Many services running on the firewall machine need to be able to establish connections to the same machine. X11, RPC, DNS are services like that, to name a few. Blocking these services on the firewall can cause various problems, depending on what protocol is being blocked. If it is DNS, then it may take a lot longer than usual to get to a command-line prompt when logging in to the machine using Telnet or SSH. Once logged in, you won't be able to resolve any host names into addresses. If X11 is blocked, then X server and any graphic environment using it (KDE, Gnome etc.) won't start. In any case though the problem can easily be solved by adding a simple any-any rule and specifying the loopback interface of the firewall to permit all sorts of communications. As shown on Figure 14.31, this rule must specify the loopback interface, have action Accept and direction Both.
Running X11 and other complex services on the dedicated firewall machine should be discouraged. However, you may want to run a firewall to protect a server, workstation, or laptop where X11, RPC, and other services are perfectly normal.
The generated iptables commands are placed in INPUT and OUTPUT chains because packets sent by the firewall to itself never hit FORWARD chain. Options "-i lo" and "-o lo" nail interface and direction:
$IPTABLES -A INPUT -i lo -m state --state NEW -j ACCEPT $IPTABLES -A OUTPUT -o lo -m state --state NEW -j ACCEPT
For PF, we can specify interface to match but keep direction open so both "in" and "out" will match:
pass quick on lo inet from any to any keep state
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