A virtual private network ( VPN) extends a private network across a public network, and enables users to send and receive data across shared or public networks as if their computing devices were directly connected to the private network. Applications running across a VPN may therefore benefit from the functionality, security, and management of the private network.
VPN technology was developed to allow remote users and branch offices to securely access corporate applications and other resources. To ensure security, data would travel through secure tunnels and VPN users would use authentication methods – including passwords, tokens and other unique identification methods – to gain access to the VPN. In addition, Internet users may secure their transactions with a VPN, to circumvent Geo-blocking and censorship, or to connect to to protect personal identity and location to stay anonymous on the Internet. However, some Internet sites block access to known VPN technology to prevent the circumvention of their geo-restrictions, and many VPN providers have been developing strategies to get around these roadblocks.
A VPN is created by establishing a virtual point-to-point connection through the use of dedicated connections, virtual tunneling protocols, or traffic encryption. A VPN available from the public Internet can provide some of the benefits of a wide area network (WAN). From a user perspective, the resources available within the private network can be accessed remotely.
Traditional VPNs are characterized by a point-to-point topology, and they do not tend to support or connect , so services such as Microsoft Windows NetBIOS may not be fully supported or work as they would on a local area network (LAN). Designers have developed VPN variants, such as Virtual Private LAN Service (VPLS), and Layer 2 Tunneling Protocols (L2TP), to overcome this limitation.
Early data networks allowed VPN-style remote connections through dial-up modem
or through leased line
connections utilizing Frame Relay
and Asynchronous Transfer Mode (ATM) virtual circuits, provided through networks 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.
[Cisco Systems, et al. Internet working Technologies Handbook, Third Edition. Cisco Press, 2000, p. 232.]
They have been replaced by VPNs based on IP and IP/Multi-protocol Label Switching (MPLS) Networks, due to significant cost-reductions and increased bandwidth
[Lewis, Mark. Comparing, Designing. And Deploying VPNs. Cisco Press, 2006, p. 5]
provided by new technologies such as digital subscriber line (DSL)
[International Engineering Consortium. Digital Subscriber Line 2001. Intl. Engineering Consortium, 2001, p. 40.]
and fiber-optic networks.
VPNs can be either remote-access (connecting a computer to a network) or site-to-site (connecting two networks). In a corporate setting, remote-access VPNs allow employees to access their company's intranet from home or while travelling 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 may be classified by:
the tunneling protocol used to IP tunnel the traffic
the tunnel's termination point location, e.g., on the customer Edge device or network-provider edge
the type of topology of connections, such as site-to-site or network-to-network
the levels of security provided
the OSI model they present to the connecting network, such as Layer 2 circuits or Layer 3 network connectivity
the number of simultaneous connections.
VPNs cannot make online connections completely anonymous, but they can usually increase privacy and security. To prevent disclosure of private information, VPNs typically allow only authenticated remote access using tunneling protocols and encryption techniques.
The VPN security model provides:
confidentiality such that even if the network traffic is sniffed at the packet level (see Packet analyzer and deep packet inspection), an attacker would see only Encryption
sender authentication to prevent unauthorized users from accessing the VPN
message Data integrity to detect any instances of tampering with transmitted messages.
Secure VPN protocols include the following:
Internet Protocol Security (IPsec) was initially developed by the Internet Engineering Task Force (IETF) for IPv6, which was required in all standards-compliant implementations of IPv6 before made it only a recommendation.
[, "IPv6 Node Requirements", E. Jankiewicz, J. Loughney, T. Narten (December 2011)] 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 and SoftEther VPN 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 be authenticated before secure VPN tunnels can be established. User-created remote-access VPNs may use passwords
, 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 administrator.
Tunneling protocols can operate in a point-to-point network topology
that would theoretically not be considered as 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
Depending on whether a provider-provisioned VPN (PPVPN) operates in layer 2 or layer 3, the building blocks described below may be L2 only, L3 only, or combine them both. Multi-protocol label switching (MPLS) functionality blurs the L2-L3 identity.
generalized the following terms to cover L2 and L3 VPNs, but they were introduced in .
