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