target
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... NAPTR records) are used to determine the successive
DNS lookups until a desirable target is found.
For the rest of this section, refer to the set of NAPTR resource ...
... A client retrieves all the NAPTR records associated with the target
domain name (example.com, above). These are to be sorted in terms of
...
... FLAG field in S-NAPTR, the REPLACEMENT field is
used as the target of the next DNS lookup -- for NAPTR RRs ...
... NAPTR processing rules. In the case of an "S" flag, the
REPLACEMENT field is used as the target of a DNS query for SRV RRs ...
... SRV processing is applied. In the case of an "A" flag, an
address record is sought for the REPLACEMENT field target (and the
default protocol port is assumed).
...
...
As shown in the example set above, it is possible to have multiple
possible targets for a single application service+protocol pair.
These are to be pursued in order until a server is successfully
...
... DNS lookup fails to yield expected results
-- e.g., no A RR for an "A" target, no SRV record for an "S"
target ...
... target, no SRV record for an "S"
target, or no NAPTR record with appropriate application service
...
... preference, or on the PREF ranking in the first set of NAPTR records
(i.e., those for the target named domain). However, the chosen
protocol MUST be listed in that first NAPTR ...
... RRs alone) for naming
service targets, without requiring each application protocol (or
service ...
... NAPTR. Specifically, under what circumstances should the client
retry a target that was found via S-NAPTR? What should it consider a
failure that causes it to return to the S-NAPTR ...
... failure that causes it to return to the S-NAPTR process to determine
the next serviceable target, which by definition will have a lower
preference ranking.
...
... connection refused" message from a
server, should it retry for some (protocol-dependent) period of time?
Or should it try the next-preferred target in the S-NAPTR chain of
resolution? Should it only try the next-preferred target ...
... target in the S-NAPTR chain of
resolution? Should it only try the next-preferred target if it
receives a protocol-specific permanent error message ...
... As noted earlier, failure to provide appropriate credentials to
identify the server as being authoritative for the original target
domain is always considered a failure condition.
...
... tree could be quite deep, and retracing the tree to retry
other targets can become expensive if the tree has many branches.
...
... NAPTR and SRV records, provide
different targets with varying preferences where appropriate
(e.g., to provide backup services) but don't look for reasons to
...
... 4].
However, the most important aspect to keep in mind is that if the
application cannot successfully connect to one target, the
application will be expected to continue through the S-NAPTR tree ...
... services
for delivery to a target.
Two particular features of this hypothetical extensible messaging
...
... So the client looks up SRV records for that target, ultimately
making the request of the NS for example.com.
...
... application protocol standards use domain names to
identify server targets and stipulate that clients should look up SRV
...
... This enables a distinction between naming an application service
target and actually hosting the server. It also increases
flexibility in hosting ...
... hosting the server. It also increases
flexibility in hosting the target service, as follows:
...
... DDDS specification and use of NAPTR allows
multiple levels of redirection before the target server machine with
an SRV record is located, this proposal requires only a subset of
...
... others as backups".
o Target servers by "service" (e.g., "ldap") and "protocol" (e.g.,
"tcp") in a given domain ...
... NAPTR replacements,
SRV targets, or CNAME changes). In certain cases where the
application protocol ...
... A.1. Finding the First (Best) Target ...
... service, the following pseudocode outlines the expected process to
find the first (best) target for the client, using S-NAPTR.
...
... ; now, whether it was an "S" or an "A" in the NAPTR, we
; have the target for an A record lookup
...
... A.2. Finding Subsequent Targets ...