Virtual private network
||This article may be confusing or unclear to readers. (November 2009)|
A virtual private network (VPN) extends a private network across public networks like the Internet. It enables a host computer to send and receive data across shared or public networks as if they were an integral part of the private network with all the functionality, security and management policies of the private network. This is done by establishing a virtual point-to-point connection through the use of dedicated connections, encryption, or a combination of the two.
The VPN connection across the Internet is technically a wide area network (WAN) link between the sites. From a user perspective, the extended network resources are accessed in the same way as resources available from the private network—hence the name "virtual private network".
Types of VPN 
Early data networks allowed VPN-style remote connectivity through dial-up modems or through leased line connections utilizing Frame Relay and Asynchronous Transfer Mode (ATM) virtual circuits, provisioned through a network owned and operated by telecommunication carriers such as AT&T or Verizon. These networks are not considered true VPNs because they passively secure the data being transmitted by the creation of logical data streams. They have given way to VPNs based on IP and IP/Multiprotocol Label Switching Networks (MPLS), due to significant cost-reductions and increased bandwidth provided by new technologies such as Digital Subscriber Line (DSL) and fiber-optic networks.
VPNs can be either remote-access (connecting an individual computer to a network) or site-to-site (connecting two networks together). In a corporate setting, remote-access VPNs allow employees to access their company's intranet from home or while traveling outside the office, and site-to-site VPNs allow employees in geographically disparate offices to share one cohesive virtual network. A VPN can also be used to interconnect two similar networks over a dissimilar middle network; for example, two IPv6 networks over an IPv4 network.
VPN systems can be classified by:
- the protocols used to tunnel the traffic
- the tunnel's termination point, i.e., customer edge or network-provider edge
- whether they offer site-to-site or remote-access connectivity
- the levels of security provided
- the OSI layer they present to the connecting network, such as Layer 2 circuits or Layer 3 network connectivity
Security mechanisms 
To prevent disclosure of private information, VPNs typically allow only authenticated remote access and make use of encryption techniques.
- confidentiality such that even if the network traffic is sniffed at the packet level (see network sniffer and Deep packet inspection), an attacker would only see encrypted data
- sender authentication to prevent unauthorized users from accessing the VPN.
- message integrity to detect any instances of tampering with transmitted messages
Secure VPN protocols include the following:
- Internet Protocol Security (IPsec) as initially developed by the Internet Engineering Task Force (IETF) for IPv6, which was required in all standards-compliant implementations of IPv6 before RFC 6434 made it only a recommendation. This standards-based security protocol is also widely used with IPv4 and the Layer 2 Tunneling Protocol. Its design meets most security goals: authentication, integrity, and confidentiality. IPsec uses encryption, encapsulating an IP packet inside an IPsec packet. De-encapsulation happens at the end of the tunnel, where the original IP packet is decrypted and forwarded to its intended destination.
- Transport Layer Security (SSL/TLS) can tunnel an entire network's traffic (as it does in the OpenVPN project) or secure an individual connection. A number of vendors provide remote-access VPN capabilities through SSL. An SSL VPN can connect from locations where IPsec runs into trouble with Network Address Translation and firewall rules.
- Datagram Transport Layer Security (DTLS) - used in Cisco AnyConnect VPN and in OpenConnect VPN to solve the issues SSL/TLS has with tunneling over UDP.
- Microsoft Point-to-Point Encryption (MPPE) works with the Point-to-Point Tunneling Protocol and in several compatible implementations on other platforms.
- Microsoft Secure Socket Tunneling Protocol (SSTP) tunnels Point-to-Point Protocol (PPP) or Layer 2 Tunneling Protocol traffic through an SSL 3.0 channel. (SSTP was introduced in Windows Server 2008 and in Windows Vista Service Pack 1.)
- Multi Path Virtual Private Network (MPVPN). Ragula Systems Development Company owns the registered trademark "MPVPN".
- Secure Shell (SSH) VPN - OpenSSH offers VPN tunneling (distinct from port forwarding) to secure remote connections to a network or to inter-network links. OpenSSH server provides a limited number of concurrent tunnels. The VPN feature itself does not support personal authentication.
Tunnel endpoints must authenticate before secure VPN tunnels can be established.
Network-to-network tunnels often use passwords or digital certificates. They permanently store the key to allow the tunnel to establish automatically, without intervention from the user.
Example use of a VPN tunnel 
The following steps illustrate the principles of a VPN client-server interaction in simple terms.
