SSH Login No Password Prompt

Sometimes on Linux, you want to automatically login to a machine using ssh without being prompted for a password. How do you set this up?

First you need to generate a public/private key pair on the server you will be connecting from, lets call it

SERVERFROM

ssh-keygen -t rsa

You could also use -t dsa as well.

Next you will be prompted for the directory to create the key pair in, just hit enter to accept the default.

[root@serverfrom ~]# ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/root/.ssh/id_rsa):

Enter passphrase (empty for no passphrase):
Enter same passphrase again:
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:
c5:72:ee:10:f3:8e:ca:98:da:46:85:01:11:08:ba:75 root@serverfrom

Now you have a public/private key pair. Your private key is in the file:

/root/.ssh/id_rsa

& your public key file is in

/root/.ssh/id_rsa.pub

Now, in order to get ssh to work without prompting for a password, you need to copy your public key to the server you want to connect to. Lets say the server you want to connect to is called SERVERTO, then on SERVERFROM you would do:

scp /root/.ssh/id_rsa.pub  root@SERVERTO:/root/.ssh/authorized_keys2

If you already have an authorized_keys2 file on SERVERTO, then just append the new key to the end of it by copying the key to SERVERTO and then appending it like:

scp /root/.ssh/id_rsa.pub  root@serverto:/root/.ssh/id_rsa.pub

Then on the SERVERTO server, just concatenate the file id_rsa.pub to the authorized_keys2 file like:

cd /root/.ssh
cat id_rsa.pub >> authorized_keys2

Now, you should be able to scp or ssh from SERVERFROM to SERVERTO without supplying a password:

[root@serverfrom ~]# ssh serverto
Last login: Wed Jan  6 09:09:21 2010 from serverfrom.domain.org
[root@serverto ~]#