[dns-operations] "off label" use of PTR records for fanout

[Viktor Dukhovni]

On Thu, Jun 15, 2023 at 01:05:50PM -0700, Fred Morris wrote:

> I'm using DNS to retrieve some distributed telemetry data from multiple
> servers. To facilitate this I have an FQDN which resolves to a set of
> PTR records. If there's a generally accepted better option, let me know.
> 
> If you just want to bike shed this fine, I invite you to email me
> directly as I think this is already tangential to the purpose of this list.

I see, this is then just a way to publish multiple names in one RRset.

There is nothing wrong with doing this.  Unlike CNAME RRs you don't get
any implicit indirection, the application has to make explicit
subsequent queries for each name, as it sees fit.

PTR records also have the advantage of being "old enough" to support
name compression, so the representation will be reasonably efficient
(will share common suffixes) in most cases.

    myserverlist.example. IN PTR server1.example.
    myserverlist.example. IN PTR server2.example.
    ...
    myserverlist.example. IN PTR server19.example.

The main limitation is that RRset in its entirety has to ultimately fit
in a single DNS message, so you're typically "limited" to O(100) names,
with a list of ~10k names becoming impractical.

Of course there already exist more elaborate ways to list target servers
in the form of SRV, SVCB and HTTPS RRs, but if all you need is a name
list, and you don't expect generic support for such lists outside your
own application, then sure, PTR will do.

-- 
    Viktor.