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How to block icmp messages with a cisco router

Posted on 2011-03-17
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Last Modified: 2012-05-11
We have some hosts or a host that seems to disconnect all the vpn client on my network.

I took a look at the server logs and here what is shows

22116      11:01:17.531532      MY_IP      DEST_IP      TCP      53025 > pptp [SYN]
Seq=0 Win=8192 Len=0 MSS=1460 WS=2 TSV=4447122 TSER=0

So it seems that some clients seend some icmp destination unreachable on the server. The server somehow close all the connections.

Is it possible to block those icmp send messages with a cisco 800 series.

Like deny icmp all all ? Is it going to work ??

Thanks
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Question by:caclement
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Expert Comment

by:Jimmy Larsson, CISSP, CEH
ID: 35163053
What you see in the logs is not icmp-traffic, it is tcp-traffic to your pptp-port (1723). Unless you use pptp-based vpn you can simply block all incoming pptp-traffic.

access-list YOURINBOUNDACL deny tcp any SERVERIP eq 1723.

Best regards
Kvistofta
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Accepted Solution

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caclement earned 0 total points
ID: 35191129
My messages dont show all the log and the wireshark packets, but i realy want to block sent icmp packets. I want to allow the PPTP and GRE tunels. Some bugged PPTP clients send some ICMP destination unreachable to remote  poptop and pptp linux server wich i do not own and control. The server close all the connection simultaneously.

I added the following ACL

access-list 101 deny icmp any any

It seems to works.
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Author Closing Comment

by:caclement
ID: 35225383
access-list 101 deny icmp any any Blocks any ICMP sent messages.
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