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DNS SRV Records

Sean Conner sean at conman.org
Fri May 15 22:32:51 BST 2020
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It was thus said that the Great solderpunk once stated:
> On Fri, May 15, 2020 at 05:40:58PM +0000, colecmac at protonmail.com wrote:
> 
> Hello!
> 
> 
> 
> DNS SRV records are pretty interesting, but in my opinion this would
> 
> unnecessarily complicate the spec.
> 
> I don't think this would necessarily need to clutter up the main spec -
> this is a "companion spec" kind of thing, like robots.txt or other
> things like that.

  Kind of.  One issue is:  given the URL:

	gemini://gemini.example.com/

does that mean a client has to look up the SRV RR for 'gemini.example.com'before defaulting to port 1965?  Also, it's not like there can only be oneSRV RR for a given service.  Hypothetically, a client could get this back:

			         P    W  port  host-------------------------------------------------------------_gemini._tcp.example.net. IN SRV 10    1 1965 backupserver3.example.net.			  IN SRV 0     0 1965 server3.example.net.			  IN SRV 0  5000 1965 server1.example.net.			  IN SRV 10    2 1966 backupserver2.example.net.			  IN SRV 10    3 1967 backupserver1.example.net.			  IN SRV 5     0 1965 redundant1.example.net.			  IN SRV 0  1000 1965 server2.example.net.			  IN SRV 0  5000 5691 server3.example.net.			  IN SRV 5     0 1965 redundant2.example.net.			  IN SRV 5     0 2020 redundant3.example.net.


		P = priority - lower values first 		W = weight - higher values first

  A client would need to sort the list based upon priority (lower first),then by weight (higher first), then pick a server at random among those withthe same priority and weight.  So according to this list, a client wouldhave to first attempt to contact

		pick one at random			server1.example.net:1965			server3.example.net:6591

and last

		backupserver3.example.net:1965

It's all spelled out in RFC-2782 [1].

  Is this a pathological case?  Yes.  Will most SRV records have one entry? Probably, but that's a gut feeling, I don't have actual numbers to look at.And unlike looking up an A or AA RR with the "standard" POSIX functiongetaddrinfo(), there is not simple API that I am aware of for the SRV RR.Also, an SRV RR query might require a second A/AAAA RR lookup [2].

  -spc (Just some things to keep in mind ... )

[1]	Shameless plug---I wote a DNS library in C:

		https://github.com/spc476/SPCDNS

	Yeah, that's why I know this stuff.

[2]	Depends upon the DNS server.  Some will return the answer along with	additional records to avoid further queries, some won't.