Manual: rsyslogd
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 documentation, because the man pages only covers basic aspects of op‐
eration. 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 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 stan‐
dard syslog.conf and act like the original syslogd. However, an original syslogd will not work correctly with a rsys‐
log-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 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 towards developers.
-d 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 up‐
per case!), so "-iNONE".
-n 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
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 , QUIT
Rsyslogd will die.
INT Rsyslogd will die. This signal is only processed if rsyslogd was started with the -d debug option, if the
shutdown.enable.ctl-c global config parameter is set, or if running as PID 1 (e.g. in a container).
USR1 Switch debugging on/off. This option can only be used if rsyslogd is started with the -d debug option.
CHLD Wait for children if some were born, because of wall'ing messages.
SECURITY THREATS
There is the potential for the rsyslogd daemon to be used as a conduit for a denial of service attack. A rogue pro‐
gram(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 sys‐
tem 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 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.
DEBUGGING
When debugging is turned on using the -d option, rsyslogd produces debugging information according to the RSYSLOG_DE‐
BUG 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 op‐
tions (i.e. -d) and as such enables earlier debugging output. Mutually exclusive 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 exiting 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. export RSYSLOG_DEBUG="LogFuncFlow FileTrace=vm.c FileTrace=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 deadlocks 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.enjellic.com) and Martin Schulze (joey@linux.de) for the fine sysklogd package.
Rainer Gerhards
Adiscon GmbH
Grossrinderfeld, Germany
rgerhards@adiscon.com
Version 8.2502.0 18 February 2025 RSYSLOGD(8)