scp files via intermediate host

ProxyJump

New in OpenSSH 7.3:

A$ scp -oProxyJump=B thefile C:destination

(Behind the scenes, this just uses ProxyCommand and ssh -W.)

ProxyCommand

Updated to include -W from other answers:

A$ scp -oProxyCommand="ssh -W %h:%p B" thefile C:destination

If A has a very old SSH client installed (without -W support), or if B is configured to disallow TCP forwarding (but still allows shell commands), use alternatives:

A$ scp -oProxyCommand="ssh B socat stdio tcp:%h:%p" thefile C:destination
A$ scp -oProxyCommand="ssh B nc %h %p" thefile C:destination

Pipes

A$ tar cf - thefile anotherfile | ssh B "ssh C \"cd destination && tar xvf -\""
A$ (echo thefile; echo anotherfile) | cpio -o | ssh B "ssh C \"cd destination && cpio -i\""

For just one file:

A$ ssh B "ssh C \"cd destination && cat > thefile\"" < thefile

"Tunnel" through B

A$ ssh -f -N -L 4567:C:22 B
(continues running in background)

A$ scp -P 4567 thefile localhost:destinationPath

When you're done, don't forget to kill the previously started ssh process (which has dropped to background due to -f -N).

• -f Requests ssh to go to background just before command execution. This is useful if ssh is going to ask for passwords or passphrases, but the user wants it in the background. This implies -n.
• -N Do not execute a remote command. This is useful for just forwarding ports.

Reverse "tunnel" through B to A

Doesn't always work though:

A$ ssh -f -N -R 4567:localhost:22 B
(now you can reach A from B, by using localhost:4567)

B$ scp -P 4567 localhost:thefile C:destination

• -R Specifies that connections to the given TCP port or Unix socket on the remote (server) host are to be forwarded to the given host and port, or Unix socket, on the local side.

Versions of scp from early 2011 and later may have a "-3" option:

 -3      Copies between two remote hosts are transferred through the local
         host.  Without this option the data is copied directly between
         the two remote hosts.  Note that this option disables the
         progress meter.

If you have this you can just run:

B$ scp -3 A:file C:file

Nearly all have been already said but here is my last penny: I use ProxyCommand variant without nc nor soc. Based on OpenSSH Proxies and Jumphost Cookbook I crafted a following configuration:

1. So we have following players:

   • HOME_HOST: it is from where we copy a file to the target host
   • HOP_HOST: we copy through this host (logged as HOP_USER)
   • TARGET_HOST: it is our destination (authenticated as TARGET_USER)

2. First I added my local public key from my home host .ssh/id_dsa.pub to .ssh/authorized_keys at both hop and target hosts. Yes, the same public key from the home host to both of them. Usually you would expect it is the HOP public key you have to add to the TARGET one.

3. Then I tweaked .ssh/config a little by adding following entry:

   Host TARGET_HOST
      User TARGET_USER
      ProxyCommand ssh -W %h:%p HOP_USER@HOP_HOST
   

4. After that the copy operation is as simple as: scp FILE TARGET_HOST:. It displays double banners from both the hop and target nodes but it works.

Of course you may use above to ssh directly to the target: ssh TARGET_HOST. It works with scp and ssh.

Another more general option might be sshuttle utility which appears to be a kind of transparent proxy (vpn over ssh). So in your case of A->B<->C it allows to connect to each node at C's network: A->B-[CDEFG]. It does not need admin but it requires Python 2.7 (3.5 also OK) which is not always what we have. It is worth of trying it.

ssh -L 4321:hostC:22 youruser@hostB

in another shell:

scp -P 4321 localfile youruser@127.0.0.1

This is using port forwarding. The only limitation here is host B needs to be configured to allow port forwarding. Otherwise this should work fine.

In the way of explanation, -L and -R allow you to forward ports. In -L, the first port given is the port ssh will begin listening on the originating machine (host A), and it'll forward anything it receives on that port over your SSH connection to host B, then route to host C on port 22.

edit

I messed up the syntax slightly. It sets up a forward on your LOCAL machine.