The settings in server.properties that matter.

Where the file is and how to edit it

server.properties sits in the same folder as the server JAR. It is a plain text file of key=value lines with # comments. Edit it while the server is stopped; changes are read at startup.

Two rules prevent most mistakes. First, do not add spaces around the =. Second, keep the keys exactly as written: they are case-sensitive and an unknown key is ignored without an error. If a change seems to do nothing, check the spelling.

Network and access

If online-mode is false, anyone using an offline client can join under any name, and permissions tied to a username become unreliable. Keep it true unless the server is behind a proxy that handles authentication.

World settings

level-seed only matters when the world does not exist yet. If you set a new seed on an existing world, nothing happens: the world is already generated. To change it you need a new world folder, which means starting over or moving players with an inventory tool.

Performance-related settings

Two keys explain most of the difference between a smooth server and a struggling one:

• view-distance — how far chunks are sent to clients. Defaults around 10. Lowering it to 8 or 6 reduces bandwidth and CPU a lot.
• simulation-distance — how far the server ticks farms, mobs and redstone. Lowering it to 6 or 4 helps a busy world more than it hurts gameplay.

These are not the same setting. View distance is about what players can see; simulation distance is about what the server actually updates. Cutting simulation distance is the cheaper way to reduce load without making the world look empty.

Security and moderation

A whitelist is not a substitute for permissions. It controls who can connect, not what a connected player can do. Use a permissions plugin to decide which commands and plugin features each group gets.

Gameplay toggles

• gamemode — survival, creative, adventure or spectator for new players.
• force-gamemode — forces the gamemode each time a player joins.
• pvp — enables player-versus-player damage.
• spawn-monsters / spawn-animals / spawn-npcs — world spawn toggles.
• generate-structures — whether structures generate in new chunks.
• max-world-size — caps the world border radius in blocks.

Some of these only affect newly generated chunks. Turning off structure generation on an existing world does not remove structures that already exist, and turning it back on does not add them to chunks that were already generated.

RCON and remote console

RCON lets an external tool send console commands over a TCP port. If you use it, set a long random rcon.password, keep rcon.port closed to the public internet, and enable rcon only while you need it.

enable-rcon=true
rcon.port=25575
rcon.password=change-this-to-a-long-random-value

An open RCON port with a weak password is a full server takeover. If you do not use RCON, leave it disabled.

Common mistakes

1. Editing while the server runs. The file is read at startup; your change is ignored until a restart, and the server may overwrite the file on shutdown.
2. Changing level-name to "reset" a world. It points at a different folder; it does not delete anything.
3. Setting max-players to a number the hardware cannot carry. The cap does not create RAM.
4. Disabling online-mode to "let friends in". That is a security change, not a connectivity fix.
5. Copying a whole properties file from an old version. New keys will be missing and old values may be invalid.

The short version

Keep the defaults until something needs changing. Set a realistic max-players, enable white-list for public servers, keep online-mode=true, and tune view-distance and simulation-distance before you buy more RAM. Back up the file before editing so you can compare what you changed.

Resource packs

A server can offer or require a resource pack. The server sends the pack URL to the client, and the client downloads it before joining.

A pack hosted on a slow or private URL is a common cause of long join times. Test the URL from outside your network before enabling require-resource-pack.

A minimal safe starting point

For a small private server that friends will join, these values cover the important decisions. Leave the rest at their defaults.

online-mode=true
white-list=true
enforce-whitelist=true
max-players=8
view-distance=8
simulation-distance=6
difficulty=normal
pvp=true
allow-flight=false
enable-command-block=false
enable-rcon=false

Add yourself to the whitelist before enabling it, or you will lock yourself out of your own server. Then test with a second account and read the log once more before you announce the address.