Configure SSH Tunneling

Last modified

November 13, 2024

To connect to a remote host, TU Delft uses a proxy server, know as bastion. To reach a remote host, a user has to connect first to the bastion and from there to the remote host. However, a user can connect directly to the remote host using ssh tunneling.

Set a Default SSH Tunneling for a Host (Linux Terminal)

  1. On your local machine, edit the ~/.ssh/config file and add the following confuration:
Host <host-nickname>
    HostName <target-host>
    User <target-username>
    ProxyCommand ssh <bastion-username>@linux-bastion-ex.tudelft.nl -W %h:%p

Replace: : a name for you choice for the targe host, e.g., my-server : the actual name of the target host (FQDM), e.g, server.tudelft.nl : the username used to login to the target host : the username used to login to the bastion server

  1. Create a key-pair on the local machine.
$ ssh-keygen -f ~/.ssh/<my-keyname> -t rsa -b 4096

You will be promted to crate a passphrase, we recommend you to add one to make the connection more secure. The passphrase will be asked every time you connect to the target host.

A private and public keys will be added to ~/.ssh. The public key is in the <my-keyname>.pub

  1. Copy the content of the public key to the ~/.ssh/authorized_keys file in the target host.

  2. Connect to the target host using ssh tunneling. Use your bastion-password when asked.

$ ssh <host-nickname>

If you encounter problems with the connection. Use the debug mode ssh -vvv <host-nickname> to find out what might have gone wrong.

Tunneling with WinSCP

WinSCP is a GUI that makes it very easy to inspect, edit and transfer files on the webserver. Instructions for setting this up on a hypothetical server from the CiTG faculty are provided below. General documentation on tunneling with WinSCP are here: https://winscp.net/eng/docs/ui_login_tunnel

These instructions were tested with an existing id_ed25519 key, assume you already have WinSCP installed and can modify a text file on the server in your user directory using the terminal.

Do the following:

  1. As explained above, log in to the server and add your public key to the file /home/<username>/.ssh/authorized_keys (this only needs to be done once).

It should look like this with your own keys XXXXXXX and NetID filled between the <...> (note the <XXXXXXX> is much longer in reality):

ssh-rsa <XXXXXXX> ICT-SYSTEMS-<NETID>
ssh-rsa <XXXXXXX> ICT-SYSTEMS-<NETID>
ssh-ed25519 <XXXXXXX> <NETID>@tudelft.nl
  1. Using WinSCP, the following fields should be entered:

On the main login settings page: - File protocol: SFTP - Host name: <server>.citg.tudelft.nl - User name: your NetID

  1. Open the “Advanced…” window

  2. On the page “Tunnel” (under heading “Connection,” still in the Advanced window):

  • Host name: linux-bastion-ex.tudelft.nl
  • User name: your NetID
  1. On the page “Authentication” (under heading “SSH,” still in the Advanced window):
  • Private key file: select your private key file, for example C:..../<username>/.ssh/id_ed25519
  • Note that the app may ask you to convert your existing key to a Putty format (for example “Do you want to convert OpenSSH private key to PuTTY format?”). Click “OK” then make sure you select the new PuTTY file (e.g., C:..../<username>/.ssh/id_ed25519.ppk)
  1. Save the setting and click “Login”, using your NetID password to authenticate.

Using WinSCP with sudo rights

If you have sudo rights on the webserver you can use this via WinSCP as follows:

  1. Once agin go to the “Advanced…” window to the “SFTP” page under heading “Environment”
  2. In field “SFTP server” enter the following: sudo /usr/lib/openssh/sftp-server
  3. Save the changes
  4. Use with caution!

Note that the path to sftp-server may be different but can be easily checked and arranged. This will not work if you change the setting and continue to use an open session.