Problem

I want to access remote machine without typing password.

Term.

Remote machine: target machine to which I want to access through ssh.
Local machine: local machine from which I try to access to the remote machine.

Solution

In a word, We can do it by using public-private key.

• Private key should be in local machine and it must not be exposed (keep it secret).
• Public key will be in remote machine not to prompt password in every ssh access.
  

1. Key generation in local machine (If you already have one, you don’t need to regenerate key. Check ~/.ssh/id_rsa.pub.)
Use ssh-keygen.

$ ssh-keygen
Generating public/private rsa key pair.
Enter file in which to save the key (/home/yulistic/.ssh/id_rsa): 
Created directory '/home/yulistic/.ssh'.
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /home/yulistic/.ssh/id_rsa.
Your public key has been saved in /home/yulistic/.ssh/id_rsa.pub.
The key fingerprint is:
7a:5a:8e:14:3b:5d:10:03:14:39:01:18:5c:5a:50:b7 yulistic@hostname
The key's randomart image is:
+--[ RSA 2048]----+
| .o*=o*=o        |
|  oo .o. o       |
|  .   E..        |
|         .       |
|      . S .      |
|       = .       |
|      = +        |
|     . B         |
|      o .        |
+-----------------+ 

2. Copy the generated public key to remote machine.

$ ssh-copy-id yulistic@remote.server.com
The authenticity of host 'remote.server.com (123.456.789.111)' can't be established.
RSA key fingerprint is b0:6c:87:0b:2f:ae:5c:09:21:d6:0a:d5:01:fe:28:db.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'remote.server.com,123.456.789.111' (RSA) to the list of known hosts.
yulistic@remote.server.com's password: 
Now try logging into the machine, with "ssh 'yulistic@123.456.789.111'", and check in:

  .ssh/authorized_keys

to make sure we haven't added extra keys that you weren't expecting.

3. Access to the remote machine through ssh. If it does not prompt to input password, then it is successful.

4. If it still ask password, check your ~/.ssh directory permission at the remote machine. Refer to link.

5. It might have another problem if you are using SELinux.

Reference

• link (Korean)