Recent news from the Department of Unintended Consequences: RFC 6724 changed the IPv4/IPv6 source/destination address selection rules a decade ago, and it seems that the common interpretation of those rules makes IPv6 Unique Local Addresses (ULA) less preferred than the IPv4 addresses, at least according to the recent Unintended Operational Issues With ULA draft by Nick Buraglio, Chris Cummings and Russ White.
End result: If you use only ULA addresses in your dual-stack network1, IPv6 won’t be used at all. Even worse, if you use ULA addresses together with global IPv6 addresses (GUA) as a fallback mechanism, there might be hidden gotchas that you won’t discover until you turn off IPv4. Looks like someone did a Truly Great Job, and ULA stands for Useless Local Addresses.
For more details:
- Listen to The Foibles and Frailties of IPv6 ULA episode of the Modem Podcast
- Read the IETF draft
- Enjoy the recent discussion on v6ops mailing list. I ran out of popcorn and patience and dropped out.
Do you care? Maybe.
Someone asked a question along the lines “does anyone know of a good ULA use case” at the end of the Modem Podcast episode I mentioned above, and there happens to be one that might be relevant to a few mid-sized enterprise networks: site-to-site VPN with local Internet exit. I wrote about it in 2013 and as far as I know nothing changed in the meantime (apart from ULA addresses becoming useless). Here are the links to the (somewhat) relevant blog posts:
- To ULA or not to ULA, That’s the Question (September 2013)
- PA, PI or ULA IPv6 Address Space? It depends (January 2014)
- IPv6 Legends and Myths: More Opinions than Data Points (April 2012)
- Why Can’t We All Use Provider-Independent IPv6 Addresses? (April 2018)
Finally, let me conclude with an awesome quote from Randy Bush (made in October 2012):
It is cheering to see that the IPv6 ivory tower still stands despite years of attack by reality.