Network Design Manual: Cisco Network Admission Control, Volume II: NAC Deployment and Troubleshooting

Chapter 1: NAC Solution and Technology Overview

This chapter covers the following topics:

One of the biggest challenges corporations face today is securing the internal network. When the words network security are mentioned, most people immediately associate this phrase with protecting their network from external threats. Few people think of the internal threats that already exist. Unpatched end-host systems, out-of-date antivirus signatures, and disabled or nonexistent personal firewalls all weaken the internal security of corporate networks and make them vulnerable to data theft and attacks. Preventing or limiting these hosts' access to the corporate network has been difficult to do until now.

Cisco Systems has launched the Self-Defending Network Initiative (SDNI) to dramatically improve the network's capability to identify, prevent, and adapt to threats. A key part of this initiative is Network Admission Control (NAC) [NWC editor's note: Not to be confused with Network Access Control, also known as NAC]. NAC is a multipart solution that validates the security posture of the endpoint before admitting it on the network. If admitted, NAC can also be used to define what resources the endpoint has access to, based on the endpoint's overall security posture.

This chapter is meant to provide you with an overall review of the NAC Framework solution. We start by covering what NAC is and why companies would want to deploy it. Then we cover an architectural overview of the initial NAC solution (NAC Phase I), followed by an architectural overview of the current NAC solution (NAC Phase II). In the remainder of the chapter, we provide an overview of the individual components that make up NAC. Each component has a dedicated chapter in this book where we cover the installation, configuration, and steps to troubleshoot that component in the NAC solution. After reading this chapter, you should be familiar with the concepts and components that make up the NAC Framework and should be ready to start installing and configuring NAC in your network.If you are unfamiliar with NAC or are interested in learning more about the architecture of the NAC solution, we invite you to read Cisco Network Admission Control, Volume I: NAC Architecture and Design (ISBN 1587052415), published by Cisco Press.

Reports of data and identity theft have become hot topics in the news recently. Unfortunately, they have also become fairly common, often resulting in millions of dollars' worth of damage to the companies affected. Traditionally, network security professionals have focused much of their time securing the front door to their networked companies—their Internet presence. Stateful firewalls often sit at the gateways, and, in most cases, these are supplemented with inline intrusion-prevention devices (IPS), antivirus scanners, and denial-of-service (DoS) mitigation devices. Behind this virtual fortress of protection sit hardened servers, which serve up the corporate web presence. Many companies are proud of their investment in this type of security and advertise this fact. Now, don't get me wrong—this type of security is important. However, sometimes in the zeal to make the web presence secure, we forget that a huge threat exists from within.It is becoming mandatory these days for employees to have access to the Internet; often it is a critical component of their jobs. However, have you thought about devices that your employees are using to access the Internet? How secure are they? If they are corporate assets, they should have the corporate antivirus software installed and possibly a personal firewall. But how do you know the employee has not disabled one or more of these and thereby reduced the security of not only the device, but also your internal network, and opened it up to threats?

While you are pondering that thought, let me give you another. How many noncorporate assets connect to your network? How many employees bring in their personal laptop, their personal digital assistant (PDA), or even their cell phone and connect it to the corporate network? What about partners and outside vendors? How much control do you have over these devices? Imagine what could happen if a rootkit or some other Trojan back door was installed on one of these devices and now has access to your internal network. How many confidential documents or corporate secrets could be stolen by attackers within?

It is often easier to consider the mistakes or ignorance of others, but how many times have you been guilty of letting the security of your own PC lapse? How many times have you been notified of a new critical security patch for your laptop or desktop and clicked the Not Now button, choosing instead to install it later? I am sure all of us are guilty of this; I know I am.

Installing security patches, especially to the operating system, usually results in the mandatory reboot. This usually comes at the worst time of the day, when shutting down your applications and rebooting is not an option. So we make a mental note to install the patches when we leave for the day, but how many times do we actually follow through? More often than not, weeks or months could go by before we find the time to install the patches. During this time, the PC remains susceptible to the targeted attack.

Although I have highlighted only a few of the common threats to the internal security of your corporate network, I am sure you can think of many more. Home users connecting via a VPN tunnel, remote sales forces connecting from the local hotspot or hotel, partners with direct site-to-site tunnels to your company—the list goes on. These are the types of threats NAC was designed to protect you against and eliminate.NAC is a Cisco-led, multivendor initiative focused on eliminating threats to the corporate network caused by insecure endpoints attaching to the network. In its simplest form, NAC defines a set of policies that are used to evaluate the security posture of an endpoint that wants to join the network. The endpoint can be a PC, a PDA, a server, an IP phone, a printer, and so on. Based on the security posture of the endpoint, it can be given unrestricted access to the network—if it meets all the security requirements. Devices that fail to fully satisfy the security requirements can be quarantined where autoremediation ensues. (Remediation servers can automatically push out patches and updates to software running on the endpoints to improve their security posture.) Alternatively, devices can be denied access to the network altogether, or they can be placed in their own VLAN and given limited access to the network. All of these actions are fully configurable, along with the security policy to be enforced.

