RFC 2052 (rfc2052) - Page 3 of 10
A DNS RR for specifying the location of services (DNS SRV)
Alternative Format: Original Text Document
RFC 2052 DNS SRV RR October 1996
Weight
Load balancing mechanism. When selecting a target host among
the those that have the same priority, the chance of trying this
one first SHOULD be proportional to its weight. The range of
this number is 1-65535. Domain administrators are urged to use
Weight 0 when there isn't any load balancing to do, to make the
RR easier to read for humans (less noisy).
Port
The port on this target host of this service. The range is
0-65535. This is often as specified in Assigned Numbers but
need not be.
Target
As for MX, the domain name of the target host. There MUST be
one or more A records for this name. Implementors are urged, but
not required, to return the A record(s) in the Additional Data
section. Name compression is to be used for this field.
A Target of "." means that the service is decidedly not
available at this domain.
Domain administrator advice
Asking everyone to update their telnet (for example) clients when the
first internet site adds a SRV RR for Telnet/TCP is futile (even if
desirable). Therefore SRV will have to coexist with A record lookups
for a long time, and DNS administrators should try to provide A
records to support old clients:
- Where the services for a single domain are spread over several
hosts, it seems advisable to have a list of A RRs at the same
DNS node as the SRV RR, listing reasonable (if perhaps
suboptimal) fallback hosts for Telnet, NNTP and other protocols
likely to be used with this name. Note that some programs only
try the first address they get back from e.g. gethostbyname(),
and we don't know how widespread this behaviour is.
- Where one service is provided by several hosts, one can either
provide A records for all the hosts (in which case the round-
robin mechanism, where available, will share the load equally)
or just for one (presumably the fastest).
- If a host is intended to provide a service only when the main
server(s) is/are down, it probably shouldn't be listed in A
records.
Gulbrandsen & Vixie Experimental