PPTP and L2TP

What this chapter covers

This chapter covers two tunnelling protocols that let a remote user or a branch router join a private network.

PPTP and L2TP both carry PPP sessions across an IP network. They use the profiles, secrets and pools from the previous chapter, so those ideas are assumed here.

You will learn how the protocols differ, and how to build a working server and client for each.

The chapter is practical. Every configuration page shows the console commands, the settings that matter and the checks that prove the tunnel works.

How the topics fit together

The theory page comes first, and the two configuration pages put it into practice.

The pieces follow a sensible order:

• Theory and comparison explains the ports, the protocols and how each one protects traffic.
• PPTP configuration builds a tunnel with MPPE encryption and shows the firewall needs.
• L2TP configuration builds a tunnel and adds IPsec, which supplies the encryption.

A tunnel is only the start. The remote LAN still needs a route that points into the tunnel, and the firewall still has to allow the right ports.

How to use these pages

Each topic page has the same shape, so you can skim for what you need.

• What it is: the idea in plain words.
• How it works: the mechanism, with diagrams and example commands.
• What to remember for the exam: the facts to memorise.
• Common mistakes: the classic traps.
• Check yourself: three questions to test your understanding.

Read the theory page first and fix the ports in your mind. Then build each tunnel in a small lab with two routers, and break it on purpose by changing a key, a service value or a firewall rule. Seeing each failure helps you spot it in an exam question.

Keep a short note of the ports for each protocol as you go. Questions on this chapter often turn on a single port number or a single missing firewall rule, so those details repay a little memorising.