Harshith,
I think you need to understand the proportionality here: the *vast*majority* of
the time, the client already knows the port (because ports tend to be
pre-assigned for specific services), and only needs to resolve the FQDN to one
or more address records (A and/or AAAA records), in order to make a connection.
This isn't really a burden, or complicated logic in the client software, since
it can just call a generic hostname lookup function (e.g. the traditional
gethostbyname() or the more modern getaddrinfo()). It's not like every
application programmer needs to deal with parsing packet contents, handling
retries, dealing with label compression, etc. This is all handled in the
library routine.
Only in a small minority of cases do clients go through the whole NAPTR or SRV
process. Some of the main scenarios for that include:
* Microsoft Active Directory. The clients use SRV records as part of
the so-called "dc locator" process to find a domain controller that provides a
specific service
* Certain IM clients (like Lync) use SRV records to find SIP services
* SIP telephony clients use NAPTR records
Note that SMTP clients use MX records rather than SRV records. MX records can
be considered the simpler, mail-specific precursor to SRV records.
Theoretically, SMTP _could_ be switched over to use SRV records, but the use of
MX is so ingrained that it's probably not worth the (massive) effort.
If you look at nameserver query statistics, you'll see that the volume of SRV
and/or NAPTR queries on a typical nameserver/resolver is a miniscule fraction
of the A/AAAA traffic. Where MX-record traffic falls between those extremes,
will depend a lot on whether the nameserver/resolver is internal or external,
whether the particular domain is heavily used for mail, etc.
- Kevin
From: bind-users-boun...@lists.isc.org
[mailto:bind-users-boun...@lists.isc.org] On Behalf Of Chris Buxton
Sent: Tuesday, October 13, 2015 7:58 AM
To: Harshith Mulky
Cc: BIND Users
Subject: Re: SRV Request to DNS
On Oct 5, 2015, at 11:51 PM, Harshith Mulky
<harshith.mu...@outlook.com<mailto:harshith.mu...@outlook.com>> wrote:
Let us say we are having a FQDN and we need to Resolve it. It goes through the
procedure of determining the IP and Port using NAPTR/SRV/A query mechanisms
The question I have is if I have a FQDN with a Port Number already determined,
will it go through the Procedure of NAPTR/SRV/A query (or) simply do a A query
(or) Is this left to the client to apply the Logic?
The client must supply the logic. DNS is conceptually a simple database service
- ask a question, get an answer. The logic of using NAPTR records, SRV records,
A records, AAAA records, and CNAME records is mostly handled by the client.
(CNAME and DNAME records are the primary exception, triggering extra processing
on the recursive name server.)
Chris
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