ICMP unreachables, code 9,10,13

Bill Nash billn at billn.net
Wed Mar 28 23:13:00 UTC 2007

On Wed, 28 Mar 2007, Christos Papadopoulos wrote:

> My next question is about responses to ICMP pings (echo request),
> when they return ICMP UNREACHABLE with codes 9,10 or 13.
> Responses with these codes seem to imply the presence of a firewall.
> Is this assumption correct or are these codes meaningless?

They do have meaning, and you do see them in production (generally in 
traceroute responses.) These can indicate the presence of either a 
firewall, or an ACL. Both traffic barriers are typically configurable, and 
whether or not you get a response is very often dictated by how hardcore 
the network engineer or security engineer is about giving up information 
about their network.

> If this a configurable parameter, how to you typically decide what
> to set it to?

See previous comment about relative values of hardcore. Arguably, use of 
these options is telling the end user things about your network 
configuration, including, very specifically, which device is blocking 
their traffic. Depending on your security stance and requirements, this 
may be good or bad.

Personally, I simply drop the offending packets into the bitbucket and let 
the user wonder.

- billn

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