[Snort-users] WHy no alerts using eth0_ADDRESS?

Hutchinson, Andrew Andrew.Hutchinson at ...3639...
Tue Jan 15 13:09:03 EST 2002

I believe that the issue is this:

when you use


then your $HOME_NET is set to the _single_ ip address of eth0.  For instance, if eth0 is, then snort will _only_ alert when the ICMP packet is coming from or headed to eth0 on the sensor itself.  So, if your ping was from the sensor, I would expect alerts, whereas if the ping is simply passing through the sensor, the $HOME_NET is not matched and thus no alert generated.

However, when you have 


or the like, the entire subnet is matched by $HOME_NET, and the signature is matched and an alert generated.

Hope this helps,


Andrew Hutchinson CNE MCSE
Informatics/NCS/Network Security
Vanderbilt University Medical Center
615.936.2856 - voice
615.936.0643 - fax
andrew.hutchinson at ...3295...

-----Original Message-----
From: Dr. Richard W. Tibbs [mailto:ccamp at ...4532...]
Sent: Tuesday, January 15, 2002 2:38 PM
To: snort-users at lists.sourceforge.net
Subject: [Snort-users] WHy no alerts using eth0_ADDRESS?

I am puzzled mildly by some remarks in the snort.conf  file:
# or use global variable $<interfacename>_ADDRESS
# which will be always initialized to IP address and
# netmask of the network interface which you run
# snort at.
# var HOME_NET $eth0_ADDRESS
# You can specify lists of IP addresses for HOME_NET
# by separating the IPs with commas like this:
# var HOME_NET [,]
# or you can specify the variable to be any IP address
# like this:
#var HOME_NET any

By the comments above, I am led to believe that snort will always
initialize the $eth0_ADDRESS variable to the home net.
Snort has always worked "out of the shrink wrap" with no mods to 
snort.conf and finds eth0, my only active NIC;
snort -v reports packet as usual.

However ......

In an exploration with snort, I tried
output alert_unixsock
alert icmp $HOME_NET any -> any any (msg: "OUT" ;)
alert icmp any any -> $HOME_NET any (msg: "IN" ;)

and I, when I ping another machine I get no alerts,
although the snort summary output counts as many packets as ping sends & 
receives. (i.e., the snort output is like:
Breakdown by protocol:               Action Stats:
...                                   Akerts: 0
  ICMP: 12

But when I use
output alert_unixsock
alert icmp $HOME_NET any  -> any any (msg: "OUT" ;)
alert icmp any any  -> $HOME_NET any (msg: "IN" ;)

I get the appropriate equal amounts of INs and OUTs alerted to the socket.

How come no alerts in the first case?
Do I actually have to set the eth0_ADDRESS variable myself?

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