Install a Modrinth modpack on a Minecraft server

A modpack is a ready-made set of mods and settings. Running one on a server used to mean downloading dozens of files, matching versions by hand and hoping nothing was missing. Modrinth's pack format lists every file, its address and its checksum, so a host can do it for you.

What a modpack needs from the server

Every pack is made for one loader (Fabric, Quilt, Forge or NeoForge) and one Minecraft version. The server must match both exactly. A pack for NeoForge 1.21.4 will not run on Fabric, or on NeoForge 1.21.1. That is why the first step is always finding out what the pack needs.

Do it on Saryno

Press Make a server and find the box Start from a modpack.
Type the pack's name as it appears in its Modrinth address, for example fabulously-optimized from modrinth.com/modpack/fabulously-optimized.
Press Find it. Saryno shows what the pack is, which loader and Minecraft version it needs and how big it is, and fills in the setup for you with 4 GB of memory.
Create the server. It opens on the Mods tab with an Install it now button.
Press it. If the server is asleep it wakes first. Saryno downloads every mod the pack lists for servers, checks each file against the pack's checksum, and adds the pack's configuration.
Restart the game when it finishes.

Files the pack marks as client-only, like shaders and minimaps, are skipped, since a server cannot use them.

Safety

Packs can only download from Modrinth's own file host and a few well-known code hosts, and every file must match its checksum or the install stops. A pack cannot write outside the game folder or over server.properties and the platform's own files.

Players need the pack too

Players join with the same pack on their computer. Modrinth's launcher or any launcher that supports .mrpack files installs it with one click. If a player sees "mismatched mods", their version is not the server's: ask them to update the pack.

If the install fails

"Made for another loader or version." The server you picked does not match. Make a new server from the pack instead of reusing an old one.
A mod is not allowed to download. Some packs link to sites other than Modrinth. Upload those mods yourself with SFTP.
The server will not start. Press Check for problems in the Performance tab: it reads the log and names the mod. See also how to read a crash report.

Memory

Big packs need memory. 4 GB is a good start, and 6 GB or more for packs with hundreds of mods. See how much RAM a Minecraft server needs.

How to make a Fabric server covers adding single mods, and Paper, Vanilla, Purpur or Fabric helps you choose.