DNSCrypt
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]
Contents
Installation
Install the dnscrypt-proxy package.
Configuration
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
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
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
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
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
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 dig command line tool from the bind-tools package 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"
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.