Assume a remote host with public IP address 220.127.116.11 wishes to connect to a server found inside a company network. The server has internal address 192.168.1.10 and is not reachable publicly. Before the client can reach this server, it needs to go through a VPN server / firewall device that has public IP address 18.104.22.168 and an internal address of 192.168.1.1. All data between the client and the server will need to be kept confidential, hence a secure VPN is used.
- The VPN client connects to a VPN server via an external network interface.
- The VPN server assigns an IP address to the VPN client from the VPN server's subnet. The client gets internal IP address 192.168.1.50, for example, and creates a virtual network interface through which it will send encrypted packets to the other tunnel endpoint (the device at the other end of the tunnel). (This interface also gets the address 192.168.1.50.)
- When the VPN client wishes to communicate with the company server, it prepares a packet addressed to 192.168.1.10, encrypts it and encapsulates it in an outer VPN packet, say an IPSec packet. This packet is then sent to the VPN server at IP address 22.214.171.124 over the public Internet. The inner packet is encrypted so that even if someone intercepts the packet over the Internet, they cannot get any information from it. They can see that the remote host is communicating with a server/firewall, but none of the contents of the communication. The inner encrypted packet has source address 192.168.1.50 and destination address 192.168.1.10. The outer packet has source address 126.96.36.199 and destination address 188.8.131.52.
- When the packet reaches the VPN server from the Internet, the VPN server decapsulates the inner packet, decrypts it, finds the destination address to be 192.168.1.10, and forwards it to the intended server at 192.168.1.10.
- After some time, the VPN server receives a reply packet from 192.168.1.10, intended for 192.168.1.50. The VPN server consults its routing table, and sees this packet is intended for a remote host that must go through VPN.
- The VPN server encrypts this reply packet, encapsulates it in a VPN packet and sends it out over the Internet. The inner encrypted packet has source address 192.168.1.10 and destination address 192.168.1.50. The outer VPN packet has source address 184.108.40.206 and destination address 220.127.116.11.
- The remote host receives the packet. The VPN client decapsulates the inner packet, decrypts it, and passes it to the appropriate software at upper layers.
Overall, it is as if the remote computer and company server are on the same 192.168.1.0/24 network.
Tunneling protocols can operate in a point-to-point network topology that would theoretically not be considered a VPN, because a VPN by definition is expected to support arbitrary and changing sets of network nodes. But since most router implementations support a software-defined tunnel interface, customer-provisioned VPNs often are simply defined tunnels running conventional routing protocols.
Provider Provisioned VPN (PPVPN) building-blocks 
Depending on whether the PPVPN[clarification needed] runs in layer 2 or layer 3, the building blocks described below may be L2 only, L3 only, or combine them both. Multiprotocol label switching (MPLS) functionality blurs the L2-L3 identity.
- Customer (C) devices
A device that is within a customer's network and not directly connected to the service provider's network. C devices are not aware of the VPN.
- Customer Edge device (CE)
A device at the edge of the customer's network which provides access to the PPVPN. Sometimes it's just a demarcation point between provider and customer responsibility. Other providers allow customers to configure it.
- Provider edge device (PE)
A PE is a device, or set of devices, at the edge of the provider network which connects to customer networks through CE devices and presents the provider's view of the customer site. PEs are aware of the VPNs that connect through them, and maintain VPN state.
- Provider device (P)
A P device operates inside the provider's core network and does not directly interface to any customer endpoint. It might, for example, provide routing for many provider-operated tunnels that belong to different customers' PPVPNs. While the P device is a key part of implementing PPVPNs, it is not itself VPN-aware and does not maintain VPN state. Its principal role is allowing the service provider to scale its PPVPN offerings, for example, by acting as an aggregation point for multiple PEs. P-to-P connections, in such a role, often are high-capacity optical links between major locations of providers.
User-visible PPVPN services 
This section deals with the types of VPN considered in the IETF.
OSI Layer 1 services 
Virtual private wire and private line services (VPWS and VPLS) 
OSI Layer 2 services 
- Virtual LAN
A Layer 2 technique that allows for the coexistence of multiple LAN broadcast domains, interconnected via trunks using the IEEE 802.1Q trunking protocol. Other trunking protocols have been used but have become obsolete, including Inter-Switch Link (ISL), IEEE 802.10 (originally a security protocol but a subset was introduced for trunking), and ATM LAN Emulation (LANE).
