Data types

nft describe

You can use nft describe to get information about a data type, to find out the data type of a particular selector, and to list predefined symbolic constants for that selector. Some examples:

% nft describe iif
meta expression, datatype iface_index (network interface index) (basetype integer), 32 bits

% nft describe iifname
meta expression, datatype ifname (network interface name) (basetype string), 16 characters

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

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

List of data types

Date and time types

Date and time types
Data Type	Description	nft Expressions	Notes
day	Day of week of packet reception (8 bit integer, with pre-defined symbolic constants): Sunday Monday Tuesday Wednesday Thursday Friday Saturday	meta day	Sunday = 0, Saturday = 6. Symbolic constants are case insensitive, and unique abbreviations are accepted: Sun = sun = Sunday = 0.
hour	Hour of day of packet reception (32 bit integer). Specify as string in 24-hour format, hh:mm[:ss].	meta hour	Seconds are optional: 17:00 = 17:00:00.
time	Relative time of packet reception (64 bit integer).	meta time	Can be specified as a date in ISO format, i.e. "2019-06-06 17:00". Hour and seconds are optional and can be omitted if desired. If omitted, midnight will be assumed. The following three are equivalent: "2019-06-06" = "2019-06-06 00:00" = "2019-06-06 00:00:00". When an integer is specified, it is assumed to be a UNIX timestamp.

Network interface types

Network interface types
Data Type	Description	nft Expressions	Notes
devgroup	Device group (32 bit integer).	meta {iifgroup | oifgroup}	Can be specified numerically or as symbolic name defined in /etc/iproute2/group.
iface_index	Interface index (32 bit integer).	meta {iif | oif}	Can be specified numerically or as name of an existing interface. Use ifname instead for interfaces whose name and/or index can change (i.e. those that appear / disappear dynamically).
iface_type	Interface type (16 bit integer, with pre-defined symbolic constants): ether ppp ipip ipip6 loopback sit ipgre	meta {iiftype | oiftype}
ifkind	Interface kind name (16 byte string).	meta {iifkind | oifkind}	dev->rtnl_link_ops->kind The man 8 ip-link TYPES section lists valid ifkinds. It's missing at least one: tun.
ifname	Interface name (16 byte string).	meta {iifname | oifname}	Does not have to exist. Slower than iface_index but good for interfaces that can dynamically appear / disappear.

Ethernet types

Ethernet types
Data Type	Description	nft Expressions	Notes
ether_addr	Ethernet address (48 bit integer).	ether {saddr | daddr} arp {saddr | daddr} ether
ether_type	EtherType (16 bit integer, with pre-defined symbolic constants): arp ip ip6 vlan	meta protocol	ether.h has known types. NOTE that ether.h lists EtherTypes in network order, while nft uses little-endian order on x86. (Check output of nft describe ether_type.)

IP types

IP types
Data Type	Description	nft Expressions	Notes
inet_proto	Internet protocol (8 bit integer, with pre-defined symbolic constants): tcp udp udplite esp ah icmp icmpv6 comp dccp sctp	ip protocol ip6 nexthdr ah nexthdr comp nexthdr ct {original | reply} protocol	in.h has known types.
inet_service	Network service port number (16 bit integer).	udp {sport | dport} tcp {sport | dport} udplite {sport | dport} sctp {sport | dport} dccp {sport | dport}
ipv4_addr	IPv4 address (32 bit integer).	ip {saddr | daddr} arp {saddr | daddr} ip ct {original | reply} ip {saddr | daddr} rt ip nexthop ipsec {in | out} ip {saddr | daddr}
ipv6_addr	IPv6 address (128 bit integer).	ip6 {saddr | daddr} ct {original | reply} ip6 {saddr | daddr} rt ip6 nexthop ipsec {in | out} ip6 {saddr | daddr}

Other types

Other types
Data Type	Description	nft Expressions	Notes
gid	Group ID (32 bit integer).	meta skgid	Can be specified numerically or as group name.
mark	Packet mark (32 bit integer).	meta mark socket mark fib mark . {saddr | daddr | iif | oif} [. ...] {oif | oifname | type} ct mark
pkt_type	Packet type (8 bit integer, with pre-defined symbolic constants): host or unicast - addressed to local host broadcast - to all multicast - to group other - addressed to another host	meta pkttype
realm	Routing Realm (32 bit integer).	meta rtclassid	Can be specified numerically or as symbolic name defined in /etc/iproute2/rt_realms. Routing realm references: linux-ip.net policyrouting.org
uid	User ID (32 bit integer).	meta skuid	Can be specified numerically or as user name.