[Snort-users] tcp port 0 rule

Bob Van Cleef vancleef at ...211...
Fri Oct 11 09:52:06 EDT 2002

Saw the following scan entry in my alert log:

[**] [1:524:3] BAD TRAFFIC tcp port 0 traffic [**]
[Classification: Misc activity] [Priority: 3]
10/09-02:34:18.941743 ->
TCP TTL:242 TOS:0x0 ID:65259 IpLen:20 DgmLen:40
*****R** Seq: 0xE59FF019  Ack: 0xE5B5A1E5  Win: 0xF018  TcpLen: 20

However, when I looked into the rules I noted the following:

bad-traffic.rules:alert tcp $EXTERNAL_NET any <> $HOME_NET 0 (msg:"BAD 
TRAFFIC tcp port 0 traffic"; reference:cve,CVE-1999-0675; 
reference:nessus,10074; classtype:misc-activity; sid:524; rev:4;)

The test is for TCP port 0, however the CVE-1999-0675 reference states:


"Firewall-1 can be subjected to a denial of service via UDP packets that 
are sent through VPN-1 to port 0 of a host."

So, two questions:

1 - if the attack is UDP based, why is the rule looking at TCP?

2 - is there anything wrong with TCP port 0 connection attempts?

   The process can let the system automatically assign a port.  For
   both the Internet domain and the XNS domain, specifying a port
   number of 0 before calling bind() requests the system to do this.


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