The rocket-fast system for log processing

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.

One thought on “Using the syslog receiver module

Comments are closed.