Virtual private network

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VPN connectivity overview

A virtual private network (VPN) extends a private network across a public network, such as the Internet. It enables a computer to send and receive data across shared or public networks as if it were directly connected to the private network, while benefiting from the functionality, security and management policies of the private network.[1] This is done by establishing a virtual point-to-point connection through the use of dedicated connections, encryption, or a combination of the two.

A VPN connection across the Internet is similar to 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.[2]

VPNs allow employees to securely access their company's intranet while traveling outside the office. Similarly, VPNs securely and cost-effectively connect geographically disparate offices of an organization, creating one cohesive virtual network. VPN technology is also used by ordinary Internet users to connect to proxy servers for the purpose of protecting one's identity.

Types[edit]

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. These networks are not considered true VPNs because they passively secure the data being transmitted by the creation of logical data streams.[3] They have given way to VPNs based on IP and IP/Multiprotocol Label Switching Networks (MPLS), due to significant cost-reductions and increased bandwidth[4] provided by new technologies such as Digital Subscriber Line (DSL)[5] 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.[6]

VPN systems may be classified by:

Security mechanisms[edit]

To prevent disclosure of private information, VPNs typically allow only authenticated remote access and make use of encryption techniques.

VPNs provide security by the use of tunneling protocols and through security procedures[7] such as encryption. The VPN security model provides:

Secure VPN protocols include the following:

Authentication[edit]

Tunnel endpoints must authenticate before secure VPN tunnels can be established.

User-created remote-access VPNs may use passwords, biometrics, two-factor authentication or other cryptographic methods.

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[edit]

The following steps[14] illustrate the principles of a VPN client-server interaction in simple terms are:

Assume a remote host with public IP address 1.2.3.4 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 5.6.7.8 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.

  1. The VPN client connects to a VPN server via an external network interface.
  2. 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).[15] (This interface also gets the address 192.168.1.50.)
  3. 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 5.6.7.8 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 1.2.3.4 and destination address 5.6.7.8.
  4. When the packet reaches the VPN server from the Internet, the VPN server extracts 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.
  5. 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.
  6. 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 5.6.7.8 and destination address 1.2.3.4.
  7. The remote host receives the packet. The VPN client extracts 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.

Routing[edit]

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 building-blocks[edit]

Depending on whether a provider-provisioned VPN (PPVPN)[clarification needed] operates 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.[citation needed][original research?]

RFC 4026 generalized the following terms to cover L2 and L3 VPNs, but they were introduced in RFC 2547.[16] More information on the devices below can also be found in Lewis, Cisco Press.[17]

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[edit]

This section deals with the types of VPN considered in the IETF.

OSI Layer 2 services[edit]

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.[18]

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[edit]

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.[19] 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,[20][21] 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[edit]

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[edit]

Trusted VPNs do not use cryptographic tunneling, and instead rely on the security of a single provider's network to protect the traffic.[22]

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[edit]

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.[26] 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.[27] They are also used in field service management and by healthcare organizations,[28] among other industries.

Increasingly, mobile VPNs are being adopted by mobile professionals who need reliable connections.[28] 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,[26] or fail, or even cause the computing device itself to crash.[28]

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.[26] 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[edit]

References[edit]

  1. ^ Mason, Andrew G. Cisco Secure Virtual Private Network. Cisco Press, 2002, p. 7
  2. ^ Microsoft Technet. "Virtual Private Networking: An Overview". 
  3. ^ Cisco Systems, et al.. Internet working Technologies Handbook, Third Edition. Cisco Press, 2000, p. 232.
  4. ^ Lewis, Mark. Comparing, Designing. And Deploying VPNs. Cisco Press, 20069, p. 5
  5. ^ International Engineering Consortium. Digital Subscriber Line 2001. Intl. Engineering Consortium, 2001, p. 40.
  6. ^ Technet Lab. "IPv6 traffic over VPN connections". 
  7. ^ VPN Consortium. "VPN Technologies". 
  8. ^ RFC 6434, "IPv6 Node Requirements", E. Jankiewicz, J. Loughney, T. Narten (December 2011)
  9. ^ "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." 
  10. ^ Trademark Applications and Registrations Retrieval (TARR)
  11. ^ OpenBSD ssh manual page, VPN section
  12. ^ Unix Toolbox section on SSH VPN
  13. ^ Ubuntu SSH VPN how-to
  14. ^ "VPN - Virtual Private Network and OpenVPN". 
  15. ^ "TunTap Project". 
  16. ^ E. Rosen & Y. Rekhter (March 1999). "RFC 2547 BGP/MPLS VPNs". Internet Engineering Task Forc (IETF). 
  17. ^ Lewis, Mark (2006). Comparing, designing, and deploying VPNs (1st print. ed.). Indianapolis, Ind.: Cisco Press. pp. 5–6. ISBN 1587051796. 
  18. ^ Ethernet Bridging (OpenVPN) 
  19. ^ Address Allocation for Private Internets, RFC 1918, Y. Rekhter et al.,February 1996
  20. ^ RFC 2917, A Core MPLS IP VPN Architecture
  21. ^ RFC 2918, E. Chen (September 2000)
  22. ^ 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." 
  23. ^ Layer Two Tunneling Protocol "L2TP", RFC 2661, W. Townsley et al.,August 1999
  24. ^ IP Based Virtual Private Networks, RFC 2341, A. Valencia et al., May 1998
  25. ^ Point-to-Point Tunneling Protocol (PPTP), RFC 2637, K. Hamzeh et al., July 1999
  26. ^ a b c Phifer, Lisa. "Mobile VPN: Closing the Gap", SearchMobileComputing.com, July 16, 2006.
  27. ^ Willett, Andy. "Solving the Computing Challenges of Mobile Officers", www.officer.com, May, 2006.
  28. ^ a b c Cheng, Roger. "Lost Connections", The Wall Street Journal, December 11, 2007.

Further reading[edit]

External links[edit]