Document DHCP lease vhost_net issue

[Lorin Hochstein]

There was an issue reported on serverfault.com about instances not properly processing DHCP offers:

http://serverfault.com/questions/448347/instances-in-openstack-are-not-getting-dhcp-leases

The original questioner responded with this link to some RedHat documentation:
https://access.redhat.com/knowledge/docs/en-US/Red_Hat_Enterprise_Linux/6/html/Virtualization_Host_Configuration_and_Guest_Installation_Guide/ch11s02.html

Two potential workarounds are:

1. Use virtio
2. Add an iptables rule:

iptables -A POSTROUTING -t mangle -p udp --dport 68 -j CHECKSUM --checksum-fill

It isn't clear to me whether the following setting would work for enabling virtio:

libvirt_use_virtio_for_bridges=True

[Tom Fifield (fifieldt)]

libvirt_use_virtio_for_bridges=True does work for enabling virtio

[Tom Fifield (fifieldt)]

NB - I think this is the same issues as decribed in here:

http://docs.openstack.org/trunk/openstack-compute/admin/content/network-troubleshooting.html

        </simplesect>
        <simplesect><title>KVM: Network connectivity works initially, then fails</title>
        <para>Some administrators have observed an issue with the KVM hypervisor where instances
                running Ubuntu 12.04 will sometimes lose network connectivity after functioning
                properly for a period of time. Some users have reported success with loading the
                vhost_net kernel module as a workaround for this issue (see <link
                    xlink:href="https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/997978/">bug
                    #997978</link>) . This kernel module may also <link
                    xlink:href="http://www.linux-kvm.org/page/VhostNet">improve network performance
                    on KVM</link>. To load the kernel module, as
                root:<screen><prompt>#</prompt> <userinput>modprobe vhost_net</userinput></screen></para>
            <para>Note that loading the module will have no effect on instances that are already
                running.</para></simplesect>
    </section>

and I think this was fixed in grizzly - marking fix released