Ubuntu – 2 network interfaces connected to internet. Choose the one to use according to the domain name


Question inspired from here


I live in a house near my university. That said, students wifi can reach my home so I usually use that even from home. The problem is, being a wifi managed by university sysadmins, it's not possible to connect to some dns (e.g. file uploading domains like rapidshare or megaupload).
The good news is I also have an USB internet key that I can use, but, basically that's a scam like all internet mobile plans in Italy because after 20GB they'll cut off my bandwith.

Finally…the question

To optimize my bandwith usage I want to say to my Ubuntu: if I connect to *.domain.com use this interface, otherwise use the other one. For the sake of this question let's call wifi wlan0 and usb internet key ppp0.

P.S.: it's a really specific question. Do not suggest things like "use Tor, dude". I do not want to abuse the university wifi. I actually don't know if that is even possibile with Tor…but still… 🙂

Best Answer

If your example domain "*.domain.com" has a static IP address block, you can add a static route to your routing table like:

me@thiscomputer:~$ whois domain.com

This should give you some contact information for the owners of domain.com. You can contact them to find out their network address. This address will probably be in CIDR format where: == "The set of IP addresses from up to". The number after the slash is the number of bits in the network portion of the address. This is equivalent to a netmask of

You could also use dig to look up some host addresses and attempt to deduce the netblock from that, or just add routes to discrete hosts, but I would not recommend it.

With the address information in hand, you can then add a static route like so:

me@thiscomputer:~$ sudo ip route add inet dev ppp0

IRL it will probably not work exactly like this, consult the manpage for the ip command or the route command (these are different commands which do the same thing, route is older and part of coreutils) to see how to do it right.

If the domain's address is not static (configured by DHCP for example) which seems unlikely for a site with a persistent A record, you could work the above method into a cron job running at some interval shorter than the site's DHCP lease. You'd likely have to contact the site's administrator to get this information, or just experiment. If the job runs once a day, that would probably be sufficiently frequent.

Related Question