Gathering Information about Your Current Network Infrastructure

Applies To: Windows Server 2008, Windows Server 2008 R2

Perhaps the most important aspect of planning for Windows Firewall with Advanced Security deployment is the network architecture, because IPsec is layered on the Internet Protocol itself. An incomplete or inaccurate understanding of the network can prevent any Windows Firewall with Advanced Security solution from being successful. Understanding subnet layout, IP addressing schemes, and traffic patterns are part of this effort, but accurately documenting the following components are important to completing the planning phase of this project:

  • Network segmentation. This includes IP addressing maps, showing how your routers separate each network segment. It includes information about how the routers are configured, and what security filters they impose on network traffic flowing through them.

  • Network address translation (NAT). NAT is a means of separating network segments by using a device that maps all of the IP addresses on one side of the device to a single IP address accessible on the other side.

  • Network infrastructure devices. This includes the routers, switches, hubs, and other network equipment that makes communications between the computers on the network possible.

  • Current network traffic model. This includes the quantity and the characteristics of the network traffic flowing through your network.

The goal is to have enough information to be able to identify an asset by its network location, in addition to its physical location.

Do not use a complex and poorly documented network as a starting point for the design, because it can leave too many unidentified areas that are likely to cause problems during implementation.

This guidance helps obtain the most relevant information for planning Windows Firewall with Advanced Security implementation, but it does not try to address other issues, such as TCP/IP addressing or virtual local area network (VLAN) segmentation.

Network segmentation

If your organization does not have its current network architecture documented and available for reference, such documentation should be obtained as soon as possible before you continue with the design and deployment. If the documented information is not current or has not been validated recently, you have two options:

  • Accept that the lack of accurate information can cause risk to the project.

  • Undertake a discovery project, either through manual processes or with network analysis tools that can provide the information you need to document the current network topology.

Although the required information can be presented in many different ways, a series of schematic diagrams is often the most effective method of illustrating and understanding the current network configuration. When creating network diagrams, do not include too much information. If necessary, use multiple diagrams that show different layers of detail. Use a top-level diagram that illustrates the major sites that make up your organization's network, and then break out each site into a more detailed diagram that captures a deeper level of detail. Continue until you reach the individual IP subnet level, and so have the means to identify the network location of every computer in your organization.

During this process, you might discover some network applications and services that are not compatible with IPsec. For example, IPsec breaks network-based prioritization and port/protocol-based traffic management. If traffic management or prioritization must be based on ports or protocol, the host itself must be able to perform any traffic management or prioritization.

Other examples of incompatibility include:

  • Cisco NetFlow on routers cannot analyze packets between IPsec members based on protocol or port.

  • Router-based Quality of Service (QoS) cannot use ports or protocols to prioritize traffic. However, using firewall rules that specify IP addresses to prioritize traffic are not affected by this limitation of QoS. For example, a rule that says "From anyone to anyone using port 80 prioritize" does not work, but a rule that says "From anyone to prioritize" works.

  • Weighted Fair Queuing and other flow-based router traffic priority methods might fail.

  • Devices that do not support or allow IP protocol 50, the port that is used by Encapsulating Security Payload (ESP).

  • Router access control lists (ACLs) cannot examine protocol and port fields in ESP-encrypted packets, and therefore the packets are dropped. ACLs based only on IP address are forwarded as usual. If the device cannot parse ESP, any ACLs that specify port or protocol rules will not be processed on the ESP packets. If the device has an ESP parser and uses encryption, ACLs that specify port or protocol rules will not be processed on the ESP packets.

  • Network monitoring tools might be unable to parse ESP packets that are not encrypted (ESP-Null).


