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 by 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 the -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
undefine variable
redefine variable = expr
$variable

Symbolic variables can be defined using the define statement. Variable references are expressions and can be used to initialize other variables. The scope of a definition is the current block and all blocks contained within. Symbolic variables can be undefined using the undefine statement, and modified using the redefine statement.

Using symbolic variables.

define int_if1 = eth0
define int_if2 = eth1
define int_ifs = { $int_if1, $int_if2 }
redefine int_if2 = wlan0
undefine int_if2
filter input iif $int_ifs accept

IPV4/IPV6/INET ADDRESS FAMILIES¶

The IPv4/IPv6/Inet address families handle IPv4, IPv6 or both types of packets. They contain five hooks at different packet processing stages in the network stack.

Table 1. IPv4/IPv6/Inet address family hooks

ARP ADDRESS FAMILY¶

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

Table 2. ARP address family hooks

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 the device ingress and egress path. This family allows you to filter packets of any ethertype such as ARP, VLAN 802.1q, VLAN 802.1ad (Q-in-Q) as well as IPv4 and IPv6 packets.

Table 3. Netdev address family hooks

Tunneled packets (such as vxlan) are processed by netdev family hooks both in decapsulated and encapsulated (tunneled) form. So a packet can be filtered on the overlay network as well as on the underlying network.

Note that the order of netfilter and tc is mirrored on ingress versus egress. This ensures symmetry for NAT and other packet mangling.

Ingress packets which are redirected out some other interface are only processed by netfilter on egress if they have passed through netfilter ingress processing before. Thus, ingress packets which are redirected by tc are not subjected to netfilter. But they are if they are redirected by netfilter on ingress. Conceptually, tc and netfilter can be thought of as layers, with netfilter layered above tc: If the packet hasn’t been passed up from the tc layer to the netfilter layer, it’s not subjected to netfilter on egress.

CT HELPER¶

add ct helper [family] table name { type type protocol protocol ; [l3proto family ;] }
delete ct helper [family] table name
list ct helpers

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 filter;


      tcp dport 21 ct helper set "ftp-standard"


  }
}

CT TIMEOUT¶

add ct timeout  [family] table name { protocol protocol ; policy = { state: value [, ...] } ; [l3proto family ;] }
delete ct timeout [family] table name
list ct timeouts

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

tcp connection state names that can have a specific timeout value are:

close, close_wait, established, fin_wait, last_ack, retrans, syn_recv, syn_sent, time_wait and unack.

You can use sysctl -a |grep net.netfilter.nf_conntrack_tcp_timeout_ to view and change the system-wide defaults. ct timeout allows for flow-specific settings, without changing the global timeouts.

For example, tcp port 53 could have much lower settings than other traffic.

udp state names that can have a specific timeout value are replied and unreplied.

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¶

add ct expectation  [family] table name { protocol protocol ; dport dport ; timeout timeout ; size size ; [*l3proto family ;] }
delete ct expectation  [family] table name
list ct expectations

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¶

add counter [family] table name [{ [ packets packets bytes bytes ; ] [ comment comment ; }]
delete counter [family] table name
list counters

Table 14. Counter specifications

Using named counters.

nft add counter filter http
nft add rule filter input tcp dport 80 counter name \"http\"

Using named counters with maps.

nft add counter filter http
nft add counter filter https
nft add rule filter input counter name tcp dport map { 80 : \"http\", 443 : \"https\" }

QUOTA¶

add quota [family] table name { [over|until] bytes BYTE_UNIT [ used bytes BYTE_UNIT ] ; [ comment comment ; ] }
BYTE_UNIT := bytes | kbytes | mbytes
delete quota [family] table name
list quotas

Table 15. Quota specifications

Using named quotas.

nft add quota filter user123 { over 20 mbytes }
nft add rule filter input ip saddr 192.168.10.123 quota name \"user123\"

Using named quotas with maps.

nft add quota filter user123 { over 20 mbytes }
nft add quota filter user124 { over 20 mbytes }
nft add rule filter input quota name ip saddr map { 192.168.10.123 : \"user123\", 192.168.10.124 : \"user124\" }

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¶

Table 23. overview of types used in ct expression and statement

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.

DCCP PKTTYPE TYPE¶

The DCCP packet type abstracts the different legal values of the respective four bit field in the DCCP header, as stated by RFC4340. Note that possible values 10-15 are considered reserved and therefore not allowed to be used. In iptables' dccp match, these values are aliased INVALID. With nftables, one may simply match on the numeric value range, i.e. 10-15.

Table 28. keywords may be used when specifying the DCCP packet type

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 29. Meta expression types

Table 30. Meta expression specific types

Using meta expressions.

