# DNSCrypt

DNSCrypt is a piece of software that encrypts DNS traffic between the user and a DNS resolver, preventing spying, spoofing or man-in-the-middle attacks.

## Installation

Install dnscrypt-proxy from the official repositories.

## Configuration

Tip: To automatically configure DNSCrypt and choose a resolver, use dnscrypt-autoinstall from the AUR

By default dnscrypt-proxy is pre-configured in `/etc/conf.d/dnscrypt-proxy` (read by `dnscrypt-proxy.service`) to accept incoming requests on `127.0.0.1` to an OpenDNS resolver. See the list of resolvers for alternatives.

With this setup, it will be necessary to alter your resolv.conf file and replace your current set of resolver addresses with localhost:

    nameserver 127.0.0.1

You might need to prevent other programs from overwriting it, see resolv.conf#Preserve DNS settings for details.

## Starting

Available as a systemd service: `dnscrypt-proxy.service`

## Tips and tricks

#### Using DNSCrypt in combination with Unbound

It is recommended to run DNSCrypt as a forwarder for a local DNS cache, otherwise every single query will make a round-trip to the upstream resolver.

Install unbound from the official repositories and add the following lines to the end of the `server` section in `/etc/unbound/unbound.conf`:

    do-not-query-localhost: no
    forward-zone:
      name: "."
      forward-addr: 127.0.0.1@40

Note: Port 40 is given as an example as unbound by default listens to 53, these must be different.

Start the systemd service `unbound.service`. Then configure DNScrypt to match Unbound's new `forward-zone` IP and port in `/etc/conf.d/dnscrypt-proxy`:

    DNSCRYPT_LOCALIP=127.0.0.1
    DNSCRYPT_LOCALPORT=40

Note: dnscrypt-proxy needs to start before unbound, so include `unbound.service` on a `Before=` line in the `[Unit]` section of `dnscrypt.service`.

Restart `dnscrypt-proxy.service` to reread the changes.

#### Using DNSCrypt in combination with dnsmasq

You can also use DNSCrypt with dnsmasq as a caching proxy.

Edit `/etc/dnsmasq.conf` so that it contains at least the following options:

    domain-needed
    bogus-priv
    strict-order
    no-resolv
    server=127.0.0.2
    resolv-file=/etc/resolv.dns.conf
    listen-address=127.0.0.1
    no-dhcp-interface=lo
    bind-interfaces
    proxy-dnssec

Edit `/etc/resolv.dns.conf` so that it contains just:

    nameserver 127.0.0.2

This is where dnsmasq will be sending your dns queries (dnscrypt)

Edit `/etc/resolv.conf.tail` so that it contains any secondary dns servers in case your dnscrypt isn't working for some reason. In this example you have an Opendns and a Comodo dns server as backup.

    nameserver 208.67.222.222
    nameserver 8.26.56.26

Edit `/etc/conf.d/dnscrypt-proxy` and change DNSCRYPT_LOCALIP so that it listens on 127.0.0.2 (where dnsmasq will be querying).

    DNSCRYPT_LOCALIP=127.0.0.2

If you use NetworkManager you might have to append

    dns=dnsmasq

in `/etc/NetworkManager/NetworkManager.conf` if it's not already there.

Restart `dnscrypt-proxy.service` and `dnsmasq.service` to apply the changes.

##### Test DNSCrypt and dnsmasq

To check that DNS queries are effectively sent encrypted, you can use tcpdump, and then surf on your computer. Run:

    sudo tcpdump -i eth0 dst host 208.67.222.222 or dst host 208.67.220.220 or src host 208.67.222.222 or src host 208.67.220.220 -n

You should only see remote UDP port 443 being used, not 53. For example:

    23:00:41.806710 IP 192.168.1.3.58619 > 208.67.220.220.443: UDP, length 260
    23:00:41.843235 IP 208.67.220.220.443 > 192.168.1.3.58619: UDP, length 496

#### Enable EDNS0

Extension Mechanisms for DNS that, among other things, allows a client to specify how large a reply over UDP can be.

Add the following line to your `/etc/resolv.conf`:

    options edns0

You may also wish to add the following argument to dnscrypt-proxy:

    --edns-payload-size=<bytes>

The default size being 1252 bytes, with values up to 4096 bytes being purportedly safe. A value below or equal to 512 bytes will disable this mechanism, unless a client sends a packet with an OPT section providing a payload size.

##### Test EDNS0

Make use of the DNS Reply Size Test Server, use the dig command line tool available with dnsutils from the official repositories to issue a TXT query for the name rs.dns-oarc.net:

    $ dig +short rs.dns-oarc.net txt

With EDNS0 supported, the output should look similar to this:

    rst.x3827.rs.dns-oarc.net.
    rst.x4049.x3827.rs.dns-oarc.net.
    rst.x4055.x4049.x3827.rs.dns-oarc.net.
    "2a00:d880:3:1::a6c1:2e89 DNS reply size limit is at least 4055 bytes"
    "2a00:d880:3:1::a6c1:2e89 sent EDNS buffer size 4096"

## See also

- resolv.conf
- Unbound - a validating, recursive, and caching DNS resolver.

Categories:

- Domain Name System
- Security