RSYSLOGD(8) Linux System Administration RSYSLOGD(8)NAMErsyslogd - reliable and extended syslogd
SYNOPSISrsyslogd [ -4 ] [ -6 ] [ -A ] [ -a socket ] [ -d ] [ -e ]
[ -f config file ] [ -g port,max-nbr-of-sessions ] [ -h ]
[ -i pid file ] [ -l hostlist ] [ -m interval ] [ -n ] [ -o ]
[ -p socket ] [ -q ] [ -Q ] [ -r [port] ] [ -s domainlist ]
[ -t port,max-nbr-of-sessions ] [ -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 (via UDP and TCP).
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 MySQL 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 orginal 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.
For details and configuration examples, see the rsyslog.conf (5) man
page.
OPTIONS-A When sending UDP messages, there are potentially multiple pathes
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 duplicaton. This option should
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.
-a socket
Using this argument you can specify additional sockets from that
rsyslogd has to listen to. This is needed if you're going to
let some daemon run within a chroot() environment. You can use
up to 19 additional sockets. If your environment needs even
more, you have to increase the symbol MAXFUNIX within the sys‐
logd.c source file. An example for a chroot() daemon is
described by the people from OpenBSD at
http://www.psionic.com/papers/dns.html.
-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.
-e Set the default of $RepeatedMsgReduction config option to "off".
Hine: "e" like "every message". For further information, see
there.
-f config file
Specify an alternative configuration file instead of /etc/rsys‐
log.conf, which is the default.
-g Identical to -t except that every tcp connection is authenti‐
cated using gss-api (kerberos 5). Service name may be set using
$GssListenServiceName or the default "host" will be used.
Encryption can be used if specified by the client and supported
by both sides.
-h By default rsyslogd will not forward messages it receives from
remote hosts. Specifying this switch on the command line will
cause the log daemon to forward any remote messages it receives
to forwarding hosts which have been defined.
-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.
-m interval
The rsyslogd logs a mark timestamp regularly. The default
interval between two -- MARK -- lines is 20 minutes. This can
be changed with this option. Setting the interval to zero turns
it off entirely.
-n Avoid auto-backgrounding. This is needed especially if the
rsyslogd is started and controlled by init(8).
-o Omit reading the standard local log socket. This option is most
useful for running multiple instances of rsyslogd on a single
machine. When specified, no local log socket is opened at all.
-p socket
You can specify an alternative unix domain socket instead of
/dev/log.
-q add hostname if DNS fails during ACL processing
During ACL processing, hostnames are resolved to IP addreses for
performance reasons. If DNS fails during that process, the host‐
name 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.
-r ["port"]
Activates the syslog/udp listener service. The listener will
listen to the specified port. If no port is specified, 0 is
used as port number, which in turn will lead to a lookup of the
system default syslog port. If there is no system default, 514
is used. Please note that the port must immediately follow the
-r option. Thus "-r514" is valid while "-r 514" is invalid (note
the space).
-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.
-t port,max-nbr-of-sessions
Activates the syslog/tcp listener service. The listener will
listen to the specified port. If max-nbr-of-sessions is speci‐
fied, that becomes the maximum number of concurrent tcp ses‐
sions. If not specified, the default is 200. Please note that
syslog/tcp is not standardized, but the implementation in rsys‐
logd follows common practice and is compatible with e.g. Cisco
PIX, syslog-ng and MonitorWare (Windows). Please note that the
port must immediately follow the -t option. Thus "-t514" is
valid while "-t 514" is invalid (note the space).
-v Print version and exit.
-w Supress 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`
SIGHUP 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.
SIGTERM , SIGINT , SIGQUIT
Rsyslogd will die.
SIGUSR1
Switch debugging on/off. This option can only be used if rsys‐
logd is started with the -d debug option.
SIGCHLD
Wait for childs if some were born, because of wall'ing messages.
SUPPORT FOR REMOTE LOGGING
Rsyslogd provides network support to the syslogd facility. Network
support means that messages can be forwarded from one node running
rsyslogd to another node running rsyslogd (or a compatible syslog
implementation) where they will be actually logged to a disk file.
To enable this you have to specify one of -g , -r or -t options on the
command line. The default behavior is that rsyslogd won't listen to
the network. You can also combine these options if you want rsyslogd to
listen to both TCP and UDP messages. Only one of the TCP listener
options can be used. The last one specified will take effect.
The strategy is to have rsyslogd listen on a unix domain socket for
locally generated log messages. This behavior will allow rsyslogd to
inter-operate with the syslog found in the standard C library. At the
same time rsyslogd listens on the standard syslog port for messages
forwarded from other hosts. To have this work correctly the ser‐
vices(5) files (typically found in /etc) must have the following entry:
syslog 514/udp
If this entry is missing rsyslogd will use the well known port of 514
(so in most cases, it's not really needed).
