Re: SRV RRs support in Internet Explorer?
From: Rémi Després (remi.despres_at_wanadoo.fr)
Date: 03/05/04
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Date: Fri, 5 Mar 2004 10:47:09 +0100
At this stage of the discussion the best is probably to formalize the
proposal, as it evolved during our dialogue, and to see were we are.
Rationale:
------------
- It is assumed that the need for Static Load Distribution and/or Backup, or
SLD&BU, is reasonably typical. (SLD differs from dynamic load sharing in
that times between DNS parameter updates are generally much longer than RR
TTLs, and in that no real time protocol is needed between application
servers and
DNS servers.)
- Within IETF, one tool is explicitly intended for SLD&BU, and is
particularly clean and simple for that, namely SRV RRs (a relevant extract
of RFC2052 is copied below). (SRV RRs were also designed for a different
purpose, namely facilitating new service deployments by suppressing the need
for host updates when new application ports are assigned, an interesting
progress, but there is no need to be concerned with it in the use of SRV RRs
for SLD&BU as specified below).
- A natural question is therefore: can a design be found such that SRV RRs
become a practical tool for SLD&BU, subject to the requirement that upward
compatibility is ensured in both DNS Name Servers and DNS Clients, in
particular in Web Browsers?
- The answer appears to be YES with the APSDR proposal below ("A plus SRV
DNS
Responses"), where A is taken as generic, meaning also AAAA and A6.
APSDR Specification
--------------------------
1. To become APSDR compatible, a Name Server has, when it returns a response
with A type RRs, to include in the Additional Data section of the response
the SRV records that, in its data base, match the same domain name, if any.
2. To be APSDR compatible, a DNS Client (e.g. a Web Browser) has, when it
receives SRV RRs in a response to one of its A queries, to memorize the
Priorities and Weights of hosts listed in SRV RRs, and to comply with their
SLD&BU implications (a simple piece of code).
Upward compatibility of APSDR
---------------------------------------
a. If a DNS Client which is APSDR compatible queries Name Server which is
not, the Name server will return A records only. The DNS Client works as
before.
b. If a DNS Client which is not APSDR compatible queries a Name Server which
is APSDR compatible, it will simply ignore SRV RRs, as it does for any
unknown type RR. The DNS Client works as before.
c. If both a DNS Client and a queried Name Server are APSDR compatible, and
if there is no SRV RR concerning the queried Domain name in the data base of
the server, no SRV RR is returned. Both the Name Server and the DNS Client
work as before.
d. If both DNS Client and the queried Name Server are APSDR compatible, and
if there are SRV RRs concerning the queried Domain name in the data base,
SLD&BU will apply as specified by SRV RRs. (This is the raison d'etre of
the APSDR compatible extension in DNS Clients and Name Servers).
Now that things are much clearer, I believe it would be useful to submit the
subject to a wider forum than just both of us.
You may of course feel differently (it seems you are more interested in
schemes that involve Name Servers alone, and that therefore work with
existing DNS Clients, but both approaches have their own application scopes
and can in my view usefully coexist).
I look forward to your views?
Rémi Després
****
Extract from RFC 2052 - A DNS RR for specifying the location of services
(DNS SRV)
...
The SRV RR allows administrators to use several servers for a single
domain, to move services from host to host with little fuss, and to
designate some hosts as primary servers for a service and others as
backups.
...
Priority
As for MX, the priority of this target host. A client MUST
attempt to contact the target host with the lowest-numbered
priority it can reach; target hosts with the same priority
SHOULD be tried in pseudorandom order. The range is 0-65535.
...
Weight
Load balancing mechanism. When selecting a target host among
those that have the same priority, the chance of trying this
one first SHOULD be proportional to its weight. The range of
this number is 1-65535. Domain administrators are urged to use
Weight 0 when there isn't any load balancing to do, to make the
RR easier to read for humans (less noisy).
...
****
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