Network Monitor added an ESP parser starting in version 2.1 to aid troubleshooting of unencrypted IPsec packets. The latest version of Network Monitor is available as a free download from Microsoft (

Network address translation (NAT)

Special consideration is required if NAT devices are present and separate some of the network segments from others. NAT blocks the use of Authentication Header (AH) between computers that are separated by a NAT device. If NAT devices exist on the internal network then you must specify ESP instead of AH. ESP allows you to encrypt data, but does not require encryption. ESP can be implemented by using null encryption, which provides the strongest IPsec peer-to-peer communication possible without breaking communications through NAT.

IPsec NAT traversal (NAT-T) enables IPsec peers that are behind NATs to detect the presence of NATs, negotiate IPsec security associations (SAs), and send ESP-protected data even though the addresses in the IPsec-protected IPv4 packets change. IPsec NAT-T does not support the use of AH across NAT devices.

IPsec NAT-T is supported by Windows 7, Windows Vista, Windows Server 2008, Windows Server 2008 R2, Windows Server 2003 with SP1, Windows XP with SP2, and by Windows 2000 Server with SP4 with a free Web download. For more information, see "L2TP/IPsec NAT-T update for Windows XP SP1 and Windows 2000 Server" at This is a client-side update only, and does not enable a computer that is running Windows 2000 Server to receive an incoming IPsec protected connection from a client computer that is behind a NAT device.

For detailed information about how IPsec NAT-T works, see "IPsec NAT Traversal Overview" in the August 2002 Cable Guy article at

Do not put servers behind NAT devices

Do not put servers that must be available to public IPsec clients on the private networks behind NAT devices. Windows XP with SP2 and later operating systems by default do not support establishing IPsec connection to servers that are located on the private network behind a NAT device. For more information, see "IPsec NAT-T is Not Recommended for Windows Server Computers that are Behind Network Address Translators" at

This only affects servers behind the NAT device. The article does not apply to client computers.

For more information about the challenges and risks associated with positioning servers behind NAT devices, see "Problems with Using Network Address Translators" in the Cable Guy article at

If you must locate a server on the private network behind a NAT device, then you must do the following:

  • Configure the Windows clients that must access the server to enable IPsec security associations to servers that are located behind a NAT device. For instructions about how to configure the clients for that scenario, see "L2TP/IPsec NAT-T update for Windows XP SP1 and Windows 2000 Server at The instructions apply to all later service packs of Windows XP, although the download itself is only applicable to SP1. This can be especially challenging if the client computers that need access to the server are not managed by your organization.

  • To ensure that a server is reachable from behind a NAT device for IPsec traffic, you must configure the NAT device with static translation entries that map IKE (using UDP port 500) and IPsec NAT-T (using UDP port 4500) traffic to the correct server.

Network infrastructure devices

The devices that make up the network infrastructure (routers, switches, load balancers, and firewalls) must be able communicate using IPsec after the solution is implemented. For this reason, you have to examine the following characteristics of these network devices to ensure that they can handle the technical and physical requirements of the design:

  • Make/model. You can use this information to determine the features that the device supports. In addition, check the BIOS version or software running on the device to ensure that IPsec is supported.

  • Amount of RAM. This information is useful when you are analyzing capacity or the impact of IPsec on the device.

  • Traffic analysis. Information, such as peak usage and daily orweekly trends, is helpful to have. The information helps provide a baseline snapshot of the device and how it is used over time. If problems occur after IPsec is implemented, the information can help determine whether the root cause is related to greater usage of the device.

  • Router ACLs that affect IPsec directly. ACLs directly affect the ability of specific protocols to function. For example, blocking the Kerberos V5 protocol (UDP and TCP port 88) or IP protocol 50 or 51 prevents IPsec from working. Devices must also be configured to allow IKE traffic (UDP port 500) if using NAT-T (UDP port 4500).

  • Networks/subnets connected to device interfaces. This information provides the best picture of what the internal network looks like. Defining the boundary of subnets based on an address range is straightforward and helps identify whether other addresses are either unmanaged or foreign to the internal network (such as IP addresses on the Internet).

