[Snort-users] Rules for non existent IPs

Frank Knobbe frank at ...9761...
Fri Apr 23 12:50:05 EDT 2004

On Fri, 2004-04-23 at 13:19, Marc Cozzi wrote:
> Is there any way to define a rule that will fire when an outside
> source tries to access a non existent inside IP number?  Something
> like the following does not seem to work. By access I mean
> running nmap, scanners, pings etc..
> alert tcp any any -> X.X.X.1 any (msg:"TCP port scan" ) 
> alert ucp any any -> X.X.X.1 any (msg:"UCP port scan") 
> alert icmp any any -> X.X.X.1 any (msg:"ICMP scan") 

Pardon me for replying with a prefab email, but this gets asked a couple
times a month (if not here, then in the Snortsam list). I should write a
FAQ-bot for this... :)


Alerts from the portscan preprocessor (or any other preprocessor
such as ARP spoof) are not recognized by Snortsam. It only reacts to
alerts generated by packets detected with rules.

That said, port scans can be detected. However, you will have to create
your own rules for that. There are two ways (and a combined way) of
doing this. 

a) Create rules that fire when you can packets going to unused IP
addresses. For example:

var UNUSED  [x.x.x.x, y.y.y.y, etc]
alert tcp any any -> $UNUSED any (msg:"TCP port scan"; fwsam: src, 20
alert ucp any any -> $UNUSED any (msg:"UCP port scan"; fwsam: src, 20
alert icmp any any -> $UNUSED any (msg:"ICMP scan"; fwsam: src, 20 min;)

b) If you only have one IP address, create rules that monitor the ports
around used ports. For example, if you have a server for web and email
on the same IP, use:

alert tcp any any -> $HOME_NET 1:24 (msg:"TCP port scan"; fwsam: src, 20
alert tcp any any -> $HOME_NET 26:79 (msg:"TCP port scan"; fwsam: src,
20 min;)
alert tcp any any -> $HOME_NET 81:1024 (msg:"TCP port scan"; fwsam: src,
20 min;)

Note how you watch the ports from 1 through 24, not 25 since that's used
for mail, and then 26 through 79, etc.

Repeat the same with unused UDP ports.

Detecting port scans using those rules is much more efficient since you
can detect scans by a single packet.

Warning at the Gates of Bill:  
Abandon hope, all ye who press <ENTER> here...

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