deny traceroute fail

Unanswered Question
Oct 10th, 2007

Hi all,

My objective is only to deny traceroute from to Wan interface)

Physical connection as below



interface FastEthernet0/1

ip address

ip access-group 101 in

duplex auto

speed auto

access-list 101 deny icmp host traceroute

access-list 101 permit ip any any

But result show that I can ping and traceroute from PC host

What command I wrong?

I have this problem too.
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Edison Ortiz Wed, 10/10/2007 - 20:34

Try this ACL

access-list 101 deny icmp host unreachable

access-list 101 permit ip any any

Richard Burts Thu, 10/11/2007 - 02:27

I agree with Edison that blocking the unreachable messages is a more effective way to block traceroute. In the original post Jack had the access-group as in. For this access list to work it would need to be configured for out (since the ACL now has as the source and as the destination).



Edison Ortiz Thu, 10/11/2007 - 04:41


ip access group 101 in can also be placed on the link connecting to RouterB.

I'm assuming RouterC is the host and is the subnet sitting behind RouterA.

Richard Burts Thu, 10/11/2007 - 04:55


Yes it could be placed on the interface to router B. And there is some marginal benefit in doing that since it would deny the packets before they go through the routing logic.



Pavel Bykov Thu, 10/11/2007 - 02:59

Please remember, that traceroute is usually performed using UDP, not ICMP. ICMP is used only when the packet times out in transit and ICMP unreachable is sent back.

This should work:

access-list 101 deny udp host traceroute

access-list 101 permit ip any any

Previous posts have also valid points. Depends on how you want to block it.

Richard Burts Thu, 10/11/2007 - 03:15


In my experience whether traceroute uses UDP or ICMP depends on the OS of the end station. IOS devices and _nix boxes use UDP as you mention. But Windows uses ICMP. So what to put into the access list depends on what is on the source subnet.

I worry a bit about denying all unreachable messages and what you would no longer be informed about. But I do believe that if denying traceroute is important that denying the unreachable response is more effective than trying to deny the outbound probe packets.




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