Welcome to Linux Knowledge Base and Tutorial
"The place where you learn linux"
Mercy Corps

 Create an AccountHome | Submit News | Your Account  

Tutorial Menu
Linux Tutorial Home
Table of Contents

· Introduction to Operating Systems
· Linux Basics
· Working with the System
· Shells and Utilities
· Editing Files
· Basic Administration
· The Operating System
· The X Windowing System
· The Computer Itself
· Networking
· System Monitoring
· Solving Problems
· Security
· Installing and Upgrading
· Linux and Windows

Man Pages
Linux Topics
Test Your Knowledge

Site Menu
Site Map
Copyright Info
Terms of Use
Privacy Info
Masthead / Impressum
Your Account

Private Messages

News Archive
Submit News
User Articles
Web Links


The Web

Who's Online
There are currently, 122 guest(s) and 0 member(s) that are online.

You are an Anonymous user. You can register for free by clicking here




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


       Sysklogd  provides two system utilities which provide sup­
       port for system logging and kernel message trapping.  Sup­
       port 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 pro­
       grams 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 con­
       cept 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  alter­
       native 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  http://www.psionic.com/papers/dns.html.
              on  the  command  line will cause the log daemon to
              forward any remote messages it receives to forward­
              ing 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 (``:'') sep­

       -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  espe­
              cially  if the syslogd is started and controlled by

       -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

              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

       -t     Tag forwarded messages with the sending hosts  name
              as seen by the forwarder.  Useful for relaying sys­
              log from DMZs through the firwall  to  an  internal

       -v     Print version and exit.
              The syslogd will die.

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

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

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


       Syslogd  uses a slightly different syntax for its configu­
       ration 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 exclama­
       tion mark (!), and the minus sign (-).

       The * specifies that all messages for the specified facil­
       ity 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  speci­
       fied  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
                   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


       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 stan­
       dard  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 for­
       warded from one node running syslogd to another node  run­
       ning  syslogd where they will be actually logged to a disk

       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 lis­
       tens on the standard syslog port  for  messages  forwarded
       from  other  hosts.   To have this work correctly the ser­
       vices(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.
              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  com­
       plain.   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  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

       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 respec­



       There  is  probably  one  important   consideration   when
       installing  this version of syslogd.  This version of sys­
       logd 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
       libc.so.4.[2-4].n.  The specific change was to null-termi­
       nate 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  stati­
       cally linked binaries are being used on the system.  Bina­
       ries 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,  other­
       wise  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 (jmorriso@rflab.ee.ubc.ca) 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 sock­
       ets  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

       3.     The  ext2  filesystem can be used which can be con­
              figured to limit a certain percentage of a filesys­
              tem  to  usage  by  root only.  NOTE that this will
              require syslogd to be run as  a  non-root  process.
              oil  industry  in  Western  North  Dakota and other
              locations to pump 'suck' oil from oil wells.   Sec­
              ondary uses are for the construction of cattle feed
              lots and for dealing with the  occasional  recalci­
              trant 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  for­
              mer  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


              Configuration file for syslogd.  See syslog.conf(5)
              for exact information.
              The Unix domain socket to from where  local  syslog
              messages are read.


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


       Syslogd   is   taken  from  BSD  sources,  Greg  Wettstein
       (greg@wind.enjellic.com) performed the port to Linux, Mar­
       tin Schulze (joey@linux.de) fixed some bugs and added sev­
       eral new features.  Klogd was originally written by  Steve
       Lord  (lord@cray.com),  Greg Wettstein made major improve­

       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)

More information about the site can be found in the FAQ



Security Code
Security Code
Type Security Code

Don't have an account yet? You can create one. As a registered user you have some advantages like theme manager, comments configuration and post comments with your name.

Help if you can!

Amazon Wish List

Did You Know?
The Linux Tutorial welcomes your suggestions and ideas.


Tell a Friend About Us

Bookmark and Share

Web site powered by PHP-Nuke

Is this information useful? At the very least you can help by spreading the word to your favorite newsgroups, mailing lists and forums.
All logos and trademarks in this site are property of their respective owner. The comments are property of their posters. Articles are the property of their respective owners. Unless otherwise stated in the body of the article, article content (C) 1994-2013 by James Mohr. All rights reserved. The stylized page/paper, as well as the terms "The Linux Tutorial", "The Linux Server Tutorial", "The Linux Knowledge Base and Tutorial" and "The place where you learn Linux" are service marks of James Mohr. All rights reserved.
The Linux Knowledge Base and Tutorial may contain links to sites on the Internet, which are owned and operated by third parties. The Linux Tutorial is not responsible for the content of any such third-party site. By viewing/utilizing this web site, you have agreed to our disclaimer, terms of use and privacy policy. Use of automated download software ("harvesters") such as wget, httrack, etc. causes the site to quickly exceed its bandwidth limitation and are therefore expressly prohibited. For more details on this, take a look here

PHP-Nuke Copyright © 2004 by Francisco Burzi. This is free software, and you may redistribute it under the GPL. PHP-Nuke comes with absolutely no warranty, for details, see the license.
Page Generation: 0.09 Seconds