Compatibility Notes for rsyslog v3

Written by Rainer Gerhards (2008-03-28)

Rsyslog aims to be a drop-in replacement for sysklogd. However, version 3 has some considerable enhancements, which lead to some backward compatibility issues both in regard to sysklogd and rsyslog v1 and v2. Most of these issues are avoided by default by not specifying the -c option on the rsyslog command line. That will enable backwards-compatibility mode. However, please note that things may be suboptimal in backward compatibility mode, so the advise is to work through this document, update your rsyslog.conf, remove the no longer supported startup options and then add -c3 as the first option to the rsyslog command line. That will enable native mode.

Please note that rsyslogd helps you during that process by logging appropriate messages about compatibility mode and backwards-compatibility statements automatically generated. You may want your syslogd log for those. They immediately follow rsyslogd’s startup message.


With v2 and below, inputs were automatically started together with rsyslog. In v3, inputs are optional! They come in the form of plug-in modules. At least one input module must be loaded to make rsyslog do any useful work. The config file directives doc briefly lists which config statements are available by which modules.

It is suggested that input modules be loaded in the top part of the config file. Here is an example, also highlighting the most important modules:

$ModLoad immark # provides --MARK-- message capability
$ModLoad imudp # provides UDP syslog reception
$ModLoad imtcp # provides TCP syslog reception
$ModLoad imgssapi # provides GSSAPI syslog reception
$ModLoad imuxsock # provides support for local system logging (e.g. via logger command)
$ModLoad imklog # provides kernel logging support (previously done by rklogd)

Command Line Options

A number of command line options have been removed. New config file directives have been added for them. The -h and -e option have been removed even in compatibility mode. They are ignored but an informative message is logged. Please note that -h was never supported in v2, but was silently ignored. It disappeared some time ago in the final v1 builds. It can be replaced by applying proper filtering inside syslog.conf.

-c option / Compatibility Mode

The -c option is new and tells rsyslogd about the desired backward compatibility mode. It must always be the first option on the command line, as it influences processing 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<vers> where vers is the rsyslog version that it shall be compatible to. Use -c0 to be command-line compatible to sysklogd.

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 commands 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 advise 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.

-e Option

This option is no longer supported, as the “last message repeated n times” feature is now turned off by default. We changed this default because this feature is causing a lot of trouble and we need to make it either go away or change the way it works. For more information, please see our dedicated forum thread on “last message repeated n times”. This thread also contains information on how to configure rsyslogd so that it continues to support this feature (as long as it is not totally removed).

-m Option

The -m command line option is emulated in compatibility mode. To replace it, use the following config directives (compatibility mode auto-generates them):

$ModLoad immark
$MarkMessagePeriod 1800 # 30 minutes

-r Option

Is no longer available in native mode. However, it is understood in compatibility mode (if no -c option is given). Use the $UDPSeverRun <port> config file directives. You can now also set the local address the server should listen to via $UDPServerAddress <ip> config directive.

The following example configures an UDP syslog server at the local address on port 514:

$ModLoad imudp
$UDPServerAddress # this MUST be before the $UDPServerRun directive!
$UDPServerRun 514

$UDPServerAddress \* means listen on all local interfaces. This is the default if no directive is specified.

Please note that now multiple listeners are supported. For example, you can do the following:

$ModLoad imudp
$UDPServerAddress # this MUST be before the $UDPServerRun directive!
$UDPServerRun 514
$UDPServerAddress \* # all local interfaces
$UDPServerRun 1514

These config file settings run two listeners: one at and one on port 1514, which listens on all local interfaces.

Default port for UDP (and TCP) Servers

Please note that with pre-v3 rsyslogd, a service database lookup was made when a UDP server was started and no port was configured. Only if that failed, the IANA default of 514 was used. For TCP servers, this lookup was never done and 514 always used if no specific port was configured. For consistency, both TCP and UDP now use port 514 as default. If a lookup is desired, you need to specify it in the “Run” directive, e.g. “$UDPServerRun syslog“.


klogd has (finally) been replaced by a loadable input module. To enable klogd functionality, do

$ModLoad imklog

Note that this can not be handled by the compatibility layer, as klogd was a separate binary. A limited set of klogd command line settings is now supported via rsyslog.conf. That set of configuration directives is to be expanded.

