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 ] [ -t ]
[ -v ] [ -S ]
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, nor-
mally 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.
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 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.
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.
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).
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.
Specify a domainname that should be stripped off before logging. Multiple domains may be specified using the colon (``:'') separa-
tor. 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 syslog from DMZs through the fir-
wall to an internal loghost.
-v Print version and exit.
-S Insert a special code before the hostname in each message to indicate the message's priority and facility. Having the priority and
facility encoded in each message allows a log analyzer program to monitor a single file containing all logged messages. The analyzer
program can parse the priority/facility code for each logged message to determine if it is of interest.
The priority/facility code takes the form of a priority number followed by a letter that indicates the facility. The priority number
can range from 0 to 7. The facility letter can range from 'A' to 'Y', where 'A' corresponds to facility number zero (LOG_KERN), 'B'
corresponds to facility 1 (LOG_USER), etc. See /usr/include/sys/syslog.h for a list of priority and LOG_xxx codes.
Jun 27 19:22:20 5Q:silence test: testing
The above message contains a priority/facility code of '5Q', which breaks down to a priority of 5 (LOG_NOTICE) and a facility of 16
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.
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.
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
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
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
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 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.
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:
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.
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.
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 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
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.
There is probably one important consideration when installing this version 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 libc.so.4.[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-
email@example.com) for alerting me to this potential. A rogue program(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 process. ALSO NOTE that this will prevent usage of remote logging since sys-
logd 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.
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, corresponding to the internal data structure. This tabular consists of four
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 repre-
sents 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-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 information.
The Unix domain socket to from where local syslog messages are read.
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), savelog(8)
Syslogd is taken from BSD sources, Greg Wettstein (firstname.lastname@example.org) performed the port to Linux, Martin Schulze (email@example.com)
fixed some bugs and added several new features. Klogd was originally written by Steve Lord (firstname.lastname@example.org), Greg Wettstein made major
Dr. Greg Wettstein
Enjellic Systems Development
Oncology Research Division Computing Facility
Roger Maris Cancer Center
Department of Computer Science
Edinburgh University, Scotland
Version 1.3 12 October 1998 SYSKLOGD(8)