suragu_org/tech_posts/ssh_holyness.org

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#+TITLE: SURAGU - insane stuff you can do with ssh
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#+AUTHOR: Roberto Cornell

* Insane stuff you can do with ssh

If there's a God, I think he'd probably very dissapointed with us, we
have created many awful things that can't be name because of
respect. If I were God I would be dissapointed with my creation and
probably would end it. But in 2002, a proof that God exists and love
us appeared, the first version of OpenSSH was released.

OpenSSH is a marvellous software, it is one of the most useful things
I've seen in my entire life, and when you master it, all of your
problems will disappear.

** Forwarding ports

This is a known one but that doesn't make it less of a godsend. You
can forward ports through ssh, like you have in a remote server a
software running in port 4040. And because of another reasons
(i.e. firewall, bound only to loopback) you want to have that port in
your computer.

This is possible thanks to the =-R= and =-L= ssh flags. The syntax for
both =-R= and =-L= are the same but the concept is different.

- =-L= "brings" a remote port to your device.
- =-R= "brings" a local port to a remote server.

So, to answer the question in the first paragraph, you can use this
command to bring the port 4040 running in a remote server to your
local computer:

#+begin_src bash
$ ssh -L 4040:127.0.0.1:4040 server.tld
#+end_src

This will forward the port 4040 running in remote server's
127.0.0.1:4040. So you can access the remote server from your
loopbackl address.

By default, ssh binds to the loopback address. If you want, for any
reason, bind the forwarded port to another address, prefix the local
port with the address, like this:

#+begin_src bash
$ ssh -L 0.0.0.0:4040:127.0.0.1:4040 server.tld
#+end_src

To forward a port, it must go to the another server, creating a
tunnel, if you created this forwarding rule with a firewall rule or
another software like socat or netcat, the traffic will be
unencrypted. SSH is a secure protocol, and all the outgoing and
incoming traffict that comes from the tunnel will be encrypted.

You'll notice that everytime you want to forward a port a new ssh
connection will be open, and will create another prompt, you can send
the thing to background combining the =-F= and =-n= flag.

*** Forwaring ports from an existing connection

A feature of ssh that cured my depression is the fact that if you type
"~C" (that's literally typing a tilde and a capital c in a ssh window)
it will prompt the ssh commandline, in which you can type "?" to get
the usage of the commandline.

** Creating hosts

I have this friend that edits the =/etc/hosts= file to add the
hostname of his servers to the system's DNS so he can just type =ssh
<servername>= to ssh to the server. I've told him many times that this
is stupid and he should edit the =~/.ssh/config= file.

As it name suggets, it is the ssh config file. And it is used to
configure ssh. There are an insane ammount of options that can be used
in this config file. But what's important here is that this config
file can save us many, many keystrokes in the ssh command. Here's an
example.

#+begin_src conf-space
  Host guadal
       HostName 192.168.1.57
       AddressFamily inet
       RequestTTY yes
       User diego
       SetEnv SHELL=/bin/zsh
       SetEnv ZDOTDIR=/home/diego/
       SetEnv ZSHDOTDIR=/home/diego/
       SetEnv DISPLAY=:0.0
       ForwardX11 Yes

  Host rguadal
       HostName 192.168.1.57
       AddressFamily inet
       User root
       SetEnv SHELL=/bin/zsh
#+end_src

This config file needs no commentary because you can figure what every
line is doing. After saving a config file i can just type "ssh guadal"
to login to the server as my user and "ssh rguadal" to login as root.

** Tmux and stuff

There's this software called "byobu" that is basically a tmux that is
always running in a remote server so everytime you login to your
server the same session of tmux will be there, and will prompt
automatically.

Thing is that one day I don't know what I was thinking but I decided
to install OpenBSD in my server, and Byobu is not available on the
OpenBSD ports repository and i'm a lazy motherfucker so I won't
compile it. I fixed this issue configuring regular tmux in a fancy way.

So basically tmux works with sessions, like any other terminal
multiplexer, and you can attatch to that sessions at any times, as
many times as you want and with many users you want. It's like this
impossible concept of 2 things at the same place at the same moment.

I got philosophical, first, you have to login into the server and type
=tmux -u2= command, or just =tmux=, i use the =-u2= flag because OpenBSD
is stupid and won't handle unicode alright without those flags.

This will create a tmux session in which you can do whatever you feel
like doing in a UNIX terminal.

When you're finish doing things in a UNIX terminal, you instead of
typing "exit" or "C-d" in the terminal to log off, you detatch from
the tmux session, pressing the modifier (C-b by default) and "d". Then
you exited the tmux session and can log off from the ssh session.

And when you want to get back to the tmux session you had, you can ssh
to the server and type =tmux -u2 a=.

This steps can be saved if you type (in my case because i bothered
configuring ssh, probably you have to type a different command because
you used a different name) =ssh guadal "tmux -u2"= to create a session
and =ssh guadal "tmux -u2 a"= to attatch to the other session.

** Conclusion

Mastering ssh will cure your depression.