Re: [hybi] DNS SRV for WebSocket

[Iñaki Baz Castillo <[e-mail]>]

2011/3/28 Bjoern Hoehrmann <[e-mail]>:
> * Iñaki Baz Castillo wrote:
>>So the WS client should check for the existence of a SRV record for
>>the given domain (if no port is given), but the SRV record could exist
>>or not. It's very flexible but, of course, in order to work properly,
>>it should be mandatory for WS clients.
>
> It would seem very unlikely that all client implementations will do this
> (because the feature is obscure to many, lack of test servers, lack of
> proper DNS libraries that make this easy, not getting around to it, not
> caring, and so on). As I understand it, not supporting SRV lookups is a
> common problem with simpler XMPP implementations, for instance. So, if
> this was mandatory for Websocket clients, we'd likely be looking at many
> non-compliant implementations. Putting it the other way around, usually
> people don't implement features just because they are formally required.

A WS client could not support SRV and still work. Example:


- For domain mydomain.org there are a DNS SRV record and 3 DNS A records:

  SRV record:

     _ws._tcp.mydomain.org. IN SRV 1 0 80 mydomain.org.
     _ws._tcp.mydomain.org. IN SRV 2 0 8080 backup.mydomain.org.

  A records:

      mydomain.org.                 1.1.1.1
      backup.mydomain.org.      1.1.1.2


- The web page indicates the following WS URI:  ws://mydomain.org.



case 1) SRV capable WS client
-----------------------------------------------------------------------

- It performs a SRV query for domain=mydomain.org, service=ws and transport=tcp.

- It gets the SRV record with two entries and chooses the first one
(with best priority).

- So it performs a DNS A query for domain "mydomain.org" and gets 1.1.1.1.

- Then it connects to 1.1.1.1:80.

- In case the server is unreachable, the client tries the second
location by performing a DNS A query for backup.mydomain.org and gets
1.1.1.2, so it connects to 1.1.1.2:8080 (failover, oh yeah !!!).



case 2) Non-SRV capable WS client
-----------------------------------------------------------------------

- It performs a DNS A query for domain=mydomain.org and gets 1.1.1.1.

- Then it connects to 1.1.1.1:80.

- In case the server is unreachable the client cannot do failover due
to its limitation (it doesn't support SRV, sorry).



So, as you can see, it's not fully required that the client supports
SRV, but if it does, it can take COOL advantages from it.



-- 
Iñaki Baz Castillo
<[e-mail]>