Xen guest domainU migration fails with Error: can't connect: No route to host message

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When attempting to perform the migration of a Xen domainU guest from one host to another the migration command fails as follows:

# xm migrate 7 -l
Error: can't connect: No route to host
Usage: xm migrate <Domain> <Host>

The most likely cause of this problem is that the firewall on the destination host (i.e. the server to which the guest domain is to be migrated) has not been configured to allow traffic on the port used by Xen to perform domain relocations.

By default Xen performs guest migrations using port 8002. It is possible, however, to configure another port for this service by changing the value assigned to the xend-relocation-port setting in the /etc/xen/xend-config.sxp configuration file. If this setting is commented out as shown below then the system will default to 8002:

# Port xend should use for the relocation interface, if xend-relocation-server
# is set.
#(xend-relocation-port 8002)

If, on the other hand, the line is uncommented and set to different value as shown below then the firewall needs to allow traffic on this port:

(xend-relocation-port 1234)

Once the Xen relocation port has been identified the firewall on the destination host should be configured to allow TCP traffic on that port. With this change implemented the migration should complete successfully.