A journey through the years of UNIX and Linux service management
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What happened to init scripts and why systemd is a great idea.

What happened to init scripts and why systemd is a great idea.

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A journey through the years of UNIX and Linux service management Presentation Transcript

  • 1. From /etc/init to systemd A journey through the years of UNIX and Linux service management Lubomir Rintel <lkundrak@v3.sk> BTC: 15wvWxN5QMpreKR37pYb7VBu8xLu4TiNR2
  • 2. Sixth Edition UNIX (1976) ● /etc/init (219 lines) ● ● gettys from /etc/ttys ● ● Single user shell utmp & wtmp /etc/rc (2 lines) rm -f /etc/mtab /etc/update
  • 3. UNIX System III (1980) ● /etc/init ● (486 lines) /etc/inittab "states" controlling gettys on terminals ● /etc/rc (67 lines) ● State aware, single user mode ● acct ● errdemon ● cron ● "edit to add umounts"
  • 4. 2.9 BSD UNIX (1983) ● /sbin/init (593 lines) ● ● ● Single user shell gettys from /etc/ttys /etc/rc (53 lines) ● fsck, quotacheck ● mount -a, from /etc/fstab ● savecore, ex/vi recovery ● clear /tmp, locks in /usr/spool ● update, cron, acct ● hostname
  • 5. UNIX System V Release 4 (1983) ● /etc/init ● ● /etc/inittab with runlevels, respawns /etc/rc? ● Modular init system, /etc/rc.d ● Per-daemon init scripts (8-56 lines, avg. 26) – – ● ● start & stop arguments pid from ps, stop = kill -TERM Enablement/disablement by linking into level dir Ordered by numbers
  • 6. #ident "@(#)/etc/init.d/cron.sl 1.1 4.0 10/15/90 8479 AT&T-SF" # cron control pid=`/usr/bin/ps -e | /usr/bin/grep cron | /usr/bin/sed -e 's/^ *//' -e 's/ .*//'` case $1 in 'start') if [ "${pid}" = "" ] then /usr/bin/rm -f /etc/cron.d/FIFO if [ -x /usr/bin/cron ] then /usr/bin/cron elif [ -x /usr/sbin/cron ] then /usr/sbin/cron fi fi ;; 'stop') if [ "${pid}" != "" ] then /usr/bin/kill ${pid} fi ;; *) echo "usage: /etc/init.d/cron {start|stop}" ;; esac
  • 7. 2.11 BSD UNIX (1986) ● /sbin/init (782 lines) ● ● ● Single user gettys from /etc/gettytab /etc/rc (165 lines) ● ● ● Everything 2.9BSD had Network, inetd, routed, named, lpd, rwhod /etc/rc.local ● Editable for starting local daemons
  • 8. Red Hat Enterprise Linux 5 (2007) ● SVR4-like init ● ● Some BSD elements: /etc/rc, /etc/rc.local Init scripts LSB compliant ● 61-584 lines, avg. 128 ● start, stop, status, restart, condrestart ● Pidfiles in /var/run ● Subsystem locks in /var/lock/subsys ● /etc/rc.d/functions library ● /etc/sysconfig init script configuration
  • 9. #!/bin/bash # # sendmail This shell script takes care of starting and stopping # sendmail. # # chkconfig: 2345 80 30 # description: Sendmail is a Mail Transport Agent, which is the program # that moves mail from one machine to another. # processname: sendmail # config: /etc/mail/sendmail.cf # pidfile: /var/run/sendmail.pid if [ -x /usr/bin/make -a -f /etc/mail/Makefile ]; then make all -C /etc/mail -s > /dev/null else for i in virtusertable access domaintable mailertable ; do if [ -f /etc/mail/$i ] ; then makemap hash /etc/mail/$i < /etc/mail/$i fi done fi daemon /usr/sbin/sendmail $([ "x$DAEMON" = xyes ] && echo -bd) $([ -n "$QUEUE" ] && echo -q$QUEUE) RETVAL=$? killproc sendmail -HUP RETVAL=$? echo if [ $RETVAL -eq 0 -a -f /var/run/sm-client.pid ]; then echo -n $"reloading sm-client: " killproc sm-client -HUP RETVAL=$? echo fi return $RETVAL # Source function library. . /etc/rc.d/init.d/functions # Source networking configuration. [ -f /etc/sysconfig/network ] && . /etc/sysconfig/network # Source sendmail configureation. if [ -f /etc/sysconfig/sendmail ] ; then . /etc/sysconfig/sendmail else DAEMON=no QUEUE=1h fi [ -z "$SMQUEUE" ] && SMQUEUE="$QUEUE" [ -z "$SMQUEUE" ] && SMQUEUE=1h } stop() { # Stop daemons. if test -f /var/run/sm-client.pid ; then echo -n $"Shutting down sm-client: " killproc sm-client RETVAL=$? echo [ $RETVAL -eq 0 ] && rm -f /var/run/sm-client.pid [ $RETVAL -eq 0 ] && rm -f /var/lock/subsys/sm-client fi echo -n $"Shutting down $prog: " killproc sendmail RETVAL=$? echo [ $RETVAL -eq 0 ] && rm -f /var/lock/subsys/sendmail return $RETVAL # Check that networking is up. [ "${NETWORKING}" = "no" ] && exit 0 [ -f /usr/sbin/sendmail ] || exit 0 RETVAL=0 prog="sendmail" start() { # Start daemons. echo -n $"Starting $prog: " if test -x /usr/bin/make -a -f /etc/mail/Makefile ; then make all -C /etc/mail -s > /dev/null else for i in virtusertable access domaintable mailertable ; do if [ -f /etc/mail/$i ] ; then makemap hash /etc/mail/$i < /etc/mail/$i fi done fi /usr/bin/newaliases > /dev/null 2>&1 daemon /usr/sbin/sendmail $([ "x$DAEMON" = xyes ] && echo -bd) $([ -n "$QUEUE" ] && echo -q$QUEUE) $SENDMAIL_OPTARG RETVAL=$? echo [ $RETVAL -eq 0 ] && touch /var/lock/subsys/sendmail if ! test -f /var/run/sm-client.pid ; then echo -n $"Starting sm-client: " touch /var/run/sm-client.pid chown smmsp:smmsp /var/run/sm-client.pid if [ -x /usr/sbin/selinuxenabled ] && /usr/sbin/selinuxenabled; then /sbin/restorecon /var/run/sm-client.pid fi daemon --check sm-client /usr/sbin/sendmail -L sm-msp-queue -Ac -q$SMQUEUE $SENDMAIL_OPTARG RETVAL=$? echo [ $RETVAL -eq 0 ] && touch /var/lock/subsys/sm-client fi return $RETVAL reload() { # Stop daemons.