# qualified meta expression
filter output meta oif eth0
filter forward meta iifkind { "tun", "veth" }
# unqualified meta expression
filter output oif eth0
# incoming packet was subject to ipsec processing
raw prerouting meta ipsec exists accept

SOCKET EXPRESSION¶

socket {transparent | mark | wildcard}
socket cgroupv2 level NUM

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). You can also use it to match on the socket cgroupv2 at a given ancestor level, e.g. if the socket belongs to cgroupv2 a/b, ancestor level 1 checks for a matching on cgroup a and ancestor level 2 checks for a matching on cgroup b.

Table 31. Available socket attributes

Using socket expression.

# Mark packets that correspond to a transparent socket. "socket wildcard 0"
# means that zero-bound listener sockets are NOT matched (which is usually
# exactly what you want).
table inet x {


    chain y {


        type filter hook prerouting priority mangle; policy accept;


        socket transparent 1 socket wildcard 0 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 mangle; policy accept;


        socket mark 0x0000000f nftrace set 1


    }
}
# Set packet mark to socket mark
table inet x {


    chain y {


        type filter hook prerouting priority mangle; policy accept;


        tcp dport 8080 mark set socket mark


    }
}
# Count packets for cgroupv2 "user.slice" at level 1
table inet x {


    chain y {


        type filter hook input priority filter; policy accept;


        socket cgroupv2 level 1 "user.slice" counter


    }
}

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 32. 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 filter; 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 33. 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 34. Routing expression types

Table 35. Routing expression specific types

Using routing expressions.

# IP family independent rt expression
filter output rt classid 10
# IP family dependent rt expressions
ip filter output rt nexthop 192.168.0.1
ip6 filter output rt nexthop fd00::1
inet filter output rt ip nexthop 192.168.0.1
inet filter output rt ip6 nexthop fd00::1
# outgoing packet will be encapsulated/encrypted by ipsec
filter output rt ipsec exists

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 36. Ipsec expression types

Note: When using xfrm_interface, this expression is not useable in output hook as the plain packet does not traverse it with IPsec info attached - use a chain in postrouting hook instead.

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 37. Ethernet header expression types

VLAN HEADER EXPRESSION¶

vlan {id | dei | pcp | type}

Table 38. VLAN header expression

ARP HEADER EXPRESSION¶

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

Table 39. ARP header expression

IPV4 HEADER EXPRESSION¶

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

Table 40. 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 41. 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 42. 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 43. 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 44. ICMPv6 header expression

TCP HEADER EXPRESSION¶

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

Table 45. TCP header expression

UDP HEADER EXPRESSION¶

udp {sport | dport | length | checksum}

Table 46. UDP header expression

UDP-LITE HEADER EXPRESSION¶

udplite {sport | dport | checksum}

Table 47. UDP-Lite header expression

SCTP HEADER EXPRESSION¶

sctp {sport | dport | vtag | checksum}


sctp chunk CHUNK [ FIELD ]
CHUNK := data | init | init-ack | sack | heartbeat |


           heartbeat-ack | abort | shutdown | shutdown-ack | error |


           cookie-echo | cookie-ack | ecne | cwr | shutdown-complete


           | asconf-ack | forward-tsn | asconf
FIELD := COMMON_FIELD | DATA_FIELD | INIT_FIELD | INIT_ACK_FIELD |


           SACK_FIELD | SHUTDOWN_FIELD | ECNE_FIELD | CWR_FIELD |


           ASCONF_ACK_FIELD | FORWARD_TSN_FIELD | ASCONF_FIELD
COMMON_FIELD := type | flags | length
DATA_FIELD := tsn | stream | ssn | ppid
INIT_FIELD := init-tag | a-rwnd | num-outbound-streams |


                num-inbound-streams | initial-tsn
INIT_ACK_FIELD := INIT_FIELD
SACK_FIELD := cum-tsn-ack | a-rwnd | num-gap-ack-blocks |


                num-dup-tsns
SHUTDOWN_FIELD := cum-tsn-ack
ECNE_FIELD := lowest-tsn
CWR_FIELD := lowest-tsn
ASCONF_ACK_FIELD := seqno
FORWARD_TSN_FIELD := new-cum-tsn
ASCONF_FIELD := seqno

Table 48. SCTP header expression

Table 49. SCTP chunk fields

DCCP HEADER EXPRESSION¶

dccp {sport | dport | type}

Table 50. DCCP header expression

AUTHENTICATION HEADER EXPRESSION¶

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

Table 51. AH header expression

ENCRYPTED SECURITY PAYLOAD HEADER EXPRESSION¶

esp {spi | sequence}

Table 52. ESP header expression

IPCOMP HEADER EXPRESSION¶

comp {nexthdr | flags | cpi}

Table 53. 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 54. 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¶

Extension header expressions refer to data from variable-sized protocol headers, such as IPv6 extension headers, TCP options and IPv4 options.

