[Snort-users] Doubts creating rules

Joe McAlerney joey at ...47...
Mon Oct 29 11:30:02 EST 2001


Hello Federico,

Federico wrote:
> 
> I've two "logical" problem, help me to solve it.
> 
> 1) first doubt:
> 
> I've a net: 192.168.1.0/24
> 
> >From this net, the 192.168.1.10, 192.168.1.40 and 192.168.1.50 can connect
> to 192.168.5.1 only with log, the others component of 192.168.1.0/24 must
> generate an alert... wich rule I must create?

var EXCLUDED_HOSTS [192.168.1.10/32,192.168.1.40/32,192.168.1.50/32]

log tcp $EXCLUDED_HOSTS any -> 192.168.5.1 any (msg: "connection
attempted"; flags:S;)

This one will alert on all connection attempts from any address that is
not $EXCLUDED_HOSTS.  Unfortunately, this includes addresses outside the
192.168.1.0/24 range as well.  I couldn't think of a way to specify
192.168.1.0/24 minus $EXCLUDED_HOSTS.  Maybe someone else can chime in.

alert tcp !$EXCLUDED_HOSTS any -> 192.168.5.1 any (msg: "connection
attempted"; flags:S;)
 
> 2) Second Doubt (Established Connections)
> 
> I protect my host with snort, so I've done this rule
> 
> alert tcp HOST-B any -> HOST-A any (msg: "connection attempted";)
> 
> when HOST-A connect to HOST-B:139 for netbios share, the HOST-B response the
> connection for establishing it, and snort generate an alert from it... I
> need only snort to log HOST-B attemp to HOST-A not, just established
> connection and data transfer.
> 
> How to solve this problem ?

Specify that you only wish to look for SYN packets, which indicate the
beginning of a TCP connection:

 alert tcp HOST-B any -> HOST-A any (msg: "connection attempted";
flags:S;)

Hope this helps,

-Joe M.

-- 
|   Joe McAlerney     joey at ...155...   |
| Silicon Defense - Technical Support for Snort |
|       http://www.silicondefense.com/          |
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