Migration fails with "Unable to resolve address" error

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Migration fails with "Unable to resolve address" error

Trying to migrate qemu domain fails:

# virsh migrate qemu qemu+tcp://192.168.122.12/system
error: Unable to resolve address 'fedora2' service '49155': Name or service not known

The error looks strange as we did not use "fedora2" name anywhere.

Background

During migration, libvirtd running on destination host creates a URI from an address and port where it expects to receive migration data and sends it back to libvirtd running on source host. In our case, destination host (192.168.122.12) has its name set to "fedora2". For some reason, libvirtd running on that host was not able to resolve the name to an IP address that could be sent back and still be useful and thus it sent the hostname hoping the source libvirtd would be more successful with resolving the name. This may happen if DNS is not properly configured or /etc/hosts has the hostname associated with local loopback address (127.0.0.1).

Note that the address used for migration data cannot be automatically determined from the address used for connecting to destination libvirtd (i.e., from qemu+tcp://192.168.122.12/system) because source libvirtd may need to use different network infrastructure to communicate with destination libvirtd than what virsh (possibly running on a separate machine) needs to use.

Solution

The most general and also preferred solution is to properly configure DNS so that all hosts involved in migration are able to resolve all host names.

If, for whatever reason, DNS cannot be configured properly, all hosts used for migration may be added to /etc/hosts on each of the hosts. However, keeping such lists consistent in dynamic environment is hard.

In case host names cannot be made resolvable by any mean, virsh migrate supports specifying migration host explicitly:

# virsh migrate qemu qemu+tcp://192.168.122.12/system tcp://192.168.122.12

Destination libvirtd will take the tcp://192.168.122.12 URI and append automatically generated port number. If this is not desirable (because of firewall configuration, for example), the port number can be specified explicitly:

# virsh migrate qemu qemu+tcp://192.168.122.12/system tcp://192.168.122.12:12345

Another option would be to use tunneled migration, which does not create separate connection for migration data and rather tunnels the data through the connection used for communication with destination libvirtd (i.e., qemu+tcp://192.168.122.12/system):

# virsh migrate qemu qemu+tcp://192.168.122.12/system --p2p --tunnelled