RFC 6724

Florian Weimer fweimer@redhat.com
Mon Nov 12 17:43:00 GMT 2018

* Phillip Hellewell:

> On Mon, Nov 12, 2018 at 4:20 AM Florian Weimer <fweimer@redhat.com> wrote:
>> RFC 3484 has always been rather problematic (particularly Rule 9, which
>> we do not implement).
>> I seriously doubt that there is any benefit from address sorting.  It's
>> a layering violation, and it makes renumbering harder because you now
>> need to consider the impact on address sorting.  Its core assumptions
>> are also quite wrong on many networks (e.g., private addresses often
>> have less georeplication than public Internet service, so public
>> addresses are closer by).
>> In any case, I think address sorting should be performed by the caching
>> DNS resolver, not the stub resolver.
> An administrator of a given machine may not have any control over the
> DNS server, yet they should still be able to control address order,
> e..g., to prefer IPv4 over IPv6 or vice-versa.

If you can edit /etc/gai.conf, you can install Unbound or dnsmasq, too.

>> Do you have a setup that actually relies on address sorting?  Do you
>> have any examples where DNS provides a set of addresses for a single
>> name with different labels/precedence, so that sorting the addresses
>> actually has an effect?
> Yes, this happens all the time.  Look no further than google.com.  It
> has both A and AAAA.  As a system administrator, I should be able to
> set up my prefix policy table to prefer IPv4 over IPv6 if desired.
> That is just one example.

Do you have another example that does not involve sorting merely by
protocol?  Something that you have encountered personally?

> One should be able to control address order without having to set up
> and run their own special DNS server/proxy/whatever.

To be honest, I don't see any other way to get full RFC 6724 support
because the RFC requires various things for which I do not see direct
kernel support.

If we disable or limit sorting in glibc, at least you will be able to
get RFC 6724 support with a suitable NSS service module or caching DNS


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