Linux Firewalls, Third Edition

Filtering Outgoing Packets

If your environment represents a trusted environment, filtering outgoing packets might not appear to be as critical as filtering incoming packets. Your system won't respond to incoming messages that the firewall doesn't pass through. Residential sites often take this approach. Nevertheless, even for residential sites, symmetric filtering is important, particularly if the firewall protects Microsoft Windows machines. For commercial sites, outgoing filtering is inarguably important.

If your firewall protects a LAN of Microsoft Windows systems, controlling outgoing traffic becomes much more important. Recently, tens of thousands of compromised Windows machines have been used in coordinated DoS attacks. For this reason ...