$ORIGIN example.com. IN NAPTR 100 10 "S" "SIP+D2U" "!^.*$!sip:customer-service@example.com!" _sip._udp.example.com. IN NAPTR 102 10 "S" "SIP+D2T" "!^.*$!sip:customer-service@example.com!" _sip._tcp.example.com. As in the first example, the client picks the first record because it has the lowest order value. The regular expression rule replaces the query URN, this time with the domain name _sip._udp.example.com. The flag S indicates that the resulting domain name points to a SRV record. The client thus finishes with _sip._udp.example.com, for which it can then fetch a SRV record to initiate a telephony call. Support NAPTR implementations generally also implement EDNS as responses returning multiple NAPTR records are usually larger than the normal 512 byte packet size limit and would otherwise require a less efficient fallback to TCP rather than UDP. References DNS record types