Red Hat Bugzilla – Bug 453254
DHCP fails if running on a DHCP server
Last modified: 2009-07-14 12:40:10 EDT
Description of problem:
If libvirt is running on a machine that already provides a DHCP server for a
local network, the dnsmasq DHCP server does not seem to receive BOOTPC packets
One way round this is to define a subnet for 192.168.122.0/24 in
/etc/dhcpd.conf; however, the dhcp service starts up before libvirtd, and this
means that the interface does not yet exist. As a result it is ignored by dhcpd.
Version-Release number of selected component (if applicable):
Steps to Reproduce:
1.Install libvirt on a machine that is already acting as a DHCP server
2.Run a guest
The guest cannot obtain an IP address.
The guest obtains an IP address from 192.168.122.0/24. If necessary, this can
be achieved by removing the 'dhcp' tag from
/etc/libvirt/qemu/networks/default.xml and defining a subnet clause in
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.
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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