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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 full-
conf ] [ -C ] [ -v ]
DESCRIPTION
Rsyslogd is a system utility providing support for message logging. Support of both in-
ternet and unix domain sockets enables this utility to support both local and remote log-
ging.
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 operation. For
details and configuration examples, see the rsyslog.conf (5) man page and the online docu-
mentation 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 defi-
nition 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 de-
fault, 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 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
orignally 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 upper 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 cor-
rectness. This option is meant to verify a config file. To do so, run rsyslogd in-
teractively in foreground, specifying -f <config-file> and -N level. The level ar-
gument 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 con-
figuration 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 rsyslogd is started with
the -d debug option.
CHLD Wait for childs 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 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 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 percent-
age 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 mes-
sages 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_DEBUG environment variable and the signals received. When run in
foreground, the information is written to stdout. An additional output file can be speci-
fied 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 op-
tions 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 de-
bugging 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 Log-
FuncFlow trace is provided for all files. Set to limit it to the files spec-
ified.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 sup-
port 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 AT wind.com) and Martin Schulze
(joey AT linux.de) for the fine sysklogd package.
Rainer Gerhards
Adiscon GmbH
Grossrinderfeld, Germany
rgerhards AT adiscon.com
Version 8.1905.0 28 May 2014 RSYSLOGD(8)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
On Apache
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