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 ]  [
       -x ]

       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.

       -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.

       -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.

       -x     Disable name  lookups  when  receiving  remote  messages.   This
              avoids  deadlocks  when  the  nameserver  is running on the same
              machine that runs the syslog daemon.

       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  in further times no messages that were received from a
       remote host are sent out to another (or the same) remote host  anymore.
       If  there  are  scenarios where this doesn't make sense, please drop me
       (Joey) a line.

       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 pro-
              cess.   ALSO NOTE that this will prevent usage of remote logging
              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),

       Syslogd  is  taken  from  BSD sources, Greg Wettstein (greg@wind.enjel- performed the port to Linux,  Martin  Schulze  (
       fixed  some  bugs and added several new features.  Klogd was originally
       written by  Steve  Lord  (,  Greg  Wettstein  made  major

       Dr. Greg Wettstein
       Enjellic Systems Development
       Oncology Research Division Computing Facility
       Roger Maris Cancer Center
       Fargo, ND

       Stephen Tweedie
       Department of Computer Science
       Edinburgh University, Scotland

       Juha Virtanen

       Shane Alderton

       Martin Schulze
       Infodrom Oldenburg

Version 1.3                     12 October 1998                    SYSKLOGD(8)