A RST|ACK is created by a server when a connection request arrives and
no server process is listening.

There are only 2 times you should NOT see an ACK during 'normal' TCP:

On the first segment of a connection sent by a client
On RST segments used to abort a connection

Now, if you see a SYN|ACK from your server on TCP 27374, that's another
story!

RFC 793 is a good start [see section 3.4] but I think the Stevens text
is still the finest available. Also take a look at the O'Reilly book by
Eric Hall.

Regards.

-- 
John Bradberry
The Greentree Group
Ian Melven wrote:
> 
> Hi everyone
> 
> A question on portscans..  I've been scanned for SubSeven
> a few times.. I set up a rule to track outgoing packets
> from the default port (27374) with the ACK flag set..
> 
> this caught 2 machines sending RST|ACK packets in response
> to a SYN...
> 
> Can someone explain why these are sending RST|ACK instead
> of just a RST ? I thought RST was the standard response to a
> SYN from a closed port ? Is this because the initial incoming
> SYN had some data in it (I believe this is allowed...)
> 
> I need to get an office copy of TCP/IP Illustrated :/
> 
> thanks !
> Ian
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