doh-proxy

A fast and secure DoH (DNS-over-HTTPS) and ODoH (Oblivious DoH) server.

doh-proxy is written in Rust, and has been battle-tested in production since February 2018. It doesn't do DNS resolution on its own, but can sit in front of any DNS resolver in order to augment it with DoH support.

Installation

Option 1: precompiled binaries for Linux

Precompiled tarballs and Debian packages for Linux/x86_64 can be downloaded here.

Option 2: from source code

This requires the rust compiler to be installed.

• With built-in support for HTTPS (default):

cargo install doh-proxy

• Without built-in support for HTTPS:

cargo install doh-proxy --no-default-features

Usage

USAGE:
    doh-proxy [FLAGS] [OPTIONS]

FLAGS:
    -O, --allow-odoh-post      Allow POST queries over ODoH even if they have been disabed for DoH
    -K, --disable-keepalive    Disable keepalive
    -P, --disable-post         Disable POST queries
    -h, --help                 Prints help information
    -V, --version              Prints version information

OPTIONS:
    -E, --err-ttl <err_ttl>                          TTL for errors, in seconds [default: 2]
    -H, --hostname <hostname>                        Host name (not IP address) DoH clients will use to connect
    -l, --listen-address <listen_address>            Address to listen to [default: 127.0.0.1:3000]
    -b, --local-bind-address <local_bind_address>    Address to connect from
    -c, --max-clients <max_clients>                  Maximum number of simultaneous clients [default: 512]
    -C, --max-concurrent <max_concurrent>            Maximum number of concurrent requests per client [default: 16]
    -X, --max-ttl <max_ttl>                          Maximum TTL, in seconds [default: 604800]
    -T, --min-ttl <min_ttl>                          Minimum TTL, in seconds [default: 10]
    -p, --path <path>                                URI path [default: /dns-query]
    -g, --public-address <public_address>            External IP address DoH clients will connect to
    -j, --public-port <public_port>                  External port DoH clients will connect to, if not 443
    -u, --server-address <server_address>            Address to connect to [default: 9.9.9.9:53]
    -t, --timeout <timeout>                          Timeout, in seconds [default: 10]
    -I, --tls-cert-key-path <tls_cert_key_path>
            Path to the PEM-encoded secret keys (only required for built-in TLS)

    -i, --tls-cert-path <tls_cert_path>
            Path to the PEM/PKCS#8-encoded certificates (only required for built-in TLS)

HTTP/2 termination

The recommended way to use doh-proxy is to use a TLS termination proxy (such as hitch or relayd), a CDN or a web server with proxying abilities as a front-end.

That way, the DoH service can be exposed as a virtual host, sharing the same IP addresses as existing websites.

If doh-proxy and the HTTP/2 front-end run on the same host, using the HTTP protocol to communicate between both is fine.

If both are on distinct networks, such as when using a CDN, doh-proxy can handle HTTPS requests, provided that it was compiled with the tls feature.

The certificates and private keys must be encoded in PEM/PKCS#8 format. They can be stored in the same file.

If you are using ECDSA certificates and ECDSA private keys start with -----BEGIN EC PRIVATE KEY----- and not -----BEGIN PRIVATE KEY-----, convert them to PKCS#8 with (in this example, example.key is the original file):

openssl pkcs8 -topk8 -nocrypt -in example.key -out example.pkcs8.pem

In order to enable built-in HTTPS support, add the --tls-cert-path option to specify the location of the certificates file, as well as the private keys file using --tls-cert-key-path.

Once HTTPS is enabled, HTTP connections will not be accepted.

A sample self-signed certificate localhost.pem can be used for testing. The file also includes the private key.

acme.sh can be used to create and update TLS certificates using Let's Encrypt and other ACME-compliant providers. If you are using it to create ECDSA keys, see above for converting the secret key into PKCS#8.

The certificates path must be set to the full certificates chain (fullchain.cer) and the key path to the secret keys (the .key file):

doh-proxy -i /path/to/fullchain.cer -I /path/to/domain.key ...

Once started, doh-proxy automatically reloads the certificates as they change; there is no need to restart the server.

If clients are getting the x509: certificate signed by unknown authority error, double check that the certificate file is the full chain, not the other .cer file.

Accepting both DNSCrypt and DoH connections on port 443

DNSCrypt is an alternative encrypted DNS protocol that is faster and more lightweight than DoH.

Both DNSCrypt and DoH connections can be accepted on the same TCP port using Encrypted DNS Server.

Encrypted DNS Server forwards DoH queries to Nginx or doh-proxy when a TLS connection is detected, or directly responds to DNSCrypt queries.

It also provides DNS caching, server-side filtering, metrics, and TCP connection reuse in order to mitigate exhaustion attacks.

