Linux source address selection vs. EUI-64

Mark Smith nanog at
Sat Nov 13 15:28:04 CET 2010

On Sat, 13 Nov 2010 13:51:05 +0100
Geert Hendrickx <ghen at> wrote:

> Hi,
> my VPS hosting provider has now deployed native IPv6 on their network
> (hooray!), and assigned each customer a /96 for their VPS, together with
> stateless autoconfiguration for routing and a EUI-64 address.

SLAAC requires /64 prefix lengths, as Modified EUI-64 Interface
Identifiers (IIDs) are 64 bits in size. When you say they've given each
customer a /96, are you saying a /96 range of addresses that you
can statically configure within a /64 shared by many customers (e.g.
you get 0x0 - 0xffffffff, next customer gets 0x100000000 -
0x1ffffffff etc.), or have they actually tried to route a /96 towards
you? If it is the latter, they should have enough /64s to easily be
able to give every customer at least one, if not multiple e.g. /60
or /56. That is a fairly fundamental thing to get wrong, I'd be
wondering if they really know all that much about IPv6 at all, and
would be hesitant to trust them to operate it reliably.

> I manually configured an IP from my range on the host, but Linux seems
> to prefer the EUI-64 address for outgoing connections, because it's
> always the last added one (gets refreshed with each RA) and otherwise
> equivalent with the static address (same /64 prefix), see [1].
> I rather want to use the static address for outgoing connections (as it
> has proper reverse DNS etc), though without necessarily getting rid of
> the EUI-64 address; I could disable autoconfiguration but the hosting
> provider uses the EUI-64 addresses for monitoring and troubleshooting,
> so they prefer I don't ignore their RA's.
> I could mark the EUI-64 address as deprecated so it's no longer used for
> outgoing connections, but that would imply I have to hardcode it in my
> configuration, which defeats the purpose of having it auto-configured
> (it can change without prior notice, eg. when they move VPS's around).
> Any other suggestions to influence the source address selection on Linux?

> 	Geert
> [1]
> -- 
> Geert Hendrickx  -=-  ghen at  -=-  PGP: 0xC4BB9E9F
> This e-mail was composed using 100% recycled spam messages!

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