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 documentationin 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 athttp://www.rsyslog.com/docRsyslogd(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
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.
-D 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
-d Turns on debug mode. See the DEBUGGING section for more
-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.
-n Avoid auto-backgrounding. This is needed especially if the
rsyslogd is started and controlled by init(8).
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
-C 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.
-v Print version and exit.
Rsyslogd reacts to a set of signals. You may easily send a signal to
rsyslogd 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)
HUP This lets rsyslogd perform close all open files.
TERM , INT , QUITRsyslogd will die.
USR1 Switch debugging on/off. This option can only be used if
rsyslogd is started with the -d debug option.
CHLD Wait for childs if some were born, because of wall'ing
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
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 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.
4. Disabling inet domain sockets will limit risk to the local
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
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.
Configuration file for rsyslogd. See rsyslog.conf(5) for
The Unix domain socket to from where local syslog messages are
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
Debug 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
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
Print all debug information immediately before rsyslogd
exits (currently not implemented!)
Print mutex action as it happens. Useful for finding
deadlocks and such.
Do not prefix log lines with a timestamp (default is to
Do not emit debug messages to stdout. If
RSYSLOG_DEBUGLOG is not set, this means no messages
will be displayed at all.
Help 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
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.
This page is part of the rsyslog (reliable and exitended syslog)
project. Information about the project can be found at
⟨http://www.rsyslog.com/⟩. If you have a bug report for this manual
page, send it to email@example.com. This page was obtained
from the project's upstream Git repository
⟨https://github.com/rsyslog/rsyslog⟩ on 2017-03-13. If you discover
any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page, or
you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a mail
Version 8.6.0 02 Dec 2014 RSYSLOGD(8)