Data types

Data types used in Netfilter

The following data types are used in nft expressions to select matching packets:

Netfilter Data Types
Data Type	Description	nft Expressions	Notes
day	Either a day of week ("Monday", "Tuesday", etc.), or an integer between 0 and 6. Strings are matched case-insensitively, and a full match is not expected (e.g. "Mon" would match "Monday"). When an integer is given, 0 is Sunday and 6 is Saturday.	meta day
devgroup	Device group (32 bit integer).	meta {iifgroup | oifgroup}	Can be specified numerically or as symbolic name defined in /etc/iproute2/group.
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.)
gid	Group ID (32 bit integer).	meta skgid	Can be specified numerically or as group name.
hour	A string representing an hour in 24-hour format. Seconds can optionally be specified. For example, 17:00 and 17:00:00 would be equivalent.	meta hour
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.
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}
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
time	Either an integer or a date in ISO format. For example: "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 would be equivalent: "2019-06-06", "2019-06-06 00:00" and "2019-06-06 00:00:00". When an integer is given, it is assumed to be a UNIX timestamp.	meta time
uid	User ID (32 bit integer).	meta skuid	Can be specified numerically or as user name.

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