Bug 453254

I just ran into a slight variation of this problem.  It seems libvirt won't start dhcp for the virtual subnet if dnsmasq is already running.   I was only running dnsmasq for DNS and not for DHCP on interface=lo.   I was able to resolve the problem by adding in interface=virbr0 to my dnsmasq.conf, and appropriate dhcp settings for that interface.

It seems to me rather than trying to make libvirt handle all possible conflicts, a simple and acceptable solution would be to at least notify the user of the conflict and point them to an FAQ or such on how to solve it.

Bill

libvirt is already doing the maximum it can do to avoid conflicting, by making sure its own dnsmasq instance only binds to explicit IP addresses. If there's an existing DHCP daemon bound to all IPs, its pretty much game over for what we need to be able to provide. I'm not sure there's an easy way for us to detect the problem either - since DHCP is UDP based, we can't do a simple  'connect()' call to check if something's on the port - connect() always works for UDP regardless of whether anything's listening.

There is also no easy way to get feedback to the user, because the conflict will typically  occurr during machine startup, where there's no useful UI  - we can't block the initscript waiting to see if dnsmsaq starts OK, because that unacceptably delays the boot process.

Perhaps the answer is to make dhcpd allow configuration for interfaces that do not exist at start-up, and apply that configuration if those interfaces appear during runtime.

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