in Linux, Security, Ubuntu

Simplify Your Life With an SSH Config File


If you’re anything like me, you probably log in and out of a half dozen remote servers (or these days, local virtual machines) on a daily basis. And if you’re even more like me, you have trouble remembering all of the various usernames, remote addresses and command line options for things like specifying a non-standard connection port or forwarding local ports to the remote machine.

Shell Aliases

Let’s say that you have a remote server named dev.example.com, which has not been set up with public/private keys for password-less logins. The username to the remote account is fooey, and to reduce the number of scripted login attempts, you’ve decided to change the default SSH port to 22000 from the normal default of 22. This means that a typical command would look like:

$ ssh fooey@dev.example.com -p 22000
password: *************

Not too bad.

We can make things simpler and more secure by using a public/private key pair; I highly recommend using ssh-copy-id for moving your public keys around. It will save you quite a few folder/file permission headaches.

$ ssh fooey@dev.example.com -p 22000
# Assuming your keys are properly setup…

Now this doesn’t seem all that bad. To cut down on the verbosity you could create a simple alias in your shell as well:

$ alias dev='ssh fooey@dev.example.com -p 22000'
$ dev # To connect

This works surprisingly well: Every new server you need to connect to, just add an alias to your .bashrc (or .zshrc if you hang with the cool kids), and voilà.

~/.ssh/config

However, there’s a much more elegant and flexible solution to this problem. Enter the SSH config file:

# contents of $HOME/.ssh/config
Host dev
  HostName dev.example.com
  Port 22000
  User fooey

This means that I can simply $ ssh dev, and the options will be read from the configuration file. Easy peasy. Let’s see what else we can do with just a few simple configuration directives.

Personally, I use quite a few public/private keypairs for the various servers and services that I use, to ensure that in the event of having one of my keys compromised the damage is as restricted as possible. For example, I have a key that I use uniquely for my Github account. Let’s set it up so that that particular private key is used for all my github-related operations:

Host dev
  HostName dev.example.com
  Port 22000
  User fooey
Host github.com
  IdentityFile ~/.ssh/github.key

The use of IdentityFile allows me to specify exactly which private key I wish to use for authentification with the given host. You can, of course, simply specify this as a command line option for “normal” connections:

$ ssh -i ~/.ssh/blah.key username@host.com

but the use of a config file with IdentityFile is pretty much your only option if you want to specify which identity to use for any git commands. This also opens up the very interesting concept of further segmenting your github keys on something like a per-project or per-organization basis:

Host github-project1
  User git
  HostName github.com
  IdentityFile ~/.ssh/github.project1.key
Host github-org
  User git
  HostName github.com
  IdentityFile ~/.ssh/github.org.key
Host github.com
  User git
  IdentityFile ~/.ssh/github.key

Which means that if I want to clone a repository using my organization credentials, I would use the following:

$ git clone git@github-org:orgname/some_repository.git

Going further

As any security-conscious developer would do, I set up firewalls on all of my servers and make them as restrictive as possible; in many cases, this means that the only ports that I leave open are 80/443 (for webservers), and port 22 for SSH (or whatever I might have remapped it to for obfuscation purposes). On the surface, this seems to prevent me from using things like a desktop MySQL GUI client, which expect port 3306 to be open and accessible on the remote server in question. The informed reader will note, however, that a simple local port forward can save you:

$ ssh -f -N -L 9906:127.0.0.1:3306 coolio@database.example.com
# -f puts ssh in background
# -N makes it not execute a remote command

This will forward all local port 9906 traffic to port 3306 on the remote database.example.com server, letting me point my desktop GUI to localhost (127.0.0.1:9906) and have it behave exactly as if I had exposed port3306 on the remote server and connected directly to it.

Now I don’t know about you, but remembering that sequence of flags and options for SSH can be a complete pain. Luckily, our config file can help alleviate that:

Host tunnel
  HostName database.example.com
  IdentityFile ~/.ssh/coolio.example.key
  LocalForward 9906 127.0.0.1:3306
  User coolio

Which means I can simply do:

$ ssh -f -N tunnel

And my local port forwarding will be enabled using all of the configuration directives I set up for the tunnel host. Slick.

Source: http://nerderati.com/2011/03/17/simplify-your-life-with-an-ssh-config-file/

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