Secure Shell

Secure Shell (or SSH) is a set of standards and an associated network protocol that allows establishing a secure channel between a local and a remote computer. It uses public-key cryptography to authenticate the remote computer and (optionally) to allow the remote computer to authenticate the user.

SSH without passwords

• Step 1: Generate keys (public and private) and leave passphrase blank if you want password-less logins:

ssh-keygen -t dsa

Or,

ssh-keygen -t dsa -b 2048 -f /home/bob/my-key

• Step 2: Copy public key to remote server (Important: Only the public key!):

scp ~/.ssh/id_dsa.pub username@hostname:.ssh/authorized_keys

• Step 3: Set directory/file permissions (if not already set):

chmod 700 ~/.ssh
chmod 600 ~/.ssh/authorized_keys

• Step 4: Now, SSH into your remote server (password will be required the first time):

ssh username@hostname

That's it! You are now free to log into your remote server without entering a password. This is useful for automating file transfers. However, it must be used with care. If not executed properly, it is a potential security risk.

Making SSH even more secure

Note: All of the following settings will be implemented in your /etc/ssh/sshd_config file.

• Disable SSH protocol 1. Make sure no lines reads Protocol 1. If so, change it to:

Protocol 2

• Enable key-based logins (see above for how to do this):

PubkeyAuthentication yes
AuthorizedKeysFile .ssh/authorized_keys

• Disable password-based logins (Only do this if you first enable key-based logins!):

PasswordAuthentication no

• Run on ports other than 22

Port 1717  # any free port above 1024

You will then need to point to this port when SSHing into your remote machine

ssh -p 1717 remote.machine

• Disable root logins (Very important!):

PermitRootLogin no

See also

• SSH Filesystem (sshfs)
• Fish protocol
• rsync

External links

• Copying mass amounts of data over a network with bash, rsync, and ssh
• wikipedia:Secure_Shell