I can see the version released every time contain 4.x 5.x and 6.x. If the 6.x has developed from 4.x and then 5.x, the suricata shoud only be released with 6.x whitch is the newest. if not, what is the different between suricata6.x and suricata 4.x or 5.x
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So, the reason you see more than one version with the releases is that not everyone can or will upgrade to the newest version when it goes out. We won’t know all the reasons for this, but, for instance, for big companies and setups, it’s not a simple task to change product versions like that, so they need time to prepare, and they also tend to wait a bit to see if there will be any patches to the recently released version, for instance.
You’ll see that many maintainers keep more than one version of their products because of user adoption being a slow process - while, at the same time, the products keep evolving and getting new features, so new versions will show up, at some point. When we talk about fixes, when those are compatible, we may release a minor update, but some things will make more sense in a new version altogether.
So, there you have it. The product gets new features, those go into newer versions. But the users have a different adoption pace. So we keep at least one older stable version and warn them until when that version will be officially supported.
As for details on version changes, I think the blog post about the releases is a better source: NEW: Suricata 6.0.3 and 5.0.7 releases! - Suricata
I hope to have answered your question! =]