Running and Operating PowerDNS

PowerDNS is normally controlled via a SysV-style init.d script, often located in /etc/init.d or /etc/rc.d/init.d. For Linux distributions with systemd, a service file is provided (either in the package or in the contrib directory of the tarball).

Furthermore, PowerDNS can be run on the foreground for testing or in other init- systems that supervise processes.

Guardian

When the init-system of the Operating System does not properly supervises processes, like SysV init, it is recommended to run PowerDNS with the guardian option set to 'yes'.

When launched with guardian=yes, pdns_server wraps itself inside a 'guardian'. This guardian monitors the performance of the inner pdns_server instance which shows up in the process list of your OS as pdns_server-instance. It is also this guardian that pdns_control talks to. A STOP is interpreted by the guardian, which causes the guardian to sever the connection to the inner process and terminate it, after which it terminates itself. Requests that require data from the actual nameserver are passed to the inner process as well.

Logging to syslog on systemd-based operating systems

By default, logging to syslog is disabled in the the systemd unit file to prevent the service logging twice, as the systemd journal picks up the output from the process itself.

Removing the --disable-syslog option from the ExecStart line using systemctl edit --full pdns enables logging to syslog.

Controlling A Running PowerDNS Server

As a DNS server is critical infrastructure, downtimes should be avoided as much as possible. Even though PowerDNS (re)starts very fast, it offers a way to control it while running.

Control Socket

The controlsocket is the means to contact a running PowerDNS process. Over this socket, instructions can be sent using the pdns_control program. The control socket is called pdns.controlsocket and is created inside the socket-dir.

pdns_control

To communicate with PowerDNS Authoritative Server over the controlsocket, the pdns_control command is used. The syntax is simple: pdns_control command arguments. Currently this is most useful for telling backends to rediscover domains or to force the transmission of notifications. See Master Operation.

For all supported pdns_control commands and options, see the manpage and the output of pdns_control --help on your system.

The SysV init

This script supplied with the PowerDNS source accepts the following commands:

• monitor: Monitor is a special way to view the daemon. It executes PowerDNS in the foreground with a lot of logging turned on, which helps in determining startup problems. Besides running in the foreground, the raw PowerDNS control socket is made available. All external communication with the daemon is normally sent over this socket. While useful, the control console is not an officially supported feature. Commands which work are: QUIT, SHOW *, SHOW varname, RPING.
• start: Start PowerDNS in the background. Launches the daemon but makes no special effort to determine success, as making database connections may take a while. Use status to query success. You can safely run start many times, it will not start additional PowerDNS instances.
• restart: Restarts PowerDNS if it was running, starts it otherwise.
• status: Query PowerDNS for status. This can be used to figure out if a launch was successful. The status found is prefixed by the PID of the main PowerDNS process.
• stop: Requests that PowerDNS stop. Again, does not confirm success. Success can be ascertained with the status command.
• dump: Dumps a lot of statistics of a running PowerDNS daemon. It is also possible to single out specific variable by using the show command.
• show variable: Show a single statistic, as present in the output of the dump.
• mrtg: Dump statistics in mrtg format. See the performance monitoring documentation.

Note: Packages provided by Operating System vendors might support different or less commands.

Running in the foreground

One can run PowerDNS in the foreground by invoking the pdns_server executable. Without any options, it will load the pdns.conf and run. To make sure PowerDNS starts in the foreground, add the --daemon=no option.

All settings can be added on the commandline. e.g. to test a new database config, you could start PowerDNS like this:

pdns_server --no-config --daemon=no --local-port=5300 --launch=gmysql --gmysql-user=my_user --gmysql-password=mypassword

This starts PowerDNS without loading on-disk config, in the foreground, on all network interfaces on port 5300 and starting the gmysql backend.

Commandline Parameters

There are several important command-line switches for pdns_server. All settings can also be added as a commandline option (e.g. pdns_server --daemon=no) and will overwrite any options set in pdns.conf.

--help

Outputs all known parameters, including those of launched backends, see below.

To run on the command line, use the pdns_server binary. For example, to see options for the gpgsql backend, use the following:

      $ /usr/sbin/pdns_server --launch=gpgsql --help=gpgsql

--list-modules

Will list all available modules, both compiled in and in dynamically loadable modules.

--config

This will dump the config to standard out. Should you combine this with e.g. a launch statement (pdns_server --launch=gpgsql --config), all settings related to that backend (and their defaults) are included in the dump.

Virtual Hosting

It may be advantageous to run multiple separate PowerDNS installations on a single host, for example to make sure that different customers cannot affect each others zones. PowerDNS fully supports running multiple instances on one host.

To generate additional PowerDNS instances, create a pdns-NAME.conf in your configuration directory (usually /etc/powerdns), where NAME is the name of your virtual configuration.

Following one of the following instructions, PowerDNS will read its configuration from the pdns-NAME.conf instead of pdns.conf.

Starting virtual instances with Sysv init-scripts

Symlink the init.d script pdns to pdns-NAME, where NAME is the name of your virtual configuration. Note: NAME must not contain a '-' as this will confuse the script.

Internally, the init script calls the binary with the config-name option set to name, setting in motion the loading of separate configuration files.

When you launch a virtual instance of PowerDNS, the pid-file is saved inside socket-dir as pdns-name.pid.

Warning: Be aware however that the init.d force-stop will kill all PowerDNS instances!

Starting virtual instances with systemd

With systemd it is as simple as calling the correct service instance. Assuming your instance is called myinstance and pdns-myinstance.conf exists in the configuration directory, the following command will start the service:

systemctl start pdns@myinstance.service

Similarly you can enable it at boot:

systemctl enable pdns@myinstance.service

Internals

How PowerDNS translates DNS queries into backend queries

A DNS query is not a straightforward lookup. Many DNS queries need to check the backend for additional data, for example to determine if an unfound record should lead to an NXDOMAIN ('we know about this domain, but that record does not exist') or an unauthoritative response.

Simplified, without CNAME processing, wildcards, referrals and DNSSEC, the algorithm is like this:

When a query for a qname/qtype tuple comes in, PowerDNS queries backends to find the closest matching SOA, thus figuring out what backend owns this zone. When the right backend has been found, PowerDNS issues a qname/ANY query to the backend. If the response is empty, NXDOMAIN is concluded. If the response is not empty, any contents matching the original qtype are added to the list of records to return, and NOERROR is set.

Each of these records is now investigated to see if it needs 'additional processing'. This holds for example for MX records which may point to hosts for which the PowerDNS backends also contain data. This involves further lookups for A or AAAA records.

After all additional processing has been performed, PowerDNS sieves out all double records which may well have appeared. The resulting set of records is added to the answer packet, and sent out.

A zone transfer works by looking up the domain_id of the SOA record of the name and then listing all records of that domain_id. This is why all records in a domain need to have the same domain_id.

If no SOA was found, a REFUSED is returned.