DNSCrypt

A tool for securing communications between a client and a DNS resolver


Project maintained by jedisct1 Hosted on GitHub Pages — Theme by mattgraham

DNSCrypt

A tool for securing communications between a client and a DNS resolver.

Description

dnscrypt-proxy provides local service which can be used directly as your local resolver or as a DNS forwarder, encrypting and authenticating requests using the DNSCrypt protocol and passing them to an upstream server.

The DNSCrypt protocol uses high-speed high-security elliptic-curve cryptography and is very similar to DNSCurve, but focuses on securing communications between a client and its first-level resolver.

While not providing end-to-end security, it protects the local network, which is often the weakest point of the chain, against man-in-the-middle attacks. It also provides some confidentiality to DNS queries.

List of free, DNSCrypt-enabled resolvers

To get started, you can use any of the public DNS resolvers supporting DNSCrypt .

If you want to add DNSCrypt support to your own public or private resolver, check out DNSCrypt-Wrapper, a server-side dnscrypt proxy that works with any name resolver.

GUIs for dnscrypt-proxy

If you need a simple graphical user interface in order to start/stop the proxy and change your DNS settings, check out the following project:

Download and integrity check

DNSCrypt can be downloaded here: dnscrypt download.

After having downloaded a file, compute its SHA256 digest. For example:


$ openssl dgst -sha256 dnscrypt-proxy-1.3.3.tar.bz2
  

Verify this digest against the expected one, that can be retrieved using a simple DNS query:


$ drill -D TXT dnscrypt-proxy-1.3.3.tar.bz2.download.dnscrypt.org
  
or:

$ dig +short +dnssec TXT dnscrypt-proxy-1.3.3.tar.bz2.download.dnscrypt.org
  

If the content of the TXT record doesn't match the SHA256 digest you computed, please file a bug report on Github as soon as possible and don't go any further.

Installation

The daemon is known to work on recent versions of OSX, OpenBSD, Bitrig, NetBSD, Dragonfly BSD, FreeBSD, Linux, iOS (requires a jailbroken device), Android (rooted), Solaris (SmartOS) and Windows (requires MingW).

Install libsodium. There might be a pre-built binary package for your operating system already. On Linux, don't forget to run ldconfig if you installed it from source.

Download the latest dnscrypt-proxy version and extract it:

$ bunzip2 -cd dnscrypt-proxy-*.tar.bz2 | tar xvf -
$ cd dnscrypt-proxy-*

Compile and install it using the standard procedure:

$ ./configure && make -j2
# make install

Replace -j2 with whatever number of CPU cores you want to use for the compilation process.

Running make -j2 test in the src/libnacl directory is also highly recommended.

The proxy will be installed as /usr/local/sbin/dnscrypt-proxy by default.

Command-line switches are documented in the dnscrypt-proxy(8) man page.

Usage

Having a dedicated system user, with no privileges and with an empty home directory, is highly recommended. For extra security, DNSCrypt will chroot() to this user's home directory and drop root privileges for this user's uid as soon as possible.

The easiest way to start the daemon is:

# dnscrypt-proxy --daemonize \
      --provider-name=2.dnscrypt-cert.opendns.com \
      --provider-key=B735:1140:206F:225D:3E2B:D822:D7FD:691E:A1C3:3CC8:D666:8D0C:BE04:BFAB:CA43:FB79 \
      --resolver-address=208.67.220.220:443

The proxy will accept incoming requests on 127.0.0.1, tag them with an authentication code, forward them to OpenDNS resolvers, and validate each answer before passing it to the client.

Given such a setup, in order to actually start using DNSCrypt, you need to update your /etc/resolv.conf file and replace your current set of resolvers with:

nameserver 127.0.0.1

Other common command-line switches include:

The --resolver-address=<ip>[:port], --provider-name=<certificate provider FQDN> and --provider-key=<provider public key> switches should be specified in order to use a specific DNSCrypt-enabled recursive DNS service..

Installation as a service (Windows only)

The proxy can be installed as a Windows service.

Copy the dnscrypt-proxy.exe file to any location, as well as the .dll files. They all should be in the same location. Then open a terminal and type (eventually with the full path to dnscrypt-proxy.exe):

dnscrypt-proxy.exe --install

It will install a new service named dnscrypt-proxy.

After being stopped, the service can be removed with:

dnscrypt-proxy.exe --uninstall

See the Using DNSCrypt on Windows file for more info on this topic.

Using DNSCrypt in combination with a DNS cache

The DNSCrypt proxy is not a DNS cache. This means that incoming queries will not be cached and every single query will require a round-trip to the upstream resolver.

For optimal performance, the recommended way of running DNSCrypt is to run it as a forwarder for a local DNS cache, like unbound or powerdns-recursor.

Both can safely run on the same machine as long as they are listening to different IP addresses (preferred) or different ports.

If your DNS cache is unbound, all you need is to edit the unbound.conf file and add the following lines at the end of the server section:

do-not-query-localhost: no

forward-zone:
  name: "."
  forward-addr: 127.0.0.1@40

The first line is not required if you are using different IP addresses instead of different ports.

Then start dnscrypt-proxy, telling it to use a specific port (40, in this example):

# dnscrypt-proxy --local-address=127.0.0.1:40 --daemonize

IPv6 support

IPv6 is fully supported. IPv6 addresses with a port number should be specified as [ip]:port

# dnscrypt-proxy --local-address='[::1]:40' --daemonize

Queries using nonstandard ports / over TCP

Some routers and firewalls can block outgoing DNS queries or transparently redirect them to their own resolver. This especially happens on public Wifi hotspots, such as coffee shops.

As a workaround, the port number can be changed using the --resolver-port=<port> option. For example, OpenDNS servers reply to queries sent to ports 53, 443 and 5353.

By default, dnscrypt-proxy sends outgoing queries to UDP port 443.

In addition, the DNSCrypt proxy can force outgoing queries to be sent over TCP. For example, TCP port 443, which is commonly used for communication over HTTPS, may not be filtered.

The --tcp-only command-line switch forces this behavior. When an incoming query is received, the daemon immediately replies with a "response truncated" message, forcing the client to retry over TCP. The daemon then authenticates the query and forwards it over TCP to the resolver.

--tcp-only is slower than UDP, and this workaround should never be used except when bypassing a filter is actually required. Moreover, multiple queries over a single TCP connections aren't supported yet.

EDNS payload size

DNS packets sent over UDP have been historically limited to 512 bytes, which is usually fine for queries, but sometimes a bit short for replies.

Most modern authoritative servers, resolvers and stub resolvers support the Extension Mechanism for DNS (EDNS) that, among other things, allows a client to specify how large a reply over UDP can be.

Unfortunately, this feature is disabled by default on a lot of operating systems. It has to be explicitly enabled, for example by adding options edns0 to the /etc/resolv.conf file on most Unix-like operating systems.

dnscrypt-proxy can transparently rewrite outgoing packets before authenticating them, in order to add the EDNS0 mechanism. By default, a conservative payload size of 1252 bytes is advertised.

This size can be made larger by starting the proxy with the --edns-payload-size=<bytes> command-line switch. Values up to 4096 are usually safe.

A value below or equal to 512 will disable this mechanism, unless a client sends a packet with an OPT section providing a payload size.

The hostip utility

The DNSCrypt proxy ships with a simple tool named hostip that resolves a name to IPv4 or IPv6 addresses.

This tool can be useful for starting some services before dnscrypt-proxy.

Queries made by hostip are not authenticated.

Additional documentation