Computer Science
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 interval ] [ -n ] [ -p socket ] [ -r ]
[ -s domainlist ] [ -v ]
DESCRIPTION
Sysklogd provides two system utilities which provide sup-
port for system logging and kernel message trapping. Sup-
port 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 pro-
grams 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 con-
cept 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 alter-
native 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 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 forward-
ing 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 (``:'') sep-
arator.
-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. Remember that
the first match is used, not the best.
-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, other-
wise 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 configu-
ration 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 exclama-
tion mark (!), and the minus sign (-).
The * specifies that all messages for the specified facil-
ity 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 speci-
fied 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 priori-
ties. This affects all (!) possibilities of specifying
priorities.
For example the following lines would log all mes-
sages 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 stan-
dard syslog(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 for-
warded from one node running syslogd to another node run-
ning 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 lis-
tens on the standard syslog port for messages forwarded
from other hosts. To have this work correctly the ser-
vices(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 com-
plain. 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 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 log-
ging 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 respec-
tively.
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 debug-
ging. Note that the fifo must be created with the mkfifo
command before syslogd is started.
The following configuration file routes debug mes-
sages 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 sys-
logd 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-termi-
nate 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 stati-
cally linked binaries are being used on the system. Bina-
ries 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, other-
wise 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 sock-
ets 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 con-
figured to limit a certain percentage of a filesys-
tem 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. Sec-
ondary uses are for the construction of cattle feed
lots and for dealing with the occasional recalci-
trant 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 facil-
ity (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 col-
umn 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(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-log-
ging this is the used console; for tty-logging this
is the specified tty; wall has no additional argu-
ments.
FILES
/etc/syslog.conf
Configuration file for syslogd. See syslog(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 pass-
words.
SEE ALSO
syslog(5), klogd(8), logger(1), syslog(2), syslog(3),
services(5), savelog(8)
COLLABORATORS
Syslogd is taken from BSD sources, Greg Wettstein
(greg@wind.enjellic.com) performed the port to Linux, Mar-
tin Schulze (joey@linux.de) fixed some bugs and added sev-
eral new features. Klogd was originally written by Steve
Lord (lord@cray.com), Greg Wettstein made major improve-
ments.
Dr. Greg Wettstein
Enjellic Systems Development
Oncology Research Division Computing Facility
Roger Maris Cancer Center
Fargo, ND
greg@wind.enjellic.com
Stephen Tweedie
Department of Computer Science
Edinburgh University, Scotland
sct@dcs.ed.ac.uk
Juha Virtanen
jiivee@hut.fi
Shane Alderton
shane@ion.apana.org.au
Martin Schulze
Infodrom Oldenburg
joey@linux.de
Version 1.3 12 October 1998 1
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