RSYSLOGD(8) Linux System Administration RSYSLOGD(8)
NAME
rsyslogd - reliable and extended syslogd
SYNOPSIS
rsyslogd [ -d ] [ -D ] [ -f config file ] [ -i pid file ] [ -n ] [ -N level ] [
-C ] [ -v ]
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 ad‐
vanced features, you need to look at the HTML documentation, because the man
pages only covers basic aspects of operation. For details and configuration ex‐
amples, see the rsyslog.conf (5) man page and the online documentation at
https://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 sup‐
ports precise timestamps and writing directly to databases. If the database op‐
tion 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 er‐
ror element is ignored. It is tried to parse the rest of the line.
OPTIONS
-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. 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 ma‐
chine. To disable writing a pid file, use the reserved name "NONE" (all
upper case!), so "-iNONE".
-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).
-C This prevents rsyslogd from changing to the root directory. This is al‐
most never a good idea in production use. This option was introduced in
support of the internal testbed.
-v Print version and exit.
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.
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 de‐
nial of service attack. A rogue program(mer) could very easily flood the rsys‐
logd 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 individu‐
als 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 cer‐
tain 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 rsys‐
logd 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 informa‐
tion obtained from the packets for malicious things. Also, an attacker might re‐
play 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 au‐
thentication and encryption. Be sure to think about syslog network security be‐
fore enabling it.
DEBUGGING
When debugging is turned on using the -d option, rsyslogd produces debugging in‐
formation according to the RSYSLOG_DEBUG environment variable and the signals
received. When run in foreground, the information is written to stdout. An addi‐
tional output file can be specified using the RSYSLOG_DEBUGLOG environment vari‐
able.
FILES
/etc/rsyslog.conf
Configuration file for rsyslogd. See rsyslog.conf(5) for exact informa‐
tion.
/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 en‐
ables earlier debugging output. Mutually exclusive with DebugOnDe‐
mand.
DebugOnDemand
Enables debugging but turns off debug output. The output can be
toggled by sending SIGUSR1. Mutually exclusive with Debug.
LogFuncFlow
Print out the logical flow of functions (entering and exiting
them)
FileTrace
Specifies which files to trace LogFuncFlow. If not set (the de‐
fault), a LogFuncFlow trace is provided for all files. Set to
limit it to the files specified.FileTrace may be specified multi‐
ple 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 annoy‐
ances.
Further Information
Please visit https://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
Version 8.6.0 02 Dec 2014 RSYSLOGD(8)