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[tomato.git] / release / src / router / dnscrypt / README.markdown

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[DNSCrypt](http://dnscrypt.org)
===============================

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

Disclaimer
----------

`dnscrypt-proxy` verifies that responses you get from a DNS provider have been
actually sent by that provider, and haven't been tampered with.

This is not a VPN. It doesn't mask your IP address, and if you are
using it with a public DNS service, be aware that it will (and has to)
decrypt your queries.

If you are using it for privacy, it might do the opposite of what you are
trying to achieve. It you are using it to prevent VPN "leaks", this
isn't the right tool either: the proper way to prevent VPN "leaks" is
to avoid sending data to yet another third party: use a VPN service that
operates its own DNS resolvers.

Description
-----------

`dnscrypt-proxy` provides local service which can be used directly as
your local resolver or as a DNS forwarder, 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](http://dnscurve.org/),
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.

Download and integrity check
----------------------------

dnscrypt-proxy can be downloaded here:
[dnscrypt-proxy download](http://download.dnscrypt.org/dnscrypt-proxy/)

Note: dnscrypt.org is now blocked by the Great Firewall of China. But
the site can be accessed at dnscrypt.bit instead (requires a resolver
with Namecoin support). Or if your current DNS resolver doesn't
support Namecoin yet, the source code can also be downloaded on
Github, in the "releases" section.

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

    $ openssl dgst -sha256 dnscrypt-proxy-1.5.0.tar.bz2

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

    $ drill -D TXT dnscrypt-proxy-1.5.0.tar.bz2.download.dnscrypt.org

or

    $ dig +dnssec TXT dnscrypt-proxy-1.5.0.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 (requires a rooted device), and
Windows (requires MingW).

Install [libsodium](https://github.com/jedisct1/libsodium).
On Linux, don't forget to run `ldconfig` if you installed it from
source.

A "minimal" build of libsodium (`--enable-minimal`) works equally well
as a full build with this proxy.

On Fedora, RHEL and CentOS, you may need to add `/usr/local/lib` to
the paths the dynamic linker is going to look at. Before issuing
`ldconfig`, type:

    # echo /usr/local/lib > /etc/ld.so.conf.d/usr_local_lib.conf

Now, 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.

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.

*Note:* gcc 3.4.6 (and probably other similar versions) is known to
produce broken code on Mips targets with the -Os optimization level.
Use a different level (-O and -O2 are fine) or upgrade the compiler.
Thanks to Adrian Kotelba for reporting this.

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:

- [DNSCrypt WinClient](https://github.com/Noxwizard/dnscrypt-winclient):
Easily enable/disable DNSCrypt on multiple adapters. Supports
different ports and protocols, IPv6, parental controls and the proxy
can act as a gateway service. Windows only, written in .NET.

- [DNSCrypt Windows Service Manager](http://simonclausen.dk/projects/dnscrypt-winservicemgr/):
Assists in setting up DNSCrypt as a service, configure it and change network adapter DNS
settings to use DNSCrypt. It includes the option to use TCP/UDP protocol, IPV4/IPV6
connectivity, choice of network adapter to configure, as well as configurations for currently
available DNSCrypt providers.

- [DNSCrypt OSXClient](https://github.com/alterstep/dnscrypt-osxclient):
Mac OSX application to control the DNSCrypt Proxy.

- [DNSCrypt Tools for Linux](http://opendesktop.org/content/show.php/DNScrypt+Tools?content=164488):
A set of tools for `dnscrypt-proxy`. Features a start and stop button as well as options to enable
or disable from startup. Developed for Porteus Linux.

DNSCrypt-enabled resolvers
--------------------------

To get started, you can use any of the
[public DNS resolvers supporting DNSCrypt](https://github.com/jedisct1/dnscrypt-proxy/blob/master/dnscrypt-resolvers.csv).

This file is constantly updated, and its [minisign](https://jedisct1.github.io/minisign/)
[signature](https://raw.githubusercontent.com/jedisct1/dnscrypt-proxy/master/dnscrypt-resolvers.csv.minisig)
can be verified with the following command:

    minisign -VP RWQf6LRCGA9i53mlYecO4IzT51TGPpvWucNSCh1CBM0QTaLn73Y7GFO3 -m dnscrypt-resolvers.csv

If you want to add DNSCrypt support to your own public or private
resolver, check out
[DNSCrypt-Wrapper](https://github.com/Cofyc/dnscrypt-wrapper), a
server-side dnscrypt proxy that works with any name resolver.

A [DNSCrypt server](https://github.com/jedisct1/dnscrypt-server-docker) Docker
image is also available to deploy a non-logging, DNSSEC and
DNSCrypt-capable resolver without having to manually compile or
configure anything.

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 --resolver-name=<resolver name>

Replace `<resolver name>` with the name of the resolver you want to
use (the first column in the list of public resolvers).

