Why tunnel Librarian using ngrok

You can run Librarian completely on your local network, with it being completely in-accessible over the internet, but the problem is installing iOS builds.

Unlike android, where APK files can directly be downloaded using the mobile browser and installed, iOS requires IPA files to be served using a manifest.plist file. The tricky part, however, is that this manifest file needs to be served over HTTPS, otherwise iOS refuses to install the build.

There are ways to make a local server use HTTPs, almost all of them are tedious to set up, and the certificates served have to be manually trusted by each client connecting to them.

That's why Librarian uses ngrok to tunnel it's webserver over HTTPs (or it's manifest server in private web mode).

If you're worried about people gaining access to your builds / information over the internet, you can always run Librarian in private mode, where only a separate manifest server is tunnelled using ngrok, and your build assets are still completely local and accessible only on the local network.