#+title: reverse ssh tunnels
#+rss_title: reverse ssh tunnels

You have 3 machines:

- ~home~: some computer you want to access while away
- ~vps~: a vps that you have so it can be used to access ~home~
- ~laptop~: a laptop you want to use to access ~home~

The problem is that ~home~ is behind some network you don't control. You can use a reverse ssh tunnel to get access. We will use 22222 as our example port on ~vps~.

** [[#h-a3854f2f-36e7-43a8-8b45-b4a491923ba4][From ~home~]]
:PROPERTIES:
:CUSTOM_ID: h-a3854f2f-36e7-43a8-8b45-b4a491923ba4
:END:

#+begin_src
ssh -R 22222:localhost:22 username@vps
#+end_src

This will set a shim on ~vps:22222~ -> ~home:22~ through the ssh connection. Some more detail and visualizations may be found [[https://unix.stackexchange.com/questions/46235/how-does-reverse-ssh-tunneling-work][here]].

** [[#h-56d8bbe1-5940-4d97-b817-a71979240c19][From ~laptop~]]
:PROPERTIES:
:CUSTOM_ID: h-56d8bbe1-5940-4d97-b817-a71979240c19
:END:

#+begin_src sh
# to access home after ssh'ing into vps:
ssh localhost -p 22222

# to access home in one go from laptop:
ssh -t vps 'ssh localhost -p 22222'

# to Jump from the laptop (-J is the jumpbox target):
ssh -J username@vps:22 localhost -p 22222
#+end_src

** [[#h-76dc15ec-0aab-464c-a674-60474010c8f0][TRAMP syntax]]
:PROPERTIES:
:CUSTOM_ID: h-76dc15ec-0aab-464c-a674-60474010c8f0
:END:

emacs TRAMP syntax for jumping across a local reference on a different port:

#+begin_src
find-file /ssh:username@vps|ssh:username@localhost#22222:/path/to/some/file
#+end_src

and then here's a cool trick for providing the connection info to a remote shell through the environment with ~$TRAMP_INFO~: (this will also do the jumping for you!)

#+begin_src emacs-lisp
(let* ((tramp-connection-path "/ssh:username@vps|ssh:username@localhost#22222:")
        (default-directory (concat tramp-connection-path "/home/username"))
        (process-environment (cons (format "TRAMP_INFO=%s" tramp-connection-path) process-environment)))
  (shell))
#+end_src

** [[#h-49b5aea4-b058-45ae-ac2a-d2ebc04bacc4][Other]]
:PROPERTIES:
:CUSTOM_ID: h-49b5aea4-b058-45ae-ac2a-d2ebc04bacc4
:END:

https://en.wikibooks.org/wiki/OpenSSH > Cookbook