Saturday 6 April 2013

Traceroute through Cisco PIX / ASA

I recently had to clear and redeploy a PIX firewall to a new location, and realised that I had forgotten some of the subtleties involved in getting management and troubleshooting tools to work properly. So this is more of a note to self....

Windows tracert is fairly straight forward and uses pure ICMP with incrementing TTL values. Linux traceroute with the -I switch works the same way.

The firewall is required to allow the following:
- Echo Request

- Echo Reply
- Time-Exceeded (needed for TTL=0 responses)

Cisco and Linux traceroute by default uses incrementing UDP ports (from 33434) and incrementing TTL values.

The firewall is required to allow the following:
- UDP ports 33434 - 33464

- Time-Exceeded (needed for TTL=0 responses)
- Destination Unreachable (needed for the final hop port-not-found response)

Putting it all together we get a rule set that looks something like this:

object-group icmp-type ICMP-returns
 description Legit ICMP responses
 icmp-object echo-reply
 icmp-object time-exceeded
 icmp-object unreachable

object-group service Cisco_Traceroute_udp udp
 port-object range 33434 33464

access-list outside_access_in extended permit icmp any object-group External_nets object-group ICMP-returns log disable

access-list inside_access_in remark Permit outbound pings
access-list inside_access_in extended permit icmp object-group Internal_nets any echo log disable

access-list inside_access_in remark Permit traceroute from Cisco devices
access-list inside_access_in extended permit udp object-group Internal_nets any object-group Cisco_Traceroute_udp log disable

Obviously, this assumes sensible values for Internal_nets (e.g. and External_nets (i.e. public IP ranges assigned to your external interface)

As an addendum, the Firewall is not (strictly speaking) a router, and therefore in many cases will not decrement the TTL. I have found this unnecessary in most cases, but if needed, can be enabled as follows:

policy-map global_policy
 class class-default
  set connection decrement-ttl

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