[Snort-users] BAD TRAFFIC data in TCP SYN packet

Matt Kettler mkettler at ...4108...
Mon Jan 14 09:08:13 EST 2002

Well, the port 29291 is just a random local port.. This is a syn packet 
remember, so the service being used is on destination end, and is port 53 

so, has decided that might be a dns server, and 
has attempted to connect to it via TCP (it is unusual, but legal for a DNS 
server to be contacted via tcp instead of UDP).

I've seen some similar traffic myself from a pair of DNS servers directed 
at the local DNS server here.. the TCP syn packets contain several bytes of 
data which are all 00's. It is strange (AFAIK it is not legal to send data 
with a syn packet.. you haven't negotiated a connection yet), but it 
appears to be an artifact of a buggy tcp/ip implementation.. Or who knows, 
it may be an artifact of some obscure, buggy worm  or scanning tool that 
looks at DNS servers and uses raw sockets instead of the local TCP/IP 
stack. Even if it is from some obscure hacking tool, the syn packets 
themselves appear harmless.

At 07:39 AM 1/14/2002 +0100, you wrote:
>I get a lot of
>01/14-02:24:17.089098  [**] [1:526:3] BAD TRAFFIC data in TCP SYN packet
>[**] [Classification: Misc activity] [Priority: 3] {TCP}
> is my DNS server, but why would clients put data in the syn
>packets? According to RIPE, the source address is "ALLOCATED UNSPECIFIED",
>so I can't find out who's doing this. It comes from a limited number of
>addresses, they all seem to be 207.xx.xxx.xxx.

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