Developing rsyslog modules (outdated)
Written by `Rainer Gerhards <https://rainer.gerhards.net>`_ (2007-07-28)
This document is outdated and primarily contains historical information. Do not trust it to build code. It currently is under review.
In theory, modules provide input and output, among other functions, in rsyslog. In practice, modules are only utilized for output in the current release. The module interface is not yet completed and a moving target. We do not recommend to write a module based on the current specification. If you do, please be prepared that future released of rsyslog will probably break your module.
A goal of modularization is to provide an easy to use plug-in interface. However, this goal is not yet reached and all modules must be statically linked.
There is a lot of plumbing that is always the same in all modules. For example, the interface definitions, answering function pointer queries and such. To get rid of these laborious things, I generate most of them automatically from a single file. This file is named module-template.h. It also contains the current best description of the interface “specification”.
One thing that can also be achieved with it is the capability to cope with a rapidly changing interface specification. The module interface is evolving. Currently, it is far from being finished. As I moved the monolithic code to modules, I needed (and still need) to make many “non-clean” code hacks, just to get it working. These things are now gradually being removed. However, this requires frequent changes to the interfaces, as things move in and out while working towards a clean interface. All the interim is necessary to reach the goal. This volatility of specifications is the number one reasons I currently advise against implementing your own modules (hint: if you do, be sure to use module-template.h and be prepared to fix newly appearing and disappearing data elements).
Output modules, and only output modules, should start with a file name of “om” (e.g. “omfile.c”, “omshell.c”). Similarly, input modules will use “im” and filter modules “fm”. The third character shall not be a hyphen.
Modules are directly loaded into rsyslog’s address space. As such, any module is provided a big level of trust. Please note that further module interfaces might provide a way to load a module into an isolated address space. This, however, is far from being completed. So the number one rule about module security is to run only code that you know you can trust.
To minimize the security risks associated with modules, rsyslog provides only the most minimalistic access to data structures to its modules. For that reason, the output modules do not receive any direct pointers to the selector_t structure, the syslogd action structures and - most importantly - the msg structure itself. Access to these structures would enable modules to access data that is none of their business, creating a potential security weakness.
Not having access to these structures also simplifies further queueing and error handling cases. As we do not need to provide e.g. full access to the msg object itself, we do not need to serialize and cache it. Instead, strings needed by the module are created by syslogd and then the final result is provided to the module. That, for example, means that in a queued case $NOW is the actual timestamp of when the message was processed, which may be even days before it being dequeued. Think about it: If we wouldn’t cache the resulting string, $NOW would be the actual date if the action were suspended and messages queued for some time. That could potentially result in big confusion.
It is thought that if an output module actually needs access to the while msg object, we will (then) introduce a way to serialize it (e.g. to XML) in the property replacer. Then, the output module can work with this serialized object. The key point is that output modules never deal directly with msg objects (and other internal structures). Besides security, this also greatly simplifies the job of the output module developer.
Modules (and rsyslog) need to know when they are called. For this, there must an action identification in selector lines. There are two syntaxes: the single-character syntax, where a single characters identifies a module (e.g. “*” for a wall message) and the modules designator syntax, where the module name is given between colons (e.g. “:ommysql:”). The single character syntax is depreciated and should not be used for new plugins.
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