Ssh-agent is a program that contains private keys and passphrases used to connect to an SSH server. Without ssh-agent, when making a connection to an SSH server using a public / private key pair, there will be a prompt to enter the passphrase associated with the public / private key pair.
Enter passphrase for key '/root/.ssh/id_dsa':
The ssh-agent command without any options will display basic information about the ssh-agent deamon, such as the socket and PID being used by ssh-agent. In this example, the socket is 30025 and the PID is 30026.
[root@server1 ~]# ssh-agent SSH_AUTH_SOCK=/tmp/ssh-fXi6UEtKUojG/agent.30025; export SSH_AUTH_SOCK; SSH_AGENT_PID=30026; export SSH_AGENT_PID; echo Agent pid 30026
The ssh-add command can be used to add your passphrase to ssh-agent. Before using ssh-add, you will want to start the ssh-agent.
[root@server1 ~]# eval `ssh-agent -s` Agent pid 2023
Now add your passphrase.
[root@server1 ~]# ssh-add Enter passphrase for /root/.ssh/id_rsa:
After you enter the correct passphrase, Identity added will be displayed.
Identity added: /root/.ssh/id_rsa (/root/.ssh/id_rsa)
Now when connecting to the SSH server, there will be no prompt to enter the passphrase associated with the public / private key pair.
[root@server1 ~]# ssh root@server2 Last login: Sun Jan 29 12:53:23 2017 from 192.168.0.141
It is important to note that your passphrase will not be permanently associated with the public / private key pair via ssh-agent. If you log out of the shell, reset the environment, or restart the machine, you will need to use ssh-add again to add your passphrase to ssh-agent. If you frequently connect to an SSH server, it would be preferrable to make this permanent via your BASH profile.