How To A virtual private network (VPN)

History of virtual private network (VPN)

A virtual private network (VPN) is a computer network that is layered on top of an underlying computer network. The private nature of a VPN means that the data travelling over the VPN is not generally visible to, or is encapsulated from, the underlying network traffic. This is done with strong encryption, as VPN's are commonly deployed to be high-security "network tunnels". Similarly, the traffic within the VPN appears to the underlying network as just another traffic stream to be passed. If you can envision secured "pipe" within the wire that is your connection, you would be well on your way to picturing a VPN deployment, if perhaps oversimplified.

In more technical terms, the link layer protocols of the virtual network are said to be tunneled through the underlying transport network.

The term VPN can be used to describe many different network configurations and protocols. As such, it can become complex when trying to generalise about the characteristics of a VPN. Some of the more common uses of VPNs are described below, along with more detail about the various classification schemes and VPN models.

VPN classifications

VPN technologies are not easily compared, due to the myriad of protocols, terminologies and marketing influences that have defined them. For example, VPN technologies can differ:

In the protocols they use to tunnel the traffic over the underlying network;

By the location of tunnel termination, such as the customer edge or network provider edge;

Whether they offer site-to-site or remote access connectivity;

In the levels of security provided;

By the OSI layer which they present to the connecting network, such as Layer 2 circuits or Layer 3 network connectivity.

Some classification schemes are discussed in the following sections.

Secure VPN vs Trusted VPN

The industry group 'Virtual Private Networking Consortium' have defined two types of VPN classifications, Secure VPNs and Trusted VPNs[1]. The consortium includes members such as Cisco, D-Link, Juniper and many others[2].

Secure VPNs explicitly provide mechanisms for authentication of the tunnel endpoints during tunnel setup, and encryption of the traffic in transit. Often secure VPNs are used to protect traffic when using the Internet as the underlying backbone, but equally they may be used in any environment when the security level of the underlying network differs from the traffic within the VPN.

Secure VPNs may be implemented by organizations wishing to provide remote access facilities to their employees or by organizations wishing to connect multiple networks together securely using the Internet to carry the traffic. A common use for secure VPNs is in remote access scenarios, where VPN client software on an end user system is used to connect to a remote office network securely. Secure VPN protocols include IPSec, L2TP (with IPsec for traffic encryption), SSL/TLS VPN (with SSL/TLS) or PPTP (with MPPE).

Trusted VPNs are commonly created by carriers and large organizations and are used for traffic segmentation on large core networks. They often provide quality of service guarantees and other carrier-grade features. Trusted VPNs may be implemented by network carriers wishing to multiplex multiple customer connections transparently over an existing core network or by large organizations wishing to segregate traffic flows from each other in the network. Trusted VPN protocols include MPLS, ATM or Frame Relay.

Trusted VPNs differ from secure VPNs in that they do not provide security features such as data confidentiality through encryption. Secure VPNs however do not offer the level of control of the data flows that a trusted VPN can provide such as bandwidth guarantees or routing.

From a customer perspective, a trusted VPN may act as a logical wire connecting two networks. The underlying carrier network is not visible to the customer, nor is the customer aware of the presence of other customers traversing the same backbone. Interference between customers, or interference with the backbone itself, is not possible from within a trusted VPN.

Some Internet service providers offer managed VPN service for business customers who want the security and convenience of a VPN but prefer not to undertake administering a VPN server themselves. Managed secure VPNs are again a hybrid of the two major VPN models, and are a contracted security solution that can reach into hosts. In addition to providing remote workers with secure access to their employer's internal network, other security and management services are sometimes included as part of the package. Examples include keeping anti-virus and anti-spyware programs updated on each connecting computer or ensuring particular software patches are installed before connection is permitted.

Categorization by user administrative relationships

The Internet Engineering Task Force (IETF) has categorized a variety of VPNs, some of which, such as Virtual LANs (VLAN) are the standardization responsibility of other organizations, such as the Institute of Electrical and Electronics Engineers (IEEE) Project 802, Workgroup 802.1 (architecture). Originally, wide area network (WAN) links from a telecommunications service provider interconnected network nodes within a single enterprise. With the advent of LANs, enterprises could interconnect their nodes with links that they owned. While the original WANs used dedicated lines and layer 2 multiplexed services such as Frame Relay, IP-based layer 3 networks, such as the ARPANET, Internet, military IP networks (NIPRNet, SIPRNet, JWICS, etc.), became common interconnection media. VPNs began to be defined over IP networks.[3] The military networks may themselves be implemented as VPNs on common transmission equipment, but with separate encryption and perhaps routers.

