TechNet網路技術: VPN

摘自: https://technet.microsoft.com/en-us/library/cc739294(v=ws.10).aspx

https://technet.microsoft.com/en-us/library/bb742458.aspx

Synology NAS VPN設定: https://walker-a.com/archives/2501/2

DrayTek FAQ: https://www.draytek.com/zh/faq/faq-vpn/vpn.lan-to-lan/%E5%A6%82%E4%BD%95%E5%BB%BA%E7%AB%8B-vigor-lan-to-lan-ipsec-%E9%80%9A%E9%81%93/

What is VPN ?

VPN Scenarios

Virtual private networks are point-to-point connections across a private or public network such as the Internet. A VPN client uses special TCP/IP-based protocols, called tunneling protocols, to make a virtual call to a virtual port on a VPN server. In a typical VPN deployment, a client initiates a virtual point-to-point connection to a remote access server over the Internet. The remote access server answers the call, authenticates the caller, and transfers data between the VPN client and the organization’s private network.

To emulate a point-to-point link, data is encapsulated, or wrapped, with a header. The header provides routing information that enables the data to traverse the shared or public network to reach its endpoint. To emulate a private link, the data being sent is encrypted for confidentiality. Packets that are intercepted on the shared or public network are indecipherable without the encryption keys. The link in which the private data is encapsulated and encrypted is known as a VPN connection.

A VPN Connection

There are two types of VPN connections:

• Remote access VPN
• Site-to-site VPN

Remote Access VPN

Remote access VPN connections enable users working at home or on the road to access a server on a private network using the infrastructure provided by a public network, such as the Internet. From the user’s perspective, the VPN is a point-to-point connection between the computer (the VPN client) and an organization’s server. The exact infrastructure of the shared or public network is irrelevant because it appears logically as if the data is sent over a dedicated private link.

Site-to-Site VPN

Site-to-site VPN connections (also known as router-to-router VPN connections) enable organizations to have routed connections between separate offices or with other organizations over a public network while helping to maintain secure communications. A routed VPN connection across the Internet logically operates as a dedicated WAN link. When networks are connected over the Internet, as shown in the following figure, a router forwards packets to another router across a VPN connection. To the routers, the VPN connection operates as a data-link layer link.

A site-to-site VPN connection connects two portions of a private network. The VPN server provides a routed connection to the network to which the VPN server is attached. The calling router (the VPN client) authenticates itself to the answering router (the VPN server), and, for mutual authentication, the answering router authenticates itself to the calling router. In a site-to site VPN connection, the packets sent from either router across the VPN connection typically do not originate at the routers.

VPN Connecting Two Remote Sites Across the Internet

VPN Connection Properties

PPTP-based VPN and L2TP/IPSec-based VPN connection properties are described in the following sections.

Encapsulation

VPN technology provides a way of encapsulating private data with a header that allows the data to traverse the network.

Authentication

There are three types of authentication for VPN connections:

User authentication

For the VPN connection to be established, the VPN server authenticates the VPN client attempting the connection and verifies that the VPN client has the appropriate permissions. If mutual authentication is being used, the VPN client also authenticates the VPN server, providing protection against masquerading VPN servers.

The user attempting the PPTP or L2TP/IPSec connection is authenticated using Point-to-Point (PPP)-based user authentication protocols such as Extensible Authentication Protocol-Transport Layer Security (EAP-TLS), Microsoft Challenge-Handshake Authentication Protocol (MS-CHAP), Microsoft Challenge-Handshake Authentication Protocol version 2 (MS-CHAP v2), Shiva Password Authentication Protocol (SPAP), and Password Authentication Protocol (PAP). For PPTP connections, you must use EAP-TLS, MS-CHAP, or MS-CHAP v2. EAP-TLS using smart cards or MS-CHAP v2 is highly recommended, as they provide mutual authentication and are the most secure methods of exchanging credentials.

Computer authentication with L2TP/IPSec

By performing computer-level authentication with IPSec, L2TP/IPSec connections also verify that the remote access client computer is trusted.

Data authentication and integrity

To verify that the data being sent on an L2TP/IPSec VPN connection originated at the other end of the connection and was not modified in transit, L2TP/IPSec packets include a cryptographic checksum based on an encryption key known only to the sender and the receiver.

Data Encryption

Data can be encrypted for protection between the endpoints of the VPN connection. Data encryption should always be used for VPN connections where private data is sent across a public network such as the Internet. Data that is not encrypted is vulnerable to unauthorized interception. For VPN connections, Routing and Remote Access uses Microsoft Point-to-Point Encryption (MPPE) with PPTP and IPSec encryption with L2TP.

Address and Name Server Allocation

When a VPN server is configured, it creates a virtual interface that represents the interface on which all VPN connections are made. When a VPN client establishes a VPN connection, a virtual interface is created on the VPN client that represents the interface connected to the VPN server. The virtual interface on the VPN client is connected to the virtual interface on the VPN server, creating the point-to-point VPN connection.