Unless the front-end is a CDN, an ideal setup is to use doh-proxy behind Encrypted DNS Server.

Oblivious DoH (ODoH)

Oblivious DoH is similar to Anonymized DNSCrypt, but for DoH. It requires relays, but also upstream DoH servers that support the protocol.

This proxy supports ODoH termination (not relaying) out of the box.

However, ephemeral keys are currently only stored in memory. In a load-balanced configuration, sticky sessions must be used.

Currently available ODoH relays only use POST queries. So, POST queries have been disabled for regular DoH queries, accepting them is required to be compatible with ODoH relays.

This can be achieved with the --allow-odoh-post command-line switch.

Operational recommendations

• DoH can be easily detected and blocked using SNI inspection. As a mitigation, DoH endpoints should preferably share the same virtual host as existing, popular websites, rather than being on dedicated virtual hosts.
• When using DoH, DNS stamps should include a resolver IP address in order to remove a dependency on non-encrypted, non-authenticated, easy-to-block resolvers.
• Unlike DNSCrypt where users must explicitly trust a DNS server's public key, the security of DoH relies on traditional public Certificate Authorities. Additional root certificates (required by governments, security software, enterprise gateways) installed on a client immediately make DoH vulnerable to MITM. In order to prevent this, DNS stamps should include the hash of the parent certificate.
• TLS certificates are tied to host names. But domains expire, get reassigned and switch hands all the time. If a domain originally used for a DoH service gets a new, possibly malicious owner, clients still configured to use the service will blindly keep trusting it if the CA is the same. As a mitigation, the CA should sign an intermediate certificate (the only one present in the stamp), itself used to sign the name used by the DoH server. While commercial CAs offer this, Let's Encrypt currently doesn't.
• Make sure that the front-end supports HTTP/2 and TLS 1.3.
• Internal DoH servers still require TLS certificates. So, if you are planning to deploy an internal server, you need to set up an internal CA, or add self-signed certificates to every single client.

Example usage with encrypted-dns-server

Add the following section to the configuration file:

[tls]
upstream_addr = "127.0.0.1:3000"

Example usage with nginx

In an existing server, a /doh endpoint can be exposed that way:

location /doh {
  proxy_pass http://127.0.0.1:3000;
}

This example assumes that the DoH proxy is listening locally to port 3000.

HTTP caching can be added (see the proxy_cache_path and proxy_cache directives in the Nginx documentation), but be aware that a DoH server will quickly create a gigantic amount of files.

DNS Stamp and certificate hashes

Use the online DNS stamp calculator to compute the stamp for your server.

Add it to the [static] section of dnscrypt-proxy and check that everything works as expected.

Then, start dnscrypt-proxy with the -show-certs command-line flag to print the hashes for your certificate chain.

Here is an example output:

[NOTICE] Advertised cert: [CN=dohtrial.att.net,O=AT&T Services\, Inc.,L=Dallas,ST=Texas,C=US] [f679e8451940f06141854dc94e1eb79fa5e04463c15b88f3b392da793c16c353]
[NOTICE] Advertised cert: [CN=DigiCert Global CA G2,O=DigiCert Inc,C=US] [f61e576877da9650294cccb5f96c75fcb71bda1bbc4646367c4ebeda89d7318f]

The first printed certificate is the certificate of the server itself. The next line is the one that signed that certificate. As you keep going down, you are getting closer to the certificate authority.

Unless you are using intermediate certificates, your safest option is probably to include the last printed hash certificate in your DNS stamp.

Go back to the online DNS stamp calculator, and copy&paste the hash (in this example: f61e576877da9650294cccb5f96c75fcb71bda1bbc4646367c4ebeda89d7318f).

If you are using Let's Encrypt, the last line is likely to be:

Advertised cert: [CN=Let's Encrypt Authority R3,O=Let's Encrypt,C=US] [444ebd67bb83f8807b3921e938ac9178b882bd50aadb11231f044cf5f08df7ce]

There you have it. Your certificate hash is 444ebd67bb83f8807b3921e938ac9178b882bd50aadb11231f044cf5f08df7ce.

This Go code snippet can also compute the hash of certificates given a .der file.

Common certificate hashes

• Let's Encrypt R3:
  • 444ebd67bb83f8807b3921e938ac9178b882bd50aadb11231f044cf5f08df7ce
• Let's Encrypt E1:
  • cc1060d39c8329b62b6fbc7d0d6df9309869b981e7e6392d5cd8fa408f4d80e6

Clients

doh-proxy can be used with dnscrypt-proxy as a client.

doh-proxy is used in production for the doh.crypto.sx public DNS resolver and many others.

An extensive list of public DoH servers can be found here: public encrypted DNS servers.