Affordable web design - Figure 9.7: The rules flow for a received
Figure 9.7: The rules flow for a received TCP datagram for telnet The user-defined chains do not have default policies. When all rules in a user-defined chain have been tested, and none have matched, the firewall code acts as though a RETURN rule were present, so if this isn’t what you want, you should ensure you supply a rule at the end of the user-defined chain that takes whatever action you wish. In our example, our testing returns to the rule in the input ruleset immediately following the one that moved us to our user-defined chain. Eventually we reach the end of the input chain, which does have a default policy and our datagram is denied. This example is very simple, but illustrates our point. A more practical use of IP chains would be much more complex. A slightly more sophisticated example is provided in the following list of commands: # # Set default forwarding policy to REJECT ipchains -P forward REJECT # # create our user-defined chains ipchains -N sshin ipchains -N sshout ipchains -N wwwin ipchains -N wwwout # # Ensure we reject connections coming the wrong way ipchains -A wwwin -p tcp -s 172.16.0.0/16 -y -j REJECT ipchains -A wwwout -p tcp -d 172.16.0.0/16 -y -j REJECT ipchains -A sshin -p tcp -s 172.16.0.0/16 -y -j REJECT ipchains -A sshout -p tcp -d 172.16.0.0/16 -y -j REJECT # # Ensure that anything reaching the end of a user-defined chain is re- jected. ipchains -A sshin -j REJECT ipchains -A sshout -j REJECT ipchains -A wwwin -j REJECT ipchains -A wwwout -j REJECT # # divert www and ssh services to the relevant user-defined chain ipchains -A forward -p tcp -d 172.16.0.0/16 ssh -b -j sshin ipchains -A forward -p tcp -s 172.16.0.0/16 -d 0/0 ssh -b -j sshout ipchains -A forward -p tcp -d 172.16.0.0/16 www -b -j wwwin ipchains -A forward -p tcp -s 172.16.0.0/16 -d 0/0 www -b -j wwwout # # Insert our rules to match hosts at position two in our user-defined chains. ipchains -I wwwin 2 -d 172.16.1.2 -b -j ACCEPT ipchains -I wwwout 2 -s 172.16.1.0/24 -b -j ACCEPT ipchains -I sshin 2 -d 172.16.1.4 -b -j ACCEPT ipchains -I sshout 2 -s 172.16.1.4 -b -j ACCEPT ipchains -I sshout 2 -s 172.16.1.6 -b -j ACCEPT #