Forward SSH traffic through a middle machine

Question

SSH tunneling is very confusing to me. I am wondering if I can do this in Linux.

I have 3 machines..

A. My local machine at home.
B. Machine at work that I can SSH into (middle man).
C. My desktop at work that I can only SSH into from machine B.

So I can SSH from A -> B and from B -> C, but not from A -> C.

Is there a way to setup an SSH tunnel from A through B, so when I run other SSH commands it they just work from my local machine A? I am basically trying to clone a git repo from work to home (and I cannot install git on machine B).

Also, once setup.. How would I unset it as well?

Answer 1

Place this in your .ssh/config file on hostA (see man 5 ssh_config for details):

# .ssh/config on hostA:
Host hostC
    ProxyCommand ssh hostB -W %h:%p

Now the following command will automatically tunnel through hostB

hostA:~$ ssh hostC

You may like to add options like -oCiphers=arcfour and -oClearAllForwardings=yes to speed things up, since wrapping ssh inside ssh is computationally more expensive and the extra effort and the wrapper doesn't need to be as secure when it's tunneling already-encrypted traffic.

---

If you are using OpenSSH earlier than 5.3, the -W option is not available. In this case you can implement the above using netcat (nc):

ProxyCommand ssh hostB nc %h %p  # or netcat or whatever you have on hostB

Answer 2

Edit: This is the wrong approach. See ephemient's answer instead. This answer will work, but is potentially less secure and definitely less awesome.

It sounds like you want a solution like the following:

ssh -L localhost:22:machinec:22 machineb

This will get you a shell on machineb. Leave this alone; minimize the terminal window.

Now, whenever you make an ssh connection to localhost, you will actually be connected to machinec through machineb. When you're done with the tunnel, just close the terminal in which you ran the above command.

Note that you'll need superuser privileges to run the command.

Answer 3

Sounds like you want a shell alias on A that causes ssh to occur on C

1. I assume that on A, you can type ssh me@b "ssh me@c hostname" and get back "C"
2. Make an alias sshc which expands sshc foo into ssh me@b "ssh me@c foo"
3. For exact syntax of creating the alias, consult superuser.com

Answer 4

For interactive shell you can use this simple command:

ssh -J <user>@<hostB> <user>@<hostC>

The -J options is for jump.

Answer 5

If your employer provides a VPN, I'd recommend using that instead.

That way, you won't have to configure any applications specially (even ssh), and you can see any machine behind the firewall. Additionally, all of your traffic will be encrypted by the VPN software, which will add security to any inadvertently or deliberately unencrypted traffic.

Answer 6

YASS Yet Another Simple Solution

ssh -f -L 2222:HostC_IP_or_Name:22 userOnB@hostB sleep 10 &&
    ssh -o HostKeyAlias=HostC -p 2222 userOnC@localhost

• First command open a ssh connection to HostB and tell HostB to forward connections from localhost:2222 to HostC:22.
• the -f parameter tell SSH to go to background once connection established
• Second command open simply a client connection to localhost:2222
• Option HostKeyAlias are not required, but could help to prevent connection to wrong host
• Nota: command sleep 10 are needed to maintain connection until second ssh command use forwarded port. Then first ssh will close when second ssh leave forwarded port.

you could now run subsequent ssh sessions:

ssh -o HostKeyAlias=HostC -p 2222 userOnC@localhost

Variant:

ssh -f -L 2222:HostC_IP_or_Name:22 userOnB@hostB sleep 10 &&
    ssh -M -S ~/.ssh/ssh_HostC22userOnC.sock -o HostKeyAlias=HostC -p 2222 userOnC@localhost

subsequent ssh sessions could be open by running:

ssh -S ~/.ssh/ssh_HostC22userOnC.sock userOnC@localhost

The main advantage of using -M and -S param is that only one connection is open from HostA to HostC, subsequent session won't authenticate again and run a lot quicker.

Answer 7

Special case, mixed nix platforms:

  hostA (linux) -> HostB (solaris) -> HostC (linux)

If need an X application on hostC, and the intermediate hop is on Solaris box... in this case I found the netcat (nc) needed on the ProxyCommand like so:

hostA:~$ vi .ssh/config:

Host hostC
    ProxyCommand ssh hostB nc %h %p       # where nc is netcat

Then automatic tunneling works:

hostA:~$ ssh hostC