IP Security (IPSec) is another new feature in Windows 2000. IPSec features and implementation details are very complex and are described in detail in a series of RFCs and IETF drafts and in other Microsoft white papers. IPSec uses cryptography-based security to provide access control, connectionless integrity, data origin authentication, protection against replays, confidentiality, and limited traffic-flow confidentiality. Because IPSec is provided at the IP layer, its services are available to the upper-layer protocols in the stack and, transparently, to existing applications.
IPSec enables a system to select security protocols, decide which algorithm(s) to use for the service(s), and establish and maintain cryptographic keys for each security relationship. IPSec can protect paths between hosts, between security gateways, or between hosts and security gateways. The services available and required for traffic are configured using IPSec policy. IPSec policy may be configured locally on a computer or can be assigned through Windows 2000 Group Policy mechanisms using the Active Directory™ services. When using the Active Directory, hosts detect policy assignment at startup, retrieve the policy, and then periodically check for policy updates. The IPSec policy specifies how computers trust each other. IPSec can use either certificates or Kerberos as an authentication method. The easiest trust to use is the Windows 2000 domain trust based on Kerberos. Predefined IPSec policies are configured to trust computers in the same or other trusted Windows 2000 domains.
Each IP datagram processed at the IP layer is compared to a set of filters that are provided by the security policy, which is maintained by an administrator for a computer that belongs to a domain. IP can do one of three things with any datagram:
Provide IPSec services to it.
Allow it to pass unmodified.
An IPSec policy contains a filter, filter action, authentication, tunnel setting, and connection type. For example, two stand-alone computers in the same Windows 2000 domain can be configured to use IPSec between them and activate the secure server policy. If the two computers are not members of the same or a trusted domain, trust must be configured using a certificate or preshared key in a secure server mode by:
Setting up a filter that specifies all traffic between the two hosts
Choosing an authentication method
Selecting a negotiation policy (secure server in this case, indicating that all traffic matching the filter(s) must use IPSec)
Specifying a connection type (LAN, dial-up, or all)
Once the policy has been put in place, traffic that matches the filters uses the services provided by IPSec. When IP traffic (including something as simple as a ping in this case) is directed at one host by another, a Security Association (SA) is established through a short conversation over UDP port 500, through Internet Key Exchange service (IKE), and then the traffic begins to flow. The following network trace illustrates setting up a TCP connection between two such IPSec-enabled hosts. The only parts of the IP datagram that are unencrypted and visible to Netmon after the SA is established are the media access control and IP headers:
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