SYSKLOGDSection: Linux System Administration (8)
Updated: 12 October 1998
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NAMEsysklogd - Linux system logging utilities.
SYNOPSISsyslogd [ -a socket ] [ -d ] [ -f config file ] [ -h ] [ -l hostlist ] [ -m interval ] [ -n ] [ -p socket ] [ -r ] [ -s domainlist ] [ -v ]
DESCRIPTIONSysklogd 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.
SIGNALSSyslogd reacts to a set of signals. You may easily send a signal to syslogd using the following:
CONFIGURATION FILE SYNTAX DIFFERENCESSyslogd 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.
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.
The ! is used to exclude logging of the specified priorities. This affects all (!) possibilities of specifying priorities.
You may use it intuitively as an exception specifier. The above mentioned 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 somewhat 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 LOGGINGThese 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 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 forwarded 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 @.
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 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 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.
INSTALLATION CONCERNSThere 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.
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 THREATSThere 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:
DEBUGGINGWhen 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:
BUGSIf 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 ALSOsyslog.conf(5), klogd(8), logger(1), syslog(2), syslog(3), services(5), savelog(8)
COLLABORATORSSyslogd 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.
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Time: 15:49:38 GMT, February 23, 2024