Mastering OpenVPN

Table of Contents

Mastering OpenVPN

Credits

About the Authors

About the Reviewers

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Support files, eBooks, discount offers, and more

Why subscribe?

Free access for Packt account holders

Preface

What this book covers

What you need for this book

Who this book is for

Conventions

Reader feedback

Customer support

Downloading the example code

Errata

Piracy

Questions

1. Introduction to OpenVPN

What is a VPN?

Types of VPNs

PPTP

IPSec

SSL-based VPNs

OpenVPN

Comparison of VPNs

Advantages and disadvantages of PPTP

Advantages and disadvantages of IPSec

Advantages and disadvantages of SSL-based VPNs

Advantages and disadvantages of OpenVPN

History of OpenVPN

OpenVPN packages

The open source (community) version

The closed source (commercial) Access Server

The mobile platform (mixed) OpenVPN/OpenVPN Connect

Other platforms

OpenVPN internals

The tun/tap driver

The UDP and TCP modes

The encryption protocol

The control and data channels

Ciphers and hashing algorithms

OpenSSL versus PolarSSL

Summary

2. Point-to-point Mode

Pros and cons of the key mode

The first example

TCP protocol and different ports

The TAP mode

The topology subnet

The cleartext tunnel

OpenVPN secret keys

Using multiple keys

Using different encryption and authentication algorithms

Routing

Configuration files versus the command line

The complete setup

Advanced IP-less setup

Three-way routing

Route, net_gateway, vpn_gateway, and metrics

Bridged tap adapter on both ends

Removing the bridges

Combining point-to-point mode with certificates

Summary

3. PKIs and Certificates

An overview of PKI

PKI using Easy-RSA

Building the CA

Certificate revocation list

Server certificates

Client certificates

PKI using ssl-admin

OpenVPN server certificates

OpenVPN client certificates

Other features

Multiple CAs and CRLs

Extra security – hardware tokens, smart cards, and PKCS#11

Background information

Supported platforms

Initializing a hardware token

Generating a certificate/private key pair

Generating a private key on a token

Generating a certificate request

Writing an X.509 certificate to the token

Getting a hardware token ID

Using a hardware token with OpenVPN

Summary

4. Client/Server Mode with tun Devices

Understanding the client/server mode

Setting up the Public Key Infrastructure

Initial setup of the client/server mode

Detailed explanation of the configuration files

Topology subnet versus topology net30

Adding extra security

Using tls-auth keys

Generating a tls-auth key

Checking certificate key usage attributes

Basic production-level configuration files

TCP-based configuration

Configuration files for Windows

Routing and server-side routing

Special parameters for the route option

Masquerading

Redirecting the default gateway

Client-specific configuration – CCD files

How to determine whether a CCD file is properly processed

CCD files and topology net30

Client-side routing

In-depth explanation of the client-config-dir configuration

Client-to-client traffic

The OpenVPN status file

Reliable connection tracking for UDP mode

The OpenVPN management interface

Session key renegotiation

A note on PKCS#11 devices

Using IPv6

Protected IPv6 traffic

Using IPv6 as transit

Advanced configuration options

Proxy ARP

How does Proxy ARP work?

Assigning public IP addresses to clients

Summary

5. Advanced Deployment Scenarios in tun Mode

Enabling file sharing over VPN

Using NetBIOS names

Using nbtstat to troubleshoot connection problems

Using LDAP as a backend authentication mechanism

Troubleshooting the LDAP backend authentication

Filtering OpenVPN

FreeBSD example

A Windows example

Policy-based routing

Windows network locations – public versus private

Background

Changing the TAP-Win adapter location using the redirect-gateway

Using the Group Policy editor to force an adapter to be private

Changing the TAP-Win adapter location using extra gateways

Redirecting all traffic in combination with extra gateways

Using OpenVPN with HTTP or SOCKS proxies

HTTP proxies

SOCKS proxies

Summary

6. Client/Server Mode with tap Devices

The basic setup

Enabling client-to-client traffic

Filtering traffic between clients

Disadvantage of the proxy_arp_pvlan method

Filtering traffic using the pf filter of OpenVPN

Using the tap device (bridging)

