Its in the reserve IP list file on the server machine... It can be named whatever as its defined in the config file as RipFile. The easiest way to set this is if you have admin access to the primary server, you just connect up with the relay client, and then while the relay is connected, on the primary server go to the admin interface and click on the link in the Reserve IP column.
code:
; RipFile is the text file sc_serv reads and writes to/from
; for the list of client IPs which are *ALWAYS* permitted
; to connect to this server (useful for relay servers).
; This file is automatically generated via the web
; interface. Note that if your server is FULL, and someone
; from a Reserved IP connects, the DNAS will force the person
; listening for the longest time off to make room for the new
; connection.
; RipFile=sc_serv.rip