In these exrcises we'll show how you can eliminate passwords by using ssh key authentication and make this process more convenient.
Choose the version of the exercises depending on what OS you are running on your laptop.
Remember: the $ character before commands indicates that they are to be
run as your normal login user, not as root.
Download the following onto your desktop or into a downloads folder:
For this class you can download each of these items by going to:
http://wsnoc.nsrc.org/wiki/Putty
Outside of class you can go to http://www.chiark.greenend.org.uk/~sgtatham/putty/download.html (Or you can try the installer bundle which gets them all)
Double-click on puttygen.exe
At the bottom of the dialog box, under "Parameters":
Click on "Generate". Move the mouse randomly over the blank area until the progress bar reaches 100%
Now you will have some additional fields in the puttygen.exe window:
Key comment: [Your Name <your@email.address> ]
Key passphrase: [chooose a passphrase ]
Confirm passphrase: [choose same passphrase ]
The passphrase is used to keep your private key encrypted on disk. It can be pretty much anything you want and as long as you want - including spaces - but if you forget it, your key becomes worthless. For now pick something that you will easily remember. You can change it at any time you want in the future.
Click "Save public key". Give a filename of "id_rsa.pub" (please save files into the same directory as where the executables are)
Click "Save private key". Give a filename of "id_rsa.ppk"
Use the mouse to highlight all the text in the box "Public key for pasting into OpenSSH authorized_keys file", and copy it to the clipboard.
Exit puttygen.
NOTE: Key generation is a one-off exercise. The more you deploy your public key, the more work it will be if you were to lose it and have to start again with a new one. We suggest you keep a secure backup of it somewhere, e.g. on a CD-ROM, a safe USB key that you lock away, an encrypted file somewhere else, etc.
You have two ways of doing this.
Use putty.exe to make a normal ssh connection to your MacMini as the 'nsrc' user. Your MacMini should be accessible as s1.ws.nsrc.org, but you use the IP address if necessary of 10.10.0.241.
You will likely see a window about the server's host key not being cached in the registry. In order to connect click on "Yes" to continue. Do you understand what this meant?
Check so if there is a .ssh directory already in the home directory for the nsrc user:
ls -laIf you do not see the directory, then you are the first one to connect. Create the .ssh directory like this:
$ mkdir .ssh
$ chmod 700 .ssh
Now you will paste the key you copied to your clipboard earlier in to a file called .ssh/authorized_keys. The way we do this ensures that the text you paste will go on the end of the file so you don't overwrite the public keys that your classmates are placing in this file as well
$ cat >>.ssh/authorized_keys
*** PASTE KEY FROM CLIPBOARD ***
*** If the cursor is still at the end of the line, hit Enter ***
*** hit ctrl-D ***
The key consists of one very long line, which looks like
ssh-rsa <lots of base64 data> <comment>
As a quick check that it hasn't been corrupted, count the lines in the file:
$ wc -l .ssh/authorized_keys
1 .ssh/authorized_keys
If you don't see "1", then you'll need to fix it (possibly with an editor). You can ask an instructor for help. Don't delete the file!
Now let's be sure that the file .ssh/authorized_keys ahs the right permissions set, otherwise the ssh server on your MacMini will not allow you to connect using your private key. To do this do:
$ chmod 600 .ssh/authorized_keys
Now log out of your MacMini
$ exit
Double-click on psftp.exe. Open a connection to your server, and upload your public key:
psftp> open s1.ws.nsrc.org
login as: nsrc
nsrc@s1.ws.nsrc.org's password: <usual one>
Remote working directory is /home/nsrc
psftp> put id_rsa.pub
local:id_rsa.pub => remote:/home/nsrc/id_rsa.pub
psftp> quit
Unfortunately, this public key is not in the format which openssh requires, so now login again using putty.exe to connect to the server again as the user nsrc. First we need to see if the .ssh directory exists in the nsrc user home directory:
ls -laIf you do not see the directory, then you are the first one to connect. Create the .ssh directory like this:
$ mkdir .ssh
$ chmod 700 .ssh
Now you can use the following command to convert the id_rsa.pub key file you uploaded to the home directory and place this key at the end of a file called authorized keys in the .ssh directory:
$ ssh-keygen -i -f id_rsa.pub >>.ssh/authorized_keys
Now let's be sure that the file .ssh/authorized_keys has the right permissions set, otherwise the ssh server on your MacMini will not allow you to connect using your private key. To do this do:
$ chmod 600 .ssh/authorized_keys
We should remove our id_rsa.pub file sitting in the /home/nsrc directory:
$ rm id_rsa.pub
Now log out of your MacMini:
$ exit
Start putty.exe again. Enter the hostname as usual, but before clicking Open, browse in the left hand column to Connection > SSH > Auth
[-] Connection
|
[-] SSH
| |- Keyex
| |- Auth <--- CLICK HERE
Next to "Private key for authentication", click Browse. Find your id_rsa.ppk file, open it, then go back to the Open option to start the connection. Remember Host Name or IP address is s1.ws.nsrc.org or 10.10.0.241
You should be prompted for your username as before (nsrc), but then instead of being prompted for a password, you are asked for the passphrase for your private key. Enter it, and you should be logged in.
