rsyslogd - reliable and extended syslogd
rsyslogd [ -d ] [ -D ] [ -f config file ]
[ -i pid file ] [ -n ] [ -N level ] [
-o fullconf ] [ -C ] [ -v ]
Rsyslogd is a system utility providing support for message logging.
Support of both internet and unix domain sockets enables this utility to
support both local and remote logging.
Note that this version of rsyslog ships with extensive
documentation in HTML format. This is provided in the ./doc subdirectory
and probably in a separate package if you installed rsyslog via a packaging
system. To use rsyslog's advanced features, you need to look at the
HTML documentation, because the man pages only covers basic aspects of
operation. For details and configuration examples, see the rsyslog.conf
(5) man page and the online documentation at
Rsyslogd(8) is derived from the sysklogd package which in
turn is derived from the stock BSD sources.
Rsyslogd 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. The rsyslog package supports free definition of
output formats via templates. It also supports precise timestamps and
writing directly to databases. If the database option is used, tools like
phpLogCon can be used to view the log data.
While the rsyslogd sources have been heavily modified a
couple of notes are in order. First of all there has been a systematic
attempt to ensure that rsyslogd follows its default, standard BSD behavior.
Of course, some configuration file changes are necessary in order to support
the template system. However, rsyslogd should be able to use a standard
syslog.conf and act like the original syslogd. However, an original syslogd
will not work correctly with a rsyslog-enhanced configuration file. At best,
it will generate funny looking file names. The second important concept to
note is that this version of rsyslogd 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/rsyslog.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 error element is ignored. It is tried to
parse the rest of the line.
Rsyslogd reacts to a set of signals. You may easily send a signal to
rsyslogd using the following:
- Runs the Bison config parser in debug mode. This may help when hard to
find syntax errors are reported. Please note that the output generated is
deeply technical and orignally targeted towards developers.
- Turns on debug mode. See the DEBUGGING section for more information.
- -f config file
- Specify an alternative configuration file instead of
/etc/rsyslog.conf, which is the default.
- -i pid file
- Specify an alternative pid file instead of the default one. This option
must be used if multiple instances of rsyslogd should run on a single
machine. To disable writing a pid file, use the reserved name
"NONE" (all upper case!), so "-iNONE".
- Avoid auto-backgrounding. This is needed especially if the rsyslogd
is started and controlled by init(8).
- -N level
- Do a config check. Do NOT run in regular mode, just check configuration
file correctness. This option is meant to verify a config file. To do so,
run rsyslogd interactively in foreground, specifying -f
<config-file> and -N level. The level argument modifies behaviour.
Currently, 0 is the same as not specifying the -N option at all (so this
makes limited sense) and 1 actually activates the code. Later, higher
levels will mean more verbosity (this is a forward-compatibility
- -o fullconf
- Generates a consolidated config file fullconf that contains all of
rsyslog's configuration in a single file. Include files are exploded into
that file in exactly the way rsyslog sees them. This option is useful for
troubleshooting, especially if problems with the order of action
processing is suspected. It may also be used to check for
"unexepectedly" included config content.
- This prevents rsyslogd from changing to the root directory. This is almost
never a good idea in production use. This option was introduced in support
of the internal testbed.
- Print version and exit.
kill -SIGNAL $(cat /var/run/rsyslogd.pid)
Note that -SIGNAL must be replaced with the actual signal you are
trying to send, e.g. with HUP. So it then becomes:
kill -HUP $(cat /var/run/rsyslogd.pid)
There is the potential for the rsyslogd daemon to be used as a conduit for a
denial of service attack. A rogue program(mer) could very easily flood the
rsyslogd 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.
- This lets rsyslogd perform close all open files.
- TERM , INT , QUIT
- Rsyslogd will die.
- Switch debugging on/off. This option can only be used if rsyslogd
is started with the -d debug option.
- Wait for children if some were born, because of wall'ing messages.
There are a number of methods of protecting a machine:
If remote logging is enabled, messages can easily be spoofed and replayed. As
the messages are transmitted in clear-text, an attacker might use the
information obtained from the packets for malicious things. Also, an attacker
might replay recorded messages or spoof a sender's IP address, which could
lead to a wrong perception of system activity. These can be prevented by using
GSS-API authentication and encryption. Be sure to think about syslog network
security before enabling it.
When debugging is turned on using the -d option, rsyslogd produces
debugging information according to the RSYSLOG_DEBUG environment
variable and the signals received. When run in foreground, the information is
written to stdout. An additional output file can be specified using the
RSYSLOG_DEBUGLOG environment variable.
- Implement kernel firewalling to limit which hosts or networks have access
to the 514/UDP socket.
- Logging can be directed to an isolated or non-root filesystem which, if
filled, will not impair the machine.
- 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 rsyslogd to be run as a non-root process. ALSO NOTE
that this will prevent usage of remote logging on the default port since
rsyslogd will be unable to bind to the 514/UDP socket.
- Disabling inet domain sockets will limit risk to the local machine.
- Configuration file for rsyslogd. See rsyslog.conf(5) for
- The Unix domain socket to from where local syslog messages are read.
- The file containing the process id of rsyslogd.
- Default directory for rsyslogd modules. The prefix is
specified during compilation (e.g. /usr/local).
- Controls runtime debug support. It contains an option string with the
following options possible (all are case insensitive):
- Turns on debugging and prevents forking. This is processed earlier in the
startup than command line options (i.e. -d) and as such enables earlier
debugging output. Mutually exclusive with DebugOnDemand.
- Enables debugging but turns off debug output. The output can be toggled by
sending SIGUSR1. Mutually exclusive with Debug.
- Print out the logical flow of functions (entering and exiting them)
- Specifies which files to trace LogFuncFlow. If not set (the default), a
LogFuncFlow trace is provided for all files. Set to limit it to the files
specified.FileTrace may be specified multiple times, one file each (e.g.
export RSYSLOG_DEBUG="LogFuncFlow FileTrace=vm.c
- Print the content of the debug function database whenever debug
information is printed (e.g. abort case)!
- Print all debug information immediately before rsyslogd exits (currently
- Print mutex action as it happens. Useful for finding deadlocks and
- Do not prefix log lines with a timestamp (default is to do that).
- Do not emit debug messages to stdout. If RSYSLOG_DEBUGLOG is not set, this
means no messages will be displayed at all.
- Display a very short list of commands - hopefully a life saver if you
can't access the documentation...
Please review the file BUGS for up-to-date information on known bugs and
Please visit https://www.rsyslog.com/doc/ for additional information,
tutorials and a support forum.
rsyslogd is derived from sysklogd sources, which in turn was taken from
the BSD sources. Special thanks to Greg Wettstein (email@example.com) and
Martin Schulze (firstname.lastname@example.org) for the fine sysklogd package.
- If set, writes (almost) all debug message to the specified log file in
addition to stdout.
- Provides the default directory in which loadable modules reside.
Compared to syslogd(8), rsyslogd cannot be forced to reread its
configuration file with the SIGHUP signal and therefore svcadm refresh
is not sufficient to update the configuration. To do that, you must restart
rsyslogd with svcadm restart.
- Rainer Gerhards
- Adiscon GmbH
- Grossrinderfeld, Germany