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SYSKLOGD(8) Linux System Administration SYSKLOGD(8)
NAME
sysklogd - Linux system logging utilities.
SYNOPSIS
syslogd [ -a socket ] [ -d ] [ -f config file ] [ -h ] [ -l hostlist ] [ -m inter-
val ] [ -n ] [ -p socket ] [ -r ] [ -s domainlist ] [ -v ] [ -x ]
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.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 sec-
tion 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 for-
ward any remote messages it receives to forwarding 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 (‘‘:’’)
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.
-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.
-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 dae-
mon.
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 origi-
nal 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 dae-
mon 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 addi-
tion 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 mes-
sages 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 inter-
pretation 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 sys-
log.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 sup-
port 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 gener-
ated log messages. This behavior will allow syslogd to inter-operate with the sys-
log found in the standard C library. At the same time syslogd listens on the stan-
dard 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 sys-
log.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. Sys-
logd will retry to resolve the name ten times and then complain. Another possibil-
ity 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 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 host-
names. 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 sys-
logd. 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 mes-
sage.
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 AT rflab.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 pro-
gram/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 recal-
citrant 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 repre-
sents 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 sys-
log(3)).
action This field describes the particular action that takes place whenever a mes-
sage is received that matches the pattern. Refer to the syslog.conf(5) man-
page 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)
COLLABORATORS
Syslogd is taken from BSD sources, Greg Wettstein (greg AT wind.com) per-
formed the port to Linux, Martin Schulze (joey AT linux.de) fixed some bugs and added
several new features. Klogd was originally written by Steve Lord (lord AT cray.com),
Greg Wettstein made major improvements.
Dr. Greg Wettstein
Enjellic Systems Development
Oncology Research Division Computing Facility
Roger Maris Cancer Center
Fargo, ND
greg AT wind.com
Stephen Tweedie
Department of Computer Science
Edinburgh University, Scotland
sct AT dcs.uk
Juha Virtanen
jiivee AT hut.fi
Shane Alderton
shane AT ion.au
Martin Schulze
Infodrom Oldenburg
joey AT linux.de
Version 1.3 12 October 1998 SYSKLOGD(8)
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