netdev - Re: FYI: tftp-hpa breaks when used on secondary ip addresses

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Message-ID: <4D719254.8000903@zytor.com>
Date:	Fri, 04 Mar 2011 17:31:00 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	netdev@...r.kernel.org, Joel Sing <jsing@...gle.com>,
	David Miller <davem@...emloft.net>
Subject: Re: FYI: tftp-hpa breaks when used on secondary ip addresses

On 03/04/2011 04:58 PM, Eric W. Biederman wrote:
> 
> Which if fails now if you happen to be running tftp-hpa on a secondary
> ip address on the same subnet as your first ip.  Because pref_source
> in the routing table points at the first ip.
> 
> The change in kernel behavior appears to be from the commit below to
> honor the preferred source address in local connections.
> 

If this is done for all local connections (as opposed to the ones that
have been configured explicitly by the administrator to behave that
way), that's a massive lossage.  Not only does a large number of
applications use this trick to determine if an address is local (e.g.
allowing bypass) -- it is pretty much the only portable way to do it --
but it would cause applications which expect to get a reply back from
the same address they sent a request to to completely fall on their face.

tftp-hpa needs this information in order to handle clients that send
their initial request to a broadcast (or multicast) address.

> This all seems very fuzzy to me and mostly this appears to be a bug in
> tftp-hpa but since I tracked it down I figured I would let everyone
> know what happened.

This seems like a broken change to me, or at the very least having
seriously unintended consequences.

	-hpa
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