Output File Syncing

Rsyslogd tries to keep as compatible to stock syslogd as possible. As such, it retained stock syslogd’s default of syncing every file write if not specified otherwise (by placing a dash in front of the output file name). While this was a useful feature in past days where hardware was much less reliable and UPS seldom, this no longer is useful in today’s world. Instead, the syncing is a high performance hit. With it, rsyslogd writes files around 50 times slower than without it. It also affects overall system performance due to the high IO activity. In rsyslog v3, syncing has been turned off by default. This is done via a specific configuration directive


$ActionFileEnableSync on/off

which is off by default. So even if rsyslogd finds sync selector lines, it ignores them by default. In order to enable file syncing, the administrator must specify $ActionFileEnableSync on at the top of rsyslog.conf. This ensures that syncing only happens in some installations where the administrator actually wanted that (performance-intense) feature. In the fast majority of cases (if not all), this dramatically increases rsyslogd performance without any negative effects.

Output File Format

Rsyslog supports high precision RFC 3339 timestamps and puts these into local log files by default. This is a departure from previous syslogd behaviour. We decided to sacrifice some backward-compatibility in an effort to provide a better logging solution. Rsyslog has been supporting the high-precision timestamps for over three years as of this writing, but nobody used them because they were not default (one may also assume that most people didn’t even know about them). Now, we are writing the great high-precision time stamps, which greatly aid in getting the right sequence of logging events. If you do not like that, you can easily turn them off by placing

$ActionFileDefaultTemplate RSYSLOG_TraditionalFileFormat

right at the start of your rsyslog.conf. This will use the previous format. Please note that the name is case-sensitive and must be specified exactly as shown above. Please also note that you can of course use any other format of your liking. To do so, simply specify the template to use or set a new default template via the $ActionFileDefaultTemplate directive. Keep in mind, though, that templates must be defined before they are used.

Keep in mind that when receiving messages from remote hosts, the timestamp is just as precise as the remote host provided it. In most cases, this means you will only a receive a standard timestamp with second precision. If rsyslog is running at the remote end, you can configure it to provide high-precision timestamps (see below).

Forwarding Format

When forwarding messages to remote syslog servers, rsyslogd by default uses the plain old syslog format with second-level resolution inside the timestamps. We could have made it emit high precision timestamps. However, that would have broken almost all receivers, including earlier versions of rsyslog. To avoid this hassle, high-precision timestamps need to be explicitly enabled. To make this as painless as possible, rsyslog comes with a canned template that contains everything necessary.  To enable high-precision timestamps, just use:

$ActionForwardDefaultTemplate RSYSLOG_ForwardFormat # for plain TCP and UDP
$ActionGSSForwardDefaultTemplate RSYSLOG_ForwardFormat # for GSS-API

And, of course, you can always set different forwarding formats by just specifying the right template.

If you are running in a system with only rsyslog 3.12.5 and above in the receiver roles, it is suggested to add one (or both) of the above statements to the top of your rsyslog.conf (but after the $ModLoad’s!) - that will enable you to use the best in timestamp support available. Please note that when you use this format with other receivers, they will probably become pretty confused and not detect the timestamp at all. In earlier rsyslog versions, for example, that leads to duplication of timestamp and hostname fields and disables the detection of the original hostname in a relayed/NATed environment. So use the new format with care.

Queue Modes for the Main Message Queue

Either “FixedArray” or “LinkedList” is recommended. “Direct” is available, but should not be used except for a very good reason (“Direct” disables queueing and will potentially lead to message loss on the input side).

See also

Help with configuring/using Rsyslog:

See also

Contributing to Rsyslog:

Copyright 2008-2023 Rainer Gerhards (Großrinderfeld), and Others.