More information on the devices below can also be found in Lewis, Cisco Press.
- 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 is 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
OSI Layer 2 services
- Virtual LAN
(VLAN) is a Layer 2 technique that allow for the coexistence of multiple local area network (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 Institute of Electrical and Electronics Engineers, 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 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 VPLS, 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.
- Ethernet over IP tunneling
is an Ethernet over IP tunneling protocol specification. EtherIP has only packet encapsulation mechanism. It has no confidentiality nor message integrity protection. EtherIP was introduced in the FreeBSD
[Glyn M Burton: RFC 3378 EtherIP with FreeBSD, 03 February 2011]
and the SoftEther VPN
[net-security.org news: Multi-protocol SoftEther VPN becomes open source, January 2014]
- IP-only LAN-like service (IPLS)
A subset of VPLS, the CE devices must have Layer 3 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.
[ Address Allocation for Private Internets, , Y. Rekhter et al., February 1996] The provider must be able to disambiguate overlapping addresses in the multiple customers' PPVPNs.
- BGP/MPLS PPVPN
In the method defined by , 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,
[, A Core MPLS IP VPN Architecture] [, E. Chen (September 2000)]
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.
Some virtual networks use tunneling protocols without encryption for protecting the privacy of data. While VPNs often do 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 with Generic Routing Encapsulation (GRE) is a virtual private network, but neither secure nor trusted.
[: Generic Routing Encapsulation over IPv4 networks. October 1994.]
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).
[IETF (1999), , Layer Two Tunneling Protocol "L2TP"]
Trusted delivery networks
Trusted VPNs do not use cryptographic tunneling, and instead rely on the security of a single provider's network to protect the traffic.
Multi-Protocol Label Switching (MPLS) often overlays VPNs, often with quality-of-service control over a trusted delivery network.
[ Layer Two Tunneling Protocol "L2TP", , W. Townsley et al., August 1999] 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) [ IP Based Virtual Private Networks, , A. Valencia et al., May 1998] (obsolete ) and Microsoft's Point-to-Point Tunneling Protocol (PPTP). [ Point-to-Point Tunneling Protocol (PPTP), , K. Hamzeh et al., July 1999]
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
Users utilize mobile virtual private networks in settings 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
[Phifer, Lisa. "Mobile VPN: Closing the Gap", SearchMobileComputing.com, July 16, 2006. ]
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.
[Willett, Andy. "Solving the Computing Challenges of Mobile Officers", www.officer.com, May, 2006. ]
Field service management and by healthcare organizations,
Cheng, Roger. "Lost Connections", The Wall Street Journal, December 11, 2007.
among other industries, also make use of them.
Increasingly, mobile who need reliable connections are adopting mobile VPNs.
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 can not withstand 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 to 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 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.
VPN on routers
With the increasing use of VPNs, many have started deploying VPN connectivity on routers for additional security and encryption of data transmission by using various cryptographic techniques.
Home users usually deploy VPNs on their routers to protect devices, such as smart TV
s or gaming consoles, which are not supported by native VPN clients. Supported devices are not restricted to those capable of running a VPN client.
Many router manufacturers supply routers with built-in VPN clients. Some use open-source firmware such as DD-WRT, OpenWRT and Tomato, in order to support additional protocols such as OpenVPN.
Setting up VPN services on a router requires a deep knowledge of network security and careful installation. Minor misconfiguration of VPN connections can leave the network vulnerable. Performance will vary depending on the ISP.
One major limitation of traditional VPNs is that they are point-to-point, and do not tend to support or connect
. Therefore, communication, software, and networking, which are based on OSI layer
and broadcast Network packet
, such as NetBIOS
used in Windows networking, may not be fully supported or work exactly as they would on a real LAN. Variants on VPN, such as Virtual Private LAN Service (VPLS), and layer 2 tunneling protocols, are designed to overcome this limitation.
A VPN connection may not be as robust as a direct connection to a network. A VPN connection depends on the VPN provider and the ISP. If either fails, the connection fails.