config snippet

Sending messages with tags larger than 32 characters

The relevant syslog RFCs 3164 and 5424 limit the syslog tag to 32 characters max. Messages with larger tag length are malformed and may be discarded by receivers. Anyhow, some folks sometimes need to send tags longer than permitted.

To do so, a new template must be created and used when sending. The simplest way is to start with the standard forwarding template. The standard templates are hardcoded inside rsyslog. Thus they do not show up in your configuration file (but you can obtain them from the source, of course). In 5.8.6, the forwarding template is defined as follows:

template (name="ForwardFormat" type="string" string="<%PRI%>%TIMESTAMP:::date-rfc3339% %HOSTNAME%

NOTE: all templates are on one line in rsyslog.conf. They are broken here for readability.

This template is RFC-compliant. Now look at the part in red. It specifies the tag. Note that, via the property replacer, it is restricted to 32 characters (from position 1 to position 32 inclusive). This is what you need to change. To remove the limit … just remove it ;-) This leads to a template like this:

template (name="LongTagForwardFormat" type="string" string="<%PRI%>%TIMESTAMP:::date-rfc3339% %HOSTNAME%

Note that I have renamed the template in order to avoid conflicts with build-in templates. As it is a custom template, it is not hardcoded, so you need to actually configure it in your rsyslog.conf. Then, you need to use that template if you want to send messages to a remote host. This can be done via the usual way. Let’s assume you use legacy plain TCP syslog. Then the line looks as follows:


This will bind the forwarding action to the newly defined template. Now tags of any size will be forwarded. Please keep in mind that receivers may have problems with large tags and may truncate them or drop the whole message. So check twice that the receiver handles long tags well.

Rsyslog supports tags to a build-defined maximum. The current (5.8.6) default is 511 characters, but this may be different if you install from a package, use a newer version of rsyslog or use sources obtained from someone else. So double-check.

Using the syslog receiver module

We want to use rsyslog in its general purpose. We want to receive syslog. In rsyslog, we have two possibilities to achieve that.

Things to think about

First of all, we will determine, which way of syslog reception we want to use. We can receive syslog via UDP or TCP. The config statements are each a bit different in both cases.

In most cases, UDP syslog should be fully sufficient and performing well. But, you should be aware, that on large message bursts messages can be dropped. That is not the case with TCP syslog, since the sender and receiver communicate about the arrival of network packets. That makes TCP syslog more suitable for environments where log messages may not be lost or that must ensure PCI compliance (like banks).

Config Statements

module(load="imudp") # needs to be done just once
input(type="imudp" port="514")


module(load="imtcp") # needs to be done just once
input(type="imtcp" port="514")

How it works

The configuration part for the syslog server is pretty simple basically. Though, there are some parameters that can be set for both modules. But this is not necessary in most cases.

In general, first the module needs to be loaded. This is done via the directive module().

module(load="imudp / imtcp")

The module must be loaded, because the directives and functions rely on it. Just by using the right commands, rsyslog will not know where to get the functionality code from.

And next is the command that runs the syslog server itself is called input().

input(type="imudp" port="514")
input(type="imtcp" port="514")

Basically, the command says to run a syslog server on a specific port. Depending on the command, you can easily determine the UDP and the TCP server.

You can of course use both types of syslog server at the same time, too. You just need to load both modules for that and configure the server command to listen to specific ports. Then you can receive both UDP and TCP syslog.


In general, we suggest to use TCP syslog. It is way more reliable than UDP syslog and still pretty fast. The main reason is, that UDP might suffer of message loss. This happens when the syslog server must receive large bursts of messages. If the system buffer for UDP is full, all other messages will be dropped. With TCP, this will not happen. But sometimes it might be good to have a UDP server configured as well. That is, because some devices (like routers) are not able to send TCP syslog by design. In that case, you would need both syslog server types to have everything covered. If you need both syslog server types configured, please make sure they run on proper ports. By default UDP syslog is received on port 514. TCP syslog needs a different port because often the RPC service is using this port as well.

Using the Text File Input Module

Log files should be processed by rsyslog. Here is some information on how the file monitor works. This will only describe setting up the Text File Input Module. Further configuration like processing rules or output methods will not be described.

Things to think about

The configuration given here should be placed on top of the rsyslog.conf file.

Config Statements

module(load="imfile" PollingInterval="10")
# needs to be done just once. PollingInterval is a module directive and is only set once when loading the module
# File 1
input(type="imfile" File="/path/to/file1" 
# File 2
input(type="imfile" File="/path/to/file2" 
# ... and so on ...

How it works

The configuration for using the Text File Input Module is very extensive. At the beginning of your rsyslog configuration file, you always load the modules. There you need to load the module for Text File Input as well. Like all other modules, this has to be made just once. Please note that the directive PollingInterval is a module directive which needs to be set when loading the module.

module(load="imfile" PollingInterval="10")

Next up comes the input and its parameters. We configure a input of a certain type and then set the parameters to be used by this input. This is basically the same principle for all inputs:

# File 1
input(type="imfile" File="/path/to/file1" 

File specifies, the path and name of the text file that should be monitored. The file name must be absolute.

Tag will set a tag in front of each message pulled from the file. If you want a colon after the tag you must set it as well, it will not be added automatically.

