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 ] [ -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 cor-
rectly 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 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. Speci-
fying 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.
-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 daemon.
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 con-
figuration 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 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 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 (!) possi-
bilities 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
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 standard syslog.conf(5) files. You must specif-
ically 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 behav-
ior 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 mes-
sages forwarded from other hosts. To have this work correctly the services(5) files (typ-
ically 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 sys-
log.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 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 acci-
dently 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 informa-
tion 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.
kern.=debug |/usr/adm/debug
INSTALLATION CONCERNS
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 some-
where 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.
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 ser-
vice attack. Thanks go to John Morrison () for alerting me to
this potential. A rogue program(mer) could very easily flood the syslogd daemon with sys-
log messages resulting in the log files consuming all the remaining space on the filesys-
tem. 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 percent-
age 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 program/dae-
mon 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.
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 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 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.
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 con-
sole-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 () performed the
port to Linux, Martin Schulze () fixed some bugs and added several new fea-
tures. Klogd was originally written by Steve Lord (), Greg Wettstein made
major improvements.
Dr. Greg Wettstein
Enjellic Systems Development
Oncology Research Division Computing Facility
Roger Maris Cancer Center
Fargo, ND
Stephen Tweedie
Department of Computer Science
Edinburgh University, Scotland
Juha Virtanen
Shane Alderton
Martin Schulze
Infodrom Oldenburg
Version 1.3 12 October 1998 SYSKLOGD(8)
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