Linux – Is there a way to log into a machine using SSH without password

linuxsshssh-keys

I know ssh is to create a secure shell connection to a machine, and we call ssh username@ip_address to login the machine.

Is there a way that I won't need to enter the password when connecting via SSH, or let's say SCP, or a run a command using SSH on a remote machine?

I know there are ssh-keys, which allow particular users to login without entering passwords.

Is there any other way to achieve so, which doesn't involve keys? Like some change in the ssh configuration?

Best Answer

Key-based authentication is the way to go. Not only is it easier than password auth when configured, but it is much more secure and is easily integrated into automated scripts. There is tons of literature out there on the subject -- to get you started:

https://hkn.eecs.berkeley.edu/~dhsu/ssh_public_key_howto.html

Connection reuse

SSHv2 allows the same authenticated connection to establish multiple 'channels' – interactive shell, batch command, SFTP, along with the secondary ones such as agent-forwarding or TCP-forwarding. Your server probably supports connection multiplexing by default. (If your admins complain, it's not caching your password anywhere – it's caching the whole connection.)

With OpenSSH you have ControlMaster and ControlPath options (-M and -S) to make use of this:

Start a 'master' SSH connection using -M. (Since you don't have a ControlPath in your config yet, you need to specify it in command line using -S. It needs to live long, so I add the -fN options to drop to background; they're technically optional otherwise.)
```
$ ssh foo@bar.example.com -fNMS ~/.ssh/bar.socket
foo@bar.example.com's password:
```
You're back to the local shell.
Start a new connection through the master:
```
$ ssh foo@bar.example.com -S ~/.ssh/bar.socket
```
You're in.
To make this useful for Git/rsync/SFTP, you need to set up ControlPath in your configuration, because you won't be able to specify -S all the time:
```
Host *
    ControlPath ~/.ssh/S.%r@%h:%p
```

You can automate this – recent OpenSSH versions also have ControlPersist which automatically establishes a master connection in background if there isn't one yet. This allows you to skip step 1 and just use ssh as you normally would.

Configuration in ~/.ssh/config:

Host *
    ControlPath ~/.ssh/S.%r@%h:%p
    ControlMaster auto
    ControlPersist 15m

First connection asks for password:

$ ssh foo@bar.example.com
foo@bar.example.com's password:
[foo@bar:~]$ exit

The second doesn't:

$ ssh foo@bar.example.com
[foo@bar:~]$ yay

To control the multiplex master (stop it or configure TCP forwardings), use the -O option.

A similar method is supported by recent PuTTY versions.

Best Answer

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Connection reuse

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