Secure Linux Login Connection

Introduction

SSH offers two methods of authentication: password and key pair authentication.

• Password Authentication: While simple passwords are easily remembered, they are also easily compromised through brute force attacks. On the other hand, complex passwords, though safer, are challenging to remember.
• Key Pair Authentication: This method involves a combination of a public key and a private key. The public key is placed on the device that one wishes to access, while the private key is stored on the user's local machine. Only the holder of the private key can access the device, making this method secure and convenient.

Generating a Key Pair with ssh-keygen

The ssh-keygen command can be used to generate a key pair. Here is how to use it:

ssh-keygen

For a stronger key pair, use:

ssh-keygen -t rsa -b 4096 -C $comment

Note: When prompted, hit Enter for each prompt.

Uploading the Public Key to the Remote Host

There are two ways to upload the public key: manually and automatically.

Automatic Upload

To automatically upload the public key, run:

ssh-copy-id user@remoteHost

Or specify the public key and port:

ssh-copy-id -i ~/.ssh/id_rsa.pub user@remoteHost

Manual Upload

To manually upload the public key, copy the public key content:

ssh user@remoteHost 'mkdir -p .ssh && cat >> .ssh/authorized_keys' < ~/.ssh/id_rsa.pub

Next, set the correct permissions on the remote host:

chmod 700 ~/.ssh
chmod 600 ~/.ssh/authorized_keys

After creating the authorized_keys file and pasting the public key contents into it, we can log in without a password:

ssh user@remoteHost

Managing Sessions via SSH Profiles

SSH profiles are an elegant and efficient way to manage multiple remote logins. You can create several remote hosts on the SSH profiles as shown:

cat >> ~/.ssh/config << EOF
Host HOST01
    HostName 123.123.123.33
    Port 22
    User user01
    IdentityFile "~/.ssh/id_rsa"
    IdentitiesOnly yes

Host HOST02
    HostName 10.110.254.99
    Port 2222
    User user02
    IdentityFile "~/.ssh/id_ecdsa"
    IdentitiesOnly yes
EOF

Ensure that you set the correct permissions on the SSH profiles:

chmod 600 ~/.ssh/config

After setting up the SSH profiles, you can log in by simply entering the alias name:

ssh HOST01

Disabling Password Login

For security reasons, it is recommended to disable password login:

sudo sed -i "s@.*\(PasswordAuthentication \).*@\1no@" /etc/ssh/sshd_config
sudo service sshd restart

One-Key Configuration on SSH

Setting up a new remote host key login requires several steps such as key pair generation, permissions setting, public key upload, and password disabling.

However, we can upload all the public keys to Github SSH keys, and then deploy the public key with one command on the new remote host:

curl -fsSL https://github.com/$githubUser.keys >> ~/.ssh/authorized_keys

Also, disable the password and restart the SSH daemon:

sudo sed -i "s@.*\(PasswordAuthentication \).*@\1no@" /etc/ssh/sshd_config
sudo service sshd restart

Additionally, we can simplify the process using P3TERX's SSH Key Installer:

bash <(curl -fsSL git.io/key.sh) -g $githubUser -d

Deploying the Public Key

Here are some ways of getting the public key:

i. Get the public key from Github:

bash <(curl -fsSL git.io/key.sh) -g $githubUser

ii. Get the public key from a URL:

bash <(curl -fsSL git.io/key.sh) -u https://keyaddress.com/id_rsa.pub

iii. Overwrite mode will completely replace the previous key on /.ssh/authorized_keys:

bash <(curl -fsSL git.io/key.sh) -o -g $githubUser

iv. Disable password login:

bash <(bash <(curl -fsSL git.io/key.sh) -d

v. Modify the SSH port:

bash <(curl -fsSL git.io/key.sh) -p 2222

Conclusion

Whether manually or automatically, managing SSH keys involves creating a secure key pair, uploading the public key to the intended device, and managing sessions using SSH profiles. For increased security, it is advisable to disable password logins. Various tools such as P3TERX's SSH Key Installer can simplify these processes.

Reference

• 使用 ssh-keygen 和 ssh-copy-id 組態 SSH 金鑰實現免密登陸
• SSH 金鑰一鍵組態指令碼 使用教學

Related:

• SSH Tunneling
• Securely Transfer Files Over The Network