Linux man pages : syslogd (8)
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 ] [ -v ] [
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
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 inter-
acts 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 behav-
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
-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/sys-
log.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 inter-
val between two -- MARK -- lines is 20 minutes. This can be
changed with this option. Setting the interval to zero turns it
-n Avoid auto-backgrounding. This is needed especially if the sys-
logd is started and controlled by init(8).
You can specify an alternative unix domain socket instead of
-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 pack-
age. 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 (``:'') sepa-
rator. Please be advised that no sub-domains may be specified
but only entire domains. For example if -s north.de is speci-
fied and the host logging resolves to satu.infodrom.north.de no
domain would be cut, you will have to specify two domains like:
-v Print version and exit.
-x 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`
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
Switch debugging on/off. This option can only be used if sys-
logd 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 par-
ticular 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
# Sample syslog.conf
You may use it intuitively as an exception specifier. The above men-
tioned 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 run-
ning syslogd to another node running syslogd where they will be actu-
ally 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 for-
warded 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 configura-
tion 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 mes-
sages 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 quali-
fied 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 for-
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 ver-
sion 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
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
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-
firstname.lastname@example.org) for alerting me to this potential. A rogue pro-
gram(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 pro-
cess. 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
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.