1. Ente Auth

Source: Ente Auth

Most of the open source auth apps work locally on your phone  and thus self hosting is not necessary. However, you are responsible for backups of your keys in the event of loosing your device. Now, there are two ways around this problem. 

1. Synchronize a folder on your phone with a folder on your server. All these apps allow for automatic backups to a folder on your phone each time you save a new key. Now, you can choose the folder your Auth app is saving the keys to. Therefore, you can save it to a folder that is synced with your server via applications like Syncthing, Nextcloud, Seafile etcetera. Our recommendation is syncthing for its ease of use.
2. Have the auth app itself syncronize to your server without needing to set up a sync job like with syncthing. The best example of this so far is Ente Auth. With Ente Auth your users can simply use their Ente Photos account and log in to Ente Auth and backups are handles automatically for them. For users this is simple, but you as the maintainer of the server is responsible for the backup jobs. 

So in this chapter we will discuss the different open source options