- Virtual private LAN service (VPLS)
Developed by IEEE, VLANs allow multiple tagged LANs to share common trunking. VLANs frequently comprise only customer-owned facilities. Whereas VPLS as described in the above section (OSI Layer 1 services) supports emulation of both point-to-point and point-to-multipoint topologies, the method discussed here extends Layer 2 technologies such as 802.1d and 802.1q LAN trunking to run over transports such as Metro Ethernet.
As used in this context, a VPLS is a Layer 2 PPVPN, rather than a private line, emulating the full functionality of a traditional local area network (LAN). From a user standpoint, a VPLS makes it possible to interconnect several LAN segments over a packet-switched, or optical, provider core; a core transparent to the user, making the remote LAN segments behave as one single LAN.
In a VPLS, the provider network emulates a learning bridge, which optionally may include VLAN service.
- Pseudo wire (PW)
PW is similar to VPWS, but it can provide different L2 protocols at both ends. Typically, its interface is a WAN protocol such as Asynchronous Transfer Mode or Frame Relay. In contrast, when aiming to provide the appearance of a LAN contiguous between two or more locations, the Virtual Private LAN service or IPLS would be appropriate.
- IP-only LAN-like service (IPLS)
A subset of VPLS, the CE devices must have L3 capabilities; the IPLS presents packets rather than frames. It may support IPv4 or IPv6.
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. The provider must be able to disambiguate overlapping addresses in the multiple customers' PPVPNs.
- BGP/MPLS PPVPN
In the method defined by RFC 2547, BGP extensions advertise routes in the IPv4 VPN address family, which are of the form of 12-byte strings, beginning with an 8-byte Route Distinguisher (RD) and ending with a 4-byte IPv4 address. RDs disambiguate otherwise duplicate addresses in the same PE.
PEs understand the topology of each VPN, which are interconnected with MPLS tunnels, either directly or via P routers. In MPLS terminology, the P routers are Label Switch Routers without awareness of VPNs.
- Virtual router PPVPN
The Virtual Router architecture, as opposed to BGP/MPLS techniques, requires no modification to existing routing protocols such as BGP. By the provisioning of logically independent routing domains, the customer operating a VPN is completely responsible for the address space. In the various MPLS tunnels, the different PPVPNs are disambiguated by their label, but do not need routing distinguishers.
Plaintext tunnels 
Some virtual networks may not use encryption to protect the data contents. While VPNs often provide security, an unencrypted overlay network does not neatly fit within the secure or trusted categorization. For example a tunnel set up between two hosts that used Generic Routing Encapsulation (GRE) would in fact be a virtual private network, but neither secure nor trusted.
Besides the GRE example above, native plaintext tunneling protocols include Layer 2 Tunneling Protocol (L2TP) when it is set up without IPsec and Point-to-Point Tunneling Protocol (PPTP) or Microsoft Point-to-Point Encryption (MPPE).
Trusted delivery networks 
- Multi-Protocol Label Switching (MPLS) often overlays VPNs, often with quality-of-service control over a trusted delivery network.
- Layer 2 Tunneling Protocol (L2TP) which is a standards-based replacement, and a compromise taking the good features from each, for two proprietary VPN protocols: Cisco's Layer 2 Forwarding (L2F) (obsolete as of 2009[update]) and Microsoft's Point-to-Point Tunneling Protocol (PPTP).
From the security standpoint, VPNs either trust the underlying delivery network, or must enforce security with mechanisms in the VPN itself. Unless the trusted delivery network runs among physically secure sites only, both trusted and secure models need an authentication mechanism for users to gain access to the VPN.
VPNs in mobile environments 
Mobile VPNs are used in a setting where an endpoint of the VPN is not fixed to a single IP address, but instead roams across various networks such as data networks from cellular carriers or between multiple Wi-Fi access points. Mobile VPNs have been widely used in public safety, where they give law enforcement officers access to mission-critical applications, such as computer-assisted dispatch and criminal databases, while they travel between different subnets of a mobile network. They are also used in field service management and by healthcare organizations, among other industries.
Increasingly, mobile VPNs are being adopted by mobile professionals who need reliable connections. They are used for roaming seamlessly across networks and in and out of wireless-coverage areas without losing application sessions or dropping the secure VPN session. A conventional VPN cannot survive such events because the network tunnel is disrupted, causing applications to disconnect, time out, or fail, or even cause the computing device itself to crash.
