netdev - Re: [PATCH] net: ip: Do not allow connection to remote port zero

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Date:	Thu, 12 Feb 2015 08:21:58 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Tom Marshall <tom@...gn.com>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH] net: ip: Do not allow connection to remote port zero

On Wed, 2015-02-11 at 21:24 -0800, Tom Marshall wrote:
> On Wed, Feb 11, 2015 at 08:06:14PM -0800, Eric Dumazet wrote:
> > On Wed, 2015-02-11 at 16:58 -0800, Tom Marshall wrote:
> > > Port zero is reserved according to IANA.
> > > 
> > > Note UDP sendto is already disallowed if dport is zero.
> > 
> > I have no idea why we should prevent such thing.
> 
> * It is marked "reserved" by IANA.
> 
> * UDP packets with dport=0 are already prevented from being sent.
> 
> * Many routers will either drop the packets or, worse, send back a RST to a
>   SYN destined for port 0.

Some networking/supervision applications might use this knowledge to
send TCP probes with dport=0, expecting to get an RST, knowing no
listener could possibly answer with a SYNACK.

If you take a look at traceroute -p TCP, you'll be shocked that it sends
malformed TCP packets. IANA never complained about this, so far.



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