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SYSKLOGD(8)		  Linux System Administration		   SYSKLOGD(8)

       sysklogd - Linux system logging utilities.

       syslogd	[ -a socket ] [ -d ] [ -f config file ] [ -h ] [ -l hostlist ]
       [ -m interval ] [ -n ] [ -p socket ] [ -r ] [ -s domainlist ] [ -v ]

       Sysklogd provides two system utilities which provide support for system
       logging and kernel message trapping.  Support of both internet and unix
       domain sockets enables this utility package to support both  local  and
       remote logging.

       System  logging is provided by a version of syslogd(8) derived from the
       stock BSD sources.  Support for	kernel	logging	 is  provided  by  the
       klogd(8)	 utility which allows kernel logging to be conducted in either
       a standalone fashion or as a client of syslogd.

       Syslogd provides a kind of  logging  that  many	modern	programs  use.
       Every  logged  message  contains	 at least a time and a hostname field,
       normally a program name field, too, but that depends on how trusty  the
       logging program is.

       While  the syslogd sources have been heavily modified a couple of notes
       are in order.  First of all there has  been  a  systematic  attempt  to
       insure  that  syslogd  follows its default, standard BSD behavior.  The
       second important concept to note is that this version of syslogd inter‐
       acts  transparently  with  the  version of syslog found in the standard
       libraries.  If a binary linked to the standard shared  libraries	 fails
       to  function correctly we would like an example of the anomalous behav‐

       The main configuration file /etc/syslog.conf or	an  alternative	 file,
       given  with  the	 -f  option, is read at startup.  Any lines that begin
       with the hash mark (``#'') and empty lines are ignored.	 If  an	 error
       occurs during parsing the whole line is ignored.

       -a socket
	      Using this argument you can specify additional sockets from that
	      syslogd has to listen to.	 This is needed if you're going to let
	      some  daemon  run within a chroot() environment.	You can use up
	      to 19 additional sockets.	 If your environment needs even	 more,
	      you  have	 to  increase the symbol MAXFUNIX within the syslogd.c
	      source file.  An example for a chroot() daemon is	 described  by
	      the people from OpenBSD at <>.

       -d     Turns  on	 debug mode.  Using this the daemon will not proceed a
	      fork(2) to set itself in the background, but  opposite  to  that
	      stay  in	the foreground and write much debug information on the
	      current tty.  See the DEBUGGING section for more information.

       -f config file
	      Specify an alternative configuration file instead	 of  /etc/sys‐
	      log.conf, which is the default.

       -h     By  default  syslogd  will not forward messages it receives from
	      remote hosts.  Specifying this switch on the command  line  will
	      cause  the log daemon to forward any remote messages it receives
	      to forwarding hosts which have been  defined.   This  can	 cause
	      syslog  loops  that fill up hard disks quite fast and thus needs
	      to be used with caution.

       -l hostlist
	      Specify a hostname that should be logged only  with  its	simple
	      hostname	and  not  the  fqdn.   Multiple hosts may be specified
	      using the colon (``:'') separator.

       -m interval
	      The syslogd logs a mark timestamp regularly.  The default inter‐
	      val  between  two	 --  MARK -- lines is 20 minutes.  This can be
	      changed with this option.	 Setting the interval to zero turns it
	      off  entirely.   Depending on other log messages generated these
	      lines may not be written consecutively.

       -n     Avoid auto-backgrounding.	 This is needed especially if the sys‐
	      logd is started and controlled by init(8).

       -p socket
	      You  can	specify	 an  alternative unix domain socket instead of

       -r     This option will enable the facility to receive message from the
	      network  using an internet domain socket with the syslog service
	      (see services(5)).  The default is to not receive	 any  messages
	      from the network.

	      This  option  is introduced in version 1.3 of the sysklogd pack‐
	      age.  Please note that the default behavior is the  opposite  of
	      how older versions behave, so you might have to turn this on.

