rsyslogd - reliable and extended syslogd
] [ -D
] [ -f config file
[ -i pid file
] [ -n
] [ -N level
] [ -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
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 at https://www.rsyslog.com/doc/
(8) 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.
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
- 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
/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 upper case!), so "-iNONE".
- 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
<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
- 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)
- This lets rsyslogd perform close all open files.
- TERM , INT , QUIT
- Rsyslogd will die.
- Switch debugging on/off. This option can only be used if rsyslogd
is started with the -d debug option.
- 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 -d
debugging information according to the RSYSLOG_DEBUG
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 rsyslogd. See rsyslog.conf(5) for
- The Unix domain socket to from where local syslog messages are read.
- 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):
- 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 FileTrace=vm.c
- 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
- 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
Please visit https://www.rsyslog.com/doc/
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