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8 Remote Access and Securing and Optimizing the Network
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As its name suggests, a Virtual Private Network (VPN) is a grouping of technologies that facilitate connections across point-to-point networks It enables a VPN client to connect to a VPN server After authentication and authorization, the VPN client computer can access resources on the VPN server s network as if it were locally present Although a VPN is point-topoint from a user s perspective, after the user connects via VPN, he can connect to any resource on the connected network so that it seems like a point-to-many connection A tunnel is created between the client and the VPN server All traf c between the client and the VPN server network is encapsulated in a special type of packet and sent over the tunnel in a compressed, encrypted format This means the internal network does not need many types of exceptions in its rewall; all the traf c is sent over the port used for the VPN no matter what service is accessed on the internal network VPNs are used widely in the industry for people to gain access to resources at the of ce from home, the road, or a customer site This is a Remote Access VPN connection because it provides a single user with remote access to your internal network via VPN, as shown in Figure 8-1
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Computer appears as if on a private link on the internal network thanks to the tunnel provided by the VPN
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FIGURE 8-1 Typical Remote Access VPN The other type of VPN is typically used between corporate locations, and constitutes a site-to-site VPN It is essentially the same process as a Remote Access VPN But consider the case in which a remote of ce with
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20 desktop computers does not have a dedicated line back to corporate Instead, the remote of ce has a business-class broadband connection to the Internet via a typical Internet service provider (ISP) It would be possible for each desktop computer to VPN into the corporate environment, but doing so is a hassle for each user and a waste of connections on the VPN server Instead, a server at the remote site can create a VPN connection to the central VPN server that acts as a router so that all the clients at the remote site can access corporate resources via this single VPN connection In effect, a dedicated WAN link is created between the remote of ce and the central of ce via the VPN connection, as shown in Figure 82 All the clients at the remote site can access resources at the central of ce without taking any special actions
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8 REMOTE ACCESS AND SECURING AND OPTIMIZING THE NETWORK
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FIGURE 8-2 An example of site-to-site VPN The number of situations needing VPN connections has decreased as technology has evolved Because of the many technologies supporting connectivity over typically open ports (such as HTTPS), you now have more options For example, Outlook can now talk to its Exchange server over HTTPS (remote procedure call [RPC] over HTTPS) And, as already mentioned, TS connections can now operate over HTTPS thanks to TS Gateway Many organizations use SharePoint for collaboration and information sharing SharePoint operates as a web site, so it is already Internet friendly But there are still times VPN connections are desirable Unfortunately they are not always available because until now VPN connections operated over speci c ports that corporate locations commonly block due to their potential security implications
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8 Remote Access and Securing and Optimizing the Network
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What are the security implications In essence, when you connect via VPN, you connect via a private link to the network If a vulnerability exists on that VPN network, your client machine is now open to that vulnerability The reverse argument is also valid When you offer a VPN service you need to ensure that your security is airtight As soon as someone VPNs in to your network, it s as if they are sitting at a cubicle inside your building They are virtually plugged in and can access anything that entails Clients with a virus or vulnerability connecting via VPN can spread it over your network as if they were directly connected That is why, as part of your VPN solution, you need to check the health of VPN clients You need to protect your network If many of the VPN connections are to be made from users home computers, you could have a real problem A home computer is far less likely to have up-to-date patches and virus de nitions than your of ce equipment, and probably sees far more colorful web sites thanks to little Billy exposing it to more attacks Because of these security concerns and the importance of the data going over the VPN tunnel, three areas of functionality are offered via VPN solutions The first is encapsulation, as discussed earlier Encapsulation wraps any data for transmission over the VPN with information needed for its routing The form of this encapsulation varies depending on the type of VPN tunnel you use The next area of functionality is authentication, and this is vital You don t want just anyone connecting via your VPN, so you need to be certain whom you are authenticating and maintain this checking in case a VPN session is hijacked over the Internet There are various options available for authentication Point-to-Point Protocol (PPP) is the protocol most commonly used It performs a user authentication at the start to con rm that a user is who he says he is, using an authentication protocol such as Password Authentication Protocol (PAP) or Extensible Authentication Protocol (EAP) Another option is mutual authentication, which is where the VPN server also con rms its identity to the client This avoids the possibility of a server masquerading as the VPN server, harvesting credentials, and potentially giving false information You might have heard of PPP from a dial-up perspective in which it sends data over phone lines and has largely replaced Serial Line Internet Protocol (SLIP) Other types of authentication are possible at a computer-level using Internet Key Exchange (IKE), which uses certi cates or some shared key for mutual authentication A similar approach is data origin authentication in which a cryptographic checksum based on a shared encryption key (shared between the VPN client and server) is added to the data being sent, proving the data s integrity
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