} echo -n $"reloading $prog: " /usr/bin/newaliases > /dev/null 2>&1 } # See how we were called. case "$1" in start) start ;; stop) stop ;; reload) reload RETVAL=$? ;; restart) stop start RETVAL=$? ;; condrestart) if [ -f /var/lock/subsys/sendmail ]; then stop start RETVAL=$? fi ;; status) status sendmail RETVAL=$? ;; *) echo $"Usage: $0 {start|stop|restart|condrestart|status}" exit 1 esac exit $RETVAL
  • 10. ·· ddish): hket (Yi far∙pot∙s pronoun  a result ruined as letely  erfection 1. comp inor imp o f ix a m ttempts t of a
  • 11. Issues ● inittab ● service control inflexible Not actually used very much ● No way to monitor services ● Error output is lost ● Inflexible startup ordering ● Single-threaded startup ● ● ● Slow Malfunctional startup script can lock up boot Limited to single instance of a service
  • 12. Issues (cont.) ● No way to track processes belonging to a service ● ● No way to reliably terminate a service ● No way to tell whether the service is operational ● ● They double-fork to daemonize Resource hogging Inconsistent ● Who drops privileges? ● Who writes PID files? ● Who chroots?
  • 13. SVR4 & LSB init scripts ● Unbelievably shitty ● Unreliably work around issues mentioned ● Very long and ugly ● Lots of code duplication ● ● Races, improper subsystem locks Insecure PID determination
  • 14. How did UNIX address this ● Sun Solaris 10: SMF ● ● Multiple service instances Apple Darwin: launchd ● ● On demand startup of network daemons (inetd) ● ● System and User sessions Job scheduler Common: ● ● Dependency management, service ordering ● ● Parallel startup Monitoring/restarts of services Various others: upstart, daemontools, Monit, ...
  • 15. Linux: systemd ● Heavily inspired by concepts from launchd ● Already used in most modern Linux distributions ● Solves all of the mentioned issues! ● Leverages Linux inovations -- a lot more powerful ● Control groups, Namespaces ● Seccomp, Capabilities, SELinux ● Auditing ● Automounter ● DBus API ● Structured kernel messaging
  • 16. systemd Unit types Unit type Description/equivalent service A daemon (SVR4 init script) socket A network or UNIX socket (inetd) device UDev device instance mount Mount point (fstab) target Runlevel swap Swap space (fstab) automount Autofs path Inotify watch timer Crond snapshot Dynamically created target
  • 17. httpd.service ├─system.slice └─basic.target ├─fedora-loadmodules.service ├─paths.target ├─sockets.target │ ├─cups.socket │ ├─rpcbind.socket │ ├─systemd-initctl.socket │ └─systemd-udevd-kernel.socket ├─sysinit.target │ ├─kmod-static-nodes.service │ ├─systemd-udev-trigger.service │ ├─local-fs.target │ │ ├─-.mount │ │ ├─fedora-import-state.service │ │ ├─home.mount │ │ ├─systemd-fsck-root.service │ │ └─tmp.mount │ └─swap.target │ └─dev-disk-byx2dlabel-yolo.swap └─timers.target └─systemd-tmpfiles-clean.timer
  • 18. systemd Units ● Defined from unit files ● ● ● /lib/systemd /etc/systemd Generated automatically ● ● ● Compatibility or dynamic changes device unit appears as device appears in udev mount units generated from /etc/fstab
  • 19. Service units ● Service runs in its own control group ● Isolated from the rest of system to some extent ● A process can't escape ● Freezer control group assures reliable shutdown ● Service should not double-fork (launchd) ● systemd-journald takes care of logging ● Can depend on socket units for activation (inetd) ● DBus activation also possible
  • 20. User sessions ● Manages processes for a user session (e.g. tty or GNOME desktop) ● Session runs in separate control group ● systemd-logind replaces ConsoleKit ● Multiseat ● Reliable log-off
  • 21. Essential tools systemctl --all systemctl stop sshd.service systemctl status sshd systemctl disable sshd systemd-cgtop systemd-cgls journalctl -f systemd-analyze blame systemd-analyze critical-chain
  • 22. What else ● Takes care of system-wide events ● ● Laptop lid close ● ● Pinging watchdog Shutdown, kexec Documentation ● ● Well written manual pages for everything Lightweight virtualization (LXC) ● clone()s all namespaces
  • 23. Thanks for listening! Found this useful? My Bitcoin address: 15wvWxN5QMpreKR37pYb7VBu8xLu4TiNR2