Posted: Wed Jun 16, 2010 7:57 pm Post subject: Strange ICMP acitivity on Firewall
Greetings all, and thanks in advance for viewing and responding.
I have a strange situation, ICMP requests egressing on my firewall's WAN port are going to a China telecom mobile device.
I know that there are not any users on the system that would be doing such a thing. I realize I could create a rule to block the traffic, but I really want to track down what's going on. Below, find the info packet from my firewall logs (WAN ip address redacted):
06/16/2010 10:06:11.192 - Info - Network Access - ICMP packet allowed - Src - WAN ip address, 3, X1 - Dest - 188.8.131.52, 3, X1 - ICMP Destination Unreachable, Code: 3
Any thoughts? Also, any procedures you might suggest to track this down.
Joined: 21 Sep 2003 Posts: 16777097 Location: Portugal
Posted: Thu Jun 17, 2010 2:10 am Post subject:
Your firewall is logging ICMP type 3, code 3 packets ("destination unreachable", specifically "port unreachable"). These aren't ICMP requests; they are ICMP error responses.
ICMP port unreachable packets are sent, most notably, by the UDP protocol stack, when a connection request arrives for a port that doesn't have anything listening on it. This would be the equivalent of the TCP's RST packet.
Basically this means someone in China is trying to access some UDP port(s) on your system, and your system is replying to them with the standard "there isn't anything here" error message. They're either port scanning you, and your firewall is configured to allow unsolicited UDP through instead of dropping it, or they're trying to access a legitimate service (which is being correctly allowed by your firewall) and your server is down for some reason.
Either that, or someone on your internal network is actually generating false error messages in an attempt to somehow disrupt the Chinese telecom mobile device.
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