To cause messages to be forwarded to another host replace the normal
file line in the rsyslog.conf file with the name of the host to which
the messages is to be sent prepended with an @ (for UDP delivery) or
the sequence @@ (for TCP delivery). The host name can also be followed
by a colon and a port number, in which case the message is sent to the
specified port on the remote host.
For example, to forward ALL messages to a remote host use the
following rsyslog.conf entry:
# Sample rsyslogd configuration file to
# messages to a remote host forward all.
*.* @hostname
More samples can be found in sample.conf.
If the remote hostname cannot be resolved at startup, because
the name-server might not be accessible (it may be started after
rsyslogd) you don't have to worry. Rsyslogd will retry to
resolve the name ten times and then complain. Another possibil‐
ity to avoid this is to place the hostname in /etc/hosts.
With normal syslogds you would get syslog-loops if you send out
messages that were received from a remote host to the same host
(or more complicated to a third host that sends it back to the
first one, and so on).
To avoid this no messages that were received from a remote host
are sent out to another (or the same) remote host. You can dis‐
able this feature by the -h option.
If the remote host is located in the same domain as the host,
rsyslogd is running on, only the simple hostname will be logged
instead of the whole fqdn.
In a local network you may provide a central log server to have
all the important information kept on one machine. If the net‐
work consists of different domains you don't have to complain
about logging fully qualified names instead of simple hostnames.
You may want to use the strip-domain feature -s of this server.
You can tell rsyslogd to strip off several domains other than
the one the server is located in and only log simple hostnames.
Using the -l option there's also a possibility to define single
hosts as local machines. This, too, results in logging only
their simple hostnames and not the fqdns.
OUTPUT TO DATABASES
Rsyslogd has support for writing data to MySQL database tables. The
exact specifics are described in the rsyslog.conf (5) man page. Be sure
to read it if you plan to use database logging.
While it is often handy to have the data in a database, you must be
aware of the implications. Most importantly, database logging takes far
longer than logging to a text file. A system that can handle a large
log volume when writing to text files can most likely not handle a sim‐
ilar large volume when writing to a database table.
OUTPUT TO NAMED PIPES (FIFOs)
Rsyslogd has support for logging output to named pipes (fifos). A fifo
or named pipe can be used as a destination for log messages by prepend‐
ing a pipy symbol (``|'') to the name of the file. This is handy for
debugging. Note that the fifo must be created with the mkfifo command
before rsyslogd is started.
The following configuration file routes debug messages from the
kernel to a fifo:
# Sample configuration to route kernel debugging
# messages ONLY to /usr/adm/debug which is a
# named pipe.
kern.=debug |/usr/adm/debug
INSTALLATION CONCERNS
There is probably one important consideration when installing rsyslogd.
It is dependent on proper formatting of messages by the syslog func‐
tion. The functioning of the syslog function in the shared libraries
changed somewhere in the region of libc.so.4.[2-4].n. The specific
change was to null-terminate the message before transmitting it to the
/dev/log socket. Proper functioning of this version of rsyslogd is
dependent on null-termination of the message.
This problem will typically manifest itself if old statically linked
binaries are being used on the system. Binaries using old versions of
the syslog function will cause empty lines to be logged followed by the
message with the first character in the message removed. Relinking
these binaries to newer versions of the shared libraries will correct
this problem.
The rsyslogd(8) can be run from init(8) or started as part of the rc.*
sequence. If it is started from init the option -n must be set, other‐
wise you'll get tons of syslog daemons started. This is because
init(8) depends on the process ID.
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 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 reply 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. Whenever the con‐
figuration file is reread and re-parsed you'll see a tabular, corre‐
sponding to the internal data structure. This tabular consists of four
fields:
number This field contains a serial number starting by zero. This num‐
ber represents the position in the internal data structure (i.e.
the array). If one number is left out then there might be an
error in the corresponding line in /etc/rsyslog.conf.
pattern
This field is tricky and represents the internal structure
exactly. Every column stands for a facility (refer to sys‐
log(3)). As you can see, there are still some facilities left
free for former use, only the left most are used. Every field
in a column represents the priorities (refer to syslog(3)).
action This field describes the particular action that takes place
whenever a message is received that matches the pattern. Refer
to the syslog.conf(5) manpage for all possible actions.
arguments
This field shows additional arguments to the actions in the last
field. For file-logging this is the filename for the logfile;
for user-logging this is a list of users; for remote logging
this is the hostname of the machine to log to; for console-log‐
ging this is the used console; for tty-logging this is the spec‐
ified tty; wall has no additional arguments.
templates
There will also be a second internal structure which lists all
defined templates and there contents. This also enables you to
see the internally-defined, hardcoded templates.
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.
BUGS
Please review the file BUGS for up-to-date information on known bugs
and annouyances.
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
Michael Meckelein
Adiscon GmbH
mmeckelein@adiscon.com
Version 2.0.5 28 March 2008 RSYSLOGD(8)
