RR
Click on the red underlined text to get to the source
... broadcasts to locate servers.
The SRV RR allows administrators to use several servers for a single
domain, to move services ...
...
and retrieves the document from one of the servers in the reply. The
example zone file near the end of the memo contains answering RRs for
this query.
...
... The format of the SRV RR ...
...
Here is the format of the SRV RR, whose DNS type code is 33:
...
... Name
The domain this RR refers to. The SRV RR is unique in that the
name one searches for is not this name; the example near the end
...
... The domain this RR refers to. The SRV RR is unique in that the
name one searches for is not this name; the example near the end
shows this clearly.
...
... Weight 0 when there isn't any load balancing to do, to make the
RR easier to read for humans (less noisy).
Port ...
... single domain are spread over several
hosts, it seems advisable to have a list of A RRs at the same
DNS node ...
... DNS node as the SRV RR, listing reasonable (if perhaps
suboptimal) fallback hosts for Telnet ...
... of the service ("telnet.tcp.asdf.com" for instance); each SRV RR adds
20 bytes plus the name of the target host; each NS ...
... 20 bytes plus the name of the target host; each NS RR in the NS
section is 15 bytes plus the name of the name server ...
... name server host; and
finally each A RR in the additional data section is 20 bytes or so,
and there are A's for each SRV and NS ...
... and there are A's for each SRV and NS RR mentioned in the answer.
This size estimate is extremely crude, but shouldn't underestimate
the actual answer size by much. If an answer may be close to the
...
... SRV.
If the reply is NOERROR, ANCOUNT>0 and there is at least one SRV
RR which specifies the requested Service and Protocol in the
reply:
...
... RR's for the Target or use any
RR's found in the Additional Data secion of the
earlier SRV query ...
... SRV query, and the
Additional Data section has at least one complete RR in it, the
answer MUST be considered complete and the client resolver
...
...
- A client MUST parse all of the RR's in the reply.
- If the Additional Data section doesn't contain A RR ...
... RR's in the reply.
- If the Additional Data section doesn't contain A RR's for all
the SRV RR's and the client ...
... - If the Additional Data section doesn't contain A RR's for all
the SRV RR's and the client may want to connect to the target
host(s) involved, the client ...
... client may want to connect to the target
host(s) involved, the client MUST look up the A RR(s). (This
happens quite often when the A RR has shorter TTL ...
... client MUST look up the A RR(s). (This
happens quite often when the A RR has shorter TTL than the SRV
...
... RR's.)
- A future standard could specify that a SRV RR whose Protocol was
TCP and whose Service ...
... SMTP would override RFC 974(-> 2821prop)'s rules
with regard to the use of an MX RR. This would allow firewalled
organizations with several SMTP relays to control the load
...
... distribution using the Weight field.
- Future protocols could be designed to use SRV RR lookups as the
means by which clients ...
... SRV 10 0 8000 new-fast-box.asdf.com.
; since we want to support both http://asdf.com/ and
; http://www.asdf.com/ we need the next two RRs as well
http.tcp.www SRV 0 0 80 server.asdf.com.
...
... query string, "telnet.tcp.asdf.com."
130 bytes for 4 SRV RR's, 20 bytes each plus the lengths of "new-
fast-box", "old-slow-box", "server" and "sysadmins-box" -
...
... need to be counted again.
75 bytes for 3 NS RRs, 15 bytes each plus the lengths of
"server", "ns1.ip-provider.net." and "ns2" - again, "ip-
...
... provider.net." is quoted and only needs to be counted once.
120 bytes for the 6 A RR's mentioned by the SRV and NS RR ...
... Ullmann, R., Mockapetris, P., Mamakos, L., and C. Everhart, "New DNS RR Definitions", RFC 1183exp, November 1990. ...
...
The authors believes this RR to not cause any new security problems.
Some problems become more visible, though.
...
... DNS-security people invent new features, DNS
servers will return the relevant RRs in the Additional Data section
when answering an SRV query ...