LEXICAL CONVENTIONS¶

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

IPV4/IPV6/INET ADDRESS FAMILIES¶

Table 1. IPv4/IPv6/Inet address family hooks

ARP ADDRESS FAMILY¶

Table 2. ARP address family hooks

BRIDGE ADDRESS FAMILY¶

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

NETDEV ADDRESS FAMILY¶

Table 3. Netdev address family hooks

CT HELPER¶

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

Ct helper is used to define connection tracking helpers that can then be used in combination with the ct helper set statement. type and protocol are mandatory, l3proto is derived from the table family by default, i.e. in the inet table the kernel will try to load both the ipv4 and ipv6 helper backends, if they are supported by the kernel.

Table 11. 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"
  }
}

CT TIMEOUT¶

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

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

Table 12. conntrack timeout specifications

defining and assigning ct timeout policy.

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

testing the updated timeout policy.

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

CT EXPECTATION¶

ct expectation name { protocol protocol ; dport dport ; timeout timeout ; size size ; [*l3proto family ;] }

Ct expectation is used to create connection expectations. Expectations are assigned with the ct expectation set statement. protocol, dport, timeout and size are mandatory, l3proto is derived from the table family by default.

Table 13. conntrack expectation specifications

defining and assigning ct expectation policy.

table ip filter {
        ct expectation expect {
                protocol udp
                dport 9876
                timeout 2m
                size 8
                l3proto ip
        }
        chain input {
                type filter hook input priority filter; policy accept;
                ct expectation set "expect"
        }
}

COUNTER¶

counter [packets bytes]

Table 14. Counter specifications

QUOTA¶

quota [over | until] [used]

Table 15. Quota specifications

DESCRIBE COMMAND¶

describe expression | data type

The describe command shows information about the type of an expression and its data type. A data type may also be given, in which nft will display more information about the type.

The describe command.

$ nft describe tcp flags
payload expression, datatype tcp_flag (TCP flag) (basetype bitmask, integer), 8 bits
predefined symbolic constants:
fin                           0x01
syn                           0x02
rst                           0x04
psh                           0x08
ack                           0x10
urg                           0x20
ecn                           0x40
cwr                           0x80

INTEGER TYPE¶

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

BITMASK TYPE¶

The bitmask type (bitmask) is used for bitmasks.

STRING TYPE¶

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

String specification.

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

LINK LAYER ADDRESS TYPE¶

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 specification.

# abbreviated loopback address
filter output ip6 daddr ::1

IPv6 address specification with bracket notation.

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

BOOLEAN TYPE¶

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

Table 16. The following keywords will automatically resolve into a boolean type with given value

Table 17. expressions support a boolean comparison

Boolean specification.

# match if route exists
filter input fib daddr . iif oif exists
# match only non-fragmented packets in IPv6 traffic
filter input exthdr frag missing
# match if TCP timestamp option is present
filter input tcp option timestamp exists

ICMP TYPE TYPE¶

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

Table 18. 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.

Table 19. 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.

Table 20. 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.

Table 21. 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.

Table 22. keywords may be used when specifying the ICMPvX code

CONNTRACK TYPES¶

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

Table 24. conntrack state (ct_state)

Table 25. conntrack direction (ct_dir)

Table 26. conntrack status (ct_status)

Table 27. conntrack event bits (ct_event)

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

META EXPRESSIONS¶

meta {length | nfproto | l4proto | protocol | priority}
[meta] {mark | iif | iifname | iiftype | oif | oifname | oiftype | skuid | skgid | nftrace | rtclassid | ibrname | obrname | pkttype | cpu | iifgroup | oifgroup | cgroup | random | ipsec | iifkind | oifkind | time | hour | day }

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

There are two types of meta expressions: unqualified and qualified meta expressions. Qualified meta expressions require the meta keyword before the meta key, unqualified meta expressions can be specified by using the meta key directly or as qualified meta expressions. Meta l4proto is useful to match a particular transport protocol that is part of either an IPv4 or IPv6 packet. It will also skip any IPv6 extension headers present in an IPv6 packet.

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

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

filter input meta iif "foo"

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

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

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

Like with iptables, wildcard matching on interface name prefixes is available for iifname and oifname matches by appending an asterisk (*) character. Note however that unlike iptables, nftables does not accept interface names consisting of the wildcard character only - users are supposed to just skip those always matching expressions. In order to match on literal asterisk character, one may escape it using backslash (\).

