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DNSCrypt Plugins

Overview

dnscrypt-proxy can be extended with plugins.

A plugin can implement pre-filters and post-filters.

A DNS query received by the proxy will pass through all pre-filters before being encrypted, signed, and sent to the upstream DNS resolver.

Once a response has been received by the proxy, this response is going to be validated and decrypted before passing through all post-filters, and eventually the end result will eventually be delivered to the client.

Filters are given the packets in wire format. Every filter can inspect and alter these packets at will. A filter can also tell the proxy to totally ignore a query.

query -> pre-plugins -> encryption/authentication -> resolver

client <- post-plugins <- verification/decryption <- resolver

Usage

Technically, a plugin is just a native shared library. A good old .so file on Unix, a .dylib file on OSX and a .DLL file on Windows.

Support for plugins is disabled by default, and has to be explicitly enabled at compilation time:

./configure --enable-plugins

If the ltdl library is found on the system, it will be picked up. If not, a built-in copy will be used instead.

If you intend to distribute a binary package that should run on systems without the ltdl library (which is probably the case on Windows), add --with-included-ltdl:

./configure --enable-plugins --with-included-ltdl

Plugins can inspect and mangle packets in any way, but before reinventing the wheel, take a look at what the ldns library has to offer. ldns makes it really easy to parse and build any kind of DNS packet, can validate DNSSEC records, and is rock solid.

If ldns is available on the current system, additional example plugins will be compiled.

If the ./configure isn't given a different prefix, example plugins are installed in /usr/local/lib/dnscrypt-proxy.

dnscrypt-proxy can load any number of plugins using the --plugin switch, followed by the path to a plugin (library or libtool .la file):

dnscrypt-proxy \
    --plugin=/usr/local/lib/dnscrypt-proxy/libdcplugin_example.la \
    --plugin=/usr/local/lib/dnscrypt-proxy/libdcplugin_example2.la

A full path is actually not required for plugins sitting in the default plugins directory (/usr/local/lib/dnscrypt-proxy by default):

dnscrypt-proxy \
    --plugin=libdcplugin_example.la \
    --plugin=libdcplugin_example2.la

Filters will always be applied sequentially, in the given order.

On Unix systems, a file containing a dnscrypt-proxy plugin must be owned either by root, or by the user running the proxy.

You can relax this rule. This can be especially useful on OSX when using Homebrew, that encourages /usr/local to be owned by a non-root user. In order to relax this rule, use --enable-relaxed-plugins-permissions:

./configure --enable-plugins --enable-relaxed-plugins-permissions

When run as a Windows service, the list of plugins to load should be given as a multi-strings (REG_MULTI_SZ value).

Each plugin can optionally parse one or more arguments:

--plugin=...libdcplugin_example.la,--one,--two,--three=4
--plugin=...libdcplugin_example2.la,127.0.0.1

On Windows, example plugins relying on the ldns library require two extra DLLs:

  • libgcc_s_dw2-1.dll
  • libeay32.dll, from OpenSSL.

The plugin API

When compiled with support for plugins, dnscrypt-proxy installs development headers (in /usr/local/include/dnscrypt with the default prefix).

In addition, the dnscrypt-proxy source code ships with a few example plugins in the src/plugins directory to get you started.

The <dnscrypt/plugin.h> header file is the only one you need to include in a plugin. Feel free to take a look at its Doxygen documentation.

The bare minimum a plugin needs is to mention DCPLUGIN_MAIN(__FILE__) and to implement is a dcplugin_init() function.

This function is evaluated when the proxy starts, and can optionally parse a list of arguments:

#include <dnscrypt/plugin.h>

DCPLUGIN_MAIN(__FILE__);

int
dcplugin_init(DCPlugin * const dcplugin, int argc, char *argv[])
{
    return 0;
}

The DCPlugin type is an opaque structure that can store plugin-local data using the dcplugin_get_user_data() and dcplugin_set_user_data() macros.

A filter can implement a pre-filter, a post-filter, or both. The related functions are optional.

DCPluginSyncFilterResult
dcplugin_sync_pre_filter(DCPlugin *dcplugin, DCPluginDNSPacket *dcp_packet);

DCPluginSyncFilterResult
dcplugin_sync_post_filter(DCPlugin *dcplugin, DCPluginDNSPacket *dcp_packet);

These functions are given an opaque DCPluginDNSPacket type, storing the packet in wire format, and metadata, including the client IP address.

Avoid accessing these metadata directly, and use the macros defined in dnscrypt/plugins.h instead.

A filter can alter the content of the DNS packet, and change its length with dcplugin_set_wire_data_len(). However, the size of the packet should never be larger than the size returned by dcplugin_get_wire_data_max_len.

The return code of a filter can be either of:

  • DCP_SYNC_FILTER_RESULT_OK
  • DCP_SYNC_FILTER_RESULT_DIRECT (only in a pre-filter) to bypass the resolver and directly send the (possibly modified) packet to the client. Post-filters will be bypassed as well.
  • DCP_SYNC_FILTER_RESULT_KILL in order to drop the packet.
  • DCP_SYNC_FILTER_RESULT_ERROR to drop the packet and indicate that a non-fatal error occurred.

API documentation

For more info about the API, see the online dnscrypt API documentation.

The dnscrypt-proxy source code also ships with example plugins you may want to take a look at, in the src/plugins directory.