netdev - Re: DSCP values in TCP handshake

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Date:	Mon, 18 Apr 2011 21:16:37 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Mikael Abrahamsson <swmike@....pp.se>
Cc:	Joe Buehler <aspam@....net>, Eric Dumazet <eric.dumazet@...il.com>,
	netdev@...r.kernel.org
Subject: Re: DSCP values in TCP handshake

On Tue, 19 Apr 2011 05:50:34 +0200 (CEST)
Mikael Abrahamsson <swmike@....pp.se> wrote:

> On Mon, 18 Apr 2011, Stephen Hemminger wrote:
> 
> > If the DSCP bits are reflected, then it could allow for even better SYN 
> > flood attack. Attacker could maliciously set DSCP to elevate priority 
> > processing of his bogus SYN packets and also cause SYN-ACK on reverse 
> > path to also take priority.
> 
> Incoming, it's already too late. Outgoing, yes, that might be a problem, 
> but if you have a QoS enabled network then you might as well solve that in 
> the network, not in the host.
> 
> Does Linux internally look at DSCP when deciding what SYNs to handle 
> first? If not, I think the above reasoning is misdirected.

Linux does not look at DSCP of incoming packets (there is no queue).

Of course, you can do anything with qdisc, and iptables.
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