This is actually a repost from the tutorial I often ran onto when working with servers. Lately, the sys-ad I was working with changed users home directory and also changed their passwords, but gave me the root pass. However, I don’t want to bother him for new passwords, and I guess he setup new ones from password generator apps/scripts, which are of course, really really recommended nowadays. Since I have the root pass, and I don’t want to go back to chmod-chown each and every files/folders I created via root, I thought I can automate logging in with each account I need to work ‘as’ their respective users. Or I can use cat /etc/passwd to know their home directories.

First log in on A as user a and generate a pair of authentication keys. Do not enter a passphrase:

a@A:~> ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/home/a/.ssh/id_rsa):
Created directory '/home/a/.ssh'.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/a/.ssh/id_rsa.
Your public key has been saved in /home/a/.ssh/id_rsa.pub.
The key fingerprint is:
3e:4f:05:79:3a:9f:96:7c:3b:ad:e9:58:37:bc:37:e4 a@A

Now use ssh to create a directory ~/.ssh as user b on B. (The directory may already exist, which is fine):

a@A:~> ssh b@B mkdir -p .ssh
b@B's password:

Finally append a’s new public key to b@B:.ssh/authorized_keys and enter b’s password one last time:

a@A:~> cat .ssh/id_rsa.pub | ssh b@B 'cat >> .ssh/authorized_keys'
b@B's password:

Now I can work as ‘owner’ and ‘user’ within each user home directory.

a@A:~> ssh b@B hostname
B

Some notes, depending on your version of SSH you might also have to do the following changes:

• Put the public key in .ssh/authorized_keys2
• Change the permissions of .ssh to 700
• Change the permissions of .ssh/authorized_keys2 to 640

Source: http://linuxproblem.org/art_9.html