What does ListenAddress 0.0.0.0 & ListenAddress :: mean?

This post answers the question, "What does ListenAddress 0.0.0.0 & ListenAddress :: mean?"

What does ListenAddress 0.0.0.0 & ListenAddress :: mean?

The ListenAddress directive in the /etc/ssh/sshd_config file specifies the IP addresses on which the SSH daemon (sshd) should listen for incoming connections. Here is what the values mean:

• ListenAddress 0.0.0.0:

  • This means the SSH daemon will listen to all available IPv4 addresses on the machine.

  • If your machine has multiple network interfaces or IP addresses, 0.0.0.0 will allow SSH to accept connections on any of them.

  • It is a wildcard address representing all IPv4 addresses.

• ListenAddress ::

  • This means the SSH daemon will listen to all available IPv6 addresses on the machine.
    

  • Similar to 0.0.0.0 for IPv4, :: is a wildcard address representing all IPv6 addresses.
    

  • It ensures SSH can accept connections on any IPv6 address configured on the machine.
    

Example Scenario:

If both ListenAddress 0.0.0.0 and ListenAddress :: are set in the sshd_config file, the SSH daemon will listen for incoming connections on all IPv4 and IPv6 addresses. This setup is common and ensures that SSH is accessible via any IP address assigned to the server, whether it's using IPv4 or IPv6.

Practical Considerations:

• Security: While allowing SSH to listen on all interfaces can be convenient, it also exposes your SSH service to any network the server is connected to. Ensure proper firewall rules are in place to restrict access.

• Specific Interfaces: If you only want SSH to listen on specific IP addresses, you can specify those addresses instead of using 0.0.0.0 or ::. For example:

ListenAddress 192.168.1.10 
ListenAddress 2001:db8::1

By understanding and configuring the ListenAddress directive appropriately, you can control which network interfaces and addresses SSH listens on, enhancing the security and manageability of your SSH service.