According to an aspect, a method comprises maintaining, by a cache control unit in a first packet data network, white and black lists of data servers in a second packet data network that are allowed or unable to use a cache server in the first packet data network, respectively; monitoring data traffic to and from data servers in the second packet data network over an interface between the first and second packet data network; comparing the candidate data servers to the data servers in the white and black lists; and in response to a candidate data server being in neither list, performing the following: causing sending a cache request to the candidate data server and in response to receiving an acknowledgement to the cache request within a pre-defined time, adding the candidate data server to the white list, otherwise adding the candidate data server to the black list.