StateFile will create a file where rsyslog keeps track of the position it currently is in a file. You only need to set the filename. This file always is created in the rsyslog working directory (configurable via $WorkDirectory). This file is important so rsyslog will not pull messages from the beginning of the file when being restarted.

Severity will give all log messages of a file the same severity. This is optional. By default all mesages will be set to “notice”.

Facility gives alle log messages of a file the same facility. Again, this is optional. By default all messages will be set to “local0”.

These statements are needed for monitoring a file. There are other statements described in the doc, which you might want to use. If you want to monitor another file the statements must be repeated.

Since the files cannot be monitored in genuine real time (which generates too much processing effort) you need to set a polling interval:

PollingInterval 10

This is a module setting and it defines the interval in which the log files will be polled. By default this value is set to 10 seconds. If you want this to get more near realtime, you can decrease the value, though this is not suggested due to increasing processing load. Setting this to 0 is supported, but not suggested. Rsyslog will continue reading the file as long as there are unprocessed messages in it. The interval only takes effect once rsyslog reaches the end of the file.


The StateFile needs to be unique for every file that is monitored. If not, strange things could happen.

How to write to a local socket?

One member of the rsyslog comunity wrote:

I’d like to forward via a local UNIX domain socket, instead. I think  I understand how to configure the ‘imuxsock’ module so my unprivileged instance reads from a non-standard socket location. But I can’t figure out how to tell my root instance to forward via a local domain socket.

I didn’t figure out a completely RSyslog-native method, but another poster’s message pointed me toward ‘socat’ and ‘omprog’, which I have working, now. (It would be really nice if RSyslog could support this natively, though.)

In case anyone else wants to set this up, maybe this will save you some effort. I’m also interested in any comments/criticisms about this method, I’d love to hear suggestions for better ways to make this work.

Also, I rolled it all up into a Fedora/EL RPM spec, and I’ll send it on to anyone who’s interested–just ask.

Setup steps:

  • Install the ‘socat’ utility.
  • Build RSyslog with the ‘–enable-omprog’ ./configure flag.
  • Create two separate RSyslog config files, one for the ‘root’ instance (writes to the socket) and a second for the ‘unprivileged’ instance (reads from the socket).
  • Rewrite your RSyslog init script to start two separate daemon instances, one using each config file (and separate PID files, too).
  • Create the user ‘rsyslogd’ and the group ‘rsyslogd’.
  • Set permissions/ownerships as needed to allow the user ‘rsyslogd’ to write to the file ‘/var/log/rsyslog.log’
  • Create an executable script called
    '/usr/libexec/rsyslogd/omprog_socat' that contains the lines:
    /usr/bin/socat -t0 -T0 -lydaemon -d - UNIX-SENDTO:/dev/log
  • The ‘root’ instance config file should contain (modifying the output actions to taste):

    $ModLoad imklog
    $ModLoad omprog
    $Template FwdViaUNIXSocket,"<%pri%>%syslogtag%%msg%"
    $ActionOMProgBinary /usr/libexec/rsyslogd/omprog_socat
    *.* :omprog:;FwdViaUNIXSocket

  • The ‘unprivileged’ instance config file should contain (modifying the output actions to taste):

    $ModLoad imuxsock
    $PrivDropToUser rsyslogd
    $PrivDropToGroup rsyslogd
    *.* /var/log/rsyslog.log

    The ‘root’ daemon can only accept input from the kernel message buffer, and nothing else (especially not the syslog socket (/dev/log) or any network sockets). The unprivileged user will handle all of local and network log messages. To merge the kernel logs into the same data channel as everything else, here’s what happens:

    [During the RSyslog daemons’ startup]

A) At startup, the ‘root’ daemon’s ‘imklog’ module starts listening for kernel messages (via ‘/prog/kmsg’), and its ‘omprog’ module starts an instance of ‘socat’ (called via the ‘omprog_socat’ wrapper), establishing a persistent one-way IO connection where ‘omprog’ pipes its output to the STDIN of ‘socat’.

  • (Note that this same ‘socat’ instance remains running throughout the life of the RSyslog daemon, handling everything ‘omprog’ outputs. Contrast this, efficiency-wise, against the built-in ‘subshell’ module [the ‘^/path/to/program’ action], which runs a separate instance instance of the child program for each message.)

B) At startup, the ‘unprivileged’ daemon’s ‘imuxsock’ module opens the system logging socket (‘/dev/log’) and starts listening for incoming log messages from other programs.

  • [During normal operation]1) The kernel buffer produces a message string on ‘/proc/kmsg’.2) The ‘root’ RSyslog daemon reads the message from ‘/proc/kmsg’, assigning it the priority number of ‘’ and the string tag ‘kernel’.3) The ‘root’ daemon prepends the priority number and tag as a header to the message string, and then passes it to the ‘omprog’ module for output (via persistent pipe) to the running ‘socat’ instance.4) The ‘socat’ instance receives the header-framed message and sends it to the system logging socket (‘/dev/log’).

    5) The ‘unprivileged’ RSyslog daemon reads the message from ‘/dev/log’, assigning it the priority and tag given in the message header, plus all of the other properties (timestamp, hostname, etc.) a message object should have.

    6) The ‘unprivileged’ daemon formats the message and writes it to the output file.

    The only real difference I can see in the forwarded messages is that the ‘source’ property is set to ‘imuxsock’ instead of ‘imklog’. I don’t think that’s a real problem, though, since the priority and tag are still distinct.

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