       -s domainlist
	      Specify a domainname that should be stripped off before logging.
	      Multiple domains may be specified using the colon (``:'')	 sepa‐
	      rator.   Please  be advised that no sub-domains may be specified
	      but only entire domains.	For example if -s  is	speci‐
	      fied  and the host logging resolves to no
	      domain would be cut, you will have to specify two domains	 like:

       -v     Print version and exit.

       Syslogd	reacts	to  a set of signals.  You may easily send a signal to
       syslogd using the following:

	      kill -SIGNAL `cat /var/run/`

       SIGHUP This lets syslogd perform a re-initialization.  All  open	 files
	      are closed, the configuration file (default is /etc/syslog.conf)
	      will be reread and the syslog(3) facility is started again.

	      The syslogd will die.

	      If debugging is enabled these  are  ignored,  otherwise  syslogd
	      will die.

	      Switch  debugging	 on/off.  This option can only be used if sys‐
	      logd is started with the -d debug option.

	      Wait for childs if some were born, because of wall'ing messages.

       Syslogd uses a slightly different syntax	 for  its  configuration  file
       than  the  original BSD sources.	 Originally all messages of a specific
       priority and above were forwarded to the log file.

	      For example the following line caused ALL	 output	 from  daemons
	      using  the  daemon  facilities (debug is the lowest priority, so
	      every higher will also match) to go into /usr/adm/daemons:

		   # Sample syslog.conf
		   daemon.debug		    /usr/adm/daemons

       Under the new scheme this behavior remains the same.  The difference is
       the  addition  of  four	new specifiers, the asterisk (*) wildcard, the
       equation sign (=), the exclamation mark (!), and the minus sign (-).

       The * specifies that all messages for the specified facility are to  be
       directed	 to  the  destination.	 Note that this behavior is degenerate
       with specifying a priority level of debug.  Users have  indicated  that
       the asterisk notation is more intuitive.

       The  =  wildcard	 is used to restrict logging to the specified priority
       class.  This allows, for example, routing only debug messages to a par‐
       ticular logging source.

	      For example the following line in syslog.conf would direct debug
	      messages from all sources to the /usr/adm/debug file.

		   # Sample syslog.conf
		   *.=debug	       /usr/adm/debug

       The ! is used to exclude logging of  the	 specified  priorities.	  This
       affects all (!) possibilities of specifying priorities.

	      For  example  the	 following lines would log all messages of the
	      facility mail  except  those  with  the  priority	 info  to  the
	      /usr/adm/mail file.  And all messages from (including)
	      to news.crit (excluding) would be logged	to  the	 /usr/adm/news

		   # Sample syslog.conf
		   mail.*;mail.!=info	    /usr/adm/mail;news.!crit	    /usr/adm/news

       You  may	 use it intuitively as an exception specifier.	The above men‐
       tioned interpretation is simply inverted.  Doing that you may use


       to skip every message that comes with a mail facility.  There  is  much
       room to play with it. :-)

       The  -  may  only  be  used  to	prefix	a filename if you want to omit
       sync'ing the file after every write to it.

       This may take some acclimatization for those individuals	 used  to  the
       pure  BSD behavior but testers have indicated that this syntax is some‐
       what more flexible than the BSD	behavior.   Note  that	these  changes
       should not affect standard syslog.conf(5) files.	 You must specifically
       modify the configuration files to obtain the enhanced behavior.

       These modifications provide network support to  the  syslogd  facility.
       Network support means that messages can be forwarded from one node run‐
       ning syslogd to another node running syslogd where they will  be	 actu‐
       ally logged to a disk file.

       To  enable  this you have to specify the -r option on the command line.
       The default behavior is that syslogd won't listen to the network.

       The strategy is to have syslogd listen on  a  unix  domain  socket  for
       locally	generated  log	messages.  This behavior will allow syslogd to
       inter-operate with the syslog found in the standard C library.  At  the
       same time syslogd listens on the standard syslog port for messages for‐
       warded from other hosts.	 To have this work correctly  the  services(5)
       files (typically found in /etc) must have the following entry:

		   syslog	   514/udp

       If  this	 entry	is missing syslogd neither can receive remote messages
       nor send them, because the UDP port cant be  opened.   Instead  syslogd
       will die immediately, blowing out an error message.

