[Dshield] Valid TCP Flags?

TILLEY, Alex Alex.TILLEY at suncorp.com.au
Fri Jun 2 01:36:05 GMT 2006

Many times I've seen mainframes doing SYN-RST. It freaks firewalls out I
can tell you! :)

-----Original Message-----
From: Chris Brenton [mailto:cbrenton at chrisbrenton.org] 
Sent: Thursday, 1 June 2006 2:23 AM
To: General DShield Discussion List
Subject: Re: [Dshield] Valid TCP Flags?

On Wed, 2006-05-31 at 09:21 -0400, Jon R. Kibler wrote:
> I have been trying to put together a list of all possible valid
combinations of IPv4 TCP flags. From the RFCs, I gather that the list
should be:
> 	SYN
> 	ACK
> 	FIN
> 	RST

I've also see FIN-ACK-PSH in the wild and even (eek) SYN-PSH. 

> Two questions:
>    1) Can RST and/or FIN ever appear alone, without an ACK?

Send an unsolicited ACK to an open or closed port and you will get back
a plain RST. A FIN will *never* appear by itself which is why Cisco's
"established" keyword filters on ACK and/or RST packets. Only FIN/ACK is

>    2) Are there other valid combinations that I have missed? If so,
under what circumstances would you see that combination?

As mentioned I've seen pretty liberal use of the PSH bit. 


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