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Copying your public key to a remote server
To actually transmit your public key to a target server, we can use the 
ssh-copy-id
 
command, as I'm doing in the following example command:
ssh-copy-id -i ~/.ssh/id_rsa.pub 192.168.1.150

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Connecting to Networks
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With that command, replace the IP address with the actual IP address, or with the 
hostname
 of the target server. You'll be asked to log in via password first, and then 
your key will be copied over. From that point on, you'll log in via your key, falling 
back to being asked for your password if, for some reason, your key relationship is 
broken. Here's an example of what this process looks like, if I were to work through 
the process of copying my key to a server named 
myserver.mycompany.org
:
Figure 10.10: Using the ssh-copy-id command to copy a public key to a server
So, what exactly did the 
ssh-copy-id
 command do? Where is your public key copied 
to, exactly? What happens with this command is that on the target server, a 
.ssh
 
directory is created in your 
home
 directory on the target server if it didn't already 
exist. Inside that directory, a file named 
authorized_keys
 is created if it wasn't 
already present. The contents of 
~/.ssh/id_rsa.pub
 on your machine are copied 
into the 
~/.ssh/authorized_keys
 file on the target server. With each additional key 
you add (for example, you connect to that server from multiple machines), the key is 
added to the end of the 
authorized_keys
 file, one per line.
When you connect to a server that you have set up a key relationship with by adding 
your public key, SSH checks the contents of the 
~/.ssh/authorized_keys
 file on 
that server, looking for a key that mathematically matches the private key (
~/.ssh/
id_rsa
) on your machine. If the two keys are an appropriate match, you are allowed 
access. If you set up a passphrase, you'll be asked to enter it in order to open your 
public key.
Using the 
ssh-copy-id
 command is merely a matter of 
convenience, there's nothing stopping you from copying the 
contents of your 
id_rsa.pub
 file and manually pasting it into 
the 
authorized_keys
 file of the target server. That method will 
actually work just fine as well.

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Chapter 10