omudpspoof: UDP spoofing output module — rsyslog 8.2502.0 documentation

# omudpspoof: UDP spoofing output module

Module Name: | omudpspoof
Author: | David Lang <david@lang.hm> and Rainer Gerhards <rgerhards@adiscon.com>
Available Since: | 5.1.3

## Purpose

This module is similar to the regular UDP forwarder, but permits to spoof the sender address. Also, it enables to circle through a number of source ports.

Important: This module requires root permissions. This is a hard requirement because raw socket access is necessary to fake UDP sender addresses. As such, rsyslog cannot drop privileges if this module is to be used. Ensure that you do not use $PrivDropToUser or $PrivDropToGroup. Many distro default configurations (notably Ubuntu) contain these statements. You need to remove or comment them out if you want to use omudpspoof.

## Configuration Parameters

Note

Parameter names are case-insensitive.

### Module Parameters

#### Template

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
word | RSYSLOG_TraditionalForwardFormat | no | none

This setting instructs omudpspoof to use a template different from the default template for all of its actions that do not have a template specified explicitly.

### Action Parameters

#### Target

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
word | none | yes | `$ActionOMUDPSpoofTargetHost`

Host that the messages shall be sent to.

#### Port

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
word | 514 | no | `$ActionOMUDPSpoofTargetPort`

Remote port that the messages shall be sent to. Default is 514.

#### SourceTemplate

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
word | RSYSLOG_omudpspoofDfltSourceTpl | no | `$ActionOMUDPSpoofSourceNameTemplate`

This is the name of the template that contains a numerical IP address that is to be used as the source system IP address. While it may often be a constant value, it can be generated as usual via the property replacer, as long as it is a valid IPv4 address. If not specified, the build-in default template RSYSLOG_omudpspoofDfltSourceTpl is used. This template is defined as follows: $template RSYSLOG_omudpspoofDfltSourceTpl,”%fromhost-ip%” So in essence, the default template spoofs the address of the system the message was received from. This is considered the most important use case.

#### SourcePort.start

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
integer | 32000 | no | `$ActionOMUDPSpoofSourcePortStart`

Specify the start value for circling the source ports. Start must be less than or equal to sourcePort.End.

#### SourcePort.End

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
integer | 42000 | no | `$ActionOMUDPSpoofSourcePortEnd`

Specify the end value for circling the source ports. End must be equal to or more than sourcePort.Start.

#### MTU

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
integer | 1500 | no | none

Maximum packet length to send.

#### Template

type | default | mandatory | `obsolete legacy` directive
--- | --- | --- | ---
word | RSYSLOG_TraditionalForwardFormat | no | `$ActionOMUDPSpoofDefaultTemplate`

This setting instructs omudpspoof to use a template different from the default template for all of its actions that do not have a template specified explicitly.

## Caveats/Known Bugs

- IPv6 is currently not supported. If you need this capability, please let us know via the rsyslog mailing list.

- Throughput is MUCH smaller than when using omfwd module.

## Examples

### Forwarding message through multiple ports

Forward the message to 192.168.1.1, using original source and port between 10000 and 19999.

    Action (
      type="omudpspoof"
      target="192.168.1.1"
      sourceport.start="10000"
      sourceport.end="19999"
    )

### Forwarding message using another source address

Forward the message to 192.168.1.1, using source address 192.168.111.111 and default ports.

    Module (
      load="omudpspoof"
    )
    Template (
      name="spoofaddr"
      type="string"
      string="192.168.111.111"
    )
    Action (
      type="omudpspoof"
      target="192.168.1.1"
      sourcetemplate="spoofaddr"
    )

See also

Help with configuring/using `Rsyslog`:

- Mailing list - best route for general questions

- GitHub: rsyslog source project - detailed questions, reporting issues that are believed to be bugs with `Rsyslog`

See also

Contributing to `Rsyslog`:

- Source project: rsyslog project README.

- Documentation: rsyslog-doc project README

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