Firefox, DNS over HTTPS, and us

September 18, 2019

The news of the time interval is that Mozilla will soon start rolling out DNS over HTTPS for US users, where by 'rolling out' Mozilla means 'enabling by default'. To their minimum credit, Mozilla says that they will explicitly notify people of this change and give them the opportunity to opt out. I hope and assume that this will work much like how Mozilla rolled out various tracking protection measures, including with how thoroughly informative that was.

(Clearly notifying people and giving them the chance to opt out is the obvious right thing to do, but Mozilla's track record on doing the obvious right thing is somewhat mixed.)

Since we're not in the US, this doesn't immediately affect people here; however, I have to assume that Mozilla is going to start rolling DNS over HTTPS out more broadly than just the USA. Given things like GDPR, Mozilla may not push this to Europe any time soon, but there probably aren't many roadblocks for rolling it out in Canada. My overall views on this remain unchanged; there are tradeoffs in either direction, and I have no idea what the right choice is in general.

For my department in particular, Firefox switching to DNS over HTTPS presents a potential problem because we have a split horizon DNS setup where some names resolve to different IPs internally than they do externally. According to Mozilla's blog post, the Firefox DoH implementation has some heuristics to detect a split horizon DNS environment, but from the vague descriptions we have so far it's not clear if they would reliably trigger for our users. If people here wind up with Firefox configured to use DNS over HTTPS and Mozilla's split horizon DNS heuristics don't trigger, they won't be able to connect to some of our hosts. We could theoretically say that this is people's fault in the same way that setting their machine to always use one of the public resolvers is, but this is the wrong answer, since Mozilla will have made this setting for them.

Mozilla currently supports a way for networks to explicitly disable DNS over HTTPS, by making your local resolver return NXDOMAIN for a canary domain. This is easy to do in Unbound, which we use on our local OpenBSD resolvers (see here or here). We could preemptively deploy this, but I tentatively think that we should wait to see if the Firefox split horizon detection heuristics work in our environment. Working heuristics would be the best answer for various reasons (including that Mozilla may find too many people abusing the canary domain and start paying less attention to it).

For work, there's probably no point in adding DNS over HTTPS to our local resolving DNS servers, even once it's supported on the OpenBSD version of Unbound. As far as I know, people here would have to specifically configure their Firefox to talk to our servers, and then their configuration would break when they moved outside of our network and could no longer reach our resolving DNS servers.

For my own personal use, I may eventually add DNS over HTTPS support to my resolving Unbound instances, because apparently DoH is the only way to get encrypted SNI. Unfortunately it also apparently normally requires DNSSEC, so unless I can get my Unbound to lie about that (or Firefox to not care), I may be out of luck. I do wish I could tell Firefox that a resolver on localhost was trusted even without DoH, but I suspect that I can't.

(This does raise long term issues about encrypted SNI support for our users, but perhaps in the long term people will come up with answers. Hopefully ones that don't involve DNSSEC.)

Written on 18 September 2019.
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Last modified: Wed Sep 18 23:37:31 2019
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