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The Design and Implementation
of the NetBSD rc.d system

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Luke Mewburn


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NetBSD Core Team

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<lukem@netbsd.org>


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Wasabi Systems

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<lukem@wasabisystems.com>

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Overview

	History
	Design Considerations
	Aftermath
	Technical details
	Future work
	Conclusion

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History (1/3)

	Examine 4.4BSD (the parent of NetBSD), Solaris 7 (a common SVR4), and NetBSD prior to 1.5

	4.4BSD
		At boot, kernel runs
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 init(8)
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		init runs /etc/rc, which calls /etc/netstart (to configure network) and /etc/rc.local for local services
		At shutdown where init gets SIGTERM, init sends SIGHUP to each process with a controlling terminal


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History (2/3)

	Solaris 7
		System can be in one of 8 run states, changeable at run time
		At boot, kernel runs init, which spawns processes listed in /etc/inittab that match the current run state
		Default run state defined in /etc/inittab
		/sbin/rcX controls the run state change for state X; executes all of /etc/rcX.d/K* with "stop", then /etc/rcX.d/S* with "start"

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History (3/3)

	NetBSD before 1.5
		Derived from 4.4BSD
		NetBSD 1.3 added /etc/rc.conf
			Contains variables to control each service
			Separates scripts from configuration information
		NetBSD 1.4 added /etc/rc.shutdown
			Run by shutdown (not halt or reboot)
			Similar implementation to /etc/rc

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Design Considerations: Problems

	No control over individual services
		"/etc/rc.d/foo start" is easier than the manual process
	No dependency ordering
	Little support for third party services, especially starting them elsewhere in the boot sequence

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Design Considerations: Requirements

	Manipulation of individual services
	Dependency ordering
		Independence from lexigraphical filename ordering
		Ability to insert third party scripts anywhere into the sequence
		Not bloating /bin and /sbin (i.e., '/')
		Use dynamic dependency ordering
			make or tsort not robust enough, required moving to /
			Chose dedicated program - rcorder

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Requirements (2/4)

	Support third party scripts
	Maintain /etc/rc.conf
		Pre-existing mechanism, and popular
		SYSV init.d mechanism of creating links to control service startup is difficult to manage
	Promote code reuse
	Support service shutdown
		Via manual manipulation (q.v.)
		At shutdown time (a la /etc/rc.shutdown)

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Requirements (3/4)

	Avoid mandatory run states and /etc/inittab
		Too contentious
		Lack of consistency between existing implementations
		Have equivalents for most SYSV run states in existing system

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Requirements (4/4)

	Other design issues
		Used /etc/rc.d instead of /etc/init.d
			System isn't exactly SYSV, different name highlights this
			A symlink from /etc/init.d to rc.d can provide compatibility
		Originally supported three schemes; /etc/rc, SYSV style /etc/rcN.d, and /etc/rc.d
			Multiple systems is a support and maintenance nightmare
			Lack of dynamic dependency ordering in the first two schemes
		This decision was contentious
			"Nochangeniks" are a very vocal minority and the most skilled to privately maintain their preferred mechanism 
			Those who want SYSV init.d can trivially add to rc.d

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Configuration improvements

	/etc/rc.conf mechanism was enhanced
		Default system settings were moved to /etc/defaults/rc.conf
			FreeBSD had prior art on this, and their experience was favourable
		The distributed rc.conf reads defaults/rc.conf, and local config settings are added to the end of rc.conf to override the defaults
		An optional per-service config override /etc/rc.conf.d/foo, which is read after rc.conf
			Useful for automated tools to control service startup
			Retains rc.conf semantics - no consensus was reached to remove these

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Aftermath: Human issues

	Initial CVS commits were made, but existing infrastructure wasn't changed.
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This was a mistake!
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	We didn't announce our integration intentions:
		Users perceived this as a stealth attack without consultation
		We felt there had been enough discussion already (5 years worth), and didn't want to delay any further

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Technical & 'religious' complaints

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	"the use of magic functions is bad"
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		Code reuse was a goal

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	"switching from /etc/rc isn't the BSD way"
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		A common 'religious' argument.  Robert Elz (a long-time BSD contributer) says:
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"[the BSD way is to] find something that looks
to be better (in the opinion of the small group
 deciding such things), implement it, and ship it"
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		In this case, the 'small group' was the NetBSD core team

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Technical & 'religious' complaints (cont.)

