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Passwordless SSH - 4 Steps
Author - Nic Maurel

SSH stands for very secure shell. It's function is exactly it's name, can encrypt a remote connecion to another UNIX or LINUX pc's shell using port 22 as a default. But so what you say? Well what if you don't want to be around when 2 computers connect? This could be useful in many different ways and make your workload a LOT lighter. For instance :  
The above examples are just a few things I can think off of the top of my head, the possibilities are endless. First of all we have to know how the 2 pcs authenticate with each other. Usually if you are sitting at your local pc and you want to connect to a remote pc with ssh you need the following
         # ps -ef | grep sshd
            root      3138     1  0 Mar20 ?        00:00:07 /usr/sbin/sshd
            root       727  3138  0 11:15 ?        00:00:00 /usr/sbin/sshd
            root       770   729  0 11:15 pts/0    00:00:00 grep sshd

Please note: if the ssh command and grep for sshd did not work it is probably either not installed on your system or not running. The best thing to do would be to consult your documentation or download a binary or source from the openssh website. Okay now that we have established that sshd is running we should now be able to connect to a remote pc from the local one. For reference I will be refering to the local pc as [Local]# and the remote pc as [remote]#

Step number 1-Connecting to remote
First try connecting -
            [root@local]# ssh root@remote

This will only happen the first time you connect -
            The authenticity of host 'remote (' can't be established.
            RSA key fingerprint is 9f:61:f3:c6:f5:f5:37:e4:fb:59:eb:ed:4d:a9:62:45.
            Are you sure you want to continue connecting (yes/no)? yes
            Warning: Permanently added 'remote' (RSA) to the list of known hosts.
            root@remote's password:

Type your password and if you have typed it correctly you should be prompted with a remote root shell something like this -    

Well done you have successfully connected to the remote machine. Now for the fun part!

Step number 2-Creating the keys
Setting up Public key encryption is tricky but not that difficult. Here is what we are aiming for:
local has its Private Key -----> remote has locals's Public key

Time to create Local's keys. Running this command as root means that only root on local will be able to use these keys
            [root@local]# ssh-keygen -t rsa -P ""

I have chosen to create rsa keys as personal preference, if you prefer dsa then that is fine, they both have very strong encryption but the only marginal difference is that dsa is faster at generating keys and rsa is faster at authenticating, but this is a millisecond time difference. You will then be prompted to specify where to put the keys, press <enter> to accept the default ~/.ssh/id_rsa otherwise you can specify elsewhere.     
TIP: ~ stands for the user's home directory, so if you are root this would most likely be /root
From the sshkeygen command 2 keys are generated id_rsa and PLEASE NEVER EXPOSE the id_rsa! This could lead to dire consequences.

Step number 3-Copying Public key
                  [root@local]# cd ~/.ssh
There are a number of ways we can copy it accross as openssh also comes with sftp and scp. Before we do that one must understand the consequences of using passwordless ssh, if someone else has access to you local machine then they are going to have access to your remote host (if the person realises it). From a security perspective it is good practice to use a more restricted user than root, so at least there is some damage control. So lets create a user on remote (you can skip this step if you still want to use root but I don't recommend it)                                     
            [root@remote]# useradd -d /home/bob -c "SSH USER" bob    <---- create user bob
            [root@remote]# passwd bob                                                           <---- create your password for bob
            [root@remote]# su - bob                                                                 <---- change to bob 
            [bob@remote]$ mkdir .ssh                                                              <---- create place for public key

This would basically create the place for the public key. Now that we have bob we need to copy the public key from local to remote into /home/bob/.ssh . I will show you two ways to copy the key across - once again it is personal preference.

            [root@local]# scp ~/.ssh/ bob@remote:/home/bob/.ssh/
            bob@remote's password:
           100% |*****************************|   235       00:00

                            -- OR --

            [root@local]# sftp bob@remote

            bob@remote's password:
            sftp> lcd ~/.ssh
            sftp> cd /home/bob/.ssh
            sftp> put
            Uploading to /home/bob/.ssh/

Once the has copied to /home/bob/.ssh/, you need to place the key into the .ssh/authorized_key2 file. You can do this from the .ssh directory with: cat ./ >> ./authorized_keys2
Step number 4-Setup Permissions
This is probably the trickiest part of passwordless ssh and this is where most people give up. Don't be discouraged this is the final step and we are almost there.
            [root@remote]# cd /home/bob

Because of the nature of ssh the .ssh directory
            [root@remote]# chmod -R 700 ./.ssh

Bob must own the file and directory as well
            [root@remote]# chown -R bob ./.ssh        
Here is the tricky part where most people get caught out
            [root@remote]# ls -al
                drwx------    5 bob      bob          4096 May 12 13:46 .
                drwxr-xr-x    5 root     root         4096 May 12 13:45 ..       ------------> Note the 755 permission
                -rwx------    1 bob      bob            22 May 12 13:46 .bash_history
                -rwx------    1 bob      bob            24 May 12 13:45 .bash_logout
                -rwx------    1 bob      bob           191 May 12 13:45 .bash_profile
                -rwx------    1 bob      bob           124 May 12 13:45 .bashrc
                -rwx------    1 bob      bob           120 May 12 13:45 .gtkrc
                drwx------    2 bob      bob          4096 May 12 13:53 .ssh

This 755 permission is a permission of the directory below  -/home. The directory below must be set at 755 or better or passwordless ssh will not work. And that's it! You are now ready to ssh passwordless onto remote from local. Here is a quick test
            [root@local]# ssh bob@remote ls -al

if the directory listing shows up without a password prompt then you have done it! Well done!

Here's a final tip to leave you with you troubleshoot with this command to see the steps that ssh takes
            [root@local]# ssh -vv bob@remote         <----- the "-vv" shows verbose connection