4proto | protocol | priority} [meta] {mark | iif | iifname | iiftype | oif | oifname | oiftype | skuid | skgid | nftrace | rtclassid | ibriport | obriport | pkttype | cpu | iifgroup | oifgroup | cgroup | random}

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 expression types

Keyword Description Type
length Length of the packet in bytes integer (32 bit)
nfproto real hook protocol family, useful only in inet table integer (32 bit)
protocol Ethertype protocol value ether_type
priority TC packet priority tc_handle
mark Packet mark mark
iif Input interface index iface_index
iifname Input interface name string
iiftype Input interface type iface_type
oif Output interface index iface_index
oifname Output interface name string
oiftype Output interface hardware type iface_type
skuid UID associated with originating socket uid
skgid GID associated with originating socket gid
rtclassid Routing realm realm
ibriport Input bridge interface name string
obriport Output bridge interface name string
pkttype packet type pkt_type
cpu cpu number processing the packet integer (32 bits)
iifgroup incoming device group devgroup
oifgroup outgoing device group devgroup
cgroup control group id integer (32 bits)
random pseudo-random number integer (32 bits)

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: Unicast (addressed to local host), Broadcast (to all), Multicast (to group).

Using meta expressions

# qualified meta expression
filter output meta oif eth0

# unqualified meta expression
filter output oif eth0

Fib Expressions

fib {saddr | daddr | [mark | iif | oif]} {oif | oifname | type}

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

fib expression specific types

Keyword Description Type
oif Output interface index integer (32 bit)
oifname Output interface name string
type Address type fib_addrtype

Using fib expressions

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

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

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

Routing Expressions

rt {classid | nexthop}

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

Routing expression types

Keyword Description Type
classid Routing realm realm
nexthop Routing nexthop ipv4_addr/ipv6_addr
mtu TCP maximum segment size of route integer (16 bit)

Routing expression specific types

Type Description
realm Routing Realm (32 bit number). Can be specified numerically or as symbolic name defined in /etc/iproute2/rt_realms.

Using routing expressions

# IP family independent rt expression
filter output rt classid 10

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

Payload Expressions

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

Ethernet Header Expression

ether [ethernet header field]

Ethernet header expression types

Keyword Description Type
daddr Destination MAC address ether_addr
saddr Source MAC address ether_addr
type EtherType ether_type

Vlan Header Expression

vlan [VLAN header field]

VLAN header expression

Keyword Description Type
id VLAN ID (VID) integer (12 bit)
cfi Canonical Format Indicator integer (1 bit)
pcp Priority code point integer (3 bit)
type EtherType ether_type

ARP Header Expression

arp [ARP header field]

ARP header expression

Keyword Description Type
htype ARP hardware type integer (16 bit)
ptype EtherType ether_type
hlen Hardware address len integer (8 bit)
plen Protocol address len integer (8 bit)
operation Operation arp_op

IPv4 Header Expression

ip [IPv4 header field]

IPv4 header expression

Keyword Description Type
version IP header version (4) integer (4 bit)
hdrlength IP header length including options integer (4 bit) FIXME scaling
dscp Differentiated Services Code Point dscp
ecn Explicit Congestion Notification ecn
length Total packet length integer (16 bit)
id IP ID integer (16 bit)
frag-off Fragment offset integer (16 bit)
ttl Time to live integer (8 bit)
protocol Upper layer protocol inet_proto
checksum IP header checksum integer (16 bit)
saddr Source address ipv4_addr
daddr Destination address ipv4_addr

ICMP Header Expression

icmp [ICMP header field]

ICMP header expression

Keyword Description Type
type ICMP type field icmp_type
code ICMP code field integer (8 bit)
checksum ICMP checksum field integer (16 bit)
id ID of echo request/response integer (16 bit)
sequence sequence number of echo request/response integer (16 bit)
gateway gateway of redirects integer (32 bit)
mtu MTU of path MTU discovery integer (16 bit)

IPv6 Header Expression

ip6 [IPv6 header field]

