Hosting Custom Maps
Most important server parameter you need for hosting a custom map is
+server.levelurl [url]. Server reads the [url] argument and starts downloading the map. Keep in mind that other startup parameters that are responsible for procedural maps like
+server.seed should be removed.
Your map file should be hosted on a public web-site that works 24/7, since new players of your server will download the map from that URL, not from your Rust server. If your URL link doesn't work then players that haven't downloaded the map yet won't be able to join the server.
The URL can be literally anything as long as it's a direct download link. You can check if your link is correct or not by accessing it in your browser - if download starts automatically, then it's a direct download link. If it loads a page that requires user to click something to start the download, then this URL won't work and server will throw an error.
You can also host your map locally. In this case your URL should start with
file:/// and after this you enter a full path to your map. For example,
file:///path/to/your/cool.map. Keep in mind that other players won't be able to connect because of locally hosted map.
For list of common issues and their fixes please visit this page.