rsyslogd - reliable and extended syslogd


rsyslogd [ -4 ] [ -6 ] [ -A ] [ -d ] [ -f config file ] [ -i pid file ] [ -l hostlist ] [ -n ] [ -N level ] [ -q ] [ -Q ] [ -s domainlist ] [ -u userlevel ] [ -v ] [ -w ] [ -x ]


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 cover basic aspects of operation. For details and configuration examples, see the rsyslog.conf (5) man page and the online documentation at

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 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.


Note that in version 3 of rsyslog a number of command line options have been deprecated and replaced with config file directives. The -c option controls the backward compatibility mode in use.
-A When sending UDP messages, there are potentially multiple paths to the target destination. By default, rsyslogd only sends to the first target it can successfully send to. If -A is given, messages are sent to all targets. This may improve reliability, but may also cause message duplication. This option should be enabled only if it is fully understood.
-4 Causes rsyslogd to listen to IPv4 addresses only. If neither -4 nor -6 is given, rsyslogd listens to all configured addresses of the system.
-6 Causes rsyslogd to listen to IPv6 addresses only. If neither -4 nor -6 is given, rsyslogd listens to all configured addresses of the system.
-c version
 Selects the desired backward compatibility mode. It must always be the first option on the command line, as it influences processing of the other options. To use the rsyslog v3 native interface, specify -c3. To use compatibility mode , either do not use -c at all or use -c<version> where version is the rsyslog version that it shall be compatible with. Using -c0 tells rsyslog to be command-line compatible to sysklogd, which is the default if -c is not given. Please note that rsyslogd issues warning messages if the -c3 command line option is not given. This is to alert you that your are running in compatibility mode. Compatibility mode interferes with your rsyslog.conf commands and may cause some undesired side-effects. It is meant to be used with a plain old rsyslog.conf - if you use new features, things become messy. So the best advice is to work through this document, convert your options and config file and then use rsyslog in native mode. In order to aid you in this process, rsyslog logs every compatibility-mode config file directive it has generated. So you can simply copy them from your logfile and paste them to the config.
-d Turns on debug mode. Using this the daemon will not proceed a fork(2) to set itself in the background, but opposite to that stay in the foreground and write much debug information on the current tty. 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.
-l hostlist
 Specify a hostname that should be logged only with its simple hostname and not the fqdn. Multiple hosts may be specified using the colon (‘‘:’’) separator.
-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 <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). rsyslogd is started and controlled by init(8).
-q add hostname if DNS fails during ACL processing
 During ACL processing, hostnames are resolved to IP addresses for performance reasons. If DNS fails during that process, the hostname is added as wildcard text, which results in proper, but somewhat slower operation once DNS is up again.
-Q do not resolve hostnames during ACL processing
 Do not resolve hostnames to IP addresses during ACL processing.
-s domainlist
 Specify a domainname that should be stripped off before logging. Multiple domains may be specified using the colon (‘‘:’’) separator. Please be advised that no sub-domains may be specified but only entire domains. For example if -s is specified and the host logging resolves to no domain would be cut, you will have to specify two domains like: -s
-u userlevel
 This is a "catch all" option for some very seldomly-used user settings. The "userlevel" variable selects multiple things. Add the specific values to get the combined effect of them. A value of 1 prevents rsyslogd from parsing hostnames and tags inside messages. A value of 2 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. To combine these two features, use a userlevel of 3 (1+2). Whenever you use an -u option, make sure you really understand what you do and why you do it.
-v Print version and exit.
-w Suppress warnings issued when messages are received from non-authorized machines (those, that are in no AllowedSender list).
-x Disable DNS for remote messages.


Rsyslogd reacts to a set of signals. You may easily send a signal to rsyslogd using the following:
kill -SIGNAL $(cat /var/run/
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/
HUP This lets rsyslogd perform a re-initialization. All open files are closed, the configuration file (default is /etc/rsyslog.conf) will be reread and the rsyslog(3) facility is started again. Note that this means a full rsyslogd restart is done. This has, among others, the consequence that TCP and other connections are torn down. Also, if any queues are not running in disk assisted mode or are not set to persist data on shutdown, queue data is lost. HUPing rsyslogd is an extremely expensive operation and should only be done when actually necessary. Actually, it is a rsyslgod stop immediately followed by a restart. Future versions will probably include a special handling which only closes files, but will not cause any of the other effects.
 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.


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 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 -d option then rsyslogd will be very verbose by writing much of what it does on stdout.


 Configuration file for rsyslogd. See rsyslog.conf(5) for exact information.
 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):

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...

 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 annoyances.

Further Information

Please visit for additional information, tutorials and a support forum.


rsyslog.conf(5), logger(1), syslog(2), syslog(3), services(5), savelog(8)


rsyslogd is derived from sysklogd sources, which in turn was taken from the BSD sources. Special thanks to Greg Wettstein ( and Martin Schulze ( for the fine sysklogd package.

Rainer Gerhards
Adiscon GmbH
Grossrinderfeld, Germany

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