RSYSLOGD(8) Linux System Administration RSYSLOGD(8)
NAME
rsyslogd - reliable and extended syslogd
SYNOPSIS
rsyslogd [ -d ] [ -D ] [ -f config file ] [ -i pid file ] [ -n ] [ -N
level ] [ -o fullconf ] [ -C ] [ -v ]
DESCRIPTION
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 documenta-
tion, 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 https://www.rsyslog.com/doc/
Rsyslogd(8) is derived from the sysklogd package which in turn is de-
rived 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, nor-
mally a program name field, too, but that depends on how trusty the log-
ging 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 ph-
pLogCon 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 en-
sure that rsyslogd follows its default, standard BSD behavior. Of
course, some configuration file changes are necessary in order to sup-
port 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 config-
uration file. At best, it will generate funny looking file names. The
second important concept to note is that this version of rsyslogd inter-
acts transparently with the version of syslog found in the standard li-
braries. 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.
OPTIONS
-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 originally targeted to-
wards developers.
-d Turns on debug mode. See the DEBUGGING section for more informa-
tion.
-f config file
Specify an alternative configuration file instead of /etc/rsys-
log.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 re-
served name "NONE" (all upper case!), so "-iNONE".
-n Avoid auto-backgrounding. This is needed especially if the rsys-
logd is started and controlled by init(8).
-N level
Do a config check. Do NOT run in regular mode, just check config-
uration file correctness. This option is meant to verify a con-
fig 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).
-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.
-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.
SIGNALS
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 , QUIT
Rsyslogd will die.
USR1 Switch debugging on/off. This option can only be used if rsys-
logd is started with the -d debug option.
CHLD Wait for children if some were born, because of wall'ing mes-
sages.
SECURITY THREATS
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:
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 ma-
chine.
Message replay and spoofing
If remote logging is enabled, messages can easily be spoofed and re-
played. 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 en-
cryption. Be sure to think about syslog network security before enabling
it.
DEBUGGING
When debugging is turned on using the -d option, rsyslogd produces de-
bugging 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.
FILES
/etc/rsyslog.conf
Configuration file for rsyslogd. See rsyslog.conf(5) for exact
information.
/dev/log
The Unix domain socket to from where local syslog messages are
read.
/var/run/rsyslogd.pid
The file containing the process id of rsyslogd.
prefix/lib/rsyslog
Default directory for rsyslogd modules. The prefix is specified
during compilation (e.g. /usr/local).
ENVIRONMENT
RSYSLOG_DEBUG
Controls runtime debug support. It contains an option string with
the following options possible (all are case insensitive):
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 ex-
clusive with DebugOnDemand.
DebugOnDemand
Enables debugging but turns off debug output. The output
can be toggled by sending SIGUSR1. Mutually exclusive with
Debug.
LogFuncFlow
Print out the logical flow of functions (entering and ex-
iting them)
FileTrace
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. ex-
port RSYSLOG_DEBUG="LogFuncFlow FileTrace=vm.c File-
Trace=expr.c"
PrintFuncDB
Print the content of the debug function database whenever
debug information is printed (e.g. abort case)!
PrintAllDebugInfoOnExit
Print all debug information immediately before rsyslogd
exits (currently not implemented!)
PrintMutexAction
Print mutex action as it happens. Useful for finding dead-
locks and such.
NoLogTimeStamp
Do not prefix log lines with a timestamp (default is to do
that).
NoStdOut
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...
RSYSLOG_DEBUGLOG
If set, writes (almost) all debug message to the specified log
file in addition to stdout.
RSYSLOG_MODDIR
Provides the default directory in which loadable modules reside.
BUGS
Please review the file BUGS for up-to-date information on known bugs and
annoyances.
Further Information
Please visit https://www.rsyslog.com/doc/ for additional information,
tutorials and a support forum.
SEE ALSO
rsyslog.conf(5), logger(1), syslog(2), syslog(3), services(5),
savelog(8)
COLLABORATORS
rsyslogd is derived from sysklogd sources, which in turn was taken from
the BSD sources. Special thanks to Greg Wettstein (greg@wind.enjel-
lic.com) and Martin Schulze (joey@linux.de) for the fine sysklogd pack-
age.
Rainer Gerhards
Adiscon GmbH
Grossrinderfeld, Germany
rgerhards@adiscon.com
Version 8.2502.0 18 February 2025 RSYSLOGD(8)
Generated by dwww version 1.16 on Mon Dec 15 07:14:46 CET 2025.