Overview

ssh passwordless login is done using a public/private key pair.  The private key is is kept on the system you want to login from; the public key is copied to the system and user you want to login to.

Setup procedure

1. Generate a public/private key pair following the procedure on ssh key generation
2. Copy the public key to the system and user you want to login to, adding it to the user's ~/.ssh/authorized_keys file.  If the file does not exist, create it and set the permissions to read and write for the user only (rw-------).
3. For passwordless login as root to work, /etc/ssh/sshd_config on the system you want to login to must have "PermitRootLogin without password".  This is bad security practice.

Usage

ssh <username>@<host>

If username is not specified, it defaults to the current user name

Examples:

ssh root@backupserver
ssh backupserver
ssh bl@192.168.28.2

Gotcha: the first time such a command is used for a particular host, ssh prompts interactively for confirmation to proceed.  When confirmation is given, it adds the host to ~/.ssh/known_hosts.  Thereafter there is no such interactive prompt.  If the passwordless login is to be used from a script, it needs to be done manually the first time or the script will hang.

In case you prefer a password protected key or you want to "hop" from system to system, SSH Agent Forwarding is helpful.