netdev - Re: Ping Is Broken

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Message-ID: <20091013051019.GA6159@ff.dom.local>
Date:	Tue, 13 Oct 2009 05:10:19 +0000
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Brian Haley <brian.haley@...com>
Cc:	Rob.Townley@...il.com, netdev@...r.kernel.org,
	Omaha Linux User Group <olug@...g.org>,
	CentOS mailing list <centos@...tos.org>
Subject: Re: Ping Is Broken

On Mon, Oct 12, 2009 at 07:30:26PM -0400, Brian Haley wrote:
> 
> 
> Jarek Poplawski wrote:
> > Brian Haley wrote, On 10/12/2009 10:36 PM:
> > 
> >> In this case ping is doing an SO_BINDTODEVICE to eth0, so the kernel is going
> >> to force the packets out of it, even if it isn't the "correct" interface.  If
> >> you ran tcpdump you'd probably see an ARP resolution failure, or an ICMP from
> >> a gateway.
> > 
> > BTW, SO_BINDTODEVICE is used only to acquire a source address, not the real
> > connection (unless I miss something).
> 
> No, SO_BINDTODEVICE affects routing, as well as incoming packets - it's in macros
> like INET_MATCH(), from what I've seen it restricts a socket to only use the device
> specified, irregardless of the source address you're using.  For example, you can
> bind to 127.0.0.1 and send packets out eth0 if sk_bound_dev_if is set to it if I
> remember correctly.

I've commented your: "In this case ping is doing an SO_BINDTODEVICE to eth0",
so meant: SO_BINDTODEVICE is used *by ping* only to acquire a source address.

Jarek P.
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