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Troubleshooting OpenVPN
Troubleshooting OpenVPN

Troubleshooting OpenVPN: Get the solutions you need to troubleshoot any issue you may face to keep your OpenVPN up and running

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Profile Icon Eric F Crist
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$38.99
Paperback Mar 2017 178 pages 1st Edition
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Arrow left icon
Profile Icon Eric F Crist
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$38.99
Paperback Mar 2017 178 pages 1st Edition
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$9.99 $29.99
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Troubleshooting OpenVPN

Chapter 2. Common Problems

There is a fairly consistent theme within the OpenVPN support channels, IRC, the web forums, and the mailing list. The novice users of OpenVPN generally ask the same questions. Most common issues can be resolved by simply dutifully reading the OpenVPN manual (http://openvpn.net/man) and taking notes. Although this book aims to inform the reader of additional techniques and tools, the man page is a considerable wealth of knowledge, and the close scrutiny of its contents will be beneficial in the long run.

Based on questions found in IRC and on the forums, the novice OpenVPN administrator struggles most with identifying the root cause of a problem and is generally lost with how to find that cause. The vast majority of problems can be grouped pretty easily into a small subset of issues, often quickly realized with initial configurations:

  • Certificate problems
  • Incompatible tun/tap configuration settings between a server and a client
  • IP range conflicts with VPN...

Narrowing the focus

When your VPN fails to work the way it was expected, it's best to narrow down the cause of the problem. In the simplest case, a VPN is used by a client wanting to talk to a resource that exists on the VPN. Having a diagram or flow chart of your finished VPN topology, including the VPN, the server-side LAN, and resources that will be exposed to the clients, will help rule out working components.

Here is a sample network diagram depicting a simple corporate network with an OpenVPN server. There are a number of internal components that are inaccessible to the general Internet, an application server, and an internal website. Externally accessible are a public website, the corporate e-mail server, and the OpenVPN server.

The dotted lines show our protected or internal traffic, and the solid lines show general Internet routed traffic:

Narrowing the focus

Example network diagram used in scenario 1 and 2

Sample scenarios

The utility of the diagram mentioned earlier are best demonstrated...

Supported operating systems

Arguably, the easiest thing to resolve is identifying an operating system that has proper support for OpenVPN. If the tun or tap device is not supported, it quickly rules out OpenVPN or limits the specific features of OpenVPN, but is often overlooked. Both iOS (all versions as of this writing) and Android (also, all versions as of this writing) do not support the tap device.

There are other operating systems that don't support virtualized network devices at all. FreeBSD jails, for example, don't support the tun or tap devices without some significant configuration and startup tricks. Many embedded operating systems, generally on routers and switches, do not fully support OpenVPN. Even if your platform of choice claims to support the virtual network devices, it's best to do some digging to ensure that OpenVPN runs reliably.

Embedded devices

There is a long list of embedded devices that support OpenVPN. Snom VOIP phones have the ability to support...

IP addresses

It is important to choose an IP address range that does not have or has goods odds against, conflicting with remote client address pools. If VPN uses IP addresses from a range shared by a remote client address pool, packets meant for the client LAN may attempt to traverse the VPN to the wrong system or to a system that doesn't exist at all. Alternatively, the traffic may never leave the client LAN and be routed to a local resource, instead.

The following diagram illustrates a fairly severe case of what I'm describing. There are various resources identified with their associated LAN address on both sides.

On the left, there is a network where the VPN server resides. The LAN on the server network uses the 10.4.0.0/24 subnet. For the VPN, the 10.8.0.0/24 subnet is used. This will facilitate VPN traffic, and a route will be pushed for the server-side LAN subnet. There are two internal servers for which the VPN was created. The first is an application server using LAN...

Firewalls

Incorrectly configured firewalls are one of the most prevalent problem areas, particularly for new users. Experienced users are also apt for misunderstanding how firewall rules apply to OpenVPN routed packets. In the past, firewalls were relatively monolithic devices at or near a network edge. Today, however, firewalls exist on client devices, network devices, all along the path of a network packet. Any switch in the path can have ACLs preventing or permitting traffic. This can include both the client- and server-side LANs. The OpenVPN likely has a firewall element for translation or routing traffic, and also for preventing or permitting said traffic.

The monolithic firewall also still exists and can be a pain point when troubleshooting traffic flow. The current high-end firewalls also permit deep packet inspection, SSL decryption, and what some vendors term zero-day patching. The latter generally requires SSL decryption be configured and permits the firewall vendor to detect vulnerable...