The virtual interfaces of the VPN client and the VPN server must be assigned IP addresses. The assignment of these addresses is done by the VPN server. By default, the VPN server obtains IP addresses for itself and VPN clients using the Dynamic Host Configuration Protocol (DHCP). Otherwise, a static pool of IP addresses can be configured to define one or more address ranges, with each range defined by an IP network ID and a subnet mask or start and end IP addresses.

Name server assignment, the assignment of Domain Name System (DNS) and Windows Internet Name Service (WINS) servers to the VPN connection, also occurs during the process of establishing the VPN connection

Protocols for Secure Network Communications

Over the past few years, a number of protocols have emerged that are categorized as VPN protocols and that encrypt communications. These include:

• Internet Protocol Security (IPSec)—an architecture, protocol, and related Internet Key Exchange (IKE) protocol, which are described by IETF RFCs 2401-2409.
• Layer 2 Forwarding (IPSec)—created by Cisco Systems.
• Layer 2 Tunneling Protocol (L2TP)—a combination of PPTP and L2F, which evolved through the IETF standards process.
• Point-to-Point Tunneling Protocol (PPTP)—Created by the PPTP Industry Forum (US Robotics(now 3Com), 3Com/Primary Access, Ascend, Microsoft, and ECI Telematics).

While IPSec, L2TP, and PPTP are viewed by many as competing technologies, these protocols offer different capabilities that are appropriate for different uses. To understand this, it is useful to consider the design goals and technical differences of the protocols.

IPSec Design Goals and Overview

IPSec provides integrity protection, authentication, and (optional) privacy and replay protection services for IP traffic. IPSec packets are of two types:

• IP protocol 50 called the Encapsulating Security Payload (ESP) format, which provides privacy, authenticity, and integrity.
• IP protocol 51 called the Authentication Header (AH) format, which only provides integrity and authenticity for packets, but not privacy

IPSec can be used in two modes; transport mode which secures an existing IP packet from source to destination, and tunnel mode which puts an existing IP packet inside a new IP packet that is sent to a tunnel end point in the IPSec format. Both transport and tunnel mode can be encapsulated in ESP or AH headers.

IPSec transport mode was designed to provide security for IP traffic end-to-end between two communicating systems, for example to secure a TCP connection or a UDP datagram. IPSec tunnel mode was designed primarily for network midpoints, routers, or gateways, to secure other IP traffic inside an IPSec tunnel that connects one private IP network to another private IP network over a public or untrusted IP network (for example, the Internet). In both cases, a complex security negotiation is performed between the two computers through the Internet Key Exchange (IKE), normally using PKI certificates for mutual authentication.

The IETF RFC IPSec tunnel protocol specifications did not include mechanisms suitable for remote access VPN clients. Omitted features include user authentication options or client IP address configuration. To use IPSec tunnel mode for remote access, some vendors chose to extend the protocol in proprietary ways to solve these issues. While a few of these extensions are documented as Internet drafts, they lack standards status and are not generally interoperable. As a result, customers must seriously consider whether such implementations offer suitable multi-vendor interoperability.

L2TP Design Goals and Overview

L2TP is a mature IETF standards track protocol that has been widely implemented. L2TP encapsulates Point-to-Point Protocol (PPP) frames to be sent over IP, X.25, frame relay, or asynchronous transfer mode (ATM) networks. When configured to use IP as its transport, L2TP can be used as a VPN tunneling protocol over the Internet. L2TP over IP uses UDP port 1701 and includes a series of L2TP control messages for tunnel maintenance. L2TP also uses UDP to send L2TP-encapsulated PPP frames as the tunneled data. The encapsulated PPP frames can be encrypted or compressed. When L2TP tunnels appear as IP packets, they take advantage of standard IPSec security using IPSec transport mode for strong integrity, replay, authenticity, and privacy protection. L2TP was specifically designed for client connections to network access servers, as well as for gateway-to-gateway connections. Through its use of PPP, L2TP gains multi-protocol support for protocols such as IPX and Appletalk. PPP also provides a wide range of user authentication options, including CHAP, MS-CHAP, MS-CHAPv2 and Extensible Authentication Protocol (EAP) that supports token card and smart card authentication mechanisms. L2TP/IPSec therefore provides well-defined and interoperable tunneling, with the strong and interoperable security of IPSec. It is a good solution for secure remote access and secure gateway-to-gateway connections.

PPTP Design Goals and Overview

PPTP was designed to provide authenticated and encrypted communications between a client and a gateway or between two gateways—without requiring a public key infrastructure—by using a user ID and password. It was first delivered in 1996, two years before the availability of IPSec and L2TP. The design goal was simplicity, multiprotocol support, and ability to traverse a broad range of IP networks. The Point-to-Point Tunneling Protocol (PPTP) uses a TCP connection for tunnel maintenance and Generic Routing Encapsulation (GRE) encapsulated PPP frames for tunneled data. The payloads of the encapsulated PPP frames can be encrypted and/or compressed. The use of PPP provides the ability to negotiate authentication, encryption, and IP address assignment services.