RSYSLOGD(8) Linux System Administration RSYSLOGD(8)
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 pro‐
gram is. The rsyslog package supports free definition of output formats via templates. It also supports pre‐
cise 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 func‐
tion 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.
-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.info‐
drom.north.de no domain would be cut, you will have to specify two domains like: -s north.de:info‐
drom.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 pre‐
vents 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)
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 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 writ‐
ten to stdout. An additional output file can be specified 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 options 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 debugging output. Mutually exclusive with
DebugOnDemand.
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 documen‐
tation...
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 ([email protected]) and Martin Schulze ([email protected]) for the fine sysklogd package.
Rainer Gerhards
Adiscon GmbH
Grossrinderfeld, Germany
[email protected]
Version 6.4.3 16 October 2012 RSYSLOGD(8)
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