SSH Tutorial

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 AN!Wiki :: How To :: SSH Tutorial

This was taken whole-sale from the RHCS2 Tutorial. As such, much of it uses the setup from there, but the tutorial itself should still work just fine as a stand-alone tutorial.

Contents

Setting Up SSH

Setting up SSH shared keys will allow your nodes to pass files between one another and execute commands remotely without needing to enter a password. This will be needed later when we want to enable applications like libvirtd and virt-manager.

SSH is, on it's own, a very big topic. If you are not familiar with SSH, please take some time to learn about it before proceeding. A great first step is the Wikipedia entry on SSH, as well as the SSH man page; man ssh.

SSH can be a bit confusing keeping connections straight in you head. When you connect to a remote machine, you start the connection on your machine as the user you are logged in as. This is the source user. When you call the remote machine, you tell the machine what user you want to log in as. This is the remote user.

Creating SSH Keys

You will need to create an SSH key for each source user on each node, and then you will need to copy the newly generated public key to each remote machine's user directory that you want to connect to. In this example, we want to connect to either node, from either node, as the root user. So we will create a key for each node's root user and then copy the generated public key to the other node's root user's directory.

For each user, on each machine you want to connect from, run:

# The '2047' is just to screw with brute-forces a bit. :)
ssh-keygen -t rsa -N "" -b 2047 -f ~/.ssh/id_rsa
Generating public/private rsa key pair.
Created directory '/root/.ssh'.
Your identification has been saved in /root/.ssh/id_rsa.
Your public key has been saved in /root/.ssh/id_rsa.pub.
The key fingerprint is:
a1:65:a9:50:bb:15:ae:b1:6e:06:12:4a:29:d1:68:f3 root@an-node01.alteeve.com

This will create two files: the private key called ~/.ssh/id_rsa and the public key called ~/.ssh/id_rsa.pub. The private must never be group or world readable! That is, it should be set to mode 0600.

The two files should look like:

Private key:

cat ~/.ssh/id_rsa
-----BEGIN RSA PRIVATE KEY-----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-----END RSA PRIVATE KEY-----

Public key (wrapped to make it more readable):

cat ~/.ssh/id_rsa.pub
ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQBTNg6FZyDKm4GAm7c+F2enpLWy+t8Z
Zjm4Z3Q7EhX09ukqk/QmMqprtI9OsiRVjce+wGx4nZ8+Z0NHduCVuwAxG0XG7FpK
kUJC3Qb8KhyeIpKEcfYAtsDUFnWddVF8Tsz6dDOhb61tAke77d9E01NfyHp88QBx
jJ7w+ZgB2eLPBFm6j1t+K50JHwdcFfxrZFywKnAQIdH0NCs8VaW91fQZBupg4OGO
MpSBnVzoaz2ybI9bQtbZ4GwhCghzKx7Qjz20WiqhfPMfFqAZJwn0WXfjALoioMDW
avTbx+J2HM8KJ8/YkSSKdDEgZCItg0Q2fC35TDX+aJGu3xNfoaAe3lL1 root@an
-node01.alteeve.com

Using The Keys

Copy the public key and then ssh normally into the remote machine as the root user. Create a file called ~/.ssh/authorized_keys and paste in the key.

From an-node01, type:

ssh root@an-node02
The authenticity of host 'an-node02 (192.168.3.72)' can't be established.
RSA key fingerprint is 55:58:c3:32:e4:e6:5e:32:c1:db:5c:f1:36:e2:da:4b.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'an-node02,192.168.3.72' (RSA) to the list of known hosts.
Last login: Fri Mar 11 20:45:58 2011 from 192.168.1.202

You will now be logged into an-node02 as the root user. Create the ~/.ssh/authorized_keys file and paste into it the public key from an-node01. If the remote machine's user hasn't used ssh yet, their ~/.ssh directory will not exist.

(Wrapped to make it more readable)

cat ~/.ssh/authorized_keys
ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQBTNg6FZyDKm4GAm7c+F2enpLWy+t8Z
Zjm4Z3Q7EhX09ukqk/QmMqprtI9OsiRVjce+wGx4nZ8+Z0NHduCVuwAxG0XG7FpK
kUJC3Qb8KhyeIpKEcfYAtsDUFnWddVF8Tsz6dDOhb61tAke77d9E01NfyHp88QBx
jJ7w+ZgB2eLPBFm6j1t+K50JHwdcFfxrZFywKnAQIdH0NCs8VaW91fQZBupg4OGO
MpSBnVzoaz2ybI9bQtbZ4GwhCghzKx7Qjz20WiqhfPMfFqAZJwn0WXfjALoioMDW
avTbx+J2HM8KJ8/YkSSKdDEgZCItg0Q2fC35TDX+aJGu3xNfoaAe3lL1 root@an
-node01.alteeve.com

Now log out and then log back into the remote machine. This time, the connection should succeed without having entered a password!

Pre-Approving Keys

Various applications will connect to the other node using different methods and networks. Each connection, when first established, will prompt for you to confirm that you trust the authentication, as we saw above. Many programs can't handle this prompt and will simply fail to connect. So to get around this, I will ssh into both nodes using all hostnames. This will populate a file called ~/.ssh/known_hosts. Once you do this on one node, you can simply copy the known_hosts to the other nodes and user's ~/.ssh/ directories.

I simply paste this into a terminal, answering yes and then immediately exiting from the ssh session. This is a bit tedious, I admit. Take the time to check the fingerprints as they are displayed to you. It is a bad habit to blindly type yes.

Alter this to suit your host names.

ssh root@an-node01 && \
ssh root@an-node01.alteeve.com && \
ssh root@an-node01.bcn && \
ssh root@an-node01.sn && \
ssh root@an-node01.ifn && \
ssh root@an-node02 && \
ssh root@an-node02.alteeve.com && \
ssh root@an-node02.bcn && \
ssh root@an-node02.sn && \
ssh root@an-node02.ifn

And you're done!

 

Any questions, feedback, advice, complaints or meanderings are welcome.
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