How do I create a named set of strings in nftables (for interface names)?

Question

Using sets in nftables is really cool. I am currently using a lot of statements like these in my nftables.conf rulesets:

iifname {clients0, dockernet} oifname wan0 accept comment "Allow clients and Docker containers to reach the internet"

But {clients0, dockernet} is an anonymous (inline) set of strings. Instead of repetition in the rules over and over, I'd like to define a set of interfaces, called a named set in nftables. The manpage shows how to do that for several types of sets: ipv4_addr, ipv6_addr, ether_addr, inet_proto, inet_service, mark. But, not for interface name or simple primitive type of string.

So, despite I could not find a way how to do that, I've tried this, but this does not work:

1. Omitting the type:

   table inet filter {
     set myset {
       elements = {
         clients0,
         dockernet,           
       }
     }
     [...]
   }
   

   Result: Error: set definition does not specify key.

2. Specify string type:

   table inet filter {
     type string;
     set myset {
       elements = {
         clients0,
         dockernet,           
       }
     }
     [...]
   }
   

   Result: Error: unqualified key type string specified in set definition.

Is there really no way of naming the anonymous set I've shown on the top?

Answer 1

By searching in the source for version 0.9.0:

static const struct datatype *datatypes[TYPE_MAX + 1] = {

[...]

  [TYPE_IFINDEX]      = &ifindex_type,

[...]

  [TYPE_IFNAME]       = &ifname_type,
};

and then looking for where they are defined:

const struct datatype ifindex_type = {
  .type       = TYPE_IFINDEX,
  .name       = "iface_index",
  .desc       = "network interface index",

[...]

const struct datatype ifname_type = {
  .type       = TYPE_IFNAME,
  .name       = "ifname",
  .desc       = "network interface name",

it's possible to find that the needed types are iface_index (eg: iif lo) and ifname (eg iifname "lo"), even if (as of writting this answer) it's undocumented in nft's man page.

nft add set inet filter if-index-set '{ type iface_index;  }'
nft add set inet filter if-name-set '{ type ifname;  }'

nft add element inet filter if-index-set '{ lo }'
nft add element inet filter if-name-set '{ lo }'

nft add chain inet filter input '{ type filter hook input priority 0; }'
nft add rule inet filter input iif @if-index-set counter
nft add rule inet filter input iifname @if-name-set counter

Note that it appears wildcards don't work for an ifname set, as seen in this question: How to use wildcard in a list with nftables.