RSYSLOGD(8) Linux System Administration RSYSLOGD(8)
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
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 docu-
mentation at http://www.rsyslog.com/doc
Rsyslogd(8) is derived from the sysklogd package which in turn is derived from the stock
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
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 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 compatibil-
ity mode in use.
-A When sending UDP messages, there are potentially multiple paths to the target des-
tination. 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 relia-
bility, 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.
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 inter-
feres 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 con-
-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
-f config file
Specify an alternative configuration file instead of /etc/rsyslog.conf, which is
-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 (``:'') separator.
-n Avoid auto-backgrounding. This is needed especially if the rsyslogd is started and
controlled by init(8).
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
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 rea-
sons. 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 (``:'') separator. Please be advised that no sub-
domains may be specified but only entire domains. For example if -s north.de 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: -s north.de:infodrom.north.de.
This is a "catch all" option for some very seldomly-used user settings. The "user-
level" variable selects multiple things. Add the specific values to get the com-
bined 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 under-
stand 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
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. 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. HUP-
ing rsyslogd is an extremely expensive operation and should only be done when actu-
ally 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
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 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
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
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.
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
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 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"
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
Print mutex action as it happens. Useful for finding deadlocks and such.
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.
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
Provides the default directory in which loadable modules reside.
Please review the file BUGS for up-to-date information on known bugs and annoyances.
Please visit http://www.rsyslog.com/doc for additional information, tutorials and a sup-
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 (email@example.com) and Martin Schulze
(firstname.lastname@example.org) for the fine sysklogd package.
Version 3.21.1 29 July 2008 RSYSLOGD(8)