Host Security with TCP Wrappers and Systemd Sockets

You will be using an active SSH connection to the server when you log in.

1. Verify that the sshd.socket unit is not active:

    systemctl status sshd.socket

The output should show that it is not active.

1. We will need to set up an at job that will stop the sshd.service unit and start the sshd.socket for us. Run the following commands in this sequence:

    sudo at now + 3 minutes
    at> systemctl stop sshd.service
    at> systemctl start sshd.socket
    at> <EOT>

The is from the key combination Ctrl+D. After three minutes, the sshd.service will stop, and the sshd.socket unit will take over for your connection, so your remote shell should not disconnect. If it does, SSH back into the learning activity again.

1. After the time has expired on the at job, verify that the sshd.socket unit is active and running:

    systemctl status sshd.socket

Now any new secure shell connections that come into the system will utilize an on-demand socket. This way, the server does not have to keep a running secure shell running.

1. Enable the socket for SSH and disable the service for SSH:

    sudo systemctl enable sshd.socket
    sudo systemctl disable sshd.service

1. First, verify that the sshd server has been compiled to use TCP wrappers:

    ldd /usr/sbin/sshd | grep libwrap

You should see that the sshd binary is capable of being used with TCP wrappers.

1. Edit the /etc/hosts.allow file, and add the following:

    sshd2 sshd : ALL

This will permit incoming SSH connections from any network.

3. Now set up a default deny rule for TCP wrappers to deny any other incoming connections. Edit the /etc/hosts.deny file and add the following:

    ALL : ALL

Exit out of the SSH session and attempt to reconnect. Provided that the commands have been entered as described, you will be granted access back into the system.