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
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 http://www.rsyslog.com/doc
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.
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
or an alternative file, given with the
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 optionshave been deprecated and replaced with config file directives. The-c option controls the backward compatibility mode in use.
When sending UDP messages, there are potentially multiple paths to
the target destination. By default,
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.
to listen to IPv4 addresses only.
If neither -4 nor -6 is given,
listens to all configured addresses of the system.
to listen to IPv6 addresses only.
If neither -4 nor -6 is given,
listens to all configured addresses of the system.
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
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 -c3command 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.
Turns on debug mode. Using this the daemon will not proceed a
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
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.
Specify a hostname that should be logged only with its simple hostname
and not the fqdn. Multiple hosts may be specified using the colon
Avoid auto-backgrounding. This is needed especially if the
is started and controlled by
Do a coNfig check. Do NOT run in regular mode, just check configuration
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).
is started and controlled by
-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.
Specify a domainname that should be stripped off before
logging. Multiple domains may be specified using the colon (``:'')
Please be advised that no sub-domains may be specified but only entire
domains. For example if
is specified and the host logging resolves to satu.infodrom.north.de
no domain would be cut, you will have to specify two domains like:
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
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.
Print version and exit.
Suppress warnings issued when messages are received from non-authorized
machines (those, that are in no AllowedSender list).
Disable DNS for remote messages.
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)
perform close all open files.
Also, in v3 a full restart will be done in order to read changed configuration files.
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 remove
this restart functionality of HUP (it will go away in v5). So it is advised to use
HUP only for closing files, and a "real restart" (e.g. /etc/rc.d/rsyslogd restart)
to activate configuration changes.
TERM , INT , QUIT
Switch debugging on/off. This option can only be used if
is started with the
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
will be very verbose by writing much of what it does on stdout.
Configuration file for
for exact information.
The Unix domain socket to from where local syslog messages are read.
The file containing the process id of
Default directory for
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):
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
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
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 annoyances.
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.