Table 28. Meta expression types

Table 29. Meta expression specific types

Using meta expressions.

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

SOCKET EXPRESSION¶

socket {transparent | mark}

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

Table 30. Available socket attributes

Using socket expression.

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

OSF EXPRESSION¶

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

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

Table 31. Available osf attributes

Available ttl values.

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

Using osf expression.

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

FIB EXPRESSIONS¶

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

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

Table 32. fib expression specific types

Use nft describe fib_addrtype to get a list of all address types.

Using fib expressions.

# drop packets without a reverse path
filter prerouting fib saddr . iif oif missing drop
In this example, 'saddr . iif' looks up routing information based on the source address and the input interface.
oif picks the output interface index from the routing information.
If no route was found for the source address/input interface combination, the output interface index is zero.
In case the input interface is specified as part of the input key, the output interface index is always the same as the input interface index or zero.
If only 'saddr oif' is given, then oif can be any interface index or zero.
# drop packets to address not configured on incoming interface
filter prerouting fib daddr . iif type != { local, broadcast, multicast } drop
# perform lookup in a specific 'blackhole' table (0xdead, needs ip appropriate ip rule)
filter prerouting meta mark set 0xdead fib daddr . mark type vmap { blackhole : drop, prohibit : jump prohibited, unreachable : drop }

ROUTING EXPRESSIONS¶

rt [ip | ip6] {classid | nexthop | mtu | ipsec}

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

Table 33. Routing expression types

Table 34. Routing expression specific types

Using routing expressions.

# IP family independent rt expression
filter output rt classid 10
filter output rt ipsec missing
# IP family dependent rt expressions
ip filter output rt nexthop 192.168.0.1
ip6 filter output rt nexthop fd00::1
inet filter output rt ip nexthop 192.168.0.1
inet filter output rt ip6 nexthop fd00::1

IPSEC EXPRESSIONS¶

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

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

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

Table 35. Ipsec expression types

NUMGEN EXPRESSION¶

numgen {inc | random} mod NUM [ offset NUM ]

Create a number generator. The inc or random keywords control its operation mode: In inc mode, the last returned value is simply incremented. In random mode, a new random number is returned. The value after mod keyword specifies an upper boundary (read: modulus) which is not reached by returned numbers. The optional offset allows to increment the returned value by a fixed offset.

A typical use-case for numgen is load-balancing:

Using numgen expression.

# round-robin between 192.168.10.100 and 192.168.20.200:
add rule nat prerouting dnat to numgen inc mod 2 map \
        { 0 : 192.168.10.100, 1 : 192.168.20.200 }
# probability-based with odd bias using intervals:
add rule nat prerouting dnat to numgen random mod 10 map \
        { 0-2 : 192.168.10.100, 3-9 : 192.168.20.200 }

HASH EXPRESSIONS¶

jhash {ip saddr | ip6 daddr | tcp dport | udp sport | ether saddr} [. ...] mod NUM [ seed NUM ] [ offset NUM ]
symhash mod NUM [ offset NUM ]

Use a hashing function to generate a number. The functions available are jhash, known as Jenkins Hash, and symhash, for Symmetric Hash. The jhash requires an expression to determine the parameters of the packet header to apply the hashing, concatenations are possible as well. The value after mod keyword specifies an upper boundary (read: modulus) which is not reached by returned numbers. The optional seed is used to specify an init value used as seed in the hashing function. The optional offset allows to increment the returned value by a fixed offset.

A typical use-case for jhash and symhash is load-balancing:

Using hash expressions.

# load balance based on source ip between 2 ip addresses:
add rule nat prerouting dnat to jhash ip saddr mod 2 map \
        { 0 : 192.168.10.100, 1 : 192.168.20.200 }
# symmetric load balancing between 2 ip addresses:
add rule nat prerouting dnat to symhash mod 2 map \
        { 0 : 192.168.10.100, 1 : 192.168.20.200 }

ETHERNET HEADER EXPRESSION¶

ether {daddr | saddr | type}

Table 36. Ethernet header expression types

VLAN HEADER EXPRESSION¶

vlan {id | cfi | pcp | type}

Table 37. VLAN header expression

ARP HEADER EXPRESSION¶

arp {htype | ptype | hlen | plen | operation | saddr { ip | ether } | daddr { ip | ether }

Table 38. ARP header expression

IPV4 HEADER EXPRESSION¶

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

Table 39. IPv4 header expression

ICMP HEADER EXPRESSION¶

icmp {type | code | checksum | id | sequence | gateway | mtu}

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

Table 40. ICMP header expression

IGMP HEADER EXPRESSION¶

igmp {type | mrt | checksum | group}

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

Table 41. IGMP header expression

IPV6 HEADER EXPRESSION¶

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

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

Table 42. IPv6 header expression

Using ip6 header expressions.

