Following a migration of my network from FreeBSD to Linux, I'm attempting to translate some PF rules into iptables syntax.
The following rule is giving me some hard time:
pass out on vlan42 proto tcp from (vlan42) to any port 21

The thing that blocks me in the (vlan42) part. For those who are not familiar with pf syntax, it means "The IP address on the vlan42 interface". It is updated by PF each time the IP address is updated. Is there a way with iptables to describe such a thing ?

Thanks a lot.