path: root/sysklogd.8

SYSKLOGD(8)                        Linux System Administration                        SYSKLOGD(8)

NAME
       sysklogd - Linux system logging utilities.

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

DESCRIPTION
       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 de-
       fault, 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 li-
       braries.  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 op-
       tion, 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.

OPTIONS
       -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
              http://www.psionic.com/papers/dns.html.

       -d     Turns on debug mode.  Using this the daemon will not proceed a fork(2) to  set  it-
              self  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  informa-
              tion.

       -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.  Speci-
              fying  this switch on the command line will cause the log daemon to forward any re-
              mote 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 interval 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 syslogd is started and
              controlled by init(8).

       -p socket
              You can specify an alternative unix domain socket instead of /dev/log.

       -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  north.de  is
              specified  and  the host logging resolves to satu.infodrom.north.de no domain would
              be cut, you will have to specify two domains like: -s north.de:infodrom.north.de.

       -v     Print version and exit.

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

              kill -SIGNAL `cat /var/run/syslogd.pid`

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

       SIGTERM
              The syslogd will die.

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

       SIGUSR1
              Switch  debugging  on/off.  This option can only be used if syslogd is started with
              the -d debug option.

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

CONFIGURATION FILE SYNTAX DIFFERENCES
       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  fa-
              cilities (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 (!) possi-
       bilities 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
              news.info (including) to news.crit (excluding) would be logged to the /usr/adm/news
              file.

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

            mail.none
       or
            mail.!*
       or
            mail.!debug

       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 specif-
       ically modify the configuration files to obtain the enhanced behavior.

SUPPORT FOR REMOTE LOGGING
       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  behav-
       ior 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 mes-
       sages forwarded from other hosts.  To have this work correctly the services(5) files (typ-
       ically 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, be-
       cause 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 sys-
       log.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  re-
       ceived 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 informa-
       tion  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.

OUTPUT TO NAMED PIPES (FIFOs)
       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

INSTALLATION CONCERNS
       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 func-
       tion.  The functioning of the syslog function in the shared libraries changed somewhere in
       the  region  of  libc.so.4.[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 sys-
       logd 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 re-
       moved.  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.

SECURITY THREATS
       There is the potential for the syslogd daemon to be used as a conduit for a denial of ser-
       vice  attack.   Thanks  go  to John Morrison (jmorriso@rflab.ee.ubc.ca) for alerting me to
       this potential.  A rogue program(mer) could very easily flood the syslogd daemon with sys-
       log  messages resulting in the log files consuming all the remaining space on the filesys-
       tem.  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 percent-
              age 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 socket.

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

       5.     Use step 4 and if the problem persists and is not secondary to a rogue program/dae-
              mon  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.

DEBUGGING
       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.

       pattern
              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 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.

       arguments
              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 con-
              sole-logging this is the used console; for tty-logging this is the  specified  tty;
              wall has no additional arguments.

FILES
       /etc/syslog.conf
              Configuration file for syslogd.  See syslog.conf(5) for exact information.
       /dev/log
              The Unix domain socket to from where local syslog messages are read.
       /var/run/syslogd.pid
              The file containing the process id of syslogd.

BUGS
       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.

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

COLLABORATORS
       Syslogd is taken from BSD sources, Greg Wettstein (greg@wind.enjellic.com)  performed  the
       port  to  Linux, Martin Schulze (joey@linux.de) fixed some bugs and added several new fea-
       tures.  Klogd was originally written by Steve Lord (lord@cray.com),  Greg  Wettstein  made
       major improvements.

       Dr. Greg Wettstein
       Enjellic Systems Development
       Oncology Research Division Computing Facility
       Roger Maris Cancer Center
       Fargo, ND
       greg@wind.enjellic.com

       Stephen Tweedie
       Department of Computer Science
       Edinburgh University, Scotland
       sct@dcs.ed.ac.uk

       Juha Virtanen
       jiivee@hut.fi

       Shane Alderton
       shane@ion.apana.org.au

       Martin Schulze
       Infodrom Oldenburg
       joey@linux.de

Version 1.3                              12 October 1998                              SYSKLOGD(8)