We want to be able to bind cluster/HA servers to 0.0.0.0, but this didn't work before as a cluster instance needed to know its own IP when joining clusters and broadcasting its availability. This has been worked around by letting a receiver of a message figure out the FROM header. When a message is received we then check the TO header, and if set, we know that it contains our own IP as seen by someone else. This is then used for later messages where we need this.

We also used to do lookup of local hostnames when no IP was specified, which would not work on systems where the local hostname is not in DNS or is registered with an IP. This has been changed so that this is no longer required.