[CentOS-mirror] IPv6 Mirroring

Kevin Stange kevin at steadfast.net
Tue Aug 2 12:42:47 EDT 2011

On 08/02/2011 10:10 AM, Matt Domsch wrote:
> On Mon, Aug 01, 2011 at 05:45:16PM -0500, Kevin Stange wrote:
>> We'd be catering to v6-capable (dual stack) clients by offering
>> mirror choices with v6 ahead of v4 if the client seems to prefer v6.
> Here's where I start to lose you.  We don't presently have a way for a
> client to indicate to anyone anywhere that it "prefers" v6 instead of
> v4, or vice versa.  From the server side, it sees a connection, either
> on v6 or on v4, and sees an address.  We would have to add a flag into
> the yum mirrorlist request URL to give the client an opportunity to
> note its preference.  Not hard technically, but the obvious
> implementation would require manual configuration by each end user,
> and that's nearly never done.
> How would you envision a client recognizing that it should prefer v6
> over v4, automatically?  Solving that cleanly, the rest is
> straightforward.

We can tell if a client prefers v6 because it will access
mirrorlist.centos.org using its IPv6 address.  The client doesn't need
to tell us explicitly because its decision to access the v6 address
means it's decided to try the AAAA first.  This is the default behavior
in most software, but can generally be changed manually if the user
recognizes they have very poor IPv6 connectivity.

>> In the meantime, if the client prefers IPv6, I think it's better to
>> offer him IPv6 mirrors as the top priority, followed by other regional
>> v4 only mirrors after that.
> You're asking for a different sort order to the results list, based on
> the presence of a "prefers_v6=1' flag in the request URL.  Not
> difficult really, MirrorManager could add that.  Right now it doesn't
> do A or AAAA lookups for its mirrors, it just returns the URLs that
> the mirror admins put in the database, and the clients then do their
> own DNS lookups, and then connect whichever way they choose.  So MM
> would need to add DNS lookups and cache that info, and use it in the
> sort order.

I suppose we could compile a list of prefers_v6 mirrors, but my thoughts
didn't extend past concluding that if the client prefers v6 by evidence
we just offer it the v6 mirrors sorted first so it can use the preferred

Do you think it's likely that mirrors will prefer avoiding v6 if they
have enabled it?

Kevin Stange
Chief Technology Officer
Steadfast Networks
Phone: 312-602-2689 ext. 203 | Fax: 312-602-2688 | Cell: 312-320-5867

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