SRV Record Server Availability

[Mark Andrews]

Well mDNS is not DNS.  It is a multicast request to all devices on the local network to respond.

To get the functionality being requested in the DNS it requires something to be polling the availability of the server listed in the SRV records and to add/remove/adjust them depending upon their load/availability.  In reality this should not be needed if people would just write client software to properly handle multi-homed servers.  There is nothing that say you must wait for a connection to timeout before you attempt to connect to a second address.

Happy Eyeballs RFC 8305 (previously RFC 6555) us about starting multiple connections across address families and only proceeding with the first that connects but there is NOTHING that says you can’t do the similar for all addresses independent of address family.

It isn’t hard to write a TCP client that attempts to connect to multiple servers simultaneously.

I will admit that it is slightly harder for UDP clients but in most cases it is not impossible.

For both protocols you do not wait seconds to get the initial response before trying the alternate addresses.  Most of the world is less that 300ms way (round trip).

Mark

> On 7 Jan 2021, at 03:42, Andrew P. <andrewemt at hotmail.com> wrote:
> 
> Isn't this sort of dynamic functionality (real-time presence or absence of SRV records) what mDNS and the avahi daemon are for?
> 
> ________________________________________
> From: bind-users <bind-users-bounces at lists.isc.org> on behalf of Matus UHLAR - fantomas <uhlar at fantomas.sk>
> Sent: Wednesday, January 6, 2021 8:51 AM
> To: bind-users at lists.isc.org
> Subject: Re: SRV Record Server Availability
> 
> On 06.01.21 21:41, Wilfred Sarmiento via bind-users wrote:
>> Your understanding is correct, i just thought that SRV can detect whose
>> server is alive so it can choose and provide an answer with the available
>> Server.
> 
> DNS is not designed to provide this functionality. While technically you can
> change contents of DNS depending on which servers are alive and which are
> not, it's almost never a good idea.
> 
> That means, BIND has nothing like this built in.
> 
>>> On Tue, Jan 5, 2021 at 4:30 AM Wilfred Sarmiento via bind-users
>>> <bind-users at lists.isc.org> wrote:
>>>> Is DNS Bind SRV record can detect the Server's availability? If yes, how?
> 
>> On Tue, 5 Jan 2021, 23:53 tale <d.lawrence at salesforce.com> wrote:
>>> Could you provide more information about your goal?  I don't fully
>>> understand the question.
>>> 
>>> For my reading, the answer is basically no, in that an SRV record just
>>> provides data about where.a particular service can be found.  It's up
>>> to other systems to fetch that data and interpret it, including
>>> whether that service is actually available at the given endpoint.  In
>>> its typical operation, BIND will just take whatever name and port the
>>> zone administrator said to provide for that SRV record, and not do any
>>> sort of availability checks on it.
>>> 
>>> However, if you go deep into a far more complicated, custom use of
>>> BIND, you could set up a process that monitors the availability and
>>> changes the SRV record accordingly.
> 
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Mark Andrews, ISC
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