[Snort-users] Everything being triggered as 1:486:4 ICMP unreachable

David Ryan David.Ryan at ...13912...
Wed May 30 06:48:31 EDT 2007


To partly reply to my own question - one of my flaws was keeping $HOME_NET as - this prevented most of my test traffic from triggering since my tests were coming from $HOME_NET and not $EXTERNAL_NET.  This was significant where the rule I was testing was of the form $EXTERNAL_NET -> $HOME_NET

I'm still getting the ICMP messages as well, so I have to presume that those packets are actually out there also, but I don't understand why they are being generated since as observeved from the fact that I can SSH and ping the host it is not actually unreachable.


From: David Ryan
Sent: 30 May 2007 11:10
To: 'snort-users at lists.sourceforge.net'
Subject: Everything being triggered as 1:486:4 ICMP unreachable

Hi all,

I built and tested a number of machines and shipped them out to remote sites recently.  Now that they are at the remote site I am trying to tweak them a bit but I have run into some pretty basic problems that I hadn't come across before.  I'm sure the devices were logging events properly before I shipped them, but now every time I try to test a rule I get back the following line -

May 30 10:35:14 <hostname> snort[5017]: [1:486:4] ICMP Destination Unreachable Communication with Destination Host is Administratively Prohibited [Classification: Misc activity] [Priority: 3]: {ICMP} <probe address> -> <my test machine address>

There are a couple of relevant points here -
$HOME_NET is defined as and $EXTERNAL_NET is defined as !$HOME_NET
The probe has an ip address assigned in a /24 10.x.x subnet
My test machine has an ip address assigned in a different /23 10.x.x subnet
I know there is no routing problem since I am SSH'd in to the probe from my test machine and I can ping in both directions

I am trying to read through the rule generating this message to see what is being triggered, but it looks pretty much like it is triggered on what it describes - an ICMP destination unreachable notification packet.

I looked back through the old logs and see the following from before I shipped it off so I know it was logging correctly -
May 15 12:08:10 <hostname> snort[2331]: [1:2189:3] BAD-TRAFFIC IP Proto 103 PIM [Classification: Detection of a non-standard protocol or event] [Priority: 2]: {PIM} <old probe address> ->

The only thing that changed is the probe IP address, but both the old and new configs used DHCP and I'm pretty sure that when I was testing before the IP address changed and it still continued to log OK.

Any ideas what is causing this trigger ?  As above, I am reasonably sure that it is not an actual ICMP destination unreachable packet because the destination is reachable . . .


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