UsingSSH

Using SSH to connect to machines and to move data

Introduction

The goal for this page is to provide a 'how-to' guide for several topics related to using the Secure Shell--SSH. The first topic is how to use public-private key pairs within SSH. The second topic is the use of port-forwarding to connect to a distant remote machine via another server, which acts as a 'stepping stone'.

Topic 1: Using Key-Pairs

Creating the Keys

First, let's create a key-pair. Start by typing:

ssh-keygen

You will see a message like:

Generating public/private rsa key pair.
Enter file in which to save the key (/home/gethin/.ssh/id_rsa):

The default filename suggested is fine, so accept it by hitting return.

Next you are prompted for a passphrase:

Enter passphrase (empty for no passphrase):

Think of a strong, yet memorable one and enter it. (One tip is to think of a phrase, saying, song lyric etc. For example "One small step for man, one giant leap for mankind." Then take the first letters from each word, perhaps substituting digits for letters, to create the passphrase, "Oss4mogl4m.") You will be prompted for your passphrase twice:

Enter same passphrase again: 

When the key-pair creation is completed, you will get some lines of text as confirmation, such as:

Your identification has been saved in /home/gethin/.ssh/id_rsa.
Your public key has been saved in /home/gethin/.ssh/id_rsa.pub.
The key fingerprint is:
37:7a:b3:81:e2:0e:fa:5e:b2:df:84:a5:fb:f9:e6:f7 

If you look inside the directory ~/.ssh you will see two files:

• id_rsa is your private key
• id_rsa.pub is your public key

Distributing Your Public Key

Now that you have your key-pair, you can copy your public key to any machine that you would like to connect to from the machine that you are currently logged into. When the keys are setup correctly, you will be able to connect without typing your password. Hurrah for the convenience!

The first step is to ensure that the permissions on your files are correct. The following commands will take care of this:

cd ~/.ssh
chmod 600 *
cd ~
chmod 700 .ssh

Now, let's copy your public key to the remote host of interest. In this case, I want to be able to login to a machine called brian, from one called dylan:

scp ~/.ssh/id_rsa.pub brian:~/.ssh/from-host.pub

(I'm assuming here that your username matches on the two machines. If not, you can prepend your username to the destination string, i.e. <username>@brian:~/.ssh/from-dylan.pub.)

Now, login to the remote host in the normal way:

ssh brian

where you will be prompted for your password, as per usual:

gethin@brian's password:

The following commands will:

1. ensure that your file permissions are correct on the remote host
2. add your public key to the list of authorized keys
3. exit from the remote host

chmod 700 ~/.ssh
cd ~/.ssh
chmod 600 *
cat from-host.pub >> authorized_keys
exit

Now, when you connect to your remote host:

ssh brian

you will be prompted for your passphrase rather than your password:

Enter passphrase for key '/home/gethin/.ssh/id_rsa':

Some progress! you may say, and at first blush you are (almost) right, but hang on a moment and we will see how we can connect to remote hosts with keys set up this way, only having to type your passphrase once.

Since you're currently logged in to your remote host, we may as well do a little tidying:

rm ~.ssh/from-host.pub
exit

Using ssh-agent

Enter ssh-agent and the potential for passwordless logins.

A quick way to try this out is to type:

ssh-agent bash

This will start a bash shell as the child of the ssh-agent process. (You may like to substitute bash with your shell of choice.)

Now type:

ssh-add

and you will be prompted for your passphrase:

Enter passphrase for /home/gethin/.ssh/id_rsa:

if you type correctly, you will get a confirmation:

Identity added: /home/gethin/.ssh/id_rsa (/home/gethin/.ssh/id_rsa)

and now when you connect to your remote host, you won't need to enter a thing! You can exit and connect again. Still no need for a password. You can start an xterm and connect from there. No password required. As you can see, all child processes can use the cached passphrase added to your agent.

Appendix: Cleaning Away Old Keys--If Required!

If you need to remove some existing keys for any reason (for example you set them up without passphrases, and now you want to create more secure versions), here is the procedure that you should follow:

1. login to the machine that holds your private key (dylan in the examples above) and remove the files .ssh/id_rsa and .ssh/id_rsa.pub.
2. Next login to the destination host for your key pair, i.e. the machine which you copied your public key onto. (brian in the examples).
3. open the file .ssh/authorized_keys (it's a text file) and delete the line corresponding to the machine that you would be connecting from, e.g.

   ssh-rsa AAAAB3NzaC1yc......== gethin@dylan

4. Now you're in a position to create some new keys.

Topic 2: Port Forwarding

Port forwarding with SSH

We would like to connect to blanc, but it is behind a firewall. One approach is to connect to each in turn, using nile as a stepping stone. Another approach is to setup port-forwarding on nile, so that you can connect, seemingly directly, to blanc. Here's how:

Two Connections are Required

Step 1

ssh -L localhost:2222:blanc:22 nile-username@nile.ggy.bris.ac.uk

Here we designate a port (we have chosen 2222) on the local machine, to act as our ssh 'gateway' to the remote machine called blanc. This is manoeuvre is mediated by the machine in the middle, in this case nile. In this step we must authenticate ourselves to nile, where 'nile-username', is your username on that machine.

Step 2

Now that we have our stepping stone (connection to nile) in place, to reach out further and open an ssh session on blanc type:

ssh -p 2222 blanc-username@localhost

where we are using port 2222 on the local machine to talk to blanc. You must authenticate yourself to blanc, of course, and the command uses 'blanc-username' accordingly.

To copy a file directly from blanc onto our local machine using scp, we would type:

scp -P 2222 blanc-username@localhost:/path/to/file .

Note a lower case p in the ssh command, but an uppercase P for the scp.