RSYSLOGD(8) Linux System Administration RSYSLOGD(8)
rsyslogd – reliable and extended syslogd
rsyslogd [ -d ] [ -D ] [ -f config file ] [ -i pid file ] [ -n ] [ -N
level ] [ -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 docu‐
mentation, because the man pages only covers basic aspects of opera‐
tion. For details and configuration examples, see the rsyslog.conf (5)
man page and the online documentation at http://www.rsyslog.com/doc
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 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 con‐
figuration 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 stan‐
dard libraries. If a binary linked to the standard shared libraries
fails to function correctly we would like an example of the anomalous
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 informa‐
-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.
-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 con‐
figuration file correctness. This option is meant to verify a
config file. To do so, run rsyslogd interactively in foreground,
modifies behaviour. Currently, 0 is the same as not specifying
the -N option at all (so this makes limited sense) and 1 actu‐
ally activates the code. Later, higher levels will mean more
verbosity (this is a forward-compatibility option). rsyslogd is
started and controlled by init(8).
-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 try‐
ing 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 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:
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 log‐
ging 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 vari‐
able 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 exact
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 insensitive):
Debug Turns on debugging and prevents forking. This is pro‐
cessed 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
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 File‐
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 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
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 reside.
Please review the file BUGS for up-to-date information on known bugs
Please visit http://www.rsyslog.com/doc for additional information,
tutorials and a support forum.
rsyslog.conf(5), logger, syslog(2), syslog(3), services(5),
rsyslogd is derived from sysklogd sources, which in turn was taken from
the BSD sources. Special thanks to Greg Wettstein (email@example.com‐
lic.com) and Martin Schulze (firstname.lastname@example.org) for the fine sysklogd pack‐
Version 8.6.0 02 Dec 2014 RSYSLOGD(8)