RR
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The SRV RR allows administrators to use several servers for a single
domain, to move services ...
... Note that where this document refers to "address records", it means A
RR's, AAAA RR's, or their most modern equivalent.
...
... as described in [ARM]. The example zone file near the end of this
memo contains answering RRs for an SRV query.
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... The format of the SRV RR ...
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Here is the format of the SRV RR, whose DNS type code is 33:
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... 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.
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... administrators SHOULD use Weight 0 when there isn't any server
selection to do, to make the RR easier to read for humans (less
noisy). In the presence of records containing weights greater
than 0, records with weight 0 should have a very small chance of
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... target hosts,
specified by the SRV RRs, will be contacted. The following
algorithm SHOULD be used to order the SRV ...
... To select a target to be contacted next, arrange all SRV RRs
(that have not been ordered yet) in any order, except that all
those with weight 0 are placed at the beginning of the list.
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... those with weight 0 are placed at the beginning of the list.
Compute the sum of the weights of those RRs, and with each RR
associate the running sum in the selected order. Then choose a
...
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Compute the sum of the weights of those RRs, and with each RR
associate the running sum in the selected order. Then choose a
uniform random number ...
... uniform random number between 0 and the sum computed
(inclusive), and select the RR whose running sum value is the
first in the selected order which is greater than or equal to
the random number ...
... random number selected. The target host specified in the
selected SRV RR is the next one to be contacted by the client.
Remove ...
... Remove this SRV RR from the set of the unordered SRV RRs and
apply the described algorithm to the unordered SRV ...
... apply the described algorithm to the unordered SRV RRs to select
the next target host. Continue the ordering process until there
...
... target host. Continue the ordering process until there
are no unordered SRV RRs. This process is repeated for each
Priority.
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... update their client applications when the first
server publishes a SRV RR is futile (even if desirable). Therefore
SRV would have to coexist with address ...
... the same DNS node as the SRV RR, listing reasonable (if perhaps
suboptimal) fallback hosts for Telnet ...
... Clients for such protocols may use or
ignore SRV RRs with Priority higher than the RR with the lowest
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... overhead plus the name
of the service ("_ldap._tcp.example.com" for instance); each SRV RR
adds 20 bytes plus the name of the target host; each NS ...
... adds 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
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... TTLs to the SRV RRs, which would limit the usefulness of the DNS
caching mechanism, thus increasing overall network ...
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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:
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... Select an element as specified above, in the
description of Weight in "The format of the SRV
RR" Section, and move it to the tail of the new
list
...
... If the Additional Data section doesn't contain address records
for all the SRV RR's and the client may want to connect to the
target host ...
... 20 bytes for the query string, "_foobar._tcp.example.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" -
"example.com" in the query ...
... 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 ...
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The authors believe this RR to not cause any new security problems.
Some problems become more visible, though.
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... The algorithm used to select from the weighted SRV RRs of equal
priority is adapted from one supplied by Dan Bernstein.
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