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How to login SSH without password

  

The answer is using private-key & public-key.
Theory is very simple, first generate pair key (Private&public key ) then put the public key into the server machine, so when another machine request login from ssh, it will compare the public key on server machine with the private-key.

Here is the example.

You want to login from computer A as root and Computer B as root.
From computer A, you generate the pair-key ( public&private key ).

[root@A ~]# mkdir .ssh

[root@A ~]# 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:
11:42:12:25:16:cc:e2:5e:32:d7:72:48:20:78:25:e2 root@A

note : Just hit enter to empty the passphrase.

[root@A ~]# cat .ssh/id_rsa

Copy all the string into clipboard first.
On Machine B, create the directory .ssh , usually this folder is already exists

[root@B ~]#  mkdir .ssh

Check B authorized key file on the sshd config

[root@B ~]# cat /etc/ssh/sshd_config | less

You will find a line contains

AuthorizedKeysFile      .ssh/authorized_keys

that means the authorized_keys is in the $HOME/.ssh/authorized_keys
Therefor you need to paste the clipboard string copied from public key machine A

And you are done. You may try to login ssh from machine A. Meanwhile keep your eyes on the log of machine B

[root@B ~]# tail -f /var/log/secure

Authentication refused: bad ownership or modes for directory /root

The common failure is about the permission, you need to change the /root , .ssh and authorized_keys into 700 on machine B.

Thats it. Thanks God … I did it.

July 31st, 2009 Posted by admin | CentOS, General, Ubuntu, ssh | no comments