This is quite painful (both locating the private key and entering the passphrase), so as the final step of the exercise we're going automate it using an agent.
Run pageant.exe
It runs in the background, and adds an icon to your task tray (a PC with a black hat at an angle). You may need to select "Show hidden icons" to see it.
Right-click on the icon, and select "Add Key". Browse to your id_rsa.ppk and open it. You will be prompted for the passphrase - enter it. (If you make a mistake, you'll be prompted again until you get it right)
Now run putty.exe again, enter your hostname, click Open, and enter your username (sysadm). You should be logged in immediately, with no prompt for either a password or a passphrase!
Try logging in again. Also try using psftp.exe (when it runs, enter "open wsnoc.nsrc.org" to start a connection). No passphrase is needed until you tell Pageant to forget the private key. This can save quite a bit of time if you are making multiple ssh connections.
With Putty you save preconfigured sessions to save steps as well as play with many other available options. Feel free to do this if you wish at your leisure.
WARNING!: If you already have a public/private key that you use, then skip the first exercise and go directly to exercise 2.
Note that the "~" character represents the path to your home directory on your laptop.
$ ssh-keygen -t rsa -b 2048
Generating public/private rsa key pair.
Enter file in which to save the key (~/.ssh/id_rsa): <HIT ENTER>
Created directory '~/.ssh'.
Enter passphrase (empty for no passphrase): <CHOOSE PASSPHRASE>
Enter same passphrase again: <SAME PASSPHRASE>
Your identification has been saved in ~/.ssh/id_rsa.
Your public key has been saved in ~/.ssh/id_rsa.pub.
The key fingerprint is:
32:2b:e3:0e:14:fb:60:38:a6:e2:73:95:53:9d:a8:0f userid@yourlaptopname
The passphrase is used to keep your private key encrypted on disk. It can be
pretty much anything you want and as long as you want - including spaces -
but if you forget it, your key becomes worthless. For now pick something
that you will easily remember. You can change it at any time you want in
the future (using ssh-keygen -p)
NOTE: Key generation is a one-off exercise. The more you deploy your public key, the more work it will be if you were to lose it and have to start again with a new one. We suggest you keep a secure backup of it somewhere, e.g. on a CD-ROM, a safe USB key that you lock away, an encrypted file somewhere else, etc.
First we need to see if ssh has been set up on your MacMini and your nsrc account is ready to accept public keys. To do this do:
$ ssh nsrc@s1.ws.nsrc.orgIf you are prompted to say Yes or No to accepting the public key from the MacMini be sure to answer "Yes" (you have to type "Yes" out fully). Do you understand what is happening? Now let's see if there is a ".ssh" directory in the nsrc home directory on your MacMini:
ls -laIf you do not see the directory, then you are the first one to connect. Create the .ssh directory and authorized_keys file. If you do see a ".ssh" directory, then please skip down to the "Copying your public key" step below. To create the directory and file do this:
$ mkdir .ssh
$ chmod 700 .ssh
Now let's create an empty .ssh/authorized_keys file with the correct permissions so that as we place our public keys in the file things will work correctly
$ touch .ssh/authorized_keys
$ chmod 600 .ssh/authorized_keys
Now log out of the MacMini to go back to your laptop
The simplest way to copy the public key is with scp and the cat commnd. We do this in this manner to avoid overwriting other people's public key in the authorized_keys file on our MacMinis.
From a terminal window on your laptop
$ cd
$ cat .ssh/id_rsa.pub | ssh nsrc@s1.ws.nsrc.org 'cat >>.ssh/authorized_keys'
Open an ssh connection to your server as normal:
$ ssh nsrc@s1.ws.nsrc.org
This time, instead of being prompted for your password, you should be prompted for the passphrase on your private key. Enter it. You should be logged in.
Entering a passphrase every time you connect would be painful, but this isn't necessary if you have an agent which decrypts the private key and keeps it in memory.
If you are running under a modern graphical environment like Gnome or Mac OS X, you probably already got a dialog box prompting you for a passphrase, and this means you're already running an agent. You should be able to logout and login to the remote server, without being prompted for your passphrase again.
