Router domain blocking
 

I am using a slackware box as a router (2 nics).

Playing with DD-WRT on a Linksys router, I noticed that it offers the ability to block certain domains. I'm not clear on the exact function of that blocking, but I assume that it looks up the DNS entry(s) for the user listed domains, and then does something.

Does anyone here know what the mechanism is for doing this?

Even better, is there a CLI string which will accomplish the same or similar thing on a system with iptables?

Thanks.

Custom applications (like the one running in the proprietary firewalls) can do a lot of stuff which may not have
a direct easy achieve-able CLIs in linux box

if you want to block certain domains, I would suggest using tproxy with squid and then using squid config to
select the domains which you want to allow/deny. This is completely transparent and no config needed on the
part of the other networked systems.