SSH

2020/04/26 by jean tran

SSH is the secure shell protocol. It allows you to attach your terminal window to a remote server and execute commands in it.

Generating new SSH key

ED25519 SSH keys

ED25519 (SSH introduced in 2014 OpenSSH 6.5)

ssh-keygen -t ed25519 -C "[email protected]"

It generate public/private ed25519 key pair in: ~/.ssh/id_ed25519)

RSA SSH keys

By default ssh-keygen command create an 1024-bit RSA key. (Minimum recommended key size of 2048).

ssh-keygen -o -t rsa -b 4096 -C "[email protected]"

It generate public/private RSA key pair in: ~/.ssh/id_rsa

Update SSH key Passphrase

Specify the SSH key you would like to change the passphrase.

ssh-keygen -p -f "~/.ssh/<path/to/ssh_key>"

SSH Gitlab

Public SSH Key

For OSX, buffer public key to clipboard by specify your specific key with .pub extension.

pbcopy < ~/.ssh/<your_generated_key>.pub

Testing your key

Quick test by running the following command:

ssh -T <your_gitlab_domain>

Verbose version:

ssh -Tvvv <your_gitlab_domain>

Specifying none default path

To change non-default file path for SSH Key pair you can run this following command:

eval $(ssh-agent -s)
ssh-add <path to private SSH key>

Configuration file

The case of multiple ssh key for different usage could be config in the file ~/.ssh/config.

The followin case illustrate an usage of 3 keys for 3 differents domain of Git:

# office domain
Host <user_office_git_domain>
  Hostname <domain>
  Preferredauthentications publickey
  IdentityFile ~/.ssh/office_id_rsa

# some client
Host <user_client_git_domain>
  Hostname <domain>
  Preferredauthentications publickey
  IdentityFile ~/.ssh/client_id_rsa

# personal
Host <user_personal_git_domain>
  Hostname <domain>
  Preferredauthentications publickey
  IdentityFile ~/.ssh/own_id_rsa

FI: It can be multiple domains using the same key ( id_rsa | id_ed25519 )

Useful link

Going further with Gitlab official documentation

SSH-AGENT

Now that you setup your ssh with a passphrase to add some level of security.

But you don't want the inconvenience to type your passphrase every single time you use your key.

When using git/ssh, you may have come across the following prompt:

Enter passphrase for key '/home/user/.ssh/id_rsa':

Pitfalls

• Make sure your keys/agent are unload when you log off your machine.

• Do not copy your private keys on somebody else computer which has root on.

• Do not run ssh-agent on somebody else computer which has root on.

Trade-off (Security vs Convenience)

If you are tempted to add the following script on your dotfile profile. You have to be aware that every instance of your Terminal will start a ssh-agent process

if [ -z "$SSH_AUTH_SOCK" ] ; then
    eval `ssh-agent`
    ssh-add
fi

• To remove current agent provide by SSH_AGENT_PID run:

eval `ssh-agent -k`

• To list running ssh-agent process (OSX):

ps x | grep ssh-agent

• Last tips for security would be to set a time to live using: -s bourne shell stdout, -t 86400 for 24 hours. It could be less according to your security policy definition.

eval `ssh-agent -s -t 86400`