SSH Server Setup on Mac

To enable the SSH server on macOS, you can use the systemsetup command:

sudo systemsetup -setremotelogin on

What This Command Does

This command enables the SSH server on your Mac, allowing you to log in remotely via SSH. SSH (Secure Shell) is a protocol used to securely connect to remote systems over a network.

How to Verify SSH is Enabled

You can verify that SSH is enabled by running:

systemsetup -getremotelogin

The output should indicate that remote login is enabled:

Remote Login: On

Security Implications

Enabling SSH on your system has several security implications that you should be aware of:

Exposure to Remote Access: By enabling SSH, you are allowing remote access to your system. Anyone with the correct credentials can log in and access your files and system resources.
Brute Force Attacks: SSH servers are often targeted by brute force attacks where an attacker tries many different password combinations to gain access. To mitigate this risk:
- Use strong, complex passwords.
- Consider using SSH keys instead of passwords for authentication.
Open Port: The SSH service typically runs on port 22. Make sure your firewall settings allow only trusted IP addresses to connect to this port.
Regular Updates: Ensure your system and SSH server are regularly updated to protect against vulnerabilities.
Monitoring and Logging: Regularly monitor SSH access logs to detect any unauthorized access attempts.

How to disable password authentication

From Disable Password Auth (SSH Server MAC)

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If there is no explicit entry for PasswordAuthentication in the SSH server configuration file (/etc/ssh/sshd_config), the default value is typically yes. This means password-based authentication is allowed by default unless it is explicitly disabled.

To ensure that password-based authentication is disabled, you should explicitly set PasswordAuthentication no in the configuration file.

How to Explicitly Disable Password Authentication

Edit the SSH Configuration File:

Open the SSH configuration file with superuser privileges:

sudo idea /etc/ssh/sshd_config

(note: IntelliJ Sudo Access)

Set PasswordAuthentication to No:

Find the PasswordAuthentication line. If it is commented out (preceded by #) or missing, add or uncomment and modify it to:
```
PasswordAuthentication no
```
Ensure Other Relevant Settings:

Ensure PubkeyAuthentication is set to yes to allow key-based authentication:
```
PubkeyAuthentication yes
```
Also, ensure ChallengeResponseAuthentication is set to no to disable keyboard-interactive authentication:
```
ChallengeResponseAuthentication no
```
Restart SSH Service:

After making these changes, restart the SSH service to apply the new settings:
```
sudo launchctl stop com.openssh.sshd
sudo launchctl start com.openssh.sshd
```

Summary of Configuration

Here is a summary of the relevant entries in /etc/ssh/sshd_config:

PasswordAuthentication no
PubkeyAuthentication yes
ChallengeResponseAuthentication no

By explicitly setting these options, you ensure that only SSH key-based authentication is allowed and password-based authentication is disabled. This enhances the security of your SSH server.

Best Practices for Secure SSH Configuration

Use SSH Keys: SSH keys provide a more secure method of authentication than passwords. Generate an SSH key pair and add the public key to ~/.ssh/authorized_keys on the server.
Disable Root Login: Prevent direct root login via SSH by setting PermitRootLogin no in the SSH configuration file (/etc/ssh/sshd_config).
Configure Firewalls: Use firewalls to restrict access to the SSH port from known, trusted IP addresses only.
Limit User Access: Only allow SSH access to necessary users by configuring AllowUsers in the SSH configuration file.
Change Default Port: Consider changing the default SSH port from 22 to a non-standard port to reduce the likelihood of automated attacks.