Hello support team.
Current version of autoregistration, from my point of view, is not correct, as it's only store hostname and later try to use it instead of IP addres from which agent sends request to server/proxy.

Here is some description:
If we have multihomed servers (servers with several IP networks) and use one of the network for administration/monitoring then we may find a trouble with name resolution.

server1:
PUB network: 1.1.1.1 (hostname: server1)
PRIV network: 2.2.2.2 (hostname: server1-priv) - use for monitoring

In case of undefined 'Hostname' in agent config file ZABBIX will register this machine as "server1" and will try to cooperate over 1.1.1.1 network and gets access deny.

I've made a patch to fix this. Also it has compatibilty layer for unpatched version of proxies.

P.S. Sorry that i have not publish this patch to forum - my account has no permission for that.