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	"why wasn't a System V init.d implemented?"
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		(polar position to the rc retainers)
		I covered this in the design goals

	Some vocal detractors gave the impression that a majority of users were against this work
		Far from the truth ... just another "holy war"
		Over one year since implementation and a lot of people support it in principle

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Technical details (1/3)

	/etc/rc
		Started by init
		Calls rcorder to order /etc/rc.d/*, skipping files tagged "nostart"
		Invokes each script in turn with an argument of "start"

	/etc/rc.shutdown
		Reverse of /etc/rc
		Calls rcorder to order /etc/rc.d/*, matching files tagged "shutdown"
		Invokes each script in turn with an argument of "stop"

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Technical details (2/3)

	/etc/rc.d/* scripts
		Individual scripts for each service
		Called by rc, rc.shutdown, and manually by users
		Must support: start, stop
		Should support: restart, status, rcvar, and other arguments as necessary
		If argument prefixed with "force", skip rc.conf check

	/etc/rc.subr
		Shell script containing helper functions
		Removes duplication in individual rc.d scripts

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Example: /etc/rc.d/dhclient


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    #!/bin/sh
    #
    # PROVIDE: dhclient
    # REQUIRE: network mountcritlocal
    #
    
    . /etc/rc.subr
    
    name="dhclient"
    rcvar=$name
    command="/sbin/${name}"
    pidfile="/var/run/${name}.pid"
    
    load_rc_config $name
    run_rc_command "$1"
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%%Technical details (3/4)
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%%	/etc/rc.d/* scripts (cont.)
%%		Provide placeholder scripts as sequence points for other scripts to require:
%%			NETWORK - basic network services, dhclient
%%			SERVERS - basic services (NETWORK, ppp, syslog) exist for services needed later (named)
%%			DAEMON - before general purpose daemons (dhcpd, lpd, ntpd)
%%			LOGIN - before user login services (inetd, sshd, xdm) and services which run user processes (cron, sendmail)
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%%
%%Technical details (3/4)
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%%		These are shell scripts, sourced by the rc.d scripts
%%			"exit" will terminate the boot sequence
%%			Not all commands will be available
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%%/etc/rc.subr functions
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%%	checkyesno var
%%		determine if rc.conf variable var is 'yes'or 'no'
%%	check_pidfile pidfile procname
%%		parse pidfile for pid, prints pid if it matches procname
%%	check_process procname
%%		prints pids matching procname
%%	load_rc_config command
%%		reads rc.conf and rc.conf.d/command
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%%/etc/rc.subr functions
%%
%%	run_rc_command arg
%%		arg is the user provided argument (e.g., "stop")
%%		The "kitchen sink" function, which is highly flexible, supporting many different service types
%%		Behaviour controlled by setting various shell variables
%%	example: rc.d/dhclient
%%		XXX pull in example

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Technical details (3/3)

	rcorder
		Parses supplied scripts for special comments which describe the dependencies
			PROVIDE - service provided
			REQUIRE - services required
			BEFORE - service to insert before
			KEYWORD - keywords to match to either explicitly keep (rcorder -k) or skip (rcorder -s)
		Allows 3rdparty scripts to be easily inserted into arbitrary points in the sequence

	/etc/defaults/rc.conf, /etc/rc.conf, /etc/rc.conf.d/*

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Current status

	Recent enhancements
		When "stop"ping a service, poll until process exits
		Optional rc.shutdown watchdog timer, which terminates rc.shutdown if the watchdog fires
		chkconfig(8) to control rc.conf.d/foo config files

	Future work
		Ability to run scripts from service A to service B
			Gives most of what people seem to want in run states
			Have sample implementation
			Need a clean way to store current "service name" (equivalent to run state #); considered using a sysctl(3)

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Conclusion

	Extremely contentious and difficult to implement. Because of this, consensus isn't necessarily achievable; a "core" technical group needs to intervene at times

	Vocal users do not necessarily represent a majority
		People usually don't comment unless they feel extremely strongly, especially in the negative
		There is often a "silent majority" in agreement

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Availability

	NetBSD 1.5
		ftp://ftp.netbsd.org/pub/NetBSD/NetBSD-1.5/

	NetBSD-current
		ftp://ftp.netbsd.org/pub/NetBSD/NetBSD-current/
		Anon CVS
		SUP

	Access via CVS web
		http://cvsweb.netbsd.org/bsdweb.cgi/basesrc/etc/