TCP packets with RST flag set but **not** ACK flag OK??

Chris Brenton cbrenton at
Mon Apr 11 21:49:21 CEST 2005

On Mon, 2005-04-11 at 15:18, Christian Seberino wrote:
> I think RFC 793 requires TCP stacks to send RSTs if they
> get packets not meant for them in some cases.  Anyone know which
> cases?

Pretty much all. The exceptions I can think of off the top of my head:
In response to a RST
In response to a FIN to an open port (MS returns a RST/ACK)

> For example, I think closed ports must send RSTs if they
> get stuff.


> Even if YES, it seems a firewall could drop any RSTs that
> don't have ACK set without damage right?

The only condition I can think of off the top of my head that will
return a RST instead of a RST/ACK is in response to an unsolicited ACK
sent to either an open or closed port. So they do happen in the wild.


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