Thanks. The answer for me is, that I have to ask my system admin ;-)
The problem is:
People are connected in company's intranet to IP address 199.xxx.xxx.xxx to the Openfire server (via cable).
Then they disconnect the cable, take their notebook and are now connected via external network (WLAN) over internet (no longer in the intranet).
Openfire is reachable from internet and intranet. So the reconnection logic needs to find/try the correct IP address.
I thought this maybe could be resolved with 2 DNS SRV entries.
