Common SSH Command Line

Common_SSH_CLI.md

Common SSH Command Line

Generate a strong SSH key

• RSA: ssh-keygen -o -t rsa -b 8192 -C marco.franssen@macbook-pro
• Ed25519: ssh-keygen -t ed25519 -a 64 -C marco.franssen@macbook-pro

Ed25519: It’s the most recommended public-key algorithm available today! It has a 256-bit length and gives equal if not better protections as a 4096-bit RSA key. Reference: https://marcofranssen.nl/upgrade-your-ssh-security

Add passphrass into existing ssh key

• Add passphrase: ssh-keygen -p -f ~/.ssh/id_rsa
• Verify it: ssh-keygen -y -f ~/.ssh/id_rsa

Troubleshooting

• ssh is failed with error port 22:2: Too many authentication failures

  Received disconnect from 192.168.56.140 port 22:2: Too many authentication failures
  Disconnected from 192.168.56.140 port 22
  

  ==> Fix it:

  • Method 1: IdentitiesOnly=yes in ~/.ssh/config

    Reference: https://www.tecmint.com/fix-ssh-too-many-authentication-failures-error/

  • Method 2: Run ssh-add -d ~/.ssh/X where X is the key you want to remove.

    To check you have this problem: Run ssh-add -l. If this list is > 5, you need to remove keys or disable the agent. Reference: https://serverfault.com/questions/580753/ssh-aborts-with-too-many-authentication-failures

• SSH authentication agent does not automatically start when using it from a remote server. This result in the following error message:

  $ git pull
  Permission denied (publickey).
  fatal: The remote end hung up unexpectedly
  $ ssh-add ~/my-ssh-key.pem
  Could not open a connection to your authentication agent.
  

  ==> To fix it requires manually starting ssh-agent:

  $ eval `ssh-agent -s`
  Agent pid 13442
  $ ssh-add ~/my-ssh-key.pem
  Identity added: /home/user/my-ssh-key.pem (/home/user/my-ssh-key.pem)
  $ git pull
  

  Reference: https://gist.github.com/mvneves/a6a69a53f039b3755900

• After connected OpenVPN Server, then can ping, but cannot ssh ==> Fix it: Add MACs hmac-sha2-256 into /etc/ssh/ssh_config or ~/.ssh/config as a workaround