How to set-up SSH and not requiring a password every time it logs into a remote machine?

How to set-up SSH and not requiring a password every time it logs into a remote machine?


It is important to note that, since the SSH protocol is a highly secure one, it does not accept just the traditional .rhosts on the users login for accepting a remote connection without password verification.

The implementation of SSH1 and SS2 are slightly different, and from OpenSSH to SSH2 vary too. Since most of the Linux machines are running OpenSSH2, we will describe here how to make the "trick" for avoiding a password every time when running openssh2 on both client and server.
For detailed information about the class of key verification and encryption that ssh uses, see the ssh and related man pages or online documentation at http://www.openssh.com/manual.html

The basic idea is, however, that in the cryptosystem that ssh uses (RSA or/and DSA) the encryption and description are done using different keys
Basically, what the user (client) needs is to generate a public/private key pair. The server will know the public key, but only the client will know the private key.
When the client connects to the server, it tells its own public key. If this key is allowed (if it is between the know public keys list on the server), the server will send a random number to the client. This encrypted number can only be decrypted if the appropriate decryption key is used, and this decryption key is the client's private one. The client then uses then its own private key and decrypt the number. If this is done correctly, the server will grant the access with no more questions. As you can see the system is safe, because the client never tells anybody about its private key; and this key cannot be inferred using the public one.

What must be done, then , is to generate a public/private key pair, and copy the public part into the appropriate place on the server side.
For doing this, on the user's home directory, on the client machine, type

local> ssh-keygen -t dsa -f .ssh/id_dsa

-t tells the type of encryption
-f tells where to store the public/private key pairs. In this case, the .ssh directory on home is being used

A password will be asked; leave this part blank, just pressing
Now, go the .ssh directory, and you will find two new files: id_dsa and id_dsa.pub. The last one is the public part. Now, copy the public key to the server machine

local> cd .ssh
local> scp id_dsa.pub user@remote:~/.ssh/id_dsa.pub

Of course, this time you will need to enter the password.
Now, login into the server machine and go to the .ssh directory on the server side

local> ssh user@remote
remote> cd .ssh

Now, add the client's public key to the know public keys on the server

remote> cat id_dsa.pub >> authorized_keys2
remote> chmod 640 authorized_keys2
remote> rm id_dsa.pub
remote> exit

and that's all.
Next time you log into the remote server, no password will be asked.
Note that this system will work while none of the machines change its IP address and for the specific user, so it is still safe.