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

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

Table 55. IPv6 extension headers

Table 56. TCP Options

TCP option matching also supports raw expression syntax to access arbitrary options:

tcp option

tcp option @number,offset,length

Table 57. IP Options

finding TCP options.

filter input tcp option sack-perm exists counter

matching TCP options.

filter input tcp option maxseg size lt 536

matching IPv6 exthdr.

ip6 filter input frag more-fragments 1 counter

finding IP option.

filter input ip option lsrr exists counter

CONNTRACK EXPRESSIONS¶

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

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

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

The conntrack-specific types in this table are described in the sub-section CONNTRACK TYPES above.

Table 58. Conntrack expressions

restrict the number of parallel connections to a server.

nft add set filter ssh_flood '{ type ipv4_addr; flags dynamic; }'
nft add rule filter input tcp dport 22 add @ssh_flood '{ ip saddr ct count over 2 }' reject

VERDICT STATEMENT¶

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

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

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

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 the TCPMSS target in iptables.

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

You can also remove tcp options via reset keyword.

remove tcp option.

tcp flags syn reset tcp option sack-perm

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 send the log through a netlink socket to the specified group. One userspace process may subscribe to the group to receive the logs, see man(8) ulogd for the Netfilter userspace log daemon and libnetfilter_log documentation for details in case you would like to develop a custom program to digest your logs.

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 59. log statement options

Table 60. 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 icmp_code |


                 icmpv6 icmpv6_code |


                 icmpx 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 61. different ICMP reject variants are meant for use in different table families

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

Note that in bridge family, reject statement is only allowed in base chains which hook into input or prerouting.

COUNTER STATEMENT¶

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

counter packets number bytes number
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 raw (-300).

Unlike iptables, where the helper assignment happens in the raw table, the helper needs to be assigned after a conntrack entry has been found, i.e. it will not work when used with hook priorities equal or before -200.

Table 62. 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 raw;


      ct zone set iif map { "eth1" : 1, "veth1" : 2 }


  }


  chain output {


      type filter hook output priority raw;


      ct zone set oif map { "eth1" : 1, "veth1" : 2 }


  }
}

restrict events reported by ctnetlink.

ct event set new,related,destroy

NOTRACK STATEMENT¶

The notrack statement allows to disable connection tracking for certain packets.

notrack

Note that for this statement to be effective, it has to be applied to packets before a conntrack lookup happens. Therefore, it needs to sit in a chain with either prerouting or output hook and a hook priority of -300 (raw) or less.

See SYNPROXY STATEMENT for an example usage.

META STATEMENT¶

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

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

A meta statement sets meta data associated with a packet.

Table 63. 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. Default burst is 5. if you specify burst, it must be non-zero value.

Table 64. limit statement values

NAT STATEMENTS¶

snat [[ip | ip6] to] ADDR_SPEC [:PORT_SPEC] [FLAGS]


dnat [[ip | ip6] to] ADDR_SPEC [:PORT_SPEC] [FLAGS]
masquerade [to :PORT_SPEC] [FLAGS]
redirect [to :PORT_SPEC] [FLAGS]
ADDR_SPEC := address | address - address
PORT_SPEC := port | port - port
FLAGS  := FLAG [, FLAGS]
FLAG  := 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.

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 65. NAT statement values

Table 66. 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 dstnat; }
add chain nat postrouting { type nat hook postrouting priority srcnat; }
# translate source addresses of all packets leaving via eth0 to address 1.2.3.4
add rule nat postrouting oif eth0 snat to 1.2.3.4
# redirect all traffic entering via eth0 to destination address 192.168.1.120
add rule nat prerouting iif eth0 dnat to 192.168.1.120
# translate source addresses of all packets leaving via eth0 to whatever
# locally generated packets would use as source to reach the same destination
add rule nat postrouting oif eth0 masquerade
# redirect incoming TCP traffic for port 22 to port 2222
add rule nat prerouting tcp dport 22 redirect to :2222
# inet family:
# handle ip dnat:
add rule inet nat prerouting dnat ip to 10.0.2.99
# handle ip6 dnat:
add rule inet nat prerouting dnat ip6 to fe80::dead
# this masquerades both ipv4 and ipv6:
add rule inet nat postrouting meta oif ppp0 masquerade

TPROXY STATEMENT¶

Tproxy redirects the packet to a local socket without changing the packet header in any way. If any of the arguments is missing the data of the incoming packet is used as parameter. Tproxy matching requires another rule that ensures the presence of transport protocol header is specified.