Instead of logically tying the endpoint of the network tunnel to the physical IP address, each tunnel is bound to a permanently associated IP address at the device. The mobile VPN software handles the necessary network authentication and maintains the network sessions in a manner transparent to the application and the user. The Host Identity Protocol (HIP), under study by the Internet Engineering Task Force, is designed to support mobility of hosts by separating the role of IP addresses for host identification from their locator functionality in an IP network. With HIP a mobile host maintains its logical connections established via the host identity identifier while associating with different IP addresses when roaming between access networks.
See also 
- Opportunistic encryption
- Split tunneling
- Mediated VPN
- Tinc (protocol)
- DMVPN (Dynamic Multipoint VPN)
- Virtual Private LAN Service over MPLS
- Ethernet Virtual Private LAN (EVP-LAN or E-LAN) defined by MEF
- Mason, Andrew G. Cisco Secure Virtual Private Network. Cisco Press, 2002, p. 7
- Microsoft Technet. "Virtual Private Networking: An Overview".
- Cisco Systems, et. al.. Internetworking Technologies Handbook, Third Edition. Cisco Press, 2000, p. 232.
- Lewis, Mark. Comparing, Designing. And Deploying VPNs. Cisco Press, 20069, p. 5
- International Engineering Consortium. Digital Subscriber Line 2001. Intl. Engineering Consortiu, 2001, p. 40.
- Technet Lab. "IPv6 traffic over VPN connections".
- VPN Consortium. "VPN Technologies".
- RFC 6434, "IPv6 Node Requirements", E. Jankiewicz, J. Loughney, T. Narten (December 2011)
- "OpenConnect". Retrieved 2013-04-08. "OpenConnect is a client for Cisco's AnyConnect SSL VPN [...] OpenConnect is not officially supported by, or associated in any way with, Cisco Systems. It just happens to interoperate with their equipment."
- Trademark Applications and Registrations Retrieval (TARR)
- OpenBSD ssh manual page, VPN section
- Unix Toolbox section on SSH VPN
- Ubuntu SSH VPN how-to
- "VPN - Virtual Private Network and OpenVPN".
- "TunTap Project".
- E. Rosen & Y. Rekhter (March 1999). "RFC 2547 BGP/MPLS VPNs". Internet Engineering Task Forc (IETF).
- Lewis, Mark (2006). Comparing, designing, and deploying VPNs (1st print. ed.). Indianapolis, Ind.: Cisco Press. pp. 5–6. ISBN 1587051796.
- Ethernet Bridging (OpenVPN)
- Address Allocation for Private Internets, RFC 1918, Y. Rekhter et al.,February 1996
- RFC 2917, A Core MPLS IP VPN Architecture
- RFC 2918, E. Chen (September 2000)
- Cisco Systems, Inc. (2004). Internetworking Technologies Handbook. Networking Technology Series (4 ed.). Cisco Press. p. 233. ISBN 9781587051197. Retrieved 2013-02-15. "[...] VPNs using dedicated circuits, such as Frame Relay [...] are sometimes called trusted VPNs, because customers trust that the network facilities operated by the service providers will not be compromised."
- Layer Two Tunneling Protocol "L2TP", RFC 2661, W. Townsley et al.,August 1999
- IP Based Virtual Private Networks, RFC 2341, A. Valencia et al., May 1998
- Point-to-Point Tunneling Protocol (PPTP), RFC 2637, K. Hamzeh et al., July 1999
- Phifer, Lisa. "Mobile VPN: Closing the Gap", SearchMobileComputing.com, July 16, 2006.
- Willett, Andy. "Solving the Computing Challenges of Mobile Officers", www.officer.com, May, 2006.
- Cheng, Roger. "Lost Connections", The Wall Street Journal, December 11, 2007.
Further reading 
- Kelly, Sean (August 2001). "Necessity is the mother of VPN invention". Communication News: 26–28. ISSN 0010-3632. Archived from the original on 2001-12-17.
- "VPN Buyers Guide". Communication News: 34–38. August 2001. ISSN 0010-3632.
- JANET UK "Different Flavours of VPN: Technology and Applications"
- Virtual Private Network Consortium - a trade association for VPN vendors
- CShip VPN-Wiki/List
- Virtual Private Networks on Microsoft TechNet
- Creating VPNs with IPsec and SSL/TLS Linux Journal article by Rami Rosen
- curvetun a lightweight curve25519-based multiuser IP tunnel / VPN
- MPLS Telecom Networks
- Using VPN to bypass internet censorship in How to Bypass Internet Censorship, a FLOSS Manual,
http://ezyvpn.com No Logs, Fast, Secure, Cheap
10 March 2011, 240 pp