The Dropbox setup process don’t support http proxy or SOCKS5. (Yes, the file says it works with http_proxy environment variable, but appears it will not.) Thanks to two great tools: tsocks and proxychains, which can start any program and manage their network communicating request through a pre-configured proxy.

Install tsocks (or proxychains) under Ubuntu. Because of Ubuntu, who made these tools programs that are conventional.
Get an SSH tunnel (in any manner you enjoy).
ssh -D 7070 my-username@my-service provider-url
Configure your Web browser to make use of the tunnel. In Firefox, choose to use SOCKS5 proxy: localhost:7070. This permits you access to the homepage of Dropbox and download the Ubuntu program.
Install the program by clicking it in Nautilus. To inspect the setup, type the command in a shell,
dropbox begin -i
You made network access restriction if you’re able to see some error messages complaining it.

Add these lines to /etc/tsocks.conf:
server_interface = 7070
If you’re using proxychains, you must change /etc/proxychains.conf or make your own ~/.proxychains.conf.

This time, using tsocks to start the Dropbox install procedure:
tsocks dropbox begin -i
Right-click Dropbox icon, select “Settings”, and establish SOCKS5 proxy like you did for Firefox. Hopefully, Dropbox begins to sync files you want now.