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

ICMPV6 HEADER EXPRESSION¶

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

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

Table 43. ICMPv6 header expression

TCP HEADER EXPRESSION¶

tcp {sport | dport | sequence | ackseq | doff | reserved | flags | window | checksum | urgptr}

Table 44. TCP header expression

UDP HEADER EXPRESSION¶

udp {sport | dport | length | checksum}

Table 45. UDP header expression

UDP-LITE HEADER EXPRESSION¶

udplite {sport | dport | checksum}

Table 46. UDP-Lite header expression

SCTP HEADER EXPRESSION¶

sctp {sport | dport | vtag | checksum}

Table 47. SCTP header expression

DCCP HEADER EXPRESSION¶

dccp {sport | dport}

Table 48. DCCP header expression

AUTHENTICATION HEADER EXPRESSION¶

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

Table 49. AH header expression

ENCRYPTED SECURITY PAYLOAD HEADER EXPRESSION¶

esp {spi | sequence}

Table 50. ESP header expression

IPCOMP HEADER EXPRESSION¶

Table 51. IPComp header expression

RAW PAYLOAD EXPRESSION¶

@base,offset,length

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

Table 52. Supported payload protocol bases

Matching destination port of both UDP and TCP.

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

The above can also be written as

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

it is more convenient, but like the raw expression notation no dependencies are created or checked. It is the users responsibility to restrict matching to those header types that have a notion of ports. Otherwise, rules using raw expressions will errnously match unrelated packets, e.g. mis-interpreting ESP packets SPI field as a port.

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

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

EXTENSION HEADER EXPRESSIONS¶

nftables currently supports matching (finding) a given ipv6 extension header, TCP option or IPv4 option.

hbh {nexthdr | hdrlength}
frag {nexthdr | frag-off | more-fragments | id}
rt {nexthdr | hdrlength | type | seg-left}
dst {nexthdr | hdrlength}
mh {nexthdr | hdrlength | checksum | type}
srh {flags | tag | sid | seg-left}
tcp option {eol | noop | maxseg | window | sack-permitted | sack | sack0 | sack1 | sack2 | sack3 | timestamp} tcp_option_field
ip option { lsrr | ra | rr | ssrr } ip_option_field

The following syntaxes are valid only in a relational expression with boolean type on right-hand side for checking header existence only:

exthdr {hbh | frag | rt | dst | mh}
tcp option {eol | noop | maxseg | window | sack-permitted | sack | sack0 | sack1 | sack2 | sack3 | timestamp}
ip option { lsrr | ra | rr | ssrr }

Table 53. IPv6 extension headers

Table 54. TCP Options

Table 55. IP Options

finding TCP options.

filter input tcp option sack-permitted kind 1 counter

matching IPv6 exthdr.

ip6 filter input frag more-fragments 1 counter

finding IP option.

filter input ip option lsrr exists counter

CONNTRACK EXPRESSIONS¶

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

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

Table 56. Conntrack expressions

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

restrict the number of parallel connections to a server.

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

VERDICT STATEMENT¶

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

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

Using verdict statements.

# process packets from eth0 and the internal network in from_lan
# chain, drop all packets from eth0 with different source addresses.
filter input iif eth0 ip saddr 192.168.0.0/24 jump from_lan
filter input iif eth0 drop

PAYLOAD STATEMENT¶

payload_expression set value

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

route some packets instead of bridging.

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

Set IPv4 DSCP header field.

ip forward ip dscp set 42

EXTENSION HEADER STATEMENT¶

extension_header_expression set value

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

change tcp mss.

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

LOG STATEMENT¶

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

The log statement enables logging of matching packets. When this statement is used from a rule, the Linux kernel will print some information on all matching packets, such as header fields, via the kernel log (where it can be read with dmesg(1) or read in the syslog).

In the second form of invocation (if nflog_group is specified), the Linux kernel will pass the packet to nfnetlink_log which will multicast the packet through a netlink socket to the specified multicast group. One or more userspace processes may subscribe to the group to receive the packets, see libnetfilter_queue documentation for details.

