Simplify your SSH experience

2012-05-15 ·

I manage too many remote servers to be able to remember their IP addresses, usernames and passwords. The key to simplify this experience is a proper SSH configuration. By adjusting .ssh/config file you can easily create a memorable alias, along with particular settings for any server you use.

TL;DR

Use public/private key authentication to avoid entering password each time you connect to the server. Create an alias for each server within .ssh/config using the syntax below

Host alias-name
  HostName my.realm.com/or-ip
  User zaiste

and connect to it like so

λ ssh alias-name

Public/Private Key Authentication

When you connect to a remote server you must enter a password in order to authenticate yourself. This step, however, can be easily avoided using public/private key authentication. The setup is pretty straightforward. First of all, you have to generate a public/private key pair:

λ ssh-keygen -t rsa -C '<your email>'

It is a good idea to choose a strong password. If you want to change the password for a specific key use ssh-keygen wth -p option. Additionaly a -f option can be use to specify a key file to use.

Next, copy the content of .ssh/id_rsa.pub file and add it to .ssh/authorized_keys on a remote server which you want to connect with. If the file (or directory) doesn't exist, simply create it. Now you should be able to connect to that server without entering your password

λ ssh zaiste@my.realm.com

It is also a good idea to have multiple keys, separated for each remote place or keys used with a specific group e.g. private, client specific, etc. This enhances the security in case any of the authentication key pairs are compromised.

Alias Configuration

Let's make now a server alias, so we don't have to remember entire domain name or IP address each time we want to connect to it. Open your .ssh/config and add something like this.

Host my
   HostName my.realm.com
   User zaiste 

Don't forget to adjust it for your case.

Such configuration will reduce ssh invocation to the following line

λ ssh my

For each entry you can adjust connection settings like port number or which private key to use. Take a look at SSH documentation for possible options

Specifing Private Key

By default for each server the same key pair is used. In order to change this behaviour we have to use IdentityFile option which specifies a private key to use for authentication with the given host, e.g.

Host client
   HostName client.server.com
   User p45dazz
   IdentityFile ~/.ssh/client.key

This can be pretty useful when you are working within different organization on GitHub.

Host ghn
  User git
  HostName github.com
  IdentityFile ~/.ssh/nukomeet.key

With such entry I can now clone from a specific organization using even more succinct command

λ git clone ghn:nukomeet/some_repository.git

Tunneling

For security reason, you can leave only a limited number of open ports (on the server which you have control of), e.g 80/443 for web and 22 for SSH. Now, In order to connect to specific services, we can simply tunnel them using the following command:

λ ssh -f -N -L 9906:127.0.0.1:27017 zaiste@db.example.com

It means that a remote port number 27017 (MongoDB server) is mapped to a local port 9906. Now, by connecting to localhost:9906 you will have access to a remote MongoDB server. There are two additional options -f which puts ssh in background and -N which does not execute a remote command. As before, if you use such command frequently, it can be saved to .ssh/config like so:

Host tunnel
  HostName db.example.com
  IdentityFile ~/.ssh/zaiste.key
  LocalForward 9906 127.0.0.1:27017
  User zaiste
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