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       sysklogd - Linux system logging utilities.


       syslogd  [ -a socket ] [ -d ] [ -f config file ] [ -h ] [ -l hostlist ]
       [ -m interval ] [ -n ] [ -p socket ] [ -r ] [ -s domainlist ] [ -u user
       ] [ -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
       interacts 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 behavior.

       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/syslog.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
              interval  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
              syslogd 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
              package.  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 (‘‘:’’)
              separator.   Please  be  advised  that  no  sub-domains  may  be
              specified  but  only entire domains.  For example if -s
              is   specified   and    the    host    logging    resolves    to
      no domain would be cut, you will have to
              specify two domains like: -s

       -u user
              The syslogd daemon runs with full root privileges by default. If
              you  specify this option, the daemon will drop its privileges to
              the given user (and the  primary  group  of  this  user)  before
              starting  up  logging. This greatly reduces the potential impact
              of exploitable security holes in syslogd.

              syslogd will still open  all  log  files  as  root  at  startup.
              However,  after  receiving  a  SIGHUP  signal  (which causes the
              daemon to restart) the log files will be reopened  as  the  non-
              privileged  user which fails if the log files are only writeable
              by root. If you need to restart the  daemon  using  the  signal,
              then  you  have to adapt the permissions of your log files to be
              writeable by the specified user (or its primary group).

       -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/syslogd.pidSIGHUP 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
              syslogd 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
       particular 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
       mentioned 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
       somewhat 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
       running syslogd to another node running  syslogd  where  they  will  be
       actually 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
       forwarded  from  other  hosts.   To  have  this  work   correctly   the
       services(5)  files  (typically  found  in /etc) must have the following

                   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
              configuration 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
       messages  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
       qualified  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
       forwarding 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
       version  of  syslogd.   This  version of syslogd is dependent on proper
       formatting 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
       functioning 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
       (  for alerting me to this potential.  A rogue
       program(mer) could very easily flood the  syslogd  daemon  with  syslog
       messages  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
              logging since syslogd will be unable  to  bind  to  the  514/UDP

       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
              Western 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
              belligerent 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
       configuration  file  is  reread  and  re-parsed  you’ll  see a tabular,
       corresponding to the internal data structure.  This tabular consists of
       four fields:

       number This  field  contains  a  serial  number starting by zero.  This
              number 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
              syslog(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

       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-
              logging  this  is  the used console; for tty-logging this is the
              specified 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.