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

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

Table 57. log statement options

Table 58. 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 REJECT_WITH ]
REJECT_WITH := icmp type icmp_code |
                 icmpv6 type icmpv6_code |
                 icmpx type icmpx_code |
                 tcp reset

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

Table 59. 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¶

counter packets number bytes number
counter { packets number | bytes number }

CONNTRACK STATEMENT¶

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

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

Table 60. 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¶

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

A meta statement sets meta data associated with a packet.

Table 61. Meta statement types

LIMIT STATEMENT¶

limit rate [over] packet_number / TIME_UNIT [burst packet_number packets]


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

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

Table 62. limit statement values

NAT STATEMENTS¶

snat to address [:port] [PRF_FLAGS]


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

The nat statements are only valid from nat chain types.

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

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

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

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

Before kernel 4.18 nat statements require both prerouting and postrouting base chains to be present since otherwise packets on the return path won’t be seen by netfilter and therefore no reverse translation will take place.

Table 63. NAT statement values

Table 64. 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
# inet family:
# handle ip dnat:
add rule inet nat prerouting dnat ip to 10.0.2.99
# handle ip6 dnat:
add rule inet nat prerouting dnat ip6 to fe80::dead
# this masquerades both ipv4 and ipv6:
add rule inet nat postrouting meta oif ppp0 masquerade

TPROXY STATEMENT¶

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

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

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

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

Table 65. tproxy attributes

Example ruleset for tproxy statement.

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

SYNPROXY STATEMENT¶

synproxy [mss mss_value] [wscale wscale_value] [SYNPROXY_FLAGS]

Table 66. synproxy statement attributes

Table 67. synproxy statement flags

Example ruleset for synproxy statement.

Determine tcp options used by backend, from an external system
              tcpdump -pni eth0 -c 1 'tcp[tcpflags] == (tcp-syn|tcp-ack)'
                  port 80 &
              telnet 192.0.2.42 80
              18:57:24.693307 IP 192.0.2.42.80 > 192.0.2.43.48757:
                  Flags [S.], seq 360414582, ack 788841994, win 14480,
                  options [mss 1460,sackOK,
                  TS val 1409056151 ecr 9690221,
                  nop,wscale 9],
                  length 0
Switch tcp_loose mode off, so conntrack will mark out-of-flow packets as state INVALID.
              echo 0 > /proc/sys/net/netfilter/nf_conntrack_tcp_loose
Make SYN packets untracked.
        table ip x {
                chain y {
                        type filter hook prerouting priority raw; policy accept;
                        tcp flags syn notrack
                }
        }
Catch UNTRACKED (SYN  packets) and INVALID (3WHS ACK packets) states and send
them to SYNPROXY. This rule will respond to SYN packets with SYN+ACK
syncookies, create ESTABLISHED for valid client response (3WHS ACK packets) and
drop incorrect cookies. Flags combinations not expected during  3WHS will not
match and continue (e.g. SYN+FIN, SYN+ACK). Finally, drop invalid packets, this
will be out-of-flow packets that were not matched by SYNPROXY.
    table ip foo {
            chain z {
                    type filter hook input priority filter; policy accept;
                    ct state { invalid, untracked } synproxy mss 1460 wscale 9 timestamp sack-perm
                    ct state invalid drop
            }
    }
The outcome ruleset of the steps above should be similar to the one below.
        table ip x {
                chain y {
                        type filter hook prerouting priority raw; policy accept;
                        tcp flags syn notrack
                }
                chain z {
                        type filter hook input priority filter; policy accept;
                        ct state { invalid, untracked } synproxy mss 1460 wscale 9 timestamp sack-perm
                        ct state invalid drop
                }
        }

FLOW STATEMENT¶

flow add @flowtable

QUEUE STATEMENT¶

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

Table 68. queue statement values

Table 69. queue statement flags

DUP STATEMENT¶

dup to device
dup to address device device

Table 70. 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¶

fwd to device

SET STATEMENT¶

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

Example for simple blacklist.

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

MAP STATEMENT¶

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

The key is a value returned by expression.

Using the map statement.

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

VMAP STATEMENT¶

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

Using the vmap statement.

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

MONITOR¶

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

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

Hit ^C to finish the monitor operation.

Listen to all events, report in native nft format.

% nft monitor

Listen to deleted rules, report in JSON format.

% nft -j monitor destroy rules

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

% nft monitor chains

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

% nft monitor ruleset