DNSCrypt

From ArchWiki
Jump to: navigation, search

DNSCrypt encrypts and authenticates DNS traffic between user and DNS resolver. While IP traffic itself is unchanged, it prevents local spoofing of DNS queries, ensuring DNS responses are sent by the server of choice. [1]

Installation

Install the dnscrypt-proxy package.

Tip: dnscrypt-proxy-guiAUR provides a GUI written in Qt to set the DNS server used by DNSCrypt.

Configuration

Tip: An example configuration file, /etc/dnscrypt-proxy.conf.example is provided, but note that systemd overrides the LocalAddress option with a socket file.

To configure dnscrypt-proxy, perform the following steps:

Select resolver

Select a resolver from /usr/share/dnscrypt-proxy/dnscrypt-resolvers.csv and edit /etc/dnscrypt-proxy.conf, using a short name from the csv file's first column, Name. For example, to select dnscrypt.eu-nl as the resolver:

ResolverName dnscrypt.eu-nl
Tip:

Modify resolv.conf

After selecting a dnscrypt resolver, modify the resolv.conf file and replace the current set of resolver addresses with address for localhost:

nameserver 127.0.0.1

Other programs may overwrite this setting; see resolv.conf#Preserve DNS settings for details.

Start systemd service

Finally, start and enable the dnscrypt-proxy.service.

Tips and tricks

Local DNS cache configuration

Note: dnscrypt can cache entries without relying on another program. Ensure that LocalCache on is in your dnscrypt configuration file to enable this feature.

It is recommended to run DNSCrypt as a forwarder for a local DNS cache if not using dnscrypt's cache feature; otherwise, every single query will make a round-trip to the upstream resolver. Any local DNS caching program should work. In addition to setting up dnscrypt-proxy, you must setup your local DNS cache program.

Change port

Note: Changing the IP address or port in /etc/dnscrypt-proxy.conf does not work when using the provided systemd unit and must be changed in the provided systemd socket as follows.

In order to forward to a local DNS cache, dnscrypt-proxy should listen on a port different from the default 53, since the DNS cache itself needs to listen on 53 and query dnscrypt-proxy on a different port. Port number 5353 is used as an example in this section. In this example, the port number is larger than 1024 so dnscrypt-proxy is not required to be run by root. Edit dnscrypt-proxy.socket with the following contents:

[Socket]
ListenStream=
ListenDatagram=
ListenStream=127.0.0.1:5353
ListenDatagram=127.0.0.1:5353
Note: UDP Port 5353 is used by Avahi (if installed and running) and can cause warnings in the journal and Avahi's mDNS unreliable.

Example local DNS cache configurations

The following configurations should work with dnscrypt-proxy and assume that it is listening on port 5353.

Unbound

Configure Unbound to your liking (in particular, see Unbound#Local DNS server) 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@5353
Tip: If you are setting up a server, add interface: 0.0.0.0@53 and access-control: your-network/subnet-mask allow inside the server: section so that the other computers can connect to the server. A client must be configured with nameserver address-of-your-server in /etc/resolv.conf.

Restart unbound.service to apply the changes.

dnsmasq

Configure dnsmasq as a local DNS cache. The basic configuration to work with DNSCrypt:

/etc/dnsmasq.conf
no-resolv
server=127.0.0.1#5353
listen-address=127.0.0.1

If you configured DNSCrypt to use a resolver with enabled DNSSEC validation, make sure to enable it also in dnsmasq:

/etc/dnsmasq.conf
proxy-dnssec

Restart dnsmasq.service to apply the changes.

pdnsd

Install pdnsd. A basic configuration to work with DNSCrypt is:

/etc/pdnsd.conf
global {
    perm_cache = 1024;
    cache_dir = "/var/cache/pdnsd";
    run_as = "pdnsd";
    server_ip = 127.0.0.1;
    status_ctl = on;
    query_method = udp_tcp;
    min_ttl = 15m;       # Retain cached entries at least 15 minutes.
    max_ttl = 1w;        # One week.
    timeout = 10;        # Global timeout option (10 seconds).
    neg_domain_pol = on;
    udpbufsize = 1024;   # Upper limit on the size of UDP messages.
}

server {
    label = "dnscrypt-proxy";
    ip = 127.0.0.1;
    port = 5353;
    timeout = 4;
    proxy_only = on;
}

source {
    owner = localhost;
    file = "/etc/hosts";
}

Restart pdnsd.service to apply the changes.

