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分类: 网络与安全

2013-08-09 17:06:03

virtual private network (VPN) is a network that uses primarily public telecommunication infrastructure, such as the Internet, to provide remote offices or traveling users an access to a central organizational network.

VPNs typically require remote users of the network to be authenticated, and often secure data with encryption technologies to prevent disclosure of private information to unauthorized parties.

VPNs may serve any network functionality that is found on any network, such as sharing of data and access to network resources, printers, databases, websites, etc. A VPN user typically experiences the central network in a manner that is identical to being connected directly to the central network. VPN technology via the public Internet has replaced the need to requisition and maintain expensive dedicated leased-line telecommunication circuits once typical in wide-area network installations.

Virtual private network technology reduces costs because it does not need physical leased lines to connect remote users to an Intranet.

(注释:虚拟专网,主要是用于通过公用的网络基础设施,比如互联网,连接到中央控制部分,经常采用加密技术保护数据安全,而且可以提供几乎任何网络服务,而且不需要租用物理链路,大大节约了成本)


Types of VPN

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

The following sections discuss some classification schemes.

(注释:VPN系统的类型:隧道传输数据的协议;隧道终端和网络提供终端;是否提供站点到站点或者远程访问的接入服务;提供的安全等级;使用OSI模型的层数,是二层链路层还是三层网络层)

[edit]Security mechanisms

Secure VPNs use cryptographic tunneling protocols to provide:

(注释:安全的VPN采用加密隧道协议,加密数据防止窃听和包捕获,发送认证信息防止身份欺诈;保证消息的完整性防止信息的修改替换)

Secure VPN protocols include the following:

  • IPsec (Internet Protocol Security) was developed by the Internet Engineering Task Force (IETF), and was initially developed for IPv6, which requires it. This standards-based security protocol is also widely used with IPv4Layer 2 Tunneling Protocol frequently runs over IPsec. Its design meets most security goals: authentication, integrity, and confidentiality. IPsec functions by summarizing an IP packet in conjunction with a surrounding packet, and encrypting the outcome.
  • (注释:IPSec原本为IPv6开发,但是也广泛用于IPv4二层隧道协议,满足大部分安全目标,认证,完整性,机密性等。IPSec通过汇总IP包并且包上外围数据包,并且加密结果,IPsec视乎和防火墙规则有些冲突,毕竟是把IP头也封装了
  • 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.
  • (注释:传输层安全可以隧道传输整个网络流量,和在OpenVPN工程中表现的一样,也可以保护一个独立的连接。部分生厂商提供通过SSL远程接入VPN的能力。IPsec运行时会和网络地址传输以及防火墙规则不兼容, SSL VPN可以运行在IPsec有这些问题的地方。)
  • Datagram Transport Layer Security (DTLS), is used in Cisco AnyConnect VPN, to solve the issues SSL/TLS has with tunneling over UDP.
  • (注释:数据包传输层安全【DTLS】,在Cisco AnyConnect VPN中使用,SSL/TLS在隧道UDP时有问题DTLS可以解决这些问题)
  • Microsoft Point-to-Point Encryption (MPPE) works with their Point-to-Point Tunneling Protocol and in several compatible implementations on other platforms.
  • Microsoft's Secure Socket Tunneling Protocol (SSTP), introduced in Windows Server 2008 and inWindows Vista Service Pack 1. SSTP tunnels Point-to-Point Protocol (PPP) or Layer 2 Tunneling Protocol traffic through an SSL 3.0 channel.
  • MPVPN (Multi Path Virtual Private Network). 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 inter-network links. OpenSSH server provides a limited number of concurrent tunnels and the VPN feature itself does not support personal authentication.
  • (注释:微软的MPPE,SSTP【Server2008和Vista SP1引进的,通过SSL3.0通道,二层】,其中SSH,OpenSSL提供VPN隧道【和端口转发不同】,提供安全访问内部网络,但是特性不支持个人认证,而且只有有限的隧道并发数量)

[edit]Authentication

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

User-created remote access VPNs may use passwordsbiometricstwo-factor authentication or other cryptographic methods.

Network-to-network tunnels often use passwords or digital certificates, as they permanently store the key to allow the tunnel to establish automatically and without intervention from the user.

(注释:认证,终端在建立VPN隧道前必须完成认证,用户创建睡到可以通过密码,生物特征、双因素认证或者其他认证技术。网络到网络隧道经常此采用密码或者数字证书,这样可以永久存储下来,这样可以用户不干预的情况下再次自动建立隧道连接)

[edit]Routing

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.

(注释:可以在点对点网络拓扑运行的隧道协议理论上不算是VPN,因为VPN毕竟被期望支持一系列的网络节点。但是既然大部分路由支持一个软件-定义隧道接口,客户认证VPNs经常简单的定义隧道运行在便捷的路由协议上)

[edit]PPVPN building-blocks

Depending on whether the PPVPN (Provider Provisioned VPN) 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.

(本来VPN还分个二层和三层,MPLS的使用模糊了L2_L3的界限)

RFC 4026 generalized the following terms to cover L2 and L3 VPNs, but they were introduced in RFC 2547.[6]

Customer edge device. (CE)

A device at the customer premises, that 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, that 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, as, 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 provider.

User-visible PPVPN services This section deals with the types of VPN considered in the IETF; some historical names were replaced by these terms.

[edit]OSI Layer 1 services

[edit]Virtual private wire and private line services (VPWS and VPLS)

In both of these services, the service provider does not offer a full routed or bridged network, but provides components to build customer-administered networks. VPWS are point-to-point while VPLS can be point-to-multipoint. They can be Layer 1 emulated circuits with no data link .

The customer determines the overall customer VPN service, which also can involve routing, bridging, or host network elements. An unfortunate acronym confusion can occur between Virtual Private Line Service and Virtual Private LAN Service; the context should make it clear whether "VPLS" means the layer 1 virtual private line or the layer 2 virtual private LAN.

[edit]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.[7]

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.

[edit]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.[8] 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,[9][10] 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.

Virtual router architectures do not need to disambiguate addresses, because rather than a PE router having awareness of all the PPVPNs, the PE contains multiple virtual router instances, which belong to one and only one VPN.

[edit]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).

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