Problems connecting to libvirtd (KVM) on remote hosts

I ran into this annoying bug trying to connect using SSH (key auth) to libvirtd (running on CentOS6) from a LMDE host.

The error I received was unhelpful.

Unable to connect to libvirt.

Cannot recv data: Value too large for defined data type

Verify that the 'libvirtd' daemon is running
on the remote host.

I was pretty sure that the problem was not with the server running libvirtd because it had been working the day before and was unchanged since then. On the other hand my LMDE install was completely fresh.

To cut the chase I don't know what the fix is (it seems to be a bug).

If you read to the end of that bug thread it seems you can work around the problem by using the hostname instead of its FQDN.

For this to work of course you need to be able to resolve the target IP address using just the hostname. Since I was on the same domain as the libvirt server this was simply a matter of defining the domain in /etc/resolv.conf on the client.

domain tuxnetworks.net

If that is not a practical solution (because your client and server on different domains) I reckon you could probably configure the server hostname as an individual entry in your /etc/hosts file too, although I have not tried that. Let me know in the comments if that works for you!

Disable DNSMASQ on KVM host

I have a fleet of servers with bridged, static IP's running as KVM guests. These servers do not require DHCP yet KVM by default starts up dnsmasq regardless.

Normally this is not an issue but I just so happened to need dnsmasq for DNS on one of the KVM hosts and it would refuse to start due to it being already invoked by libvirt.

You can't just disable the libvirt dnsmasq because it seems required for any virtual network that is active. You can however disable the unused virtual network which has the same effect.

# virsh net-destroy default
# virsh net-autostart --disable default


Then you can configure dnsmasq by editing /etc/dnsmasq.conf and it should work normally.

Shrink a KVM disk image

This only applies to images in the qcow2 format and does not apply to raw images.


First, we should clear as many unwanted files as possible from the machine.

Because simply deleting files with rm does not actually remove the bits (it removes entries in the directory table) we therefore need to convert unused space to an easily compressible state.

We can do that by writing a bunch of zero's to the disk using this command;

cat /dev/zero > zero.fill;sync;sleep 1;sync;rm -f zero.fill

Next, we use qemu to shrink the file (compress unused space)

qemu-img convert -c -f qcow2 source.img -O qcow2 dest.img