Wednesday, April 8, 2020

Putty and double bastion host tunneling

I've always found complex ssh tunneling to be a pain in Windows.  Unfortunately, I'm stuck with it due to company mandate.  This post is as much about sharing the info as it is about documenting it so that I can come back and refer to it when I forget everything here.

So, the use case is this.  You have a host that you have to hop through in order to get to other hosts, but there may be more hosts that you can't get to unless you hop through yet another host.

For example, if I want to access my Linux workstation at the office, I have to hop through two hosts, my ssh bastion host and from there to another host that also has an IP on the office network, and from there to my desktop.

Even to experienced tunnelers, that's daunting, especially from Windows.

So, here goes.  Here's what the values in my example mean:

My workstation:
The public facing bastion host:
The internal server:
My username on my workstation: myusername
My username when accessing company servers: companyusername
My WSL username: myWSLusername

Putty Configuration

  1. Create a new session in Putty.
  2. Hostname:
  3. Port: 22
  4. Category → Connection → Data
    • Auto-login username: myusername
  5. Category → Connection → Proxy
    • Proxy type: Local
    • Telnet command, or local proxy command:
      c:\progra~1\putty\plink.exe -ssh -agent -A -l companyusername -nc %host:%port -proxycmd "c:\progra~1\putty\plink.exe -l companyusername -agent -A -nc"
  6. Category → Connection → SSH
    • Enable compression
    • Notice that I didn't use the compression option -C in my plink commands in the previous step.  When tunneling ssh traffic, you should only enable compression in one place so that you're not compressing traffic only to have other segments attempting to compress the already compressed data.
  7. Category → Connection → SSH → Auth
    • Attempt authentication using Pageant
    • Allow agent forwarding
  8. Category → Connection → SSH → X11
    • With WSL and VcXsrv X Server installed, you can run gui apps
    • X11 Forwarding: Enable X11 forwarding
    • X display location: (Look in VcXsrv server's log to confirm this value.)
    • Remote X11 authentication protocol: MIT-Magic-Cookie-1
    • X authority file for local display:
    • (You might to run from WSL bash shell xauth generate $DISPLAY initially to get the x authority file seeded.)
  9. Save new Putty session
  10. Launch new Putty session
There are certainly other ways of doing this that can be a little bit simpler.  One of the reasons I used this method is because my bastion host strips X11 traffic.  It's not configured for it and doesn't have any of the required x related dependencies needed (xorg-x11-server-utils et. al.).  Doing it the way I have creates a tunnel that simply passes all traffic through to the next host keeping me well below the application layer from the perspective of the bastion host.

Workstation File Access

The other advantage to this method is that I can use this same Putty session to make additional ssh tunnels that go right to my workstation.  So, in my Putty config detailed above, I also have a local tunnel 20202: that gives me direct ssh access to my workstation by ssh'ing to here on my laptop.

So, software like WinSCP can now access my workstation over this tunnel and behaves as if it were direct access.  I use Mountain Duck, which is not free software. The end result is that it allows me to map a drive right to my Linux workstation at the office from my Windows 10 laptop at home.

(Additionally, I use the RDP tunneling method described in my previous post to make it so that RDP sessions end up originating from my workstation.)


While drive/file access works quite well, this is not fast enough to do X11 well at all.  So as much as I'd like to run gvim directly from my workstation over the tunnel, it's just not fast enough.  

But, having the option to do it was helpful for me.  There were a couple apps that I really just needed some of the settings out of so that I could set up the Windows versions of those apps to work the same way.


I haven't actually explored the capabilities of the OpenSSH that's now included with Windows 10, but regardless, for non-Windows users, note that there is a newer ProxyJump directive.  This let's you chain together any number of bastion hosts.  So, following my earlier example, you can do something like in an .ssh/config file entry:

Host workstation-tunnel
User myusername
ForwardAgent yes
ForwardX11 yes
ForwardX11Trusted yes
Compress yes
PubkeyAuthentication yes

Then you just ssh workstation-tunnel and you're good!

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