It became useful first to distinguish among different kinds of IP VPN based on the administrative relationships (rather than the technology) interconnecting the nodes. Once the relationships were defined, different technologies could be used, depending on requirements such as security and quality of service.

When an enterprise interconnects a set of nodes, all under its administrative control, through a LAN, that is termed an intranet.[4] When the interconnected nodes are under multiple administrative authorities but are hidden from the public Internet, the resulting set of nodes is called an extranet. A user organization can manage both intranets and extranets itself, or negotiate a service as a contracted (and usually customized) offering from an IP service provider. In the latter case, the user organization contracts for layer 3 services – much as it may contract for layer 1 services such as dedicated lines, or multiplexed layer 2 services such as frame relay.

IETF documents distinguish between provider-provisioned and customer-provisioned VPNs.[5] Just as an interconnected and set of providers can supply conventional WAN services, so a single service provider can supply provider-provisioned VPNs (PPVPNs), presenting a common point-of-contact to the user organization.

Internet Protocol tunnels

Main article: Tunneling protocol

Some customer-managed virtual networks may not use encryption to protect the data contents. These types of overlay networks do not neatly fit within the secure or trusted categorization. An example of such an overlay network could be a GRE tunnel, set up between two hosts. This tunneling would still be a form of virtual private network yet is neither a secure nor a trusted VPN.

Examples of native plaintext tunneling protocols include GRE, L2TP (without IPsec) and PPTP (without MPPE).

Security mechanisms

Secure VPNs use cryptographic tunneling protocols to provide the intended confidentiality (blocking intercept and thus packet sniffing), sender authentication (blocking identity spoofing), and message integrity (blocking message alteration) to achieve privacy.

Secure VPN protocols include the following:

IPsec (Internet Protocol Security) - A standards-based security protocol developed originally for IPv6, where support is mandatory, but also widely used with IPv4. For VPNs L2TP is commonly used over IPsec.

Transport Layer Security (SSL/TLS) is used either for tunneling an entire network's traffic (SSL/TLS VPN), as in the OpenVPN project, or for securing individual connection. SSL has been the foundation by a number of vendors to provide remote access VPN capabilities. A practical advantage of an SSL VPN is that it can be accessed from locations that restrict external access to SSL-based e-commerce websites without IPsec implementations. SSL-based VPNs may be vulnerable to denial-of-service attacks mounted against their TCP connections because latter are inherently unauthenticated.

Datagram Transport Layer Security (DTLS), used by Cisco for a next generation VPN product called Cisco AnyConnect VPN. DTLS solves the issues found when tunneling TCP over TCP as is the case with SSL/TLS

Microsoft Point-to-Point Encryption (MPPE) by Microsoft is used with their PPTP. Several compatible implementations on other platforms also exist.

Secure Socket Tunneling Protocol (SSTP) by Microsoft introduced in Windows Server 2008 and Windows Vista Service Pack 1. SSTP tunnels PPP or L2TP traffic through an SSL 3.0 channel.

MPVPN (Multi Path Virtual Private Network). Ragula Systems Development Company owns the registered trademark "MPVPN".[6]

SSH VPN -- OpenSSH offers VPN tunneling to secure remote connections to a network (or inter-network links). This feature (option -w) should not be confused with port forwarding (option -L/-R/-D). OpenSSH server provides limited number of concurrent tunnels and the VPN feature itself does not support personal authentication.

Authentication

Tunnel endpoints are required to authenticate themselves before secure VPN tunnels can be established. End user created tunnels, such as remote access VPNs may use passwords, biometrics, two-factor authentication or other cryptographic methods. For network-to-network tunnels, passwords or digital certificates are often used, as the key must be permanently stored and not require manual intervention for the tunnel to be established automatically.

Routing

Tunneling protocols can be used in a point-to-point topology that would generally not be considered a VPN, because a VPN is expected to support arbitrary and changing sets of network nodes. Since most router implementations support software-defined tunnel interface, customer-provisioned VPNs often comprise simply a set of tunnels over which conventional routing protocols run. PPVPNs, however, need to support the coexistence of multiple VPNs, hidden from one another, but operated by the same service provider.

OSI Layer 3 PPVPN architectures

This section discusses the main architectures for PPVPNs, one where the PE disambiguates duplicate addresses in a single routing instance, and the other, virtual router, in which the PE contains a virtual router instance per VPN. The former approach, and its variants, have gained the most attention.

One of the challenges of PPVPNs involves different customers using the same address space, especially the IPv4 private address space[11]. The provider must be able to disambiguate overlapping addresses in the multiple customers' PPVPNs.