Bridging on Linux

Tearing down the bridge

Bridging on Windows

Using an external DHCP server

Checking broadcast and non-IP traffic

Address Resolution Protocol traffic

NetBIOS traffic

Comparing tun mode to tap mode

Layer 2 versus layer 3

Routing differences and iroute

Client-to-client filtering

Broadcast traffic and chattiness of the network

Bridging

Summary

7. Scripting and Plugins

Scripting

Server-side scripts

--setenv and --setenv-safe

--script-security

--up-restart

--up

--route-up

--tls-verify

--auth-user-pass-verify

--client-connect

--learn-address

--client-disconnect

--route-pre-down

--down

Client-side scripts

--setenv and --setenv-safe

--script-security

--up-restart

--tls-verify

--ipchange

--up

--route-up

--route-pre-down

--down

Examples of server scripts

Client-connect scripts

Client authentication

Client authorization

Example 1—client-selected routes

Example 2—track client connection statistics

Example 3—disconnect user after X minutes

Examples of client scripts

Example 4—mount NFS share

Example 5—using all scripts at once

The server-side script log

Environment variables set in the server-side scripts

--up

--route-up

--tls-verify

--auth-user-pass-verify

--client-connect

--learn-address

--client-disconnect

--route-pre-down and --down

The client-side script log

Environment variables set in the client-side scripts

Plugins

Down-root

The auth-pam plugin

Summary

8. Using OpenVPN on Mobile Devices and Home Routers

Using the OpenVPN for an Android app

Creating an OpenVPN app profile

Using the PKCS#12 file

Using the OpenVPN Connect app for Android

Using the OpenVPN Connect app for iOS

Integrating smart phones into an existing VPN setup

Using a home router as a VPN client

Using a home router as a VPN server

Summary

9. Troubleshooting and Tuning

How to read the log files

Detecting a non-working setup

Fixing common configuration mistakes

Wrong CA certificate in the client configuration

How to fix

Client certificate not recognized by the server

How to fix

Client certificate and private key mismatch

How to fix

The auth and tls-auth key mismatch

How to fix

The MTU size mismatch

How to fix

The Cipher mismatch

How to fix

The Compression mismatch

How to fix

The fragment mismatch

How to fix

The tun versus tap mismatch

How to fix

The client-config-dir issues

How to fix

No access to the tun device in Linux

How to fix

Missing elevated privileges in Windows

How to fix

Troubleshooting routing issues

Drawing a detailed picture

Start in the middle and work your way outward

Find a time to temporarily disable firewall

If all else fails, use tcpdump

How to optimize performance by using ping and iperf

Using ping

Using iperf

Gigabit networking

Analyzing OpenVPN traffic by using tcpdump

Summary

10. Future Directions

Current strengths

Current weaknesses

Scaling at gigabit speeds and above

Where we are going

Improved compression support

Per-client compression

New cryptographic routines

Mixed certificate/username authentication

IPv6 support

Windows privilege separation

Summary

Index

Mastering OpenVPN

Mastering OpenVPN

Copyright © 2015 Packt Publishing

All rights reserved. No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, without the prior written permission of the publisher, except in the case of brief quotations embedded in critical articles or reviews.

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First published: August 2015

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Credits

Authors

Eric F Crist

Jan Just Keijser

Reviewers

Stefan Agner

Emmanuel Bretelle

Michael A Cossenas

Guillaume Destuynder

Commissioning Editor

Amarabha Banerjee

Acquisition Editors

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Cover Work

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About the Authors

Eric F Crist is an IT professional with experience in hardware and software systems integration. With a few others, he has had a key role in building the OpenVPN community to what it is today. He works in research and development as a principal computer system specialist for St. Jude Medical. His role involves system engineering, configuration management, and cyber security analysis for products related to the Cardiovascular Ablation Technology division.

You can find him online at the Freenode and EFNet IRC networks as ecrist. He calls the Twin Cities, Minnesota, his home and lives with his wife, DeeDee, his son, Lance, and his daughter, Taylor.

Jan Just Keijser is an open source professional from Utrecht, the Netherlands. He has a wide range of experience in IT, ranging from providing user support, system administration, and systems programming to network programming. He has worked for various IT companies since 1989. He has been working mainly on Unix/Linux platforms since 1995. He was an active USENET contributor in the early 1990s.

Currently, he is employed as a senior scientific programmer in Amsterdam, the Netherlands, at Nikhef, the institute for subatomic physics from the Dutch Foundation for Fundamental Research on Matter (FOM). He is working on multi-core and many-core computing systems, grid computing, as well as smartcard applications. His open source interests include all types of virtual private networking, including IPSec, PPTP, and of course, OpenVPN. In 2004, he discovered OpenVPN and has been using it ever since.

His first book was OpenVPN 2 Cookbook, Packt Publishing.

About the Reviewers

Stefan Agner completed his bachelor's degree in information technology from the Lucerne University of Applied Sciences and Arts in 2009 and has worked in the field of embedded systems as a software engineer since then. He focuses on driver development and system programming and prefers to work with the open source software stack. Currently, he is working to upstream Linux support for the ARM-based Freescale Vybrid SoC for his employer, Toradex AG.

