How Virtual Private Networks Work Introduction to How Virtual Private Networks Work

[badea 0]

The world has changed a lot in the last couple of decades. Instead of simply dealing with local or regional concerns, many businesses now have to think about global markets and logistics. Many companies have facilities spread out across the country or around the world, and there is one thing that all of them need: A way to maintain fast, secure and reliable communications wherever their offices are.

Until fairly recently, this has meant the use of leased lines to maintain a wide area network (WAN). Leased lines, ranging from ISDN (integrated services digital network, 128 Kbps) to OC3 (Optical Carrier-3, 155 Mbps) fiber, provided a company with a way to expand its private network beyond its immediate geographic area. A WAN had obvious advantages over a public network like the Internet when it came to reliability, performance and security. But maintaining a WAN, particularly when using leased lines, can become quite expensive and often rises in cost as the distance between the offices increases.

 

As the popularity of the Internet grew, businesses turned to it as a means of extending their own networks. First came intranets, which are password-protected sites designed for use only by company employees. Now, many companies are creating their own VPN (virtual private network) to accommodate the needs of remote employees and distant offices.

A typical VPN might have a main LAN at the corporate headquarters of a company, other LANs at remote offices or facilities and individual users connecting from out in the field.

 

 

 

Basically, a VPN is a private network that uses a public network (usually the Internet) to connect remote sites or users together. Instead of using a dedicated, real-world connection such as leased line, a VPN uses "virtual" connections routed through the Internet from the company's private network to the remote site or employee. In this article, you will gain a fundamental understanding of VPNs, and learn about basic VPN components, technologies, tunneling and security.

 

What Makes a VPN?

A well-designed VPN can greatly benefit a company. For example, it can:

 

Extend geographic connectivity

Improve security

Reduce operational costs versus traditional WAN

Reduce transit time and transportation costs for remote users

Improve productivity

Simplify network topology

Provide global networking opportunities

Provide telecommuter support

Provide broadband networking compatibility

Provide faster ROI (return on investment) than traditional WAN

What features are needed in a well-designed VPN? It should incorporate:

 

 

Security

Reliability

Scalability

Network management

Policy management

There are three types of VPN. In the next couple of sections, we'll describe them in detail.

 

Remote-Access VPN

There are two common types of VPN. Remote-access, also called a virtual private dial-up network (VPDN), is a user-to-LAN connection used by a company that has employees who need to connect to the private network from various remote locations. Typically, a corporation that wishes to set up a large remote-access VPN will outsource to an enterprise service provider (ESP). The ESP sets up a network access server (NAS) and provides the remote users with desktop client software for their computers. The telecommuters can then dial a toll-free number to reach the NAS and use their VPN client software to access the corporate network.

A good example of a company that needs a remote-access VPN would be a large firm with hundreds of sales people in the field. Remote-access VPNs permit secure, encrypted connections between a company's private network and remote users through a third-party service provider

 

Examples of the three types of VPN

 

Site-to-Site VPN

Through the use of dedicated equipment and large-scale encryption, a company can connect multiple fixed sites over a public network such as the Internet. Site-to-site VPNs can be one of two types:

Intranet-based - If a company has one or more remote locations that they wish to join in a single private network, they can create an intranet VPN to connect LAN to LAN.

 

Extranet-based - When a company has a close relationship with another company (for example, a partner, supplier or customer), they can build an extranet VPN that connects LAN to LAN, and that allows all of the various companies to work in a shared environment.

Examples of the three types of VPN

 

Analogy: Each LAN is an Island

Imagine that you live on an island in a huge ocean. There are thousands of other islands all around you, some very close and others farther away. The normal way to travel is to take a ferry from your island to whichever island you wish to visit. Of course, traveling on a ferry means that you have almost no privacy. Anything you do can be seen by someone else.

Let's say that each island represents a private LAN and the ocean is the Internet. Traveling by ferry is like connecting to a Web server or other device through the Internet. You have no control over the wires and routers that make up the Internet, just like you have no control over the other people on the ferry. This leaves you susceptible to security issues if you are trying to connect between two private networks using a public resource.

 

Continuing with our analogy, your island decides to build a bridge to another island so that there is easier, more secure and direct way for people to travel between the two. It is expensive to build and maintain the bridge, even though the island you are connecting with is very close. But the need for a reliable, secure path is so great that you do it anyway. Your island would like to connect to a second island that is much farther away but decides that the cost are simply too much to bear.

 

This is very much like having a leased line. The bridges (leased lines) are separate from the ocean (Internet), yet are able to connect the islands (LANs). Many companies have chosen this route because of the need for security and reliability in connecting their remote offices. However, if the offices are very far apart, the cost can be prohibitively high -- just like trying to build a bridge that spans a great distance.

