netdev - Re: inet_hash_connect: source port allocation

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Message-ID: <1291058487.3435.1391.camel@edumazet-laptop>
Date:	Mon, 29 Nov 2010 20:21:27 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	John Haxby <john.haxby@...cle.com>
Cc:	NetDev <netdev@...r.kernel.org>,
	Stephen Hemminger <shemminger@...tta.com>
Subject: Re: inet_hash_connect: source port allocation

Le lundi 29 novembre 2010 à 20:07 +0100, Eric Dumazet a écrit :
> Le lundi 29 novembre 2010 à 19:46 +0100, Eric Dumazet a écrit :
> > Le lundi 29 novembre 2010 à 18:29 +0000, John Haxby a écrit :
> > 
> > > Sorry,  I think I phrased my question badly.
> > > 
> > > inet_csk_get_port() starts its search for a free port with
> > > 
> > >      smallest_rover = rover = net_random() % remaining + low;
> > > 
> > > whereas __inet_hash_connect() basically misses out that call to 
> > > net_random() so you get a predictable port number.
> > > 
> > > Is there any good reason why that is the case?
> > > 
> > 
> > It seems random select was done at bind() time only in commit
> > 6df716340da3a6f ([TCP/DCCP]: Randomize port selection)
> > 
> > It probably should be done in autobind too.
> > 
> > 
> 
> I'll test following patch :

Oh well, forget this, there is something about inet_sk_port_offset()
using secure_ipv4_port_ephemeral()

We want to avoid reusing same port too fast.

http://www.tcpipguide.com/free/t_TCPIPClientEphemeralPortsandClientServerApplicatio-2.htm

Port is predictable only for same destination, and if no other
connections are attempted by other threads.



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