The proxy will accept incoming requests on 127.0.0.1, tag them with an
authentication code, forward them to the resolver, 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:

* `--daemonize` in order to run the server as a background process.
* `--local-address=<ip>[:port]` in order to locally bind a different IP
address than 127.0.0.1
* `--logfile=<file>` in order to write log data to a dedicated file. By
default, logs are sent to stdout if the server is running in foreground,
and to syslog if it is running in background.
* `--loglevel=<level>` if you need less verbosity in log files.
* `--max-active-requests=<count>` to set the maximum number of active
requests. The default value is 250.
* `--pidfile=<file>` in order to store the PID number to a file.
* `--user=<user name>` in order to chroot()/drop privileges.
* `--resolvers-list=<file>`: to specity the path to the CSV file containing
the list of available resolvers, and the parameters to use them.
* `--test` in order to check that the server-side proxy is properly
configured and that a valid certificate can be used. This is useful
for monitoring your own dnscrypt proxy. See the man page for more
information.

The
`--resolver-address=<ip>[:port]`,
`--provider-name=<certificate provider FQDN>`
and `--provider-key=<provider public key>` switches can be specified in
order to use a DNSCrypt-enabled recursive DNS service not listed in
the configuration file.

Running dnscrypt-proxy using systemd
------------------------------------

On a system using systemd, and when compiled with `--with-systemd`,
the proxy can take advantage of systemd's socket activation instead of
creating the sockets itself. The proxy will also notify systemd on successful
startup.

Two sockets need to be configured: a UDP socket (`ListenStream`) and a
TCP socket (`ListenDatagram`) sharing the same port.

The source distribution includes the `dnscrypt-proxy.socket` and
`dnscrypt-proxy.service` files that can be used as a starting point.

Installation as a service (Windows only)
----------------------------------------

The proxy can be installed as a Windows service.

See
[README-WINDOWS.markdown](https://github.com/jedisct1/dnscrypt-proxy/blob/master/README-WINDOWS.markdown)
for more information on DNSCrypt on Windows.

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' ...

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.

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 because multiple queries over a single
TCP connections aren't supported yet, and this workaround should
never be used except when bypassing a filter is actually required.

Public-key client authentication
--------------------------------

By default, dnscrypt-proxy generates non-deterministic client keys
every time it starts, or for every query (when the ephemeral keys
feature is turned on).

However, commercial DNS services may want to use DNSCrypt to
authenticate the sender of a query using public-key cryptography, i.e.
know what customer sent a query without altering the DNS query itself,
and without using shared secrets.

Resolvers that should be accessible from any IP address, but that are
supposed to be used only by specific users, can also take advantage of
DNSCrypt to only respond to queries sent using a given list of public keys.

In order to do so, dnscrypt-proxy 1.6.0 introduced the `--client-key`
(or `-K`) switch. This loads a secret client key from a file instead
of generating random keys:

    # dnscrypt-proxy --client-key=/private/client-secret.key

This file has to remain private, and its content doesn't have to be
known by the DNS service provider.

Versions 1 and 2 of the DNSCrypt protocol use Curve25519 keys, and the
format of this file for Curve25519 keys is a hexadecimal string, with
optional :, [space] and - delimiters, decoding to 34 bytes:

    01 01 || 32-byte Curve25519 secret key

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.

Plugins
-------

`dnscrypt-proxy` can be extended with plugins. A plugin acts as a
filter that can locally inspect and modify queries and responses.

The plugin API is documented in the `README-PLUGINS.markdown` file.

Any number of plugins can be combined (chained) by repeating the
`--plugin` command-line switch.

The default distribution ships with some example plugins:

* `libdcplugin_example_ldns_aaaa_blocking`: Directly return an empty
response to AAAA queries

Example usage:

    # dnscrypt-proxy ... \
    --plugin libdcplugin_example_ldns_aaaa_blocking.la

If IPv6 connectivity is not available on your network, this plugin
avoids waiting for responses about IPv6 addresses from upstream
resolvers. This can improve your web browsing experience.

* `libdcplugin_example_ldns_blocking`: Block specific domains and IP
addresses.

This plugin returns a REFUSED response if the query name is in a
list of blacklisted names, or if at least one of the returned
IP addresses happens to be in a list of blacklisted IPs.

Recognized switches are:

    --domains=<file>
    --ips=<file>

A file should list one entry per line.

IPv4 and IPv6 addresses are supported.
For names, leading and trailing wildcards (`*`) are also supported
(e.g. `*xxx*`, `*.example.com`, `ads.*`)

    # dnscrypt-proxy ... \
    --plugin libdcplugin_example,--ips=/etc/blk-ips,--domains=/etc/blk-names

* `libdcplugin_example-logging`: Log client queries

This plugin logs the client queries to the standard output (default)
or to a file.

    # dnscrypt-proxy ... \
    --plugin libdcplugin_example_logging,/var/log/dns.log

* Extra plugins

Additional plugins can be found on Github:
[Masquerade plugin](https://github.com/gchehab/dnscrypt-plugin-masquerade),
[GeoIP plugin](https://github.com/jedisct1/dnscrypt-plugin-geoip-block).