 

So how does VPN fit in? Using our analogy, we could give each inhabitant of our islands a small submarine. Let's assume that your submarine has some amazing properties:

 

It's fast.

It's easy to take with you wherever you go.

It's able to completely hide you from any other boats or submarines.

It's dependable.

It costs little to add additional submarines to your fleet once the first is purchased.

 

In our analogy, each person having a submarine is like a remote user having access to the company's private network.

 

 

 

Although they are traveling in the ocean along with other traffic, the inhabitants of our two islands could travel back and forth whenever they wanted to with privacy and security. That's essentially how a VPN works. Each remote member of your network can communicate in a secure and reliable manner using the Internet as the medium to connect to the private LAN. A VPN can grow to accommodate more users and different locations much easier than a leased line. In fact, scalability is a major advantage that VPNs have over typical leased lines. Unlike with leased lines, where the cost increases in proportion to the distances involved, the geographic locations of each office matter little in the creation of a VPN.

 

VPN Security: Firewalls

A well-designed VPN uses several methods for keeping your connection and data secure:

 

Firewalls

Encryption

IPSec

AAA Server

In the following sections, we'll discuss each of these security methods. We'll start with the firewall.

A firewall provides a strong barrier between your private network and the Internet. You can set firewalls to restrict the number of open ports, what type of packets are passed through and which protocols are allowed through. Some VPN products, such as Cisco's 1700 routers, can be upgraded to include firewall capabilities by running the appropriate Cisco IOS on them. You should already have a good firewall in place before you implement a VPN, but a firewall can also be used to terminate the VPN sessions.

 

VPN Security: Encryption

Encryption is the process of taking all the data that one computer is sending to another and encoding it into a form that only the other computer will be able to decode. Most computer encryption systems belong in one of two categories:

 

Symmetric-key encryption

Public-key encryption

In symmetric-key encryption, each computer has a secret key (code) that it can use to encrypt a packet of information before it is sent over the network to another computer. Symmetric-key requires that you know which computers will be talking to each other so you can install the key on each one. Symmetric-key encryption is essentially the same as a secret code that each of the two computers must know in order to decode the information. The code provides the key to decoding the message. Think of it like this: You create a coded message to send to a friend in which each letter is substituted with the letter that is two down from it in the alphabet. So "A" becomes "C," and "B" becomes "D". You have already told a trusted friend that the code is "Shift by 2". Your friend gets the message and decodes it. Anyone else who sees the message will see only nonsense.

 

The sending computer encrypts the document with a symmetric key, then encrypts the symmetric key with the public key of the receiving computer. The receiving computer uses its private key to decode the symmetric key. It then uses the symmetric key to decode the document.

 

Public-key encryption uses a combination of a private key and a public key. The private key is known only to your computer, while the public key is given by your computer to any computer that wants to communicate securely with it. To decode an encrypted message, a computer must use the public key, provided by the originating computer, and its own private key. A very popular public-key encryption utility is called Pretty Good Privacy (PGP), which allows you to encrypt almost anything. You can find out more about PGP at the PGP site.

 

VPN Security: IPSec

Internet Protocol Security Protocol (IPSec) provides enhanced security features such as better encryption algorithms and more comprehensive authentication.

A remote-access VPN utilizing IPSec

 

 

 

IPSec has two encryption modes: tunnel and transport. Tunnel encrypts the header and the payload of each packet while transport only encrypts the payload. Only systems that are IPSec compliant can take advantage of this protocol. Also, all devices must use a common key and the firewalls of each network must have very similar security policies set up. IPSec can encrypt data between various devices, such as:

 

Router to router

Firewall to router

PC to router

PC to server

 

 

VPN Security: AAA Servers

AAA (authentication, authorization and accounting) servers are used for more secure access in a remote-access VPN environment. When a request to establish a session comes in from a dial-up client, the request is proxied to the AAA server. AAA then checks the following:

Who you are (authentication)

What you are allowed to do (authorization)

What you actually do (accounting)

The accounting information is especially useful for tracking client use for security auditing, billing or reporting purposes.

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[t3jem 0]

Wow, you gave a very indept lesson on virtual private networks. Thank you, hopefully some people will find this useful ( I myself already know everything I want/need to know ). Thanks for sharing this with us.Note: May have been easier to put it all in one post though

Edited May 21, 2007 by t3jem (see edit history)

[rajibbd 0]

That is huge. I cant read all the stuff. Because of too many text. but any way I get a through idea about the topic(VPN). Thank you for your post. Now I am adding some thing. In Local Area Network there is a special LAN called Virtual Local Area Network. It is in same LAN there are more then one LAN[Like in a specific LAN there are 5pcs and by this tectonic we can separate pc1,3,4(VLAN1) and 2,5(VLAN2) ] and VLAN1 can not access to VLAN2. This is a special purpose stuff, and it is maintain by a special software.