Published: June 2003




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Internet Protocol Security (IPsec)


Windows Server 2003 includes support for Internet Protocol security (IPsec). IPsec features and implementation details are very complex and are described in a series of RFCs, Internet drafts, and in other Microsoft technical papers. For more information, see the Windows Server 2003 IPsec Web page.

IPsec uses cryptography-based security to provide data integrity, data origin authentication, replay protection, data confidentiality (encryption), 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 Group Policy mechanisms using the Active Directory™ directory service. When using 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 certificates, Kerberos, or preshared keys as authentication methods. The easiest trust to use is the Windows Server 2003 domain trust based on Kerberos.

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:



  • Apply IPsec protections to it.

  • Allow it to pass unmodified.

  • Discard it.

An IPsec policy contains one or more rules, each of which contain a filter, a filter action, authentication methods, a tunnel setting, and a connection type. For example, two stand-alone computers 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 a host directs IP traffic to another host (including something as simple as ping traffic), a Security Association (SA) is established through an Internet Key Exchange (IKE) negotiation using UDP port 500, 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 Network Monitor after the SA is established are the MAC and IP headers:

Source IP Dest IP Prot Description

192.168.10.47 10.197.14.91 UDP Src Port: ISAKMP, (500); Dst Port: ISAKMP (500); Length = 216 (0xD8)

10.197.14.91 192.168.10.47 UDP Src Port: ISAKMP, (500); Dst Port: ISAKMP (500); Length = 216 (0xD8)

192.168.10.47 10.197.14.91 UDP Src Port: ISAKMP, (500); Dst Port: ISAKMP (500); Length = 128 (0x80)

10.197.14.91 192.168.10.47 UDP Src Port: ISAKMP, (500); Dst Port: ISAKMP (500); Length = 128 (0x80)

192.168.10.47 10.197.14.91 UDP Src Port: ISAKMP, (500); Dst Port: ISAKMP (500); Length = 76 (0x4C)

10.197.14.91 192.168.10.47 UDP Src Port: ISAKMP, (500); Dst Port: ISAKMP (500); Length = 76 (0x4C)

192.168.10.47 10.197.14.91 UDP Src Port: ISAKMP, (500); Dst Port: ISAKMP (500); Length = 212 (0xD4)

10.197.14.91 192.168.10.47 UDP Src Port: ISAKMP, (500); Dst Port: ISAKMP (500); Length = 172 (0xAC)

192.168.10.47 10.197.14.91 UDP Src Port: ISAKMP, (500); Dst Port: ISAKMP (500); Length = 84 (0x54)

10.197.14.91 192.168.10.47 UDP Src Port: ISAKMP, (500); Dst Port: ISAKMP (500); Length = 92 (0x5C)

192.168.10.47 10.197.14.91 IP ID = 0xC906; Proto = 0x32; Len: 96

10.197.14.91 192.168.10.47 IP ID = 0xA202; Proto = 0x32; Len: 96

192.168.10.47 10.197.14.91 IP ID = 0xCA06; Proto = 0x32; Len: 88

Opening one of the IP datagrams sent after the SA is established reveals very little of what is actually in the datagram (a TCP SYN, or connection request). The only clear parts of the packet are the Ethernet and IP headers. Even the TCP header is encrypted and cannot be parsed by Network Monitor if ESP is used.

Src IP Dest IP Protoc Description

===================================================

192.168.10.47 10.197.14.91 IP ID = 0xC906; Proto = 0x32;
Len: 96

FRAME: Base frame properties

ETHERNET: ETYPE = 0x0800 : Protocol = IP: DOD Internet Protocol

IP: ID = 0xC906; Proto = 0x32; Len: 96

IP: Version = 4 (0x4)

IP: Header Length = 20 (0x14)

IP: Precedence = Routine

IP: Type of Service = Normal Service

IP: Total Length = 96 (0x60)

IP: Identification = 51462 (0xC906)

IP: Flags Summary = 2 (0x2)

IP: Fragment Offset = 0 (0x0) bytes

IP: Time to Live = 128 (0x80)

IP: Protocol = 0x32

IP: Checksum = 0xD55A

IP: Source Address = 192.168.10.47

IP: Destination Address = 10.197.14.91

IP: Data: Number of data bytes remaining = 76 (0x004C)

Using a secure server policy also restricts all other types of traffic from reaching destinations that do not understand IPsec or are not part of the same trusted group. A secure initiator policy provides settings that apply best to servers; traffic security is attempted, but if the client does not understand IPsec, the negotiation falls back to sending clear text packets.

When IPsec is used to encrypt data, network performance generally drops, due to the processing overhead of encryption. One possible method for reducing the impact of this overhead is to offload the processing to a hardware device. Because NDIS 5.1 supports task offloading, it is feasible to include encryption hardware on NICs. NICs supporting IPsec hardware offload are available from several vendors.

IPsec promises to be popular for protecting both public network traffic and internal corporate or government traffic that requires confidentiality. One common deployment is to apply secure server IPsec policies only to specific servers that are used to store and/or serve confidential information.

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Published: June 2003

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