How does modding work on multiplayer?

[TotallyNotABot]

If I were to download a mod that increases zombie spawns, and then invite my friends to my world, would it still work? Or do they need to get the mod as well?
Or do you have to set up an actual server for modded multiplayer?

 

[khzmusik]

In general, for mods (and modlets):

• If a mod uses only XML/XPath, then the XML changes get pushed from server to client, so only the server needs the mod. These mods are EAC friendly.
  
• If a mod uses XML/XPath, but also adds new resources (such as new models or textures), the resources do not get pushed from server to client, so clients also need the mod. However, these mods are still EAC friendly.
  
• If a mod uses any custom C# code (DMT, Harmony, old-school SDX, whatever), then you should assume that both client and server need the mod. These are not EAC friendly, so EAC needs to be disabled on both the server and clients.
  

You didn't say exactly how the zombie spawns were increased. But, this can be done through XML/XPath alone. If that's the case, then only the server needs to have the mod, and it will push the changes to clients.

AFAIK it doesn't make any difference if the "server" is your local machine when you host the game, or if it's a dedicated server. (If I'm wrong, someone will hopefully correct me.)