syslogd

SYSKLOGD(8)                Linux System Administration               SYSKLOGD(8)



NAME
       sysklogd - Linux system logging utilities.

SYNOPSIS
       syslogd [ -a socket ] [ -d ] [ -f config file ] [ -h ] [ -i IP address ]
       [ -l hostlist ] [ -m interval ] [ -n ] [ -p socket ] [ -r ] [ -s
       domainlist ] [ -u username ] [ -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 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.

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.guides.sk/psionic/dns/>.

       -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.

       -i IP address
              If syslogd is configured to accept log input from a UDP port,
              specify an IP address to bind to, rather than the default of
              INADDR_ANY.  The address must be in dotted quad notation, DNS host
              names are not allowed.

       -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
              /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.

       -u username
              This causes the syslogd daemon to become the named user before
              starting up logging.

              Note that when this option is in use, syslogd will open all log
              files as root when the daemon is first started; however, after a
              SIGHUP the files will be reopened as the non-privileged user.  You
              should take this into account when deciding the ownership of the
              log files.

       -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 configuration file (default is /etc/syslog.conf) will
              be reread and the syslog(3) facility 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 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 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 specifically 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 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 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 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.

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 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-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.

SECURITY THREATS
       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 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 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/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.

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 console-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).

AUTHORS
       The system log daemon syslogd is originally  taken from BSD sources, Greg
       Wettstein <greg@wind.enjellic.com> performed the port to Linux, Martin
       Schulze <joey@infodrom.org> fixed some bugs, added several new features
       and took over maintenance.




Version 1.5                        27 May 2007                       SYSKLOGD(8)