       To  cause  messages  to be forwarded to another host replace the normal
       file line in the syslog.conf file with the name of the  host  to	 which
       the messages is to be sent prepended with an @.

	      For  example,  to	 forward ALL messages to a remote host use the
	      following syslog.conf entry:

		   # Sample syslogd configuration file to
		   # messages to a remote host forward all.
		   *.*		  @hostname

	      To forward all kernel messages to a remote host  the  configura‐
	      tion file would be as follows:

		   # Sample configuration file to forward all kernel
		   # messages to a remote host.
		   kern.*	  @hostname

       If the remote hostname cannot be resolved at startup, because the name-
       server might not be accessible (it may be started  after	 syslogd)  you
       don't  have to worry.  Syslogd will retry to resolve the name ten times
       and then complain.  Another possibility to avoid this is to  place  the
       hostname in /etc/hosts.

       With  normal  syslogds  you would get syslog-loops if you send out mes‐
       sages that were received from a remote host to the same host  (or  more
       complicated to a third host that sends it back to the first one, and so
       on).  In my domain (Infodrom Oldenburg) we accidently got one  and  our
       disks filled up with the same single message. :-(

       To  avoid  this no messages received from a remote host are sent out to
       another (or the same) remote host anymore.  If you experience are setup
       in  which  you  need  this  behaviour,  please  use the -h command line
       switch.	However, this option needs to be handled with caution since  a
       syslog loop can fill up hard disks quite fast.

       If  the	remote host is located in the same domain as the host, syslogd
       is running on, only the simple hostname will be logged instead  of  the
       whole fqdn.

       In a local network you may provide a central log server to have all the
       important information kept on one machine.  If the network consists  of
       different domains you don't have to complain about logging fully quali‐
       fied names instead of simple hostnames.	You may want to use the strip-
       domain  feature	-s  of this server.  You can tell the syslogd to strip
       off several domains other than the one the server  is  located  in  and
       only log simple hostnames.

       Using  the  -l option there's also a possibility to define single hosts
       as local machines.  This, too, results in  logging  only	 their	simple
       hostnames and not the fqdns.

       The  UDP	 socket used to forward messages to remote hosts or to receive
       messages from them is only opened when it is needed.  In releases prior
       to  1.3-23  it was opened every time but not opened for reading or for‐
       warding respectively.

       This version of syslogd has support for logging output to  named	 pipes
       (fifos).	  A  fifo  or  named pipe can be used as a destination for log
       messages by prepending a pipy symbol (``|'') to the name of  the	 file.
       This  is	 handy for debugging.  Note that the fifo must be created with
       the mkfifo command before syslogd is started.

	      The following configuration file routes debug messages from  the
	      kernel to a fifo:

		   # Sample configuration to route kernel debugging
		   # messages ONLY to /usr/adm/debug which is a
		   # named pipe.
		   kern.=debug		    |/usr/adm/debug

       There is probably one important consideration when installing this ver‐
       sion of syslogd.	 This version of syslogd is dependent on  proper  for‐
       matting	of  messages  by  the syslog function.	The functioning of the
       syslog function in the shared libraries changed somewhere in the region
       of[2-4].n.	The  specific change was to null-terminate the
       message before transmitting it to the /dev/log  socket.	 Proper	 func‐
       tioning	of this version of syslogd is dependent on null-termination of
       the message.

       This problem will typically manifest itself if  old  statically	linked
       binaries	 are being used on the system.	Binaries using old versions of
       the syslog function will cause empty lines to be logged followed by the
       message	with  the  first  character in the message removed.  Relinking
       these binaries to newer versions of the shared libraries	 will  correct
       this problem.

       Both  the syslogd(8) and the klogd(8) can either be run from init(8) or
       started as part of the rc.*  sequence.  If it is started from init  the
       option  -n  must	 be  set,  otherwise you'll get tons of syslog daemons
       started.	 This is because init(8) depends on the process ID.