IPv6 header expression

Keyword Description Type
version IP header version (6) integer (4 bit)
dscp Differentiated Services Code Point dscp
ecn Explicit Congestion Notification ecn
flowlabel Flow label integer (20 bit)
length Payload length integer (16 bit)
nexthdr Nexthdr protocol inet_proto
hoplimit Hop limit integer (8 bit)
saddr Source address ipv6_addr
daddr Destination address ipv6_addr

ICMPv6 Header Expression

icmpv6 [ICMPv6 header field]

ICMPv6 header expression

Keyword Description Type
type ICMPv6 type field icmpv6_type
code ICMPv6 code field integer (8 bit)
checksum ICMPv6 checksum field integer (16 bit)
parameter-problem pointer to problem integer (32 bit)
packet-too-big oversized MTU integer (32 bit)
id ID of echo request/response integer (16 bit)
sequence sequence number of echo request/response integer (16 bit)
max-delay maximum response delay of MLD queries integer (16 bit)

TCP Header Expression

tcp [TCP header field]

TCP header expression

Keyword Description Type
sport Source port inet_service
dport Destination port inet_service
sequence Sequence number integer (32 bit)
ackseq Acknowledgement number integer (32 bit)
doff Data offset integer (4 bit) FIXME 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 [UDP header field]

UDP header expression

Keyword Description Type
sport Source port inet_service
dport Destination port inet_service
length Total packet length integer (16 bit)
checksum Checksum integer (16 bit)

UDP-Lite Header Expression

udplite [UDP-Lite header field]

UDP-Lite header expression

Keyword Description Type
sport Source port inet_service
dport Destination port inet_service
checksum Checksum integer (16 bit)

SCTP Header Expression

sctp [SCTP header field]

SCTP header expression

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

DCCP Header Expression

dccp [DCCP header field]

DCCP header expression

Keyword Description Type
sport Source port inet_service
dport Destination port inet_service

Authentication Header Expression

ah [AH header field]

AH header expression

Keyword Description Type
nexthdr Next header protocol inet_proto
hdrlength AH Header length integer (8 bit)
reserved Reserved area integer (16 bit)
spi Security Parameter Index integer (32 bit)
sequence Sequence number integer (32 bit)

Encrypted Security Payload Header Expression

esp [ESP header field]

ESP header expression

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

Ipcomp Header Expression

comp [IPComp header field]

IPComp header expression

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

Extension Header Expressions

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

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

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

IPv6 extension headers

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

TCP Options

Keyword Description TCP option fields
eol End of option list kind
noop 1 Byte TCP No-op options kind
maxseg TCP Maximum Segment Size kind, length, size
window TCP Window Scaling kind, length, count
sack-permitted TCP SACK permitted kind, length
sack TCP Selective Acknowledgement (alias of block 0) kind, length, left, right
sack0 TCP Selective Acknowledgement (block 0) kind, length, left, right
sack1 TCP Selective Acknowledgement (block 1) kind, length, left, right
sack2 TCP Selective Acknowledgement (block 2) kind, length, left, right
sack3 TCP Selective Acknowledgement (block 3) kind, length, left, right
timestamp TCP Timestamps kind, length, tsval, tsecr

finding TCP options

filter input tcp option sack-permitted kind 1 counter

matching IPv6 exthdr

ip6 filter input frag more-fragments 1 counter

Conntrack Expressions

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

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

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

Conntrack expressions

Keyword Description Type
state State of the connection ct_state
direction Direction of the packet relative to the connection ct_dir
status Status of the connection ct_status
mark Connection mark mark
expiration Connection expiration time time
helper Helper associated with the connection string
label Connection tracking label bit or symbolic name defined in connlabel.conf in the nftables include path ct_label
l3proto Layer 3 protocol of the connection nf_proto
saddr Source address of the connection for the given direction ipv4_addr/ipv6_addr
daddr Destination address of the connection for the given direction ipv4_addr/ipv6_addr
protocol Layer 4 protocol of the connection for the given direction inet_proto
proto-src Layer 4 protocol source for the given direction integer (16 bit)
proto-dst Layer 4 protocol destination for the given direction integer (16 bit)
packets packet count seen in the given direction or sum of original and reply integer (64 bit)
bytes bytecount seen, see description for packets keyword integer (64 bit)
avgpkt average bytes per packet, see description for packets keyword integer (64 bit)
zone conntrack zone integer (16 bit)

A description of conntrack-specific types listed above can be found sub-section Conntrack Types above.

