A callback to implement a DNS server. The callback function receives a DNS
request. It should then optionally add a number of answers to the reply
using the evdns_server_request_add_*_reply functions, before calling either
evdns_server_request_respond to send the reply back, or
evdns_server_request_drop to decline to answer the request.
@param req A newly received request
@param user_data A pointer that was passed to
evdns_add_server_port_with_base().
A callback to implement a DNS server. The callback function receives a DNS request. It should then optionally add a number of answers to the reply using the evdns_server_request_add_*_reply functions, before calling either evdns_server_request_respond to send the reply back, or evdns_server_request_drop to decline to answer the request.
@param req A newly received request @param user_data A pointer that was passed to evdns_add_server_port_with_base().