SSH

What is SSH for?

SSH allows two computers to exchange data securely over unsecure links (for example, the Internet).

The server can encrypt data using the public key that you will give me. Only you (with your matching private key) can decrypt the data that my server sends you.

To set this up, you have to generate a pair of ssh keys and send me your public key.

More information on ssh can be found at http://www.openssh.org/

Why use SSH

It's secure

The entire session is encrypted. This means you can work in a cafe over wifi and not worry about anyone snooping. Note that you DO have to use a computer with ssh installed on it and have a copy of your private key on it.

Lots of tools use it

Any tool that supports ssh or sftp can talk to the server.

For example, on Ubuntu (and other Linux computers that use Nautilus file manager) you click "Places"->"Connect to Server..." and you can open a file manager window and drag and drop files between your computer and my server.

Various other tools such as Dreamweaver and ActiveState Komodo have support built in to allow opening, editing and saving files directly on the server.

Of course command line tools work too (sftp, scp, ssh, rsync).

On Windows some people like Filezilla to transfer files. For a command shell on Windows I use putty.

Why use SSH key pairs?

We never have to exchange any secrets: I don't need to give you a password. We don't have to send secrets through email. I will never know what your password is since there isn't one. Only a passphrase to protect your private key.

When you connect to my server, all data transmitted over the networks is encrypted. This means you can safely work in any public places or use wireless connections without worrying who is sneaking a look at your traffic.

\* Strictly speaking you don't need to use a passphrase, but if you put the key pair on a portable computer it protects us both if the computer is lost or stolen. No passphrase means you don't have to type your passphrase to connect to my server.

Case 1. You want access to a server that I manage (HuPI.org)

Linux and possibly Mac users

• If you already have an ssh key pair then you only need send me your public key.
• If you don't have a key pair, you will need to generate a new pair.

On Linux or Mac, the ssh files live in your home directory in the hidden folder called .ssh (That's a dot followed by ssh)

If you don't have a folder called .ssh then you don't have keys either. Here are all the commands to create the folder and generate a key pair.

# move to your home directory
cd          
# create the folder                
mkdir .ssh
# protect the folder
chmod 700 .ssh
# generate keys, use a passphrase you can remember
# this will overwrite existing keys, you might not want to do that!
ssh-keygen

This will create the files id_rsa and id_rsa.pub in .ssh by default, you can specify a zillion things other on the command line such as a different name for the files like maybe 'id_rsa_hupi'

Now send me just the file id_rsa.pub in an email. (It's in your .ssh folder.)

Special notes for Macintosh

The next time I sit at my Mac I will fill in this section.

Windows

You could install Cygwin and just follow the Linux instructions, but I am assuming that you use Windows because you don't like command lines.

Case 2. Granting me access to your server

This section explains the case when you want me to do some work for you and you need to give me access to your computer.

1. You have to run an sshd server on your machine.
2. I send you my public key.
3. You add my key to the accounts that you want to allow me to have access to. For example, you can create a personal account for me with the username "bwilson" and add the file in /home/bwilson/.ssh/authorized_keys and optionally also put a copy in /root/.ssh/authorized_keys to give me root level access.

Then I can use an ssh client to connect. I enter my passphrase and the client uses the decrypted private key to authenticate with your system. If the private and public keys match, your system grants me access.

As long as I don't share my passphrase with anyone, even if I lose my laptop, no one can use my private key to access your machine.

If you want to revoke access, you remove or disable access to my public key(s).

You don't have to give me any password information on your machine, so you don't have to change the root password when we are done working together.

This is what an RSA public key looks like

ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAsfKX946JbxemrYTT55dTk9Bg98tLeH3PAi5/cpQkWdntATz4ISIvqOltXkKrssj3dlK57M5u86RA05mzIOLPel9o52WCiYUdGiCnAOJa7fS9+XBU2rtrsgl0NBttVbC/m1qH1Ff6WKcGEwjlwxZvQAMk6pdSLDf9A4bns1qpS+jdD4nBOsVjx9cF6MD+1dHd/JqpNfllXDLeFsNNBivQ8ansAMiOj4WwXyU+XgbaZGngl1LYuwc1Lk+5N8Z4wq+33ERokvKOvNl8dAe4/n0oZqMF7LU+rHqIpUiusDRryFF+O2ogb+yTCwz0TYxNwg0tkTFFZN4xqc5hjhFgWrCuOvPQ== [email protected]

Key file installation notes

The file(s) "authorized_keys" are text files containing public keys. You can add new keys from the command line or using a text editor. The ownership and permissions should be set to be owned by the user account and readable only by that user. The typical commands:

# cd /home/username/.ssh                move to user's directory
# cat /tmp/id_rsa.pub >> authorized_keys    append key to file
# chown username authorized_keys        fix ownership
# chmod 600 authorized_keys                 fix permissions