netdev - Getting a NIC's MTU size

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Date:	Fri, 14 Feb 2014 15:49:17 +0000
From:	David Howells <dhowells@...hat.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	dhowells@...hat.com, netdev@...r.kernel.org
Subject: Getting a NIC's MTU size


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?

David
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