[libvirt] Adding MTU DHCP option when specified

Daniel P. Berrangé berrange at redhat.com
Mon Dec 10 13:31:43 UTC 2018

On Fri, Dec 07, 2018 at 04:29:18PM +0100, Casey Callendrello wrote:
> Hi there,
> I was working on some MTU testing for OpenShift, and I realized that, while
> libvirt does allow specifying the MTU of the bridge interface, it doesn't
> plumb it through the DHCP server.
> This is really easy - it's just an extra line in the dnsmasq.conf file,
> e.g.:
> dhcp-option-force=option:mtu,9000
> Any interest in this as a patch? And, apologies if this has been discussed
> to death (though I searched around and didn't find anything).

I don't recall this being discussed before.

We set the MTU on the bridge in the host. This in turn means we set the
MTU on the TAP devices attached to the bridge.  The guest OS though still
uses the default MTU size.

IIUC, this DHCP option is intended to tell the guest OS to configure its
NIC with the different MTU size, completing the end-to-end config.

If so, this makese sense as something todo.

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