Re: [PATCHv2] vhost-net: add dhclient work-around from userspace

From: Anthony Liguori
Date: Wed Jun 30 2010 - 18:08:02 EST

On 06/29/2010 08:04 AM, Michael S. Tsirkin wrote:
On Tue, Jun 29, 2010 at 12:36:47AM -0700, David Miller wrote:
From: "Michael S. Tsirkin"<mst@xxxxxxxxxx>
Date: Mon, 28 Jun 2010 13:08:07 +0300

Userspace virtio server has the following hack
so guests rely on it, and we have to replicate it, too:

Use port number to detect incoming IPv4 DHCP response packets,
and fill in the checksum for these.

The issue we are solving is that on linux guests, some apps
that use recvmsg with AF_PACKET sockets, don't know how to
The interface to return the relevant information was added
in 8dc4194474159660d7f37c495e3fc3f10d0db8cc,
and older userspace does not use it.
One important user of recvmsg with AF_PACKET is dhclient,
so we add a work-around just for DHCP.

Don't bother applying the hack to IPv6 as userspace virtio does not
have a work-around for that - let's hope guests will do the right
thing wrt IPv6.

Signed-off-by: Michael S. Tsirkin<mst@xxxxxxxxxx>
Yikes, this is awful too.

Nothing in the kernel should be mucking around with procotol packets
like this by default. In particular, what the heck does port 67 mean?
Locally I can use it for whatever I want for my own purposes, I don't
have to follow the conventions for service ports as specified by the

But I can't have the packet checksum state be left alone for port 67
traffic on a box using virtio because you have this hack there.

And yes it's broken on machines using the qemu thing, but at least the
hack there is restricted to userspace.
Yes, and I think it was a mistake to add the hack there. This is what
prevented applications from using the new interface in the 3 years
since it was first introduced.

It's far more complicated than that. dhclient is part of ISC's DHCP package. They do not have a public SCM and instead require you to join their Software Guild to get access to it.

This problem was identified in one distribution and the patch was pushed upstream but because they did not have a public SCM, most other distributions did not see the fix until it appeared in a release. ISC has a pretty long release cycle historically.

ISC's had the fix for a long time but there was a 3-year gap in their releases and since their SCM isn't public, users are stuck with the last release.

This hack makes sense in QEMU as we have a few hacks like this to fix broken guests. A primary use of virtualization is to run old applications so it makes sense for us to do that.

I don't think it makes sense for vhost to do this. These guests are so old that they don't have the requisite features to achieve really high performance anyway.

I've always thought making vhost totally transparent was a bad idea and this is one of the reasons. We can do a lot of ugly things in userspace that we shouldn't be doing in the kernel.


Anthony Liguori
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