Some XMPP clients (e.g. libpurple-based clients such as Adium and Pidgin) use DNS SRV records, cf.:
http://wiki.xmpp.org/web/SRV_Records#STUN_SRV_records
_stun._tcp.example.net. TTL IN SRV priority weight port target
_stun._udp.example.net. TTL IN SRV priority weight port target
e.g.
_stun._tcp.example.net. 86400 IN SRV 5 50 3478 server1.example.net.
_stun._tcp.example.net. 86400 IN SRV 10 30 3478 server2.example.net.
_stun._tcp.example.net. 86400 IN SRV 10 10 3478 server3.example.net.
_stun._tcp.example.net. 86400 IN SRV 10 10 3478 server4.example.net.
_stun._tcp.example.net. 86400 IN SRV 15 0 3478 backup.example.net.
_stun._udp.example.net. 86400 IN SRV 5 50 3478 server1.example.net.
_stuns._tcp.example.net. 86400 IN SRV 5 50 5349 server1.example.net.
http://wiki.xmpp.org/web/SRV_Records#TURN_SRV_records
_turn._tcp.example.net. TTL IN SRV priority weight port target
_turn._udp.example.net. TTL IN SRV priority weight port target
_turns._tcp.example.net. TTL IN SRV priority weight port target
e.g.
_turn._tcp.example.net. 86400 IN SRV 5 50 3478 server1.example.net.
_turn._tcp.example.net. 86400 IN SRV 10 30 3478 server2.example.net.
_turn._tcp.example.net. 86400 IN SRV 10 10 3478 server3.example.net.
_turn._tcp.example.net. 86400 IN SRV 10 10 3478 server4.example.net.
_turn._tcp.example.net. 86400 IN SRV 15 0 3478 backup.example.net.
_turn._udp.example.net. 86400 IN SRV 5 50 3478 server1.example.net.
_turns._tcp.example.net. 86400 IN SRV 5 50 5349 server1.example.net.