What Does online-mode Do in Minecraft?
Understand online-mode in Minecraft server.properties, how authentication works, why UUIDs matter, and when proxies use different backend settings.
online-mode is one of the most important security settings in server.properties.
online-mode=true
When enabled, the server verifies that a joining player has a valid authenticated Minecraft session.
Why It Matters
| Setting | Meaning |
| --- | --- |
| online-mode=true | Server checks official authentication |
| online-mode=false | Server does not verify account ownership |
With online-mode=false, a client can claim a username without the normal official session check. On public servers, that can create impersonation risk.
Changing online-mode can change how UUIDs are handled. That can affect inventories, permissions, bans, whitelist entries, and plugin data.
Proxy Networks
Velocity and BungeeCord setups are special. Players authenticate at the proxy, then the proxy forwards identity to backend servers. In that architecture, backend settings must match the proxy documentation and backend ports should be firewalled.
Do not make backend servers reachable directly by players if they rely on the proxy for authentication.
Invalid Session Errors
If players see invalid session errors, do not immediately disable authentication. First read how to fix Invalid Session. The player session, launcher, proxy forwarding, or auth service path may be the real issue.
Should You Turn It Off?
For most servers, no. Read should you set online-mode=false before changing this setting.
FAQ
What does online-mode=true do?
It makes the server verify player sessions with Microsoft/Mojang authentication so usernames cannot be freely impersonated.
Does online-mode affect UUIDs?
Yes. Authentication mode affects player identity. Changing it can affect inventories, bans, whitelist, permissions, and player data.
Should public servers use online-mode=true?
Yes, unless players authenticate through a correctly configured proxy and backend servers are protected from direct access.
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