When a PSYC uniform has to be mapped to a host and port pair and no port is given in the uniform, a DNS SRV lookup SHOULD be performed to retrieve the real host and port number. The lookup is performed on _psyc._tcp.hostname (See also: RFC2782).

NOTE: The DNS SRV lookup SHOULD NOT return more than one host:port pair, as PSYC currently does not support mapping of hostnames to multiple IPs. Multiple hostnames of a PSYC node SHOULD NOT resolve to multiple ip:port combinations. These things have to be taken care of by the administrators of the PSYC nodes at the moment.

When UDP is selected as a transport, _psyc._udp.hostname MAY be consulted.

Should the SRV lookup fail or a port number was provided in the uniform, a forward hostname resolution of the A/AAAA or CNAME records is performed.

The resulting IP number and port of the resolution process may point to an already existing circuit. That circuit SHOULD be reused. If the root of the uniform hasn't already been verified as being a valid _target for that circuit, a _request_authorization is necessary.

SRV lookup may be circumvented by the user by allowing him to override the physical host and port numbers explicitly. This is different from restricting the user to provide a different uniform containing the port number.

A minimal application MAY leave out DNS resolution altogether, if the implications are understood and accepted. Best practice for SRV usage in 2008 is to support it in all PSYC applications, but to not deploy SRV usage in actual installations, if possible.