Which ports should I open for apt-get to work?

I installed Webmin, and then set up the firewall like this:

INPUT
SSH port ALLOWED
Webmin port ALLOWED
HTTP port (80) ALLOWED
DROP EVERYTHING ELSE

FORWARDING
no rules

OUTPUT
no rules

If I remove DROP EVERYTHING ELSE from INPUT, everything works. However, when that rule is added, apt-get doesn't work, and I can't ping or traceroute anything. Even with DROP EVERYTHING ELSE enabled, Webmin, HTTP and SSH still work. Which ports should I unblock to get apt-get working and allowed connecting to other domains from within the server? Thanks