ssh-keygen -t rsa
chmod 700 ~/.ssh
chmod 600 ~/.ssh/id_rsa
chmod 644 ~/.ssh/id_rsa.pub

Your identification has been saved in id_rsa_1s2n.
Your public key has been saved in id_rsa_1s2n.pub.
The key fingerprint is:
ae:71:3a:16:22:91:78:7b:25:1e:5f:dd:60:fc:f2:28 dang@localhost.localdomain
The key's randomart image is:
+--[ RSA 2048]----+
|         .       |
|          +      |
| . .     o +     |
|. + o . . o o    |
| . + = .S  +     |
|  o + o.E . .    |
|   o ...o.       |
|      o=         |
|     .o.         |
+-----------------+

    Host            gitserv remote.server.com
    Hostname        remote.server.com
    IdentityFile    ~/.ssh/id_rsa.github
    IdentitiesOnly yes 
    # If you have a file named ~/.ssh/id_rsa that will get tried BEFORE your ~/.ssh/id_rsa.github without this option.


Host           friendly-name
HostName       long.and.cumbersome.server.name
IdentityFile   ~/.ssh/private_ssh_file
User           username-on-remote-machine

• http://unix.stackexchange.com/questions/90853/how-can-i-run-ssh-add-automatically-without-password-prompt
• git: https://developer.github.com/guides/using-ssh-agent-forwarding/

  ssh -T git@gitlab.tubit.tu-berlin.de -vvv
  # this show some 3 keys were tried but not successful and password prompted.
  ssh-agent 
  SSH_AUTH_SOCK=/tmp/ssh-VBvlf1OVNNP5/agent.27629; export SSH_AUTH_SOCK;
  SSH_AGENT_PID=27630; export SSH_AGENT_PID;
  echo Agent pid 27630;
  [dang@dai142 NetworkManagementPaper]$ ssh-add -l
  Could not open a connection to your authentication agent.
  ssh-agent bash
  [dang@dai142 NetworkManagementPaper]$ ssh-add -l
  The agent has no identities.
  [dang@dai142 NetworkManagementPaper]$ ssh-add ~/.ssh/id_rsa-git_at_dai
  Identity added: /home/dang/.ssh/id_rsa-git_at_dai (/home/dang/.ssh/id_rsa-git_at_dai)
  ssh -T git@gitlab.tubit.tu-berlin.de
  Welcome to GitLab, Xuan Thuy Dang!