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

Joel Esler joel.esler at ...1935...
Wed May 30 07:36:11 EDT 2007


David,

Help me understand this, since you don't have any ip's in the alert.   
Since the rule is written to be any any -> any any, the alerts you  
are seeing are being generated by an outside address with responses  
coming back in?  So your rule is triggering on response packets?

So that leads me to believe that your machine is attempting to  
connect to a third party, but it is receiving Destination Unreachable  
messages.

Snort doesn't generate alerts if the traffic isn't there.  So the  
traffic is there, now you have to figure out why.



joel esler | security consultant | Sourcefire | pgp  key is public



On May 30, 2007, at 6:48 AM, David Ryan wrote:

> All,
>
>
>
> To partly reply to my own question - one of my flaws was keeping  
> $HOME_NET as 10.0.0.0/8 - 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.
>
>
>
> David
>
>
>
> 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 10.0.0.0/8 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> -> 224.0.0.13
>
>
>
> 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 . . .
>
>
>
> David
>
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