Here is a workaround if you are behind proxy and not getting the repositories. This would work for all python plugins.

1. Download the plugin directly from the repository url using your browser. For example, for CadTools, download the zip file from the here repo link
2. Unzip the file. Copy the folder(usually one directory down the extracted folder) to your python plugins directory. In windows, this would be  C:\Program Files\Quantum GIS Wroclaw\apps\qgis\python\plugins
3. Restart Qgis and now you can see this plugin installed. Just enable this plugin in the Plugin Manager.

Here is a workaround if you are behind proxy and not getting the repositories. This would work for all python plugins.

1. Download the plugin directly from the repository url using your browser. For example, for CadTools, download the zip file from the here repo link
2. Unzip the file. Copy the folder(usually one directory down the extracted folder) to your python plugins directory. In windows, this would be for QGIS1: C:\Program Files\Quantum GIS Wroclaw\apps\qgis\python\plugins, for QGIS2: C:\Users\yourusername\.qgis2\python\plugins or for QGIS3: C:\Users\yourusername\AppData\Roaming\QGIS\QGIS3\profiles\default\python\plugins and on OSX: ~/.qgis/python/plugins
3. Restart QGIS and now you can see this plugin installed. Just enable this plugin in the Plugin Manager.