He describes himself as an open source enthusiast who works with Linux and other free software not only in his professional life as a software engineer, but also in his spare time. At several smaller companies, he successfully deployed and managed OpenVPN as the primary VPN solution. For his private IT infrastructure, he runs OpernWrt-powered routers, which serve as OpenVPN servers. He also likes to blog about technical stuff, such as fascinating projects and interesting problems he comes across.

Emmanuel Bretelle has 10 years of experience in devops, systems, and network administration. He has leveraged OpenVPN, its plugin capabilities, and cross-platform compatibility to help connect employees across the globe to corporate networks.

He has also developed and open sourced two OpenVPN plugins: openvpn-mysql-auth and openvpn-ldap-auth.

When not fiddling around with new technology or automating his way out, he enjoys traveling and chilling out.

Michael A Cossenas is a Linux/network administrator from Athens, Greece.

He has been working as a network security specialist for Digital Sima, a company specializing in LAN/WAN networking. He is now employed as a subcontractor for IBM Greece and manages 50 plus SUSE-based Linux servers as one of their customers.

His first experience with Linux was way back in 1998, using RedHat 5.2. Since then, he has worked on various open source projects, including Zimbra, DRBD, KVM, and Postfix.

He is also an OpenVPN forum moderator.

He works as a subcontractor for IBM Greece in the SO (Strategic Outsourcing) department.

I would like to thank my family (my wife, Froso, my son, Antony, and my daughter, Kate) for supporting me in difficult times.

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Preface

Privacy and security on the Internet and in private networks is a growing concern and is increasingly common in the news, where there are breaches of each. Virtual private networks (VPN) were created out of a need for secured communications. The most popular and widely used open source VPN software today is OpenVPN. Mastering OpenVPN aims to educate you on deployment, troubleshooting, and configuration of OpenVPN and provide solid use cases for various scenarios.

What this book covers

Chapter 1, Introduction to OpenVPN, discusses the various types of Virtual Private Networks and some of their various strengths and weaknesses. PPTP, OpenVPN, IPSec, and other protocols are also discussed in this chapter.

Chapter 2, Point-to-point Mode, covers the OpenVPN roots, point-to-point mode, and the initially only supported mode. It also covers the tap mode in a bridged scenario and an uncommon configuration.

Chapter 3, PKIs and Certificates, explains the complex concept of X.509 certificates and PKIs with examples and a demonstration of a couple of utilities. It also covers how to create a certificate chain and deploy that chain to their VPN.

Chapter 4, Client/Server Mode with tun Devices, walks you through the most common deployment mode, a tun or routed, and its setup. It also discusses the passing of client-backed routes along with IPv4 and IPv6.

Chapter 5, Advanced Deployment Scenarios in tun Mode, covers policy-based routing and configuring OpenVPN to integrate your VPN clients with the rest of the LAN. Complex examples of tun mode are examined, showing that they are appropriate even in advanced scenarios.

Chapter 6, Client/Server Mode with tap Devices, discusses the often misused and less commonly deployed tap or bridged mode VPNs. Solid examples of broadcast and OSI layer 2 traffic are demonstrated in this chapter.

Chapter 7, Scripting and Plugins, helps you gain an understanding of the methods to extent the VPN, including authentication, routing, and protocol enhancements. This chapter helps an administrator create a local experience for a worker or a user on the move.

Chapter 8, Using OpenVPN on Mobile Devices and Home Routers, helps you learn how to use home router OSes and features to deploy OpenVPN. We understand that it's not just enterprise or commercial users looking to protect their privacy and data. Increasingly, home users desire to deploy secure connections to their home resources.

Chapter 9, Troubleshooting and Tuning, will help you become an expert in your OpenVPN deployment by learning how to troubleshoot problems and bugs. The ability to identify issues creates a solid and reliable installation and confidence in your users.

Chapter 10, Future Directions, gives you a brief history and lengthier discussion of the future direction of OpenVPN, and the mindset of the developers is revealed. It also helps you understand the reasoning and history behind the various decisions behind features and bugs.

What you need for this book

You should have the following entities for a complete experience of reading and following Mastering OpenVPN:

A Unix, Linux, or Mac OS X system

A Windows system

A server (Windows or Linux, whichever is preferred, Linux or FreeBSD suggested)

A solid understanding (101 or 201 level) of networking (UDP and TCP over IP)

An IRC client or a web browser is also helpful. When you run into trouble or have too many questions, pop in to #openvpn on irc.freenode.net, and look for @janjust or @ecrist. We look forward to talking to you!

