Set up passwordless file exchanges

SSH configuration

Most of this relies on configuration of SSHd, the SSH (Secure SHell) daemon running on the server you’re attempting to connect to. This server will be referred to as "server2.com" for demonstration purposes.

Check the “SSHd_config” on that server, typically in /etc/SSH. In some cases, these settings are not always present or set the way you need:

RSAAuthentication yes
PubkeyAuthentication yes

This enables the public/private key authentication mechanism you will use.

Public/Private key generation

Generate the keypair on the Linux box that you want to connect from. This server will be referred to as “server1.com” for demonstration purposes. It’s that box you plan to run SSH, SFTP, or Rsync.

SSH-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/home/user1/.SSH/id_rsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in .SSH/id_rsa.
Your public key has been saved in .SSH/id_rsa.pub.
The key fingerprint is:
c1:21:e 3:01:26:0d:f7:ec:52:0e:0c:90:9b:6e:d8:47 user1@server1.com

The command above generates a public/private key pair. Respond to each prompt by pressing <Enter>. Possession of the private key is sufficient to gain access to what ever resources into which you’ve placed the corresponding public key.

You must not enter a passphrase for the private key to configure an automated solution. If you do enter a passphrase, you'll need to enter it to use the private key. 

 Note: By not placing a passphrase on your private key, the security implication is that mere possession of the private key is sufficient to gain access to whatever resources into which you’ve placed the corresponding public key. Safeguard your private key.

The private key was placed in /home/user1/.SSH/id_rsa. This needs to be kept secure because of the security implication above, but it also needs to be available to the process of attempting to make an SSH, SFTP, or Rsync connection. If these tools are run under the ‘user1’ account, the tools will automatically look in the “.SSH” directory, and you won’t need to specify the private key location. Otherwise, command line options must point to the right place and key.

The public key is in /home/user1/.SSH/id_rsa.pub. This is the key that gets distributed to those places that want to grant you access.

Plant the public key

1. On the remote server, server2.com, pick any account that you want to connect to.
2. In that account’s home directory, create a .ssh subdirectory and create a new text file named authorized_keys in that directory. If a text file by this name already exists, use the existing file.
3. On the remote server, server2.com, pick any account that you want to connect to. In that account’s home directory, create a .ssh subdirectory and create a new text file named authorized_keys in that directory. If a text file by this name already exists, use the existing file.

    Note: If you create the file and/or directory, it is recommended that the directory be chmod 700 and the file 600. This way, only the owner can access the directory and the file within it.

4. Add to that file the contents of the id_rsa.pub file created above. That would be a *single line* that looks something like this:

   ssh-rsa <lots of characters> user1@server1.com
   

5. Once saved, anyone possessing the private key that matches this public key can now login as this account.

SFTP

The public key was planted in the account user2 on server2.com. So now, server1.com logged in as user1, and where the private key is stored as described above, an sftp session looks like this:

sftp user2@server2.com

“user2” specifies the remote account on server2.com to login as.

Authentication happens because the private key is matched to the public key, which indicates you are authorized to login to that account. An SFTP session is opened with no interactivity required.

 Note: If you did enter a passphrase on the private key, you would have been prompted to enter it here. This is the passphrase to unlock the private key, which is local. It has nothing to do with any passwords on the remote site

Rsync

You can use Rsync for file copy operations, which can handle intelligent compression, copy only if needed, recursion, compression attribute retention, date/time stamp, and many other operations.

If all the keys are set up as above, this Rsync command copies a file from the local machine to the remote:

rsync -e ssh file user2@server2.com:/home/user2/

Local file “file” is copied to the remote /home/user2/file after logging in as “user2” using ssh as the transport (hence the “-e ssh” option), and with that, using the private/public key pair you created for authentication with no interactivity required.

SSH

SSH works as well to open up a remote shell once the keys are in place. If all keys are set up as above, this SSH command opens a remote shell on server2, logged in as user2.

ssh user2@server2.com