netdev - Re: dhclient, checksum and tap

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Message-Id: <20100625.112152.241921019.davem@davemloft.net>
Date:	Fri, 25 Jun 2010 11:21:52 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	mst@...hat.com
Cc:	herbert.xu@...hat.com, netdev@...r.kernel.org
Subject: Re: dhclient, checksum and tap

From: "Michael S. Tsirkin" <mst@...hat.com>
Date: Fri, 25 Jun 2010 18:10:08 +0300

> I've been looking at the issue of checksum on
> dhcp packets: to recap, 8dc4194474159660d7f37c495e3fc3f10d0db8cc
> added a way for af_packet sockets to get the packet status.
> Unfortunately not all dhcp clients caught up with
> this development, so they are still broken
> when both server and client run on the same host,
> e.g. with bridge+tap.
> 
> And of course virtualization adds another way to run
> old dhcp clients, so userspace virtio net in qemu has
> a hack to detect DHCP and fill in the checksum.
> I guess we could add this in vhost, as well.
> 
> However, one wonders whether the tap driver is a better place
> for this work-around, that would help all users.
> Any objections against putting such code in tap?

We added the af_packet status as the migration path to deal with
this issue in the cleanest manner possible.  Putting a new hack
into the TAP driver works contrary to that goal.
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