Can't complete dropbox installation from behind proxy

sudo https_proxy="https://username:password@your_proxy:proxy_port" dropbox start -i

You have to run the deamon as root, and thus, you have to configure the proxy as root. Setting the httpS_proxy root env variable on invocation should be enough.

What is still missing is the installation of the Dropbox daemon. You can download and install it manually; this is documented at https://www.dropbox.com/install?os=lnx

32-bit:

cd ~ && wget -O - "https://www.dropbox.com/download?plat=lnx.x86" | tar xzf -

64-bit:

cd ~ && wget -O - "https://www.dropbox.com/download?plat=lnx.x86_64" | tar xzf -

Next, run the Dropbox daemon from the newly created .dropbox-dist folder.

~/.dropbox-dist/dropboxd

In case it helps anyone, I had an issue with the proxy settings and although I had them set they weren't enabled:

You can see if they are enabled like this:

gsettings get org.gnome.system.proxy.http enabled

And enable them if they aren't like this:

gsettings set org.gnome.system.proxy.http enabled true