Narrowing the focus


When your VPN fails to work the way it was expected, it's best to narrow down the cause of the problem. In the simplest case, a VPN is used by a client wanting to talk to a resource that exists on the VPN. Having a diagram or flow chart of your finished VPN topology, including the VPN, the server-side LAN, and resources that will be exposed to the clients, will help rule out working components.

Here is a sample network diagram depicting a simple corporate network with an OpenVPN server. There are a number of internal components that are inaccessible to the general Internet, an application server, and an internal website. Externally accessible are a public website, the corporate e-mail server, and the OpenVPN server.

The dotted lines show our protected or internal traffic, and the solid lines show general Internet routed traffic:

Example network diagram used in scenario 1 and 2

Sample scenarios

The utility of the diagram mentioned earlier are best demonstrated with some useful...

Supported operating systems


Arguably, the easiest thing to resolve is identifying an operating system that has proper support for OpenVPN. If the tun or tap device is not supported, it quickly rules out OpenVPN or limits the specific features of OpenVPN, but is often overlooked. Both iOS (all versions as of this writing) and Android (also, all versions as of this writing) do not support the tap device.

There are other operating systems that don't support virtualized network devices at all. FreeBSD jails, for example, don't support the tun or tap devices without some significant configuration and startup tricks. Many embedded operating systems, generally on routers and switches, do not fully support OpenVPN. Even if your platform of choice claims to support the virtual network devices, it's best to do some digging to ensure that OpenVPN runs reliably.

Embedded devices

There is a long list of embedded devices that support OpenVPN. Snom VOIP phones have the ability to support OpenVPN for secure...

IP addresses


It is important to choose an IP address range that does not have or has goods odds against, conflicting with remote client address pools. If VPN uses IP addresses from a range shared by a remote client address pool, packets meant for the client LAN may attempt to traverse the VPN to the wrong system or to a system that doesn't exist at all. Alternatively, the traffic may never leave the client LAN and be routed to a local resource, instead.

The following diagram illustrates a fairly severe case of what I'm describing. There are various resources identified with their associated LAN address on both sides.

On the left, there is a network where the VPN server resides. The LAN on the server network uses the 10.4.0.0/24 subnet. For the VPN, the 10.8.0.0/24 subnet is used. This will facilitate VPN traffic, and a route will be pushed for the server-side LAN subnet. There are two internal servers for which the VPN was created. The first is an application server using LAN IP 10.4.0.76...

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Key benefits

  • This is the first book on the market that resolves your issues related to troubleshooting OpenVPN
  • Ensure your organization’s private network is protected 24x7 by resolving OpenVPN issues instantly
  • Save time and costs by troubleshooting to reduce the impact on your business

Description

OpenVPN, the most widely used open source VPN package, allows you to create a secure network across systems, keeping your private data secure. Connectivity and other issues are a pain to deal with, especially if they are impacting your business. This book will help you resolve the issues faced by OpenVPN users and teach the techniques on how to troubleshoot it like a true expert. This book is a one stop solution for troubleshooting any issue related to OpenVPN. We will start by introducing you to troubleshooting techniques such as Packet Sniffing, Log Parsing, and OpenSSL. You will see how to overcome operating system specific errors. Later on, you will get to know about network and routing errors by exploring the concepts of IPv4 and IPv6 networking issues. You will discover how to overcome these issues to improve the performance of your OpenVPN deployment. By the end of the book, you will know the best practices, tips, and tricks to ensure the smooth running of your OpenVPN.

Who is this book for?

The book is for system administrators who are experienced and well versed with OpenVPN. You should possess intermediate to master level proficiency with OpenVPN. All OpenVPN users can leverage this book.

What you will learn

  • • Diagnose and remediate authentication and authorization problems in OpenVPN
  • • Overcome simple network and startup script errors
  • • Learn OpenVPN log file format and verbosity options
  • • Resolve operating system-specific errors
  • • Discover various troubleshooting techniques to resolve problems in OpenVPN
  • • Improve performance and identify bottlenecks in the network or with hardware
  • • Determine external versus internal network issues
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Table of Contents

9 Chapters
1. Troubleshooting Basics Chevron down icon Chevron up icon
2. Common Problems Chevron down icon Chevron up icon
3. Installing OpenVPN Chevron down icon Chevron up icon
4. The Log File Chevron down icon Chevron up icon
5. Client and Server Startup Chevron down icon Chevron up icon
6. Certificates and Authentication Chevron down icon Chevron up icon
7. Network and Routing Chevron down icon Chevron up icon
8. Performance Chevron down icon Chevron up icon
9. External Problems Chevron down icon Chevron up icon
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