Conntrack helpers

You can enable conntrack helpers explicitly through your ruleset. You have to attach your conntrack helper from the prerouting chain.

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"
      }
}

The example above shows how to enable the FTP conntrack helper for traffic going through port tcp/21.

You can read more on how to enable conntrack helpers in a secure way here.