Nuvola inherits network proxy settings from your system by default. Although it should satisfy the needs of most users, chances are that you might need to use a different proxy server for some applications, e.g., to bypass geographic restrictions. That's when independent per-application proxy settings come in handy.
To change Network Proxy Preferences, open Preferences dialog from the Menu button and click Network Proxy option.
https_proxyenvironment variables or from GNOME settings.
portof the HTTP proxy server in your network.
portof the SOCKS4/SOCKS5 proxy server in your network. Note that SSH tunnels also create a proxy of the SOCKS type.