DNS-over-HTTPS in Unbound

By on 14 Dec 2020

Category: Tech matters

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Privacy plays an important part in the development of NLnet Labs products. For Unbound this manifests itself by being in the front line of the development of privacy preserving features like QNAME minimization, auth-zones, and DNS-over-TLS (DoT).

DoT makes it possible to encrypt DNS messages and gives a DNS client the possibility to authenticate a resolver. As implied by the name, this is done by sending DNS messages over TLS. Unbound has handled TLS encrypted DNS messages since 2011, long before the IETF DPRIVE working group started its work on the DoT specification. Unbound’s DoT implementation can offer an encrypted service to clients, encrypt and authenticate forwarded queries, and can encrypt upstream queries. DNS clients like like getdns/Stubby, systemd-resolved, and the DNS client in Android are able to send queries using DoT.

There are, however, DNS clients that do not support DoT but are able to use DNS-over-HTTPS (DoH) instead. Where DoT sends a DNS message directly over TLS, DoH has an HTTP layer in between. Where DoT uses its own TCP port (853), DoH uses the standard HTTPS port (443). Using the standard HTTPS port makes it harder to block DoH queries, as blocking TCP traffic on port 443 will also block a lot of web traffic. Some people think this is great as this ensures that the user’s DNS queries will always be encrypted; others have concerns about DoH as they might lose control over clients in their network.

At NLnet Labs we are in favour of encrypting DNS traffic to limit the exposure of privacy-sensitive data. By adding downstream DoH support to Unbound we hope to increase the ratio of encrypted DNS traffic and increase the number of resolvers that offer encrypted services in home networks, enterprise networks, ISPs, and public resolvers.

Unbound’s DoH implementation

The DoH implementation in Unbound requires TLS, and only works over HTTP/2. The query pipelining and out-of-order processing functionality that is provided by HTTP/2 streams is needed to be able to provide performance that is on par with DoT. The HTTP/2 capability is negotiated using Application-Layer Protocol Negotiation (ALPN) TLS extension, which is supported in OpenSSL from version 1.0.2 onward.

We decided to use the amazing nghttp2 library to handle the HTTP/2 framing layer. This library does not take care of any I/O handling, which makes it possible to easily integrate it in the existing Unbound event loop and TCP handling. Adding HTTP/2 on top of the existing TCP code makes it possible to also use the existing TCP configuration options for the DoH connections. These existing options include the number of allowed incoming TCP connections, the TCP timeout settings, and the limits on TCP connections per client IP address or netblock.

The use of HTTP makes it possible to change the DNS message format by using new media types. Unbound currently only supports the application/dns-message media type, as this is the only format standardized in the IETF standards track, and the only supported format by popular DNS clients. We are keeping an eye on the new possibilities here, such as using the oblivious-dns-message media type.

The use of the HTTP layer also makes it possible to return more detailed information to a client in case of malformed requests. This can be done by using a non-successful HTTP status code, or by closing an individual stream by sending an RST_STREAM frame. The HTTP status codes that can be returned by Unbound are:

  • 200 OK: Unbound is able to process the query, and return an answer. This could be a negative answer or an error like SERVFAIL or FORMERR.
  • 404 Not Found: The request is directed to a path other than the configured endpoint in http-endpoint (default /dns-query).
  • 413 Payload Too Large: The payload received in the POST request is too large. Payloads cannot be larger than the content-length communicated in the request header. The payload length is limited to 512 bytes if harden-large-queries is enabled, and otherwise limited to the value configured in msg-buffer-size (default 65,552 bytes). To prevent the allocation of overly large buffers, the maximum size is limited to the size of the first DATA frame if no content-length is received in the request.
  • 414 URI Too Long: The base64url encoded DNS query in the GET request is too large. The DNS query length is limited to 512 bytes if harden-large-queries is enabled, and limited tomsg-buffer-size otherwise.
  • 415 Unsupported Media Type: The media type of the request is not supported. This happens if the request contains a content-type header that is set to anything but application/dns-message. Requests without content-type will be treated as application/dns-message.
  • 400 Bad Request: No valid query received, not matched by any of the above 4xx status codes.
  • 501 Not Implemented: The method used in the request is not GET or POST.

The changes to the Unbound code for the DoH implementation are collected in a pull request.

Using DoH in Unbound

As mentioned above, the nghttp2 library is required to use Unbound’s DoH functionality. Compiling and installing Unbound with libnghttp2 can be done using:

./configure --with-libnghttp2
make && make install

Enabling DoH in Unbound is as simple as configuring the TLS certificate and the corresponding private key that will be used for the connection, and configuring Unbound to listen on the HTTPS port:

tls-service-key "key.pem"
tls-service-pem: "cert.pem"

The port that Unbound will use for incoming DoH traffic is, by default, set to 443 and can be changed using the https-port configuration option.

Unbound is now ready to handle DoH queries on the default HTTP endpoint, which is /dns-query:

$ ./dohclient -s nlnetlabs.nl AAAA IN 
Request headers
:method: GET
:scheme: https
content-type: application/dns-message
:status 200
content-type application/dns-message
;; ->>HEADER<<- opcode: QUERY, rcode: NOERROR, id: 0
;; flags: qr rd ra ad ; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 1
nlnetlabs.nl. IN AAAA;; ANSWER SECTION:
nlnetlabs.nl. 10200 IN AAAA 2a04:b900::1:0:0:10
nlnetlabs.nl. 10200 IN RRSIG AAAA 8 2 10200 20200723194739 20200625194739 42393 nlnetlabs.nl. ML5NkbykTetqBPyA0xG5fuq1t/0ojsMUixgEhcewG93jZpF+vz8WhVo6czzdRMo/qq2kAmh3aFmU94wVWn+AULEEz6a/7B1Sxz9O+bXivZiWVitUopheSya68CNHO/zCl7j23QirecLGoXozbVqMIbinqG0LS32bHS+WOsJgQCQ= ;{id = 42393};; AUTHORITY SECTION:;; ADDITIONAL SECTION:
; EDNS: version: 0; flags: do ; udp: 4096
;; MSG SIZE rcvd: 241

Queries to other paths will be answered with a 404 status code. The endpoint can be changed using the http-endpoint configuration option.

The maximum number of concurrent HTTP/2 streams can be configured using the http-max-streams configuration option. The default for this option is 100, as per the HTTP/2 RFC recommended minimum. This value will be in the SETTINGS frame sent to the client, and enforced by Unbound.

Because requests can be spread out over multiple HTTP/2 frames, which can be interleaved between frames of different streams, we have to create buffers containing partial queries. A new counter is added to Unbound to limit the total memory consumed by all query buffers. The limit can be configured using the http-query-buffer-size option. New streams will be closed by sending an RST_STREAM frame when this limit is exceeded.

After Unbound is done resolving a request the DNS response will be stored in a buffer, waiting until Unbound is ready to send them back to the client using HTTP. These buffers also have a maximum amount of memory they are allowed to consume. This maximum is configurable using the http-response-buffer-size configuration option.

DoH metrics

Three DoH related metrics have been added to Unbound; num.query.https counts the number of queries that have been serviced using DoH. The mem.http.query_buffer, and mem.http.response_buffer counters keep track of the memory used for the DoH query and response buffers.

The Mozilla Open Source Support (MOSS) awards program funded the development of the Unbound DoH implementation.

This post was originally published by NLnet labs.

Ralph Dolmans is a systems and software engineer at NLnet Labs.

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The views expressed by the authors of this blog are their own and do not necessarily reflect the views of APNIC. Please note a Code of Conduct applies to this blog.

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