sysklogd - Unix, Linux Command
sysklogd - Linux system logging utilities.
syslogd [ -a socket ]
[ -d ] [ -f config file ]
[ -h ] [ -l hostlist ]
[ -m interval ]
[ -n ] [ -p socket ]
[ -r ] [ -s domainlist ]
[ -v ] [ -x ]
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
derived from the
stock BSD sources. Support for kernel logging is provided by the
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
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
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 youre 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
MAXFUNIX within the syslogd.c source file. An example for a chroot() daemon is
described by the people from OpenBSD at
Turns on debug mode. Using this the daemon will not proceed a
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.
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
-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
-m interval |
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.
Avoid auto-backgrounding. This is needed especially if the
syslogd is started and controlled by
-p socket |
You can specify an alternative unix domain socket instead of
This option will enable the facility to receive message from the
network using an internet domain socket with the syslog service (see
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 (:)
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. |
Verbose logging. If specified once, the numeric facility and priority
are logged with each locally-written message. If specified more than
once, the names of the facility and priority are logged with each
Print version and exit.
Disable name lookups when receiving remote messages.
This avoids deadlocks when the nameserver is running on
the same machine that runs the syslog daemon.
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
syslogd perform a re-initialization. All open files are closed, the
configuration file (default is
/etc/syslog.conf) will be reread and the
facility is started again.
syslogd will die.
SIGINT, SIGQUIT |
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 walling 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.
You may use it intuitively as an exception specifier. The above
mentioned interpretation is simply inverted. Doing that you may use
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
# 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
# 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
# Sample syslog.conf
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
syncing 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
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 wont 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
files (typically found in
/etc) must have the following
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
# 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 dont 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
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 doesnt 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 information kept on one machine. If the network consists
of different domains you dont have to complain about logging fully
qualified names instead of simple hostnames. You may want to use the
-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.
-l option theres 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
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
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.
syslogd(8) and the
can either be run from
or started as part of the rc.*
sequence. If it is started from init the option -n must be set,
otherwise youll get tons of syslog daemons started. This is because
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 (firstname.lastname@example.org) 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:
Implement kernel firewalling to limit which hosts or networks have
access to the 514/UDP socket.
Logging can be directed to an isolated or non-root filesystem which,
if filled, will not impair the machine.
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.
Disabling inet domain sockets will limit risk to the local machine.
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 youll see a tabular,
corresponding to the internal data structure. This tabular consists of
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
This field is tricky and represents the internal structure
exactly. Every column stands for a facility (refer to
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
This field describes the particular action that takes place whenever a
message is received that matches the pattern. Refer to the
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
Configuration file for
for exact information.
The Unix domain socket to from where local syslog messages are read.
The file containing the process id of
If an error occurs in one line the whole rule is ignored.
Syslogd doesnt 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
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
Syslogd is taken from BSD sources, Greg Wettstein (email@example.com)
performed the port to Linux, Martin Schulze (firstname.lastname@example.org)
fixed some bugs and added several new features.
Klogd was originally written by Steve Lord (email@example.com), Greg Wettstein
made major improvements.