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

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Sep 21, 2010 at 10:27:45AM +0200, xming wrote:
> > Newer versions of dhclient should also be OK: they detect
> > that checksum is missing in the packet. Try it e.g. with
> > a recent fedora guest as a client.
> 
> I don't have fedora, but with the latest release (4.1.1-P1) on isc.org
> it still behaves the same (see output at the bottom).
> 
> > To solve the problem for old clients, recent kernels and iptables have
> > support for CHECKSUM target.
> >
> > You can use this target to compute and fill in the checksum in
> > a packet that lacks a checksum.
> >
> > Typical expected use:
> > iptables -A POSTROUTING -t mangle -p udp --dport bootpc \
> > -j CHECKSUM --checksum-fill
> 
> Nice trick :D

Does it help?

> > libvirt will program these for you if it sets up the server,
> > maybe there needs a flag to tell it that server is local.
> 
> I don't use libvirt.

Does it help if you program the rule above?

> My point is, there doesn't seem to be much working client and the only
> working client is a ver very old one (pump), newer client do not work,
> as opposite to what you have explained.

Sorry if I misled you. dhclient in Fedora is ok and has been for
a long while but the bugfix does not appear to be present in released
clients from isc.org, or so it seems from examining the source.
I don't know why and whether is was ever sent their way:
it might be a good idea to talk to them on why they don't
use the PACKET_AUXDATA checksum API linux has had since 2007.
Meanwhile, you can try adding the patch from fedora.

> To repeat myself, here is the situation:
> 
> - DHCP server with vhost_net, all client w/o vhost_net work, clients
> with vhost_net do not work except pump
> - DHCP server w/o vhost_net, all clients work

Hmm are you sure? I expect vhost net client to still not work.
At least, the hack in qemu (that vhost-net disables) is for dhcp response
packets, I do not see how it will help the server ...

> - physical DHCP server, client with vhost *do* work.
> 

Yes, qemu has a hack that catches typical use and will
attach a checksum on receive. Unfortunately keeping
that around made it all too easy for vendors to keep
shipping buggy clients (and I presume server has a similar
issue).

So the only quick solution I have is to work around this
in kernel: in this case by programming the iptables rule.

-- 
MST