       There is the potential for the syslogd daemon to be used as  a  conduit
       for  a  denial  of  service  attack.  Thanks go to John Morrison (jmor‐ for alerting me to this potential.	 A rogue  pro‐
       gram(mer)  could	 very easily flood the syslogd daemon with syslog mes‐
       sages resulting in the log files consuming all the remaining  space  on
       the  filesystem.	  Activating logging over the inet domain sockets will
       of course expose a system to risks outside of programs  or  individuals
       on the local machine.

       There are a number of methods of protecting a machine:

       1.     Implement	 kernel	 firewalling  to limit which hosts or networks
	      have access to the 514/UDP socket.

       2.     Logging can be directed to an isolated  or  non-root  filesystem
	      which, if filled, will not impair the machine.

       3.     The ext2 filesystem can be used which can be configured to limit
	      a certain percentage of a filesystem  to	usage  by  root	 only.
	      NOTE  that  this	will  require  syslogd to be run as a non-root
	      process.	ALSO NOTE that this will prevent usage of remote  log‐
	      ging since syslogd will be unable to bind to the 514/UDP socket.

       4.     Disabling	 inet  domain  sockets	will  limit  risk to the local

       5.     Use step 4 and if the problem persists and is not secondary to a
	      rogue  program/daemon  get  a 3.5 ft (approx. 1 meter) length of
	      sucker rod* and have a chat with the user in question.

	      Sucker rod def. — 3/4, 7/8 or  1in.  hardened  steel  rod,  male
	      threaded	on each end.  Primary use in the oil industry in West‐
	      ern North Dakota and other locations to pump 'suck' oil from oil
	      wells.   Secondary  uses are for the construction of cattle feed
	      lots and for dealing with the occasional	recalcitrant  or  bel‐
	      ligerent individual.

       When  debugging	is turned on using -d option then syslogd will be very
       verbose by writing much of what it does on stdout.  Whenever  the  con‐
       figuration  file	 is  reread and re-parsed you'll see a tabular, corre‐
       sponding to the internal data structure.	 This tabular consists of four

       number This field contains a serial number starting by zero.  This num‐
	      ber represents the position in the internal data structure (i.e.
	      the  array).   If	 one number is left out then there might be an
	      error in the corresponding line in /etc/syslog.conf.

	      This field is  tricky  and  represents  the  internal  structure
	      exactly.	 Every	column	stands	for  a facility (refer to sys‐
	      log(3)).	As you can see, there are still some  facilities  left
	      free  for	 former use, only the left most are used.  Every field
	      in a column represents the priorities (refer to syslog(3)).

       action This field describes the	particular  action  that  takes	 place
	      whenever	a message is received that matches the pattern.	 Refer
	      to the syslog.conf(5) manpage for all possible actions.

	      This field shows additional arguments to the actions in the last
	      field.   For  file-logging this is the filename for the logfile;
	      for user-logging this is a list of  users;  for  remote  logging
	      this  is the hostname of the machine to log to; for console-log‐
	      ging this is the used console; for tty-logging this is the spec‐
	      ified tty; wall has no additional arguments.

	      Configuration  file  for	syslogd.  See syslog.conf(5) for exact
	      The Unix domain socket to from where local syslog	 messages  are
	      The file containing the process id of syslogd.

       If an error occurs in one line the whole rule is ignored.

       Syslogd	doesn't change the filemode of opened logfiles at any stage of
       process.	 If a file is created it is world readable.  If	 you  want  to
       avoid  this,  you have to create it and change permissions on your own.
       This could be done in combination  with	rotating  logfiles  using  the
       savelog(8)  program  that  is  shipped  in  the smail 3.x distribution.
       Remember that it might be a security hole if everybody is able to  read
       auth.* messages as these might contain passwords.

       syslog.conf(5), klogd(8), logger(1), syslog(2), syslog(3), services(5),

       The system log daemon syslogd is originally  taken  from	 BSD  sources,
       Greg  Wettstein	<>  performed the port to Linux,
       Martin Schulze <> fixed some bugs, added  several  new
       features and took over maintenance.

Version 1.5			  27 May 2007			   SYSKLOGD(8)

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