Server uses entity reach distance of 6 whereas client uses lower reach distance

The following is based on a decompiled version of Minecraft 1.10 using MCP 9.30.

The bug

The client uses the reach distance defined by the method net.minecraft.client.multiplayer.PlayerControllerMP.getBlockReachDistance(), which is in 1.10

• Creative: 5
• Other gamemodes: 4.5

or 6 if net.minecraft.client.multiplayer.PlayerControllerMP.extendedReach(), which tests if the player is in creative mode, is true.

The method net.minecraft.network.NetHandlerPlayServer.processUseEntity(CPacketUseEntity) uses 6 however. This means a modified client can exploit that and in case a gamemode with a higher reach distance is introduced, trying to interact with an entity 8 or more meters away will fail.

This method allows even interaction with entities behind a wall, but limits the distance to 3 blocks there.