openrc/user-guide.md

OpenRC Users Guide

Purpose and description

OpenRC is an init system for Unixoid operating systems. It takes care of startup and shutdown of the whole system, including services.

It evolved out of the Gentoo "Baselayout" package which was a custom pure-shell startup solution. (This was both hard to maintain and debug, and not very performant)

Most of the core parts are written in C99 for performance and flexibility reasons, while everything else is posix sh. The License is 2-clause BSD

Current size is about 10k LoC C, and about 4k LoC shell.

OpenRC is known to work on Linux, many BSDs (FreeBSD, OpenBSD, DragonFlyBSD at least) and HURD.

Services are stateful (i.e. start; start will lead to "it's already started")

Startup

Usually PID1 (aka. init) calls the OpenRC binary (/sbin/openrc by default). (The default setup assumes sysvinit for this)

openrc scans the runlevels (default: /etc/runlevels) and builds a dependency graph, then starts the needed service scripts, either serialized (default) or in parallel.

When all the service scripts are started openrc terminates. There is no persistent daemon. (Integration with tools like monit, runit or s6 can be done)

Shutdown

On change to runlevel 0/6 or running reboot, halt etc., openrc stops all services that are started and runs the services in the shutdown runlevel.

Modifying Service Scripts

Any service can, at any time, be started/stopped/restarted by executing rc-service someservice start, rc-service someservice stop, etc. Another, less preferred method, is to run the service script directly, e.g. /etc/init.d/service start, /etc/init.d/service stop, etc.

OpenRC will take care of dependencies, e.g starting apache will start network first, and stopping network will stop apache first.

There is a special command zap that makes OpenRC 'forget' that a service is started; this is mostly useful to reset a crashed service to stopped state without invoking the (possibly broken) stop function of the service script.

Calling openrc without any arguments will try to reset all services so that the current runlevel is satisfied; if you manually started apache it will be stopped, and if squid died but is in the current runlevel it'll be restarted.

Runlevels

OpenRC has a concept of runlevels, similar to what sysvinit historically offered. A runlevel is basically a collection of services that needs to be started. Instead of random numbers they are named, and users can create their own if needed. This allows, for example, to have a default runlevel with "everything" enabled, and a "powersaving" runlevel where some services are disabled.

The rc-status helper will print all currently active runlevels and the state of services in them:

# rc-status
 * Caching service dependencies ... [ ok ]
Runlevel: default
 modules                     [  started  ]
 lvm                         [  started  ]

All runlevels are represented as folders in /etc/runlevels/ with symlinks to the actual service scripts.

Calling openrc with an argument (openrc default) will switch to that runlevel; this will start and stop services as needed.

Managing runlevels is usually done through the rc-update helper, but could of course be done by hand if desired. e.g. rc-update add nginx default - add nginx to the default runlevel Note: This will not auto-start nginx! You'd still have to trigger rc or run the service script by hand.

FIXME: Document stacked runlevels

The default startup uses the runlevels sysinit, boot, and default, in that order. Shutdown uses the shutdown runlevel.

The Magic of conf.d

Most service scripts need default values. It would be fragile to explicitly source some arbitrary files. By convention openrc-run will source the matching file in /etc/conf.d/ for any script in /etc/init.d/

This allows you to set random startup-related things easily. Example:

conf.d/foo:
START_OPTS="--extraparameter sausage"

init.d/foo:
start() {
	/usr/sbin/foo-daemon ${START_OPTS}
}

The big advantage of this split is that most of the time editing of the service script can be avoided.

Start-Stop-Daemon

OpenRC has its own modified version of s-s-d, which is historically related and mostly syntax-compatible to Debian's s-s-d, but has been rewritten from scratch.

It helps with starting daemons, backgrounding, creating PID files and many other convenience functions related to managing daemons.

/etc/rc.conf

This file manages the default configuration for OpenRC, and it has examples of per-service-script variables.

Among these are rc_parallel (for parallelized startup), rc_log (logs all boot messages to a file), and a few others.

ulimit and CGroups

Setting ulimit and nice values per service can be done through the rc_ulimit variable.

Under Linux, OpenRC can use cgroups for process management as well. Once the kernel is configured appropriately, the rc_cgroup_mode setting in /etc/rc.conf should be used to control whether cgroups version one, two, or both are used. The default is to use both if they are available.

By changing certain settings in the service's conf.d file limits can be enforced per service. These settings are documented in detail in the default /etc/rc.conf under LINUX CGROUPS RESOURCE MANAGEMENT.

Dealing with Orphaned Processes

It is possible to get into a state where there are orphaned processes running which were part of a service. For example, if you are monitoring a service with supervise-daemon and supervise-daemon dies for an unknown reason. The way to deal with this will be different for each system.

On Linux systems with cgroups enabled, the cgroup_cleanup command is added to all services. You can run it manually, when the service is stopped, by using:

# rc-service someservice cgroup_cleanup

The rc_cgroup_cleanup setting can be changed to yes to make this happen automatically when the service is stopped.

Caching

For performance reasons OpenRC keeps a cache of pre-parsed service metadata (e.g. depend). The default location for this is /${RC_SVCDIR}/cache.

The cache uses mtime to check for file staleness. Should any service script change it'll re-source the relevant files and update the cache

Convenience functions

OpenRC has wrappers for many common output tasks in libeinfo. This allows to print colour-coded status notices and other things. To make the output consistent the bundled service scripts all use ebegin/eend to print nice messages.