Some general things and views on DNS over HTTPS

May 8, 2019

Over on Twitter I said something and then Brad Beyenhof asked me a sensible question related to DNS over HTTPS. Before I elaborate my Twitter answers to that specific question, I want to do an overview of some general views on DNS over HTTPS on the whole.

DNS over HTTPS (hereafter DoH) is what it sounds like; it's a protocol for making DNS queries over a HTTPS connection. There is also the older DNS over TLS, but my impression is that DoH has become more popular, perhaps partly because it's more likely to make it through middleware firewalls and so on. DoH and DoT are increasingly popular ideas for the straightforward reason that on the modern Internet, ISPs are one of your threats. A significant number of ISPs snoop on your DNS traffic to harvest privacy-invasive things about your Internet activities, and some of them actively tamper with DNS results. DoH (and DoT) mostly puts a stop to both DNS snooping and DNS interference.

(It's likely that DoH doesn't completely stop these because of the possibility of traffic analysis of the encrypted traffic, which has already been shown to reveal information about HTTPS browsing, but we probably won't know how serious an issue this is until DoH is enough used in the field to attract research attention.)

As far as I can tell, DoH (and DoT) are currently used and intended to be used between clients and resolving DNS servers. If there is work to protect queries between a resolving DNS server and authoritative servers, I think it's using a different protocol (Wikipedia mentions DNSCurve, but it's not widely adopted). This doesn't matter for typical users, who use someone else's DNS server, but it matters for people who run their own local resolving server that wants to query directly to authoritative servers instead of delegating to another resolving server.

DoH protects you from your ISP monitoring or tampering with your DNS queries, but it doesn't protect you from your DoH server of choice doing either. To protect against tampering, you'd need some form of signed DNS (but then see Against DNSSEC). It also doesn't currently protect you against your ISP knowing what HTTPS websites you go on to visit, because of SNI; until we move to ESNI, we're always sending the website name (but not the URL) in the clear as part of the early TLS negotiation, so your ISP can just capture it there. In fact, if your ISP wants to know what HTTPS sites you visit, harvesting the information from SNI is easier than getting your DNS queries and then correlating things.

(There is very little protection possible against your DoH server monitoring your activity; you either need to have a trustworthy DNS server provider or run your server yourself in a situation where its queries to authoritative servers will probably not be snooped on.)

More or less hard-coded use of DNS over HTTPS servers in client programs like browsers poses the same problems for sysadmins as basically any hard-coded use of an upstream resolver does, which is that it will bypass any attempts to use split-horizon DNS or other ways of providing internal DNS names and name bindings. Since these are obvious issues, I can at least optimistically hope that organizations like Mozilla are thinking about how to make this work without too much pain (see the current Mozilla wiki entry for some discussion of this; it appears that Mozilla's current setup will work for unresolvable names but not names that get a different result between internal and external DNS).

PS: There are a number of recursive DNS servers that can be configured to use DoH to an upstream recursive server; see eg this brief guide for Unbound. Unbound can also be configured to be a DoH server itself for clients, but this doesn't really help the split horizon case for reasons beyond the scope of this entry.

Written on 08 May 2019.
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Last modified: Wed May 8 23:12:48 2019
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