Provided by: rsyslog_7.4.4-1ubuntu2.7_amd64 
NAME
rsyslogd - reliable and extended syslogd
SYNOPSIS
rsyslogd [ -4 ] [ -6 ] [ -A ] [ -d ] [ -D ] [ -f config file ]
[ -i pid file ] [ -l hostlist ] [ -n ] [ -N level ]
[ -q ] [ -Q ] [ -s domainlist ] [ -u userlevel ] [ -v ] [ -w ] [ -x ]
DESCRIPTION
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 http://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 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.
OPTIONS
-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
This option has been obsoleted and has no function any longer. It is still accepted in order not
to break existing scripts. However, future versions may not support it.
-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. 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 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.
-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.
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. 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
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 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.
DEBUGGING
When debugging is turned on using -d option then rsyslogd will be very verbose by writing much of what it
does on stdout.
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 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...
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 http://www.rsyslog.com/doc for additional information, tutorials and a support 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@wind.enjellic.com) and Martin Schulze (joey@linux.de) for the fine sysklogd
package.
Rainer Gerhards
Adiscon GmbH
Grossrinderfeld, Germany
rgerhards@adiscon.com