Proof-of-concept Server-Sent CSMs
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Minetest server-sent CSM proof-of-concept


Attempts to run server-sent CSMs locally in a sandbox.

How it works

Any client with the CSM installed will automatically attempt to request SSCSMs from the server via a mod channel. If the server has this mod installed, it will reply with a few messages containing the mod name and partially minified mod code. The CSM will then create a separate environment so SSCSMs cannot mess with existing CSMs (and so CSMs do not accidentally interfere with SSCSMs), and execute the SSCSMs inside this environment. Note that it is trivial for users to modify this environment. The server-side mod sends two “built-in” SSCSMs before and after all other SSCSMs to add extra helper functions (in the sscsm namespace), to execute register_on_mods_loaded callbacks and attempt to leave the mod channel.


To create a SSCSM:

  • Install this mod onto a server.
  • Enable mod channels on the server (add enable_mod_channels = true to minetest.conf).
  • Allow CSMs to send chat messages if you haven’t already (add csm_restriction_flags = 60 to minetest.conf).
  • Create SSCSMs with the API.
  • Install the CSM (in the csm/ directory) onto clients and enable it.

The minifier preserves comments starting with “copyright” or “license”: (case-insensitive, excluding leading spaces).


-- Copyright: 1
-- License: 2
-- A normal comment.



-- Copyright: 1
-- License: 2

Server-side mod facing API

This API is subject to change.


Registers a server-provided CSM with the following definition table.

  • name (string): The name of the server-provided CSM. Please use the modname:sscsmname convention. Cannot start with a colon or contain newlines.
  • code (string): The code to be sent to clients.
  • file (string): The file to read the code from, read during the register() call.
  • depends (list): A list of SSCSMs that must be loaded before this one.

This definition table must have name and either code or file.


Registers a function to be called when a client loads SSCSMs. Note that this function will not work unless CSMs have the ability to send chat messages as it relies on the SSCSM communication API described below.

Communication with SSCSMs

SSCSM provides an API inspired by csm_com for sending private messages to clients. Unlike mod channels, only the target client gets these messages. Although the API is stable, the internal protocol will probably change in the future as better CSM API functions are added.

Note that channel names must not contain \001/U+0001.

  • sscsm.com_send(player_or_name, channel, msg): Sends msg (a JSON-compatible object) to player_or_name on the SSCSM com channel channel. Channel names should be modname or modname:name to prevent conflicts. Although the theoretical limit for server-to-client messages is 128MiB, I strongly recommend not sending large messages when not necessary.
  • sscsm.com_send_all(channel, msg): Sends msg to all clients that are running SSCSMs.
  • sscsm.register_on_com_receive(channel, function(name, msg)): Registers a function to be called when a message on channel is received from the client. msg may be any JSON-compatible type, so checking the type of this object is strongly recommended.
  • sscsm.has_sscsms_enabled(name): Returns true if name has enabled SSCSMs. This will not be true immediately after players join, however.

Maximum SSCSM size

Because of Minetest network protocol limitations, the amount of data that can be sent over mod channels is limited, and therefore the maximum SSCSM size is 65300 (to leave room for the player name and future expansion). The name of the SSCSM also counts towards this total.

Because of this size limitation, SSCSMs are passed through a primitive code minifier that removes some whitespace and comments, so even if your code is above this size limit it could still work.

Server-sent CSM facing API

SSCSMs can access most functions on client_lua_api.txt, as well as a separate sscsm namespace:

  • sscsm.global_exists(name): The same as minetest.global_exists.
  • sscsm.register_on_mods_loaded(callback): Runs the callback once all SSCSMs are loaded.
  • sscsm.register_chatcommand(...): Similar to minetest.register_chatcommand, however overrides commands starting in / instead. This can be used to make some commands have instantaneous responses. The command handler is only added once register_chatcommand has been called.
  • sscsm.unregister_chatcommand(name): Unregisters a chatcommand.
  • sscsm.get_player_control(): Alternative for minetest.localplayer:get_control() that works with Minetest 5.2.0 and below.
  • sscsm.every(interval, func, ...): Calls func every interval seconds with any extra parameters specified. Use minetest.register_globalstep instead if interval is 0.
  • sscsm.restriction_flags: The csm_restriction_flags setting set in the server’s minetest.conf.
  • sscsm.restrictions: A table based on csm_restriction_flags:
    • chat_messages: When true, SSCSMs can’t send chat messages or run server chatcommands.
    • read_itemdefs: When true, SSCSMs can’t read item definitions.
    • read_nodedefs: When true, SSCSMs can’t read node definitions.
    • lookup_nodes_limit: When true, any get_node calls are restricted.
    • read_playerinfo: When true, minetest.get_player_names() will return nil.
  • sscsm.com_send(channel, msg): Sends msg (a JSON-compatible object) to the server. Note that client-to-server messages cannot be long, for plain strings the channel and message combined must be at most 492 characters.
  • sscsm.register_on_com_receive(channel, function(msg)): Registers a function to be called when a message on channel is received from the server.

To communicate with the server-side mods, it is possible to open a mod channel.

CSM restriction flags example

minetest.register_chatcommand('m', {
    description = 'Alias for /msg',
    func = function(param)
        if sscsm.restrictions.chat_messages then
            return false, 'Sorry, csm_restriction_flags prevents chat messages'
                .. ' from being sent.'
        minetest.run_server_chatcommand('msg', param)

Note that modifying sscsm.restrictions or sscsm.restriction_flags will not add or remove restrictions and is not recommended.

Security considerations

Do not trust any input sent to the server via SSCSMs (and do not store sensitive data in SSCSM code), as malicious users can and will inspect code and modify the output from SSCSMs.

I repeat, do not trust the client and/or SSCSMs with any sensitive information and do not trust any output from the client and/or SSCSMs. Make sure to rerun any privilege checks on the server.

Other recommendations

Although it is possible to kick clients that do not support SSCSMs, this has not been implemented. Some users may not want to allow servers to automatically download and run code locally for security reasons. Please try and make sure clients without SSCSMs do not suffer from major functionality loss.