OfflineSkins is a forge plugin that will pull and cashe your minecraft skin for offline servers.
This mod allows you to show off your skins even if the server is in offline mode.
In case of your normal skin doesn't load, you can add your/others skin in the generated folder (.minecraft/cachedImages/skins), named after your username and ends with .png. This way the mod will use the provided file to load a skin for your/others character.
[This can be turned off] Crafatar will also be used to provide skins when normal skin doesn't load.
This mod is Client-Only, you don't need to install on server.
This mod was developed before SkinPort but it has been rewritten using codes from SkinPort.
There might be some bugs, reporting them to me is appreciated.