Every now and then I am asked which versions the rsyslog project officially supports. I track the current version on the rsyslog status page. To me as a surprise, I was recently asked what that really means. And I have to admit that the page actually does not spell this out in detail. So I thought it may be a good idea to write some small comments on this.
First, what does "officially supported" mean? I think the best definition is that this is the software we expect users to run and we will most probably not look into problems in older versions. At least I will usually not craft patches for older versions. Note that things are a bit different for customers with rsyslog support contracts: here Adiscon as rsyslog's prime sponsor provides support for all stable versions ever created (as long as this is technically possible, e.g., there are some things which simply can not be fixed in very old versions).
So which versions are officially supported? It is first important to note that for each major version (v4, v5, ...) there exist three branches: stable, beta and development. In practice, there usually exists only a beta and development version for the most recent major version and maybe the one before (but we try to avoid that). All other active versions are stable. An older but still useful plot of this concept can be seen in my blog post on the rsyslog family tree. The idea is that new functionality goes into the devel branch, is then slowly migrated to beta and moved to stable state once it gets sufficiently mature (read my blog post on "How Software get Stable" for more on this philosophy).
It is important to note, however, that only the current versions of each branch are officially supported (but note the extension for support contracts I stated above). Today exist far more than 100 versions of rsyslog. It is absolutely impossible to maintain all of them. So we focus on what we expect a careful user should run. For the ultra-conservative folks, we keep older versions, like v4, inside the supported range, but we can (and will) not support any interim version. For example, the some early 4.6.x versions were bugged by some really nasty problems in the TLS arena. This has been fixed in later versions, so why should we still support the known-to-be-faulty versions? This sounds like a big waste of time. Note the difference between, e.g. 4.6.0 and 4.6.5 is just bug fixes (!). So if you follow the "How Software get Stable" spirit, there is absolutely no point in running software with more bugs than necessary and so it sounds like a really bad idea to run those versions. We understand that corporate guys may still have valid reasons to use outdated versions, but that's covered by the support contracts (think about it this way: this generates a lot of technically unnecessary work, so someone needs to pay for that...).
The story is of course slightly different for version shifts from e.g. 4.4.x to 4.6.x. Note that even number denote stable versions (odd ones are either devel or beta, depending on how they are declared). Here, some new functionality is introduced, and thus there is additional risk for regressions. We try to mitigate this potential by our beta phase, which usually lasts three month. In my opinion, a beta is very close to a stable version. So for this time period, we support something like two stables in order to make sure users can migrate to the new version without risk. Of course, this doesn't outrule the chance that some regression is left uncaught. But, some risk is always involved in life...
Note that we do not generally provide patches for older stable versions once the next one is out. Even when we patch older versions for support contract customers, we usually patch exactly the problem they see, and not more (what usually is exactly within their minimal-change strategy). So to re-use our 4.x TLS example: the nasty TLS bug was fixed in 4.6.5, which was declared the current stable. So the patch is not available in 4.6.0 to 4.6.4. This also means that no 4.2.x or 4.4.x version will ever get it, because these versions were quite outdated (and unsupported) when the problem was fixed.
If you are still not convinced, just think if this: what would happen if I applied all patches to 4.6.0 that went into 4.6.5? Yes, exactly -- it would be the exact same code. This is because 4.6.5 is version 4.6.0 plus all patches ever created for that version ;)
To sum up in one sentence: the project officially supports the head versions of all supported branches (as of the rsyslog status page).