Host Security with TCP Wrappers and Systemd Sockets

A Linux system administrator is responsible for keeping their servers secure. There are a multitude of tools and software packages available to keep a networked Linux system safe from malicious intruders. In this hands-on lab, we will learn how to move away from always-on services to those that use systemd socket units. Socket units only provide access to a network service when an incoming connection requests it. To further enchance the security of the service, we will apply TCP wrappers to allow incoming connections to a specified service.

Learning Objectives

Successfully complete this lab by achieving the following learning objectives:

Configure `sshd` to use sockets.

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

Verify that the sshd.socket unit is not active:
```
systemctl status sshd.socket
```
The output should show that it is not active.
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 <EOT> 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.
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.

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

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

Set up TCP wrappers to only allow SSH.

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.
Edit the /etc/hosts.allow file, and add the following:
```
sshd2 sshd : ALL
```
This will permit incoming SSH connections from any network.
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.

What are Hands-on Labs

Hands-on Labs are real environments created by industry experts to help you learn. These environments help you gain knowledge and experience, practice without compromising your system, test without risk, destroy without fear, and let you learn from your mistakes. Hands-on Labs: practice your skills before delivering in the real world.

Host Security with TCP Wrappers and Systemd Sockets

About this Hands-on Lab

Learning Objectives

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