imudp: UDP Syslog Input Module

Module Name: imudp
Author: Rainer Gerhards <>

Provides the ability to receive syslog messages via UDP.

Multiple receivers may be configured by specifying multiple input statements.

Note that in order to enable UDP reception, Firewall rules probably need to be modified as well. Also, SELinux may need additional rules.

Configuration Parameters

Module Parameters

TimeRequery <nbr-of-times>

Default: 2

This is a performance optimization. Getting the system time is very costly. With this setting, imudp can be instructed to obtain the precise time only once every n-times. This logic is only activated if messages come in at a very fast rate, so doing less frequent time calls should usually be acceptable. The default value is two, because we have seen that even without optimization the kernel often returns twice the identical time. You can set this value as high as you like, but do so at your own risk. The higher the value, the less precise the timestamp.

Note: the timeRequery is done based on executed system calls (not messages received). So when batch sizes are used, multiple messages are received with one system call. All of these messages always receive the same timestamp, as they are effectively received at the same time. When there is very high traffic and successive system calls immediately return the next batch of messages, the time requery logic kicks in, which means that by default time is only queried for every second batch. Again, this should not cause a too-much deviation as it requires messages to come in very rapidly. However, we advise not to set the “timeRequery” parameter to a large value (larger than 10) if input batches are used.

SchedulingPolicy <rr/fifo/other>

Default: unset

Can be used the set the scheduler priority, if the necessary functionality is provided by the platform. Most useful to select “fifo” for real-time processing under Linux (and thus reduce chance of packet loss).

SchedulingPriority <number>

Default: unset

Scheduling priority to use.

batchSize <number>

Default: 32

This parameter is only meaningful if the system support recvmmsg() (newer Linux OSs do this). The parameter is silently ignored if the system does not support it. If supported, it sets the maximum number of UDP messages that can be obtained with a single OS call. For systems with high UDP traffic, a relatively high batch size can reduce system overhead and improve performance. However, this parameter should not be overdone. For each buffer, max message size bytes are statically required. Also, a too-high number leads to reduced efficiency, as some structures need to be completely initialized before the OS call is done. We would suggest to not set it above a value of 128, except if experimental results show that this is useful.

threads <number>

Available since: 7.5.5

Default: 1

Number of worker threads to process incoming messages. These threads are utilized to pull data off the network. On a busy system, additional threads (but not more than there are CPUs/Cores) can help improving performance and avoiding message loss. Note that with too many threads, performance can suffer. There is a hard upper limit on the number of threads that can be defined. Currently, this limit is set to 32. It may increase in the future when massive multicore processors become available.

Input Parameters

..index:: imudp; address (input parameter)

Address <IP>

Default: *

Local IP address (or name) the UDP server should bind to. Use “*” to bind to all of the machine’s addresses.

Port <port>

Default: 514

Specifies the port the server shall listen to.. Either a single port can be specified or an array of ports. If multiple ports are specified, a listener will be automatically started for each port. Thus, no additional inputs need to be configured.

Single port: Port=”514”

Array of ports: Port=[“514”,”515”,”10514”,”...”]

Ruleset <ruleset>

Default: RSYSLOG_DefaultRuleset

Binds the listener to a specific ruleset.

RateLimit.Interval [number]

Available since: 7.3.1

Default: 0

The rate-limiting interval in seconds. Value 0 turns off rate limiting. Set it to a number of seconds (5 recommended) to activate rate-limiting.

RateLimit.Burst [number]

Available since: 7.3.1

Default: 10000

Specifies the rate-limiting burst in number of messages.

name [name]

Available since: 8.3.3

Default: imudp

specifies the value of the inputname property. In older versions, this was always “imudp” for all listeners, which still is the default. Starting with 7.3.9 it can be set to different values for each listener. Note that when a single input statement defines multipe listner ports, the inputname will be the same for all of them. If you want to differentiate in that case, use “name.appendPort” to make them unique. Note that the “name” parameter can be an empty string. In that case, the corresponding inputname property will obviously also be the empty string. This is primarily meant to be used together with “name.appendPort” to set the inputname equal to the port.

