netdev - Re: Getting a NIC's MTU size

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Message-ID: <20140214165222.GC27343@order.stressinduktion.org>
Date:	Fri, 14 Feb 2014 17:52:22 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	David Howells <dhowells@...hat.com>
Cc:	netdev@...r.kernel.org
Subject: Re: Getting a NIC's MTU size

On Fri, Feb 14, 2014 at 03:49:17PM +0000, David Howells wrote:
> 
> One further question:  If I want to get the MTU size of the NIC through which
> packets will go to get to a particular peer, can I do:
> 
> 	struct rtable *rt;
> 	struct flowi4 fl4;
> 	unsigned if_mtu;
> 
> 	rt = ip_route_output_ports(&init_net, &fl4, NULL,
> 				   peer->srx.transport.sin.sin_addr.s_addr, 0,
> 				   htons(7000), htons(7001),
> 				   IPPROTO_UDP, 0, 0);
> 
> 	if_mtu = rt->dst->dev->mtu;
> 
> 	dst_release(&rt->dst);
> 
> Or might this go wrong if rt->dst->dev changes under me?  Can it change
> without replacing the dst record?

If it is a one-shot query this is fine. If you store the dst somewhere you need
to check dst->obsolete flags and redo the lookup if it is > 0. ->dev pointer
won't change for a created dst.

Bye,

  Hannes

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