[Snort-users] false alerts

Phil Wood cpw at ...440...
Thu Jan 24 13:17:08 EST 2002

On Thu, Jan 24, 2002 at 11:22:52AM +1100, support wrote:
> I have am having a problem with snort ...
> I apologize in advance for the nature of the question , however...
> When running Snort 1.8.3 in daemon mode with no output modules I am
> receiving false alerts from my internal network. Below is an excerpt from my
> logs
> "
> Jan 24 10:23:46 proxy snort[12568]: [1:618:1] INFO - Possible Squid Scan
> [Classification: Attempted Information Leak] [Priority: 2]: {TCP}
> ->
> Jan 24 10:23:49 proxy snort[12568]: [1:618:1] INFO - Possible Squid Scan
> [Classification: Attempted Information Leak] [Priority: 2]: {TCP}
> ->
Well, let's take a look at the rule:

% grep "Possible Squid Scan" *.rules
scan.rules:alert tcp $EXTERNAL_NET any -> $HOME_NET 3128 (msg:"INFO - Possible Squid Scan"; flags:S; classtype:attempted-recon; sid:618; rev:1;)

If this is not the rule than delete this message.  Otherwise, there is not
much meat to this rule.  The TCP packet would have to be directed at your 
HOME_NET and be the first (SYN) of the connection establishment phase trying
to contact a "service" on port 3128.  This can happen with FTP file transfers
initiated by a host on your HOME_NET (unless you inforce passive mode).
However, you should check that the value of your HOME_NET and EXTERNAL_NET
are not "any".  Your snort.conf should have the following two lines defined:

var HOME_NET []


The above is assuming that your home net is

> "
> The snort.conf file is from version 1.8.1 and defines the internal network
> both in HOME_NET and within the preprocessor portscan-ignorehosts
> Any suggestions would be greatly appreciated.
> David
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Phil Wood, cpw at ...440...

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