180 lines
6.4 KiB
Markdown
180 lines
6.4 KiB
Markdown
OpenRC Users Guide
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==================
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# Purpose and description
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OpenRC is an init system for Unixoid operating systems. It takes care of
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startup and shutdown of the whole system, including services.
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It evolved out of the Gentoo "Baselayout" package which was a custom pure-shell
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startup solution. (This was both hard to maintain and debug, and not very
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performant)
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Most of the core parts are written in C99 for performance and flexibility
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reasons, while everything else is posix sh.
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The License is 2-clause BSD
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Current size is about 10k LoC C, and about 4k LoC shell.
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OpenRC is known to work on Linux, many BSDs (FreeBSD, OpenBSD, DragonFlyBSD at
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least) and HURD.
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Services are stateful (i.e. `start`; `start` will lead to "it's already started")
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# Startup
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Usually PID1 (aka. `init`) calls the OpenRC binary (`/sbin/openrc` by default).
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(The default setup assumes sysvinit for this)
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openrc scans the runlevels (default: `/etc/runlevels`) and builds a dependency
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graph, then starts the needed service scripts, either serialized (default) or in
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parallel.
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When all the service scripts are started openrc terminates. There is no
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persistent daemon. (Integration with tools like monit, runit or s6 can be done)
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# Shutdown
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On change to runlevel 0/6 or running `reboot`, `halt` etc., openrc stops all
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services that are started and runs the services in the `shutdown` runlevel.
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# Modifying Service Scripts
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Any service can, at any time, be started/stopped/restarted by executing
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`rc-service someservice start`, `rc-service someservice stop`, etc.
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Another, less preferred method, is to run the service script directly,
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e.g. `/etc/init.d/service start`, `/etc/init.d/service stop`, etc.
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OpenRC will take care of dependencies, e.g starting apache will start network
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first, and stopping network will stop apache first.
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There is a special command `zap` that makes OpenRC 'forget' that a service is
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started; this is mostly useful to reset a crashed service to stopped state
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without invoking the (possibly broken) stop function of the service script.
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Calling `openrc` without any arguments will try to reset all services so
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that the current runlevel is satisfied; if you manually started apache it will be
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stopped, and if squid died but is in the current runlevel it'll be restarted.
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# Runlevels
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OpenRC has a concept of runlevels, similar to what sysvinit historically
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offered. A runlevel is basically a collection of services that needs to be
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started. Instead of random numbers they are named, and users can create their
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own if needed. This allows, for example, to have a default runlevel with
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"everything" enabled, and a "powersaving" runlevel where some services are
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disabled.
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The `rc-status` helper will print all currently active runlevels and the state
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of services in them:
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```
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# rc-status
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* Caching service dependencies ... [ ok ]
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Runlevel: default
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modules [ started ]
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lvm [ started ]
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```
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All runlevels are represented as folders in `/etc/runlevels/` with symlinks to
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the actual service scripts.
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Calling openrc with an argument (`openrc default`) will switch to that
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runlevel; this will start and stop services as needed.
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Managing runlevels is usually done through the `rc-update` helper, but could of
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course be done by hand if desired.
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e.g. `rc-update add nginx default` - add nginx to the default runlevel
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Note: This will not auto-start nginx! You'd still have to trigger `rc` or run
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the service script by hand.
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FIXME: Document stacked runlevels
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The default startup uses the runlevels `boot`, `sysinit` and `default`, in that
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order. Shutdown uses the `shutdown` runlevel.
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# The Magic of `conf.d`
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Most service scripts need default values. It would be fragile to
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explicitly source some arbitrary files. By convention `openrc-run` will source
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the matching file in `/etc/conf.d/` for any script in `/etc/init.d/`
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This allows you to set random startup-related things easily. Example:
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```
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conf.d/foo:
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START_OPTS="--extraparameter sausage"
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init.d/foo:
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start() {
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/usr/sbin/foo-daemon ${STARTOPTS}
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}
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```
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The big advantage of this split is that most of the time editing of the service
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script can be avoided.
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# Start-Stop-Daemon
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OpenRC has its own modified version of s-s-d, which is historically related and
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mostly syntax-compatible to Debian's s-s-d, but has been rewritten from scratch.
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It helps with starting daemons, backgrounding, creating PID files and many
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other convenience functions related to managing daemons.
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# `/etc/rc.conf`
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This file manages the default configuration for OpenRC, and it has examples of
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per-service-script variables.
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Among these are `rc_parallel` (for parallelized startup), `rc_log` (logs all boot
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messages to a file), and a few others.
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# ulimit and CGroups
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Setting `ulimit` and `nice` values per service can be done through the
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`rc_ulimit` variable.
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Under Linux, OpenRC can use cgroups for process management as well. Once
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the kernel is configured appropriately, the `rc_cgroup_mode` setting in
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/etc/rc.conf should be used to control whether cgroups version one,,
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two, or both are used. The default is to use both if they are available.
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By changing certain settings in the service's `conf.d` file limits can be
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enforced per service. These settings are documented in detail in the
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default /etc/rc.conf under `LINUX CGROUPS RESOURCE MANAGEMENT`.
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# Dealing with Orphaned Processes
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It is possible to get into a state where there are orphaned processes
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running which were part of a service. For example, if you are monitoring
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a service with supervise-daemon and supervise-daemon dies for an unknown
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reason. The way to deal with this will be different for each system.
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On Linux systems with cgroups enabled, the cgroup_cleanup command is
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added to all services. You can run it manually, when the service is
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stopped, by using:
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```
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# rc-service someservice cgroup_cleanup
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```
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The `rc_cgroup_cleanup` setting can be changed to yes to make this
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happen automatically when the service is stopped.
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# Caching
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For performance reasons OpenRC keeps a cache of pre-parsed service metadata
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(e.g. `depend`). The default location for this is `/${RC_SVCDIR}/cache`.
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The cache uses `mtime` to check for file staleness. Should any service script
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change it'll re-source the relevant files and update the cache
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# Convenience functions
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OpenRC has wrappers for many common output tasks in libeinfo.
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This allows to print colour-coded status notices and other things.
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To make the output consistent the bundled service scripts all use ebegin/eend to
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print nice messages.
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