Who this book is for

This book is really designed for anyone looking to deploy a VPN solution to any private or enterprise network. OpenVPN can be used for point-to-point tunnels, intra-network connections, and road warriors. The concepts covered in this book can be applied generally across more than just OpenVPN deployments, with the exception of configuration argument specifics.

Conventions

In this book, you will find a number of styles of text that distinguish between different kinds of information. Here are some examples of these styles, and an explanation of their meaning.

Code words in text, database table names, folder names, filenames, file extensions, pathnames, dummy URLs, user input, and Twitter handles are shown as follows: You can specify a message digest as parameter to the --auth option.

A block of code is set as follows:

proto udp

port 1194

dev tun

server 10.200.0.0 255.255.255.0

When we wish to draw your attention to a particular part of a code block, the relevant lines or items are set in bold:

proto udp

port 1194

dev tun

server 10.200.0.0 255.255.255.0

Any command-line input or output is written as follows:

#  mkdir -p /etc/openvpn/movpn

Note that first character (the prompt) is used to indicate a root-shell (#) or a user shell ($).

New terms and important words are shown in bold. Words that you see on the screen, in menus or dialog boxes for example, appear in the text like this: Launch the OpenVPN GUI application, select the configuration basic-udp-client and click on Connect.

Note

Warnings or important notes appear in a box like this.

Tip

Tips and tricks appear like this.

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Questions

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Chapter 1. Introduction to OpenVPN

The Internet in modern society is as ubiquitous as any public utility. When someone buys a home or moves into a new apartment, or a business moves into a new space, an Internet service is the first utility on the list to be ordered, followed by power, heat, trash, and maybe (but not likely) a land line or telephone service. You could even argue that the modern qualifier isn't even necessary. With programs such as One Laptop per Child, coupled with efforts by the likes of Facebook and Google, so-called third-world nations have the Internet where there is no running water, sewers, or even telephone services.

When you have such a wide-reaching service with so many individuals, at a certain point it will be necessary to secure and protect the data transmitted on that network. With most crowds and heavy concentrations of people, there is a more nefarious element looking to take advantage of those with less knowledge. Virtual Private Networks (VPNs) were created out of a greater need for secured communication across an otherwise unprotected infrastructure. The original large-scale network, ARPANET, had very little (if any) protection and authentication and all other nodes were inherently trusted. The network landscapes today are very different and even many casual, nontechnical users are aware of the lack of security of their connections.

Government agencies have long been targets for intelligence. For thousands of years, methods and procedures have been slowly perfected and tuned to protect sensitive information from enemies and other prying eyes. Initially, wax-sealed letters carried by trusted individuals meant you and the receiver could trust a message had arrived safely and untampered. As time and technology have progressed, it became easier to intercept those messages, read or alter them, and send them along their way.

World War II saw some of the greatest advances in cryptography and secure communications. From devices such as the German Enigma machine to the Navajo Code Talkers, communicating securely between troops and command was a never-ending arms race. Today, governments and militaries aren't the only groups with a desire for privacy. Corporations want to maintain data integrity and protection for payment card industry (PCI) standards to protect consumers. Family members want to discuss family matters over private channels, where the community at large isn't able to eavesdrop. Others wish to break through the national firewalls meant to oversee the populous and restrict content deemed controversial or against party politics.

Every day, most people use a VPN or have a use for a VPN, whether they realize it at the time or not. Many different VPN technologies exist, both from commercial vendors and as open source projects. One of the most popular pieces of open source VPN software is OpenVPN. The goal of this book is to make you an OpenVPN master; you will learn not just the technology behind it, but the reasoning, logic, and logistics of everything involved. While this book will mention and touch on the commercial offering from OpenVPN Technologies, Inc., Access Server, the primary focus will be on the open source/community version of OpenVPN.

What is a VPN?

Put simply, a VPN allows an administrator to create a local network between multiple computers on varying network segments. In some instances, those machines can be on the same LAN, they can be distant from each other across the vast Internet, or they can even be connected across a multitude of connection media such as wireless uplinks, satellite, dial-up-networking, and so on. The P in VPN comes from the added protection to make that virtual network private. Network traffic that is flowing over a VPN is often referred to as inside the (VPN) tunnel, compared to all the other traffic that is outside the tunnel.

In the following figure, network traffic is shown as it traditionally traverses across multiple network segments and the general Internet. Here, this traffic is relatively open to inspection and analysis. Though protected protocols such as HTTPS and SSH are less vulnerable, they are still identifiable; if an attacker is snooping network traffic, they can still see what type of connection is made from which computer to which server.

When a VPN is used, the traffic inside the tunnel is no longer identifiable.

The traffic within a VPN can be anything you would send over a local or wide-area network: web traffic,