RSYSLOGD(8) Linux System Administration RSYSLOGD(8)NAMErsyslogd - reliable and extended syslogd
SYNOPSISrsyslogd [ -4 ] [ -6 ] [ -A ] [ -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 docu‐
mentation, 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 sup‐
port 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 con‐
figuration 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 stan‐
dard 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
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 compatibility mode in use.
-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
Selects the desired backward compatibility mode. It must always
be the first option on the command line, as it influences pro‐
cessing 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 rsys‐
log 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 interferes with your rsyslog.conf com‐
mands 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.
-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/rsys‐
log.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 con‐
figuration 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 actu‐
ally 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 (``:'') sepa‐
rator. Please be advised that no sub-domains may be specified
but only entire domains. For example if -s north.de is speci‐
fied 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 test‐
bed. 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 try‐
ing to send, e.g. with HUP. So it then becomes:
kill -HUP $(cat /var/run/rsyslogd.pid)
HUP This lets rsyslogd perform a re-initialization. All open files
are closed, the configuration file (default is /etc/rsys‐
log.conf) will be reread and the rsyslog(3) facility is started
again. Note that this means a full rsyslogd restart is done.
This has, among others, the consequence that TCP and other con‐
nections 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. Actu‐
ally, it is a rsyslgod stop immediately followed by a restart.
Future versions will probably include a special handling which
only closes files, but will not cause any of the other effects.
TERM , INT , QUIT
Rsyslogd will die.
USR1 Switch debugging on/off. This option can only be used if rsys‐
logd 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 log‐
ging 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 File‐
Trace=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 ALSOrsyslog.conf(5), logger(1), syslog(2), syslog(3), services(5),
savelog(8)COLLABORATORSrsyslogd is derived from sysklogd sources, which in turn was taken from
the BSD sources. Special thanks to Greg Wettstein (greg@wind.enjel‐
lic.com) and Martin Schulze (joey@linux.de) for the fine sysklogd pack‐
age.
Rainer Gerhards
Adiscon GmbH
Grossrinderfeld, Germany
rgerhards@adiscon.com
Version 3.21.1 29 July 2008 RSYSLOGD(8)
