Scp over a proxy with one command from local machine?

You can do this with ProxyJump. put this in your ~/.ssh/config file (creating the file if it does not exist):

Host target.machine
User targetuser
HostName target.machine
ProxyJump [email protected]

After saving the file, you can just use

ssh target.machine

any time you want to connect. Scp also will work as it also respects the ssh config file. So will Nautilus, if you're using GNOME and want to use a GUI.

Old Answer (for older versions of OpenSSH)

Host target.machine
User targetuser
HostName target.machine
ProxyCommand ssh [email protected] nc %h %p 2> /dev/null

You can now* do this as a one-liner, without requiring nc anywhere:

scp -o "ProxyCommand ssh [email protected] -W %h:%p" [email protected]:file .

Explanation

pcreds and tcreds represent your proxy and target credentials if required (username, username:password, etc.).

This is possible because of a built-in netcat-like ability, removing the requirement for nc on the intermediate host. Using ssh -W host:port sets up a tunnel to the specified host and port and connects it to stdin/stdout, and then scp runs over the tunnel.

The %h and %p in the ProxyCommand are replaced with the target host and port from the outer scp command, to save you having to repeat them.

For even more convenience, you can configure the proxy in your ssh configuration:

Host target.machine
    ProxyCommand ssh [email protected] -W %h:%p

and from then on just do

scp [email protected]:file .

* since OpenSSH 5.4 - released March 2010

If you don't mind using rsync instead of scp, you can use the following one-liner:

rsync -v --rsh "ssh proxy.machine ssh" target.machine:/remote/file /local/dir/

(you'll need passwordless access to the proxy machine)