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

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

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

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

Table 67. tproxy attributes

Example ruleset for tproxy statement.

table ip x {


    chain y {


        type filter hook prerouting priority mangle; 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 mangle; 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 mangle; policy accept;


        tcp dport 321 tproxy to :ssh


        tcp dport 99 tproxy ip to 1.1.1.1:999


        udp dport 155 tproxy ip6 to [dead::beef]:smux


    }
}

SYNPROXY STATEMENT¶

This statement will process TCP three-way-handshake parallel in netfilter context to protect either local or backend system. This statement requires connection tracking because sequence numbers need to be translated.

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

Table 68. synproxy statement attributes

Table 69. 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 x {


            chain z {


                    type filter hook input priority filter; policy accept;


                    ct state invalid, untracked synproxy mss 1460 wscale 9 timestamp sack-perm


                    ct state invalid drop


            }


    }

FLOW STATEMENT¶

A flow statement allows us to select what flows you want to accelerate forwarding through layer 3 network stack bypass. You have to specify the flowtable name where you want to offload this flow.

flow add @flowtable

QUEUE STATEMENT¶

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

queue [flags QUEUE_FLAGS] [to queue_number]
queue [flags QUEUE_FLAGS] [to queue_number_from - queue_number_to]
queue [flags QUEUE_FLAGS] [to QUEUE_EXPRESSION ]
QUEUE_FLAGS := QUEUE_FLAG [, QUEUE_FLAGS]
QUEUE_FLAG  := bypass | fanout
QUEUE_EXPRESSION := numgen | hash | symhash | MAP STATEMENT

QUEUE_EXPRESSION can be used to compute a queue number at run-time with the hash or numgen expressions. It also allows to use the map statement to assign fixed queue numbers based on external inputs such as the source ip address or interface names.

Table 70. queue statement values

Table 71. queue statement flags

DUP STATEMENT¶

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

dup to device
dup to address device device

Table 72. 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
netdev ingress dup to "eth0"
dup to "eth0"
# combine with map dst addr to gateways
dup to ip daddr map { 192.168.7.1 : "eth0", 192.168.7.2 : "eth1" }

FWD STATEMENT¶

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

fwd to device

SET STATEMENT¶

The set statement is used to dynamically add or update elements in a set from the packet path. The set setname must already exist in the given table and must have been created with one or both of the dynamic and the timeout flags. The dynamic flag is required if the set statement expression includes a stateful object. The timeout flag is implied if the set is created with a timeout, and is required if the set statement updates elements, rather than adding them. Furthermore, these sets should specify both a maximum set size (to prevent memory exhaustion), and their elements should have a timeout (so their number will not grow indefinitely) either from the set definition or from the statement that adds or updates them. The set statement can be used to e.g. create dynamic blacklists.

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

Example for simple blacklist.

# declare a set, bound to table "filter", in family "ip".
# Timeout and size are mandatory because we will add elements from packet path.
# Entries will timeout after one minute, after which they might be
# re-added if limit condition persists.
nft add set ip filter blackhole \


    "{ type ipv4_addr; flags dynamic; timeout 1m; size 65536; }"
# declare a set to store the limit per saddr.
# This must be separate from blackhole since the timeout is different
nft add set ip filter flood \


    "{ type ipv4_addr; flags dynamic; timeout 10s; size 128000; }"
# 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.
nft add rule ip filter input tcp flags syn tcp dport ssh \


    add @flood { ip saddr limit rate over 10/second } \


    add @blackhole { ip saddr } \


    drop
# inspect state of the sets.
nft list set ip filter flood
nft list set ip filter blackhole
# manually add two addresses to the blackhole.
nft add element filter blackhole { 10.2.3.4, 10.23.1.42 }

MAP STATEMENT¶

The map statement is used to lookup data based on some specific input key.

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

The key is a value returned by expression.

Using the map statement.

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

VMAP STATEMENT¶

The verdict map (vmap) statement works analogous to the map statement, but contains verdicts as values.

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

Using the vmap statement.

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

MONITOR¶

The monitor command allows you to listen to Netlink events produced by the nf_tables subsystem. These are either related to creation and deletion of objects or to packets for which meta nftrace was enabled. When they occur, nft will print to stdout the monitored events in either JSON or native nft format.

monitor [new | destroy] MONITOR_OBJECT
monitor trace
MONITOR_OBJECT := tables | chains | sets | rules | elements | ruleset

To filter events related to a concrete object, use one of the keywords in MONITOR_OBJECT.

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

The second form of invocation takes no further options and exclusively prints events generated for packets with nftrace enabled.

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

Trace incoming packets from host 10.0.0.1.

% nft add rule filter input ip saddr 10.0.0.1 meta nftrace set 1
% nft monitor trace