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

Keyword Description Type
prefix Log message prefix quoted string
syslog-level Syslog level of logging string: emerg, alert, crit, err, warn [default], notice, info, debug
group NFLOG group to send messages to unsigned integer (16 bit)
snaplen Length of packet payload to include in netlink message unsigned integer (32 bit)
queue-threshold Number of packets to queue inside the kernel before sending them to userspace unsigned integer (32 bit)

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 {icmp | icmp6 | icmpx} type {icmp_type | icmp6_type | icmpx_type} ] reject [ with {tcp} {reset} ]

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

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

Variant Family Type
icmp ip icmp_code
icmp6 ip6 icmpv6_code
icmpx inet icmpx_code

For a description of the different types and a list of supported keywords refer to Data Types section above. The common default reject value is port-unreachable.

Counter Statement

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

counter {packets number } {bytes number }

Conntrack Statement

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

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

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

Conntrack statement types

Keyword Description Value
event conntrack event bits bitmask, integer (32 bit)
helper name of ct helper object to assign to the connection quoted string
mark Connection tracking mark mark
label Connection tracking label label
zone conntrack zone integer (16 bit)

save packet nfmark in conntrack

ct mark set meta mark

set zone mapped via interface

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

restrict events reported by ctnetlink

ct event set new,related,destroy

Meta Statement

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

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

A meta statement sets meta data associated with a packet.

Meta statement types

Keyword Description Value
priority TC packet priority tc_handle
mark Packet mark mark
pkttype packet type pkt_type
nftrace ruleset packet tracing on/off. Use monitor trace command to watch traces 0, 1

Limit Statement

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

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

limit statement values

Value Description Type
packet_number Number of packets unsigned integer (32 bit)
byte_number Number of bytes unsigned integer (32 bit)

Nat Statements

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

The nat statements are only valid from nat chain types.

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

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

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

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

NAT statement values

Expression Description Type
address Specifies that the source/destination address of the packet should be modified. You may specify a mapping to relate a list of tuples composed of arbitrary expression key with address value. ipv4_addr, ipv6_addr, eg. abcd::1234, or you can use a mapping, eg. meta mark map { 10 : 192.168.1.2, 20 : 192.168.1.3 }
port Specifies that the source/destination address of the packet should be modified. port number (16 bits)

NAT statement flags

Flag Description
persistent Gives a client the same source-/destination-address for each connection.
random If used then port mapping will be randomized using a random seeded MD5 hash mix using source and destination address and destination port.
fully-random If used then port mapping is 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

Queue Statement

This statement passes the packet to userspace using the nfnetlink_queue handler. The packet is put into the queue identified by its 16-bit queue number. Userspace can inspect and modify the packet if desired. Userspace must then drop or reinject the packet into the kernel. See libnetfilter_queue documentation for details.

queue [num queue_number] [bypass] queue [num queue_number_from - queue_number_to] [bypass,fanout]

queue statement values

Value Description Type
queue_number Sets queue number, default is 0. unsigned integer (16 bit)
queue_number_from Sets initial queue in the range, if fanout is used. unsigned integer (16 bit)
queue_number_to Sets closing queue in the range, if fanout is used. unsigned integer (16 bit)

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 recomendations.
fanout Distribute packets between several queues.

Additional Commands

These are some additional commands included in nft.

Monitor

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

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

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

Hit ^C to finish the monitor operation.

Listen to all events, report in native nft format

% nft monitor

Listen to added tables, report in XML format

% nft monitor new tables xml

Listen to deleted rules, report in JSON format

% nft monitor destroy rules json

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

% nft monitor chains

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

% nft monitor ruleset

Error Reporting

When an error is detected, nft shows the line(s) containing the error, the position of the erroneous parts in the input stream and marks up the erroneous parts using carrets (^). If the error results from the combination of two expressions or statements, the part imposing the constraints which are violated is marked using tildes (~).

For errors returned by the kernel, nft can't detect which parts of the input caused the error and the 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.

Info

23 October 2017