Sandboxing

Edit dnscrypt-proxy.service to include the following lines:

[Service]
CapabilityBoundingSet=CAP_IPC_LOCK CAP_SETGID CAP_SETUID
ProtectSystem=strict
ProtectHome=true
ProtectKernelTunables=true
ProtectKernelModules=true
ProtectControlGroups=true
PrivateTmp=true
PrivateDevices=true
MemoryDenyWriteExecute=true
NoNewPrivileges=true
RestrictRealtime=true
RestrictAddressFamilies=AF_INET
SystemCallArchitectures=native
SystemCallFilter=~@clock @cpu-emulation @debug @keyring @ipc @module @mount @obsolete @raw-io

See systemd.exec(5) and Systemd#Sandboxing application environments for more information. Additionally see upstream comments.

This can be combined with the additions in #dnscrypt runs with root privileges.

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 append the following to /etc/dnscrypt-proxy.conf:

EDNSPayloadSize <bytes>

Where <bytes> is a number, the default size being 1252, 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 drill command line tool to issue a TXT query for the name rs.dns-oarc.net:

$ drill rs.dns-oarc.net TXT

With EDNS0 supported, the "answer section" of 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"

Redundant DNSCrypt providers

To use several different dnscrypt providers, you may simply copy the original dnscrypt-proxy.service and dnscrypt-proxy.socket. Then in your new copy of the service change the command line parameters, either pointing to a new configuration file or naming a different resolver directly. From there change the port in the new copy of the socket. Lastly, update your local DNS cache program to point to new service's port. For example, with unbound the configuration file would look like if using ports 5353 for the original socket and 5354 for the new socket.

/etc/unbound/unbound.conf
 do-not-query-localhost: no
 forward-zone:
   name: "."
   forward-addr: 127.0.0.1@5353
   forward-addr: 127.0.0.1@5354

Create instanced systemd service

An alternative option to copying the systemd service is to used an instanced service.

Create systemd file

First, create /etc/systemd/system/dnscrypt-proxy@.service containing:

[Unit]
Description=DNSCrypt client proxy
Documentation=man:dnscrypt-proxy(8)
Requires=dnscrypt-proxy@%i.socket

[Service]
Type=notify
NonBlocking=true
ExecStart=/usr/bin/dnscrypt-proxy \
    --resolver-name=%i
Restart=always

This specifies an instanced systemd service that starts a dnscrypt-proxy using the service name specified after the @ symbol of a corresponding .socket file.

Add dnscrypt-sockets

To create multiple dnscrypt-proxy sockets, copy /usr/lib/systemd/system/dnscrypt-proxy.socket to a new file, /etc/systemd/system/dnscrypt-proxy@short-name.here.socket, replacing the socket instance name with one of the short names listed in dnscrypt-resolvers.csv and change the port. Use a different port for each instance (5353, 5354, and so forth).

Apply new systemd configuration

Now we need to reload the systemd configuration.

# systemctl daemon-reload

Since we are replacing the default service with a different name, we need to explicitly stop and disable dnscrypt-proxy.service and dnscrypt-proxy.socket.

Now start/enable the new service(s), e.g., dnscrypt-proxy@dnscrypt.eu-nl, etc.

Finally restart unbound.service.

Troubleshooting

dnscrypt runs with root privileges

See FS#49881 for more information. To work around this, create an unprivileged user manually.

Create the user as follows:

# useradd -r -d /var/dnscrypt -m -s /sbin/nologin dnscrypt

Two possible solutions Edit /etc/dnscrypt-proxy.conf, appending the new user:

User dnscrypt

Alternatively, you should use User= in in the dnscrypt-proxy.service systemd unit:

[Service]
User=dnscrypt
CapabilityBoundingSet=CAP_NET_BIND_SERVICE

This second option is useful when using a caching server like unbound and is preferred, since the unit is not exec'ed as root in the first place. If you changed the port to an unprivileged one (e.g. 5353), then CapabilityBoundingSet=CAP_NET_BIND_SERVICE is not needed. This method can be combined with #Sandboxing.