If you don't have an agent, then you can start a new subshell with ssh-agent as its parent:
$ ssh-agent bash
$ ssh-add
... prompted for your passphrase
$
To see what identities (decrypted private keys) your agent has in memory:
$ ssh-add -l
To forget all identities:
$ ssh-add -d
(By the way, if you do the above command you will be prompted for your passphrase again the next time you connect to your MacMini.)
Now the agent will handle future connections for you.
If you are running an older graphical environment, and you normally start
X using startx, then start it using ssh-agent startx instead. Then type
'ssh-add' in an xterm.
Everyone can do this exercise. You should do this once everyone has copied their public key to the /home/nsrc/.ssh/authorized_keys files on your group's MacMini, so check with your neighbor's to see if they are done with that part of the exercises.
Log in to your MacMini as the user nsrc. Once you are logged in do the following. When prompted for a password use the same one you used originally to connect to the nsrc account:
$ sudo su -
# cd /root
# mkdir .ssh
# chmod 700 .ssh
# cat /home/nsrc/.ssh/authorized_keys >> .ssh/authorized_keys
# chown root:root .ssh/authorized_keys
# chmod 600 .ssh/authorized_keys
If you wish to see how things look in the /root/.ssh directory do:
# ls -la .sshNow you can log off from the MacMini and try connecting directly as the root user. You need to type exit twice as the sudo command opened a new shell on the MacMini:
# exit
$ exit
Now try connection to the MacMini as the root user (root@s1.ws.nsrc.org) and see if you are prompted for your passphrase, or if you log in without any prompt as your ssh agent program should take care of the private/public key exchange between your machine and the MacMini.
Once you are done you can log off.
This section is for reference only. We are not going to turn off password access at this time, so please do not do this exercise.
Only do this exercise if you have successfully copied your public key to your machine and you are being prompted for your ssh private key passphrase when you log in and not your password.
Log in on your machine. Once logged in become the root user:
$ sudo su -As the root user cat the authorized_keys file to the directory /root/.ssh. This wasy we ensure we don't overwrite any other /root/.ssh/authorized_keys file that might already be in place.
# mkdir /root/.ssh
# chmod 700 /root/.ssh
# cat /home/nsrc/.ssh/authorized_keys >> /root/.ssh/authorized_keys
# chown root:root /root/.ssh/authorized_keys
# chmod 644 /root/.ssh/authorized_keys
Now log out of your machine and try to log back in, but this time as the "root" user, not as the "nsrc" user. If you are prompted for your ssh private key passphrase and not a password, then you are ready to disable password access to your machine.
Log in on your machine as the root user. Now we are going to edit the file /etc/ssh/sshd_config.
# vi /etc/ssh/sshd_configInsie the file look for the following line:
#PasswordAuthentication yes
Just after this line add a line that says:
PasswordAuthentication no
Save and exit from the file. The reload the ssh server:
# service ssh reloadBefore logging out we recommend you leave your current session open in case there are problems. You could lock yourself out of your machine. If you do let your instructor know. Now use ssh or putty on your laptop and try connecting to your server. You should get prompted for your ssh private key's passphrase and be able to log in. If you did, everthing is working. At this point all new users on your machine must use ssh keys to connect and not passwords. If you wish to verify this you can do the following:
# adduser testuserAnswer the on-screen prompts. Once the user has been created and you have given it a new password of your choosing, try to open a new ssh session to your machine as that user. You should either be rejected or your password attempts will have not affect (depends on the ssh server version).
To log in as this user you would need to copy your public ssh key to their account as we did for your nsrc user. This greatly enhances the security of your system.
Using an agent, you can access across multiple ssh hops without having to copy your key or enter your passphrase anywhere.
If you enable "agent forwarding" when you login to host X, you can then login from X to Y without any prompting (assuming Y has your public key in authorized_keys). The request to authenticate is forwarded securely back along your original ssh session to the agent running on your workstation.
Under Unix:
$ ssh -o ForwardAgent=yes user@host
If you do this frequently, it's easier to configure it in .ssh/hosts
host foo
hostname foo.example.com
user nsrc
ForwardAgent yes
Then you only have to type ssh foo to get a connection with those options.
ssh can securely carry arbitrary forwarded TCP connections and X11 graphics.
$ ssh -L8080:some.where:80 user@remote.host
... while ssh connection is open, a connection to 127.0.0.1 port 8080
... will be tunneled, and the far end will open a connection to
... some.where port 80
$ ssh -X user@remote.host
...
$ xclock # graphical output redirected back through ssh tunnel