ViaBackwards

ViaBackwards is an add-on to ViaVersion and covers the opposite direction than one would expect: it allows players with an older version of Minecraft to connect to a server running a newer version. ViaVersion itself takes care of players with a newer version, so together the pair covers both directions.

This is typical for servers that update quickly: the server moves to a new version and some players still have an old version in their launcher. Without ViaBackwards they would not be able to connect and would have to wait until they update the game.

ViaBackwards is installed alongside ViaVersion and must have the same version — that is the most important rule. When the versions diverge, players cannot connect or crash while loading.

Translation has its limits: a player on an older version will not see blocks, items, and mechanics that do not exist in their version. A substitute is shown to them. It is not a bug in the plugin but a feature of translation — and it is worth telling players in advance.

ViaBackwards alone cannot handle very old clients (1.7 and 1.8). ViaRewind is also used for those. The combination ViaVersion + ViaBackwards + ViaRewind covers practically all Java versions.

1. First, you must have ViaVersion installed and working — without it ViaBackwards does not work.
2. Find out the ViaVersion version: /viaversion version.
3. Download ViaBackwards from modrinth.com/plugin/viabackwards in the same version as ViaVersion.
4. Upload the .jar to the plugins folder — where ViaVersion is (on the proxy, if you use a proxy).
5. Restart the server.
6. Check the console: both ViaVersion and ViaBackwards must load, and their versions must match.
7. Verify that a player with an older version can connect — /viaversion list ho po pconnection will be output.
8. For very old versions (1.7, 1.8), also add ViaRewind in the same version.
9. When a player cannot connect, the first suspect is a mismatched version (ViaVersion vs. ViaBackwards vs. ViaRewind). Align them to the same version line.
10. After every Minecraft update, check for newer versions of both plugins and replace them together.

Most important rule: ViaBackwards, ViaVersion and ViaRewind must be from the same version line. Mismatched versions are the most common cause of crashes when connecting.
A player on an older version won't see new blocks and items as they are — a replacement will be shown to them. There's no way around it, only explain it.
On a proxy (Velocity, BungeeCord), ViaBackwards belongs on the proxy, not on the game servers. Otherwise double translation and errors occur.
If the server won't let older players connect after an update, verify that ViaBackwards is actually running (console at startup) and that its version matches ViaVersion.
For versions 1.7 and 1.8 add ViaRewind — ViaBackwards alone won't cover them.
When players connect but crash when switching between worlds, check that translation is running on the proxy (for proxy servers) and that all parts have the same version.