Outbound ICMP Time Exceed messages must be blocked to prevent network discovery by unauthorized users.

The trace route tool will display routes and trip times on an IP network. An attacker can use trace route responses to create a map of the subnets and hosts behind the perimeter router, just as they could do with pings. The traditional trace route relies on TTL - time exceeded responses from routers along the path and an ICMP port-unreachable message from the target host. In some Operating Systems such as UNIX, trace route will use UDP port 33400 and increment ports on each response. Since blocking these UDP ports alone will not block trace route capabilities along with blocking potentially legitimate traffic on a network, it is unnecessary to block them explicitly. Because trace routes typically rely on ICMP Type 11 - Time exceeded message, the time exceeded message will be the target for implicitly or explicitly blocking outbound from the trusted network.