InputName [name]

Available since: 7.3.9


This provides the same functionality as “name”. It is the historical parameter name and should not be used in new configurations. It is scheduled to be removed some time in the future.

name.appendPort [on/off]

Available since: 7.3.9

Default: off

Appends the port the the inputname property. Note that when no “name” is specified, the default of “imudp” is used and the port is appended to that default. So, for example, a listner port of 514 in that case will lead to an inputname of “imudp514”. The ability to append a port is most useful when multiple ports are defined for a single input and each of the inputnames shall be unique. Note that there currently is no differentiation between IPv4/v6 listeners on the same port.

InputName.AppendPort [on/off]

Available since: 7.3.9


This provides the same functionality as “name.appendPort”. It is the historical parameter name and should not be used in new configurations. It is scheduled to be removed some time in the future.

defaultTZ <timezone-info>

Default: unset

This is an experimental parameter; details may change at any time and it may also be discoutinued without any early warning. Permits to set a default timezone for this listener. This is useful when working with legacy syslog (RFC3164 et al) residing in different timezones. If set it will be used as timezone for all messages that do not contain timezone info. Currently, the format must be “+/-hh:mm”, e.g. “-05:00”, “+01:30”. Other formats, including TZ names (like EST) are NOT yet supported. Note that consequently no daylight saving settings are evaluated when working with timezones. If an invalid format is used, “interesting” things can happen, among them malformed timestamps and rsyslogd segfaults. This will obviously be changed at the time this feature becomes non-experimental.

rcvbufSize [size]

Available since: 7.5.3

Default: unset

This request a socket receive buffer of specific size from the operating system. It is an expert parameter, which should only be changed for a good reason. Note that setting this parameter disables Linux auto-tuning, which usually works pretty well. The default value is 0, which means “keep the OS buffer size unchanged”. This is a size value. So in addition to pure integer values, sizes like “256k”, “1m” and the like can be specified. Note that setting very large sizes may require root or other special privileges. Also note that the OS may slightly adjust the value or shrink it to a system-set max value if the user is not sufficiently privileged. Technically, this parameter will result in a setsockopt() call with SO_RCVBUF (and SO_RCVBUFFORCE if it is available).

See Also

Caveats/Known Bugs

  • Scheduling parameters are set after privileges have been dropped. In most cases, this means that setting them will not be possible after privilege drop. This may be worked around by using a sufficiently-privileged user account.


This sets up an UPD server on port 514:

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

The following sample is mostly equivalent to the first one, but request a larger rcvuf size. Note that 1m most probably will not be honored by the OS until the user is sufficiently privileged.

module(load="imudp") # needs to be done just once
input(type="imudp" port="514" rcvbufSize="1m")

In the next example, we set up three listeners at ports 10514, 10515 and 10516 and assign a listner name of “udp” to it, followed by the port number:

input(type="imudp" port=["10514","10515","10516"]
      inputname="udp" inputname.appendPort="on")

The next example is almost equal to the previous one, but now the inputname property will just be set to the port number. So if a message was received on port 10515, the input name will be “10515” in this example whereas it was “udp10515” in the previous one. Note that to do that we set the inputname to the empty string.

input(type="imudp" port=["10514","10515","10516"]
      inputname="" inputname.appendPort="on")

Legacy Configuration Directives

Legacy configuration parameters should not be used when crafting new configuration files. It is easy to get things wrong with them.

Directive Equivalent Parameter Requires
$UDPServerTimeRequery <nbr-of-times> TimeRequery  
$IMUDPSchedulingPolicy <rr/fifo/other> SchedulingPolicy 4.7.4+, 5.7.3+, 6.1.3+
$IMUDPSchedulingPriority <number> SchedulingPriority 4.7.4+, 5.7.3+, 6.1.3+
$UDPServerAddress <IP> Address  
$UDPServerRun <port> Port  
$InputUDPServerBindRuleset <ruleset> Ruleset 5.3.2+

Note: module parameters are given in italics. All others are input parameters.

Multiple receivers may be configured by specifying $UDPServerRun multiple times.

Legacy Sample

This sets up an UDP server on port 514:

$ModLoad imudp # needs to be done just once
$UDPServerRun 514

This documentation is part of the rsyslog project. Copyright © 2009-2014 by Rainer Gerhards and Adiscon. Released under the GNU GPL version 3 or higher.