  • VLAN segmentation. Determining how VLANs are implemented on the network can help you understand traffic patterns and security requirements, and then help to determine how IPsec might augment or interfere with these requirements.

  • The maximum transmission unit (MTU) size on device interface(s). The MTU defines the largest datagram that can be transmitted on a particular interface without being divided into smaller pieces for transmission (a process also known as fragmentation). In IPsec communications, the MTU is necessary to anticipate when fragmentation occurs. Packet fragmentation must be tracked for Internet Security Association and Key Management Protocol (ISAKMP) by the router. IPsec configures the MTU size on the session to the minimum-discovered MTU size along the communication path being used, and then set the Don't Fragment bit (DF bit) to 1.


If Path MTU (PMTU) discovery is enabled and functioning correctly, you do not have to gather the MTU size on device interfaces. Although sources, such as the Windows Server 2003 Hardening Guide, recommend disabling PMTU discovery, it must be enabled for IPsec to function correctly.

  • Intrusion detection system (IDS) in use. Your IDS must have an IPsec-compatible parser to interpret data inside secured packets. If the IDS does not have such a parser, it cannot examine data in those packets to determine whether a particular session is a potential threat.

After you obtain this information, you can quickly determine whether you must upgrade the devices to support the requirements of the project, change the ACLs, or take other measures to ensure that the devices can handle the loads needed.

Current network traffic model

After gathering the addressing and network infrastructure information, the next step is to examine the communications flow. For example, if a department such as Human Resources (HR) spans several buildings, and you want to use server isolation with encryption to help protect information in that department, you must know how those buildings are connected to determine the level of "trust" to place in the connection. A highly secured building that is connected by an unprotected cable to another building that is not secured can be compromised by an eavesdropping or information replay attack. If such an attack is considered a threat, IPsec can help by providing strong mutual authentication and traffic encryption for trusted hosts. However, the solution cannot account for the fact that a lack of physical security on trusted hosts will remain a threat.

When you examine traffic flow, look closely at how all managed and unmanaged devices interact. This includes non-Windows-based computers running Linux, UNIX, and Macintosh. Ask yourself such questions as:

  • Do specific communications occur at the port and protocol level, or are there many sessions between the same hosts across many protocols?

  • How do servers and clients communicate with each other?

  • Are there security devices or projects currently implemented or planned that could affect an isolation deployment? For example, if you use Windows Firewall on your computers to "lock down" specific ports, such as UDP 500, IKE negotiations fail.

Some of the more common applications and protocols are as follows:

  • NetBIOS over TCP/IP (NetBT) and server message block (SMB). On a LAN, it is common to have ports 137, 138, and 139 enabled for NetBT and port 445 enabled for SMB. These ports provide NetBIOS name resolution services and other features. Unfortunately, they also allow the creation of null sessions. A null session is a session that is established on a host that does not use the security context of a known user or entity. Frequently, these sessions are anonymous.

  • Remote procedure call (RPC). RPC operates by listening on a port known as the endpoint mapper, TCP port 135. The response to a query on this port is an instruction to begin communication on another port in the ephemeral range (ports numbered over 1024). In a network that is segmented by firewalls, RPC communication presents a configuration challenge because it means opening the RPC listener port and all ports greater than 1024. Opening so many ports increases the attack surface of the whole network and reduces the effectiveness of the firewalls. Computers running Windows 7, Windows Vista, Windows Server 2008, and Windows Server 2008 R2 reduce this risk by introducing stateful inspection of RPC traffic. Because many applications depend on RPC for basic functionality, any firewall and connection security policy must take RPC requirements into account.

  • Other traffic. Windows Firewall with Advanced Security can help secure transmissions between computers by providing authentication of the packets in addition to encrypting the data that they contain. The important thing to do is to identify what must be protected, and the threats that must be mitigated. Examine and model other traffic or traffic types that must be secured.

Next: Gathering Information about Your Active Directory Deployment