Section: Linux System Administration (8)
Updated: 02 Dec 2014
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rsyslogd - reliable and extended syslogd
[ -d ]
[ -D ]
[ -n ]
[ -C ]
[ -v ]
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
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 http://www.rsyslog.com/doc
is derived from the sysklogd package which in turn is derived from the
stock BSD sources.
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.
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
The main configuration file
or an alternative file, given with the
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.
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
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
is started and controlled by
- -N level
Do a coNfig check. Do NOT run in regular mode, just check configuration
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 option).
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.
reacts to a set of signals. You may easily send a signal to
using the following:
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)
perform close all open files.
- TERM , INT , QUIT
Switch debugging on/off. This option can only be used if
is started with the
Wait for childs if some were born, because of wall'ing messages.
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.
There are a number of methods of protecting a machine:
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.
Message replay and spoofing
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
produces debugging information according to the
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
Configuration file for
for exact information.
The Unix domain socket to from where local syslog messages are read.
The file containing the process id of
Default directory for
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
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 not implemented!)
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...
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.
Please review the file BUGS for up-to-date information on known
bugs and annoyances.
for additional information, tutorials and a support forum.
is derived from sysklogd sources, which in turn was taken from
the BSD sources. Special thanks to Greg Wettstein (firstname.lastname@example.org)
and Martin Schulze (email@example.com) for the fine sysklogd package.
- Rainer Gerhards
- Adiscon GmbH
- Grossrinderfeld, Germany
- SECURITY THREATS
- Message replay and spoofing
- Further Information
- SEE ALSO
This document was created by
using the manual pages.
Time: 10:48:15 GMT, September 25, 2018