Cisco rolled out NAC in a series of phases. Phase I was launched in the summer of 2004. It includes using Cisco routers as the enforcement point, running Cisco IOS Release 12.3(8)T or later. When NAC is deployed on Cisco IOS routers, it is called NAC-L3-IP because the router operates at Layer 3 (the IP layer) and contains noncompliant endpoints using Layer 3 Access Control Lists (ACLs). As endpoints attempt to access devices through the router, they are queried to determine their security posture. Based on the endpoint's security posture, a security policy for the endpoint is pushed down to the router that permits or restricts access. Figure 1-1 shows a NAC-L3-IP architecture overview.

Follow along in Figure 1-1 as we walk through each step of this process:

The protocols used in Figure 1-1 are discussed in more detail in later chapters. For now, it is important to know only that the posture credentials and security policy are carried over authenticated and encrypted tunnels for added security. Figure 1-2 illustrates the relationship among these protocols in a graphical way. The PEAP-encrypted tunnel is carried over both the RADIUS and EAPoUDP tunnels. It contains the EAP-TLVs used to determine the host's posture.Cisco launched NAC Phase II in the summer of 2005. Phase II expands on Phase I by placing NAC capabilities into several more product lines, including the Catalyst line of switches, the VPN 3000 series concentrators, the ASA 5500 series and PIX 500 series security appliances, the Aironet wireless access points, and the Wireless LAN Service Module. With these new additions, the enforcement point has moved to the network edge, providing enforcement and containment at a port (or host) level instead of at the gateway. These additions also created some new terminology:

When NAC-L2-802.1X is enabled and a PC is connected to a switch port, 802.1X authentication and posture validation occur within the same EAP transaction. The posture credentials are included within the EAP-FAST messages that are transmitted on top of the 802.1X protocol. However, unlike NAC-L3-IP and NAC-L2-IP, posture enforcement is done not through ACLs but instead solely through VLAN assignment.

Figure 1-3 illustrates NAC-L2-802.1X on a switch that uses 802.1X authentication as the Layer 2 protocol.

Follow along in Figure 1-3 as we walk through the process of what happens when an endpoint connects to a switch with NAC-L2-802.1X enabled on the port:

Periodic revalidations are built into the NAC-L3-IP and NAC-L2-IP solution. The network-access device (NAD) initiates the process by periodically polling validated endpoints to determine whether a change has been made in their posture. CTA alerts the NAD of any changes on the end host, and the NAD then issues a full revalidation and posture assessment.

This security measure prevents users from validating their host and then lowering their security posture after they have been granted access to the network.

Additionally, a separate revalidation timer requires all active hosts to be fully revalidated every 30 minutes, by default. This enables the network administrator to change the security policy on the fly. All already-validated end hosts must meet this new policy when their revalidation timer expires. The following example further illustrates this point:

Bob, the network administrator of example.com, receives a new alert about a critical security vulnerability in Microsoft Windows. Realizing the security impact that this vulnerability might have on his network, Bob immediately modifies his NAC security policy to require the hotfix that addresses this vulnerability to be applied on all end hosts on his network. Because it is during the day, most users validated their machines on the network when they arrived in the morning. Without periodic revalidation, Bob would have to wait until each user disconnects and reconnects to the network before the endpoint is revalidated. However, the revalidation timer solves this by requiring all active, validated hosts to be fully revalidated every 30 minutes (by default).

NAC Agentless Hosts

A NAC agentless host (NAH) (or a clientless endpoint) is a device that does not have CTA installed. Therefore, it cannot respond to the EAPoUDP or EAP-FAST request from the NAD. A printer, a webcam, an IP phone, and a guest PC are all examples of NAHs.Individual policies can be defined on the NAD for NAHs. The policy can be designed to exclude a specific MAC or IP address or a range of addresses. Alternatively, a global policy can be defined on Cisco Secure ACS for NAHs. After the EAPoUDP or EAP-FAST session times out, the NAD can notify Cisco Secure ACS of the NAH, and Cisco Secure ACS can apply the appropriate authorization rights. We look at NAHs in more detail in Chapters 4, "Configuring Layer 2 NAC on Network-Access Devices," through 8, "Cisco Secure Access Control Server."

Another option for NAHs (which is part of NAC Phase II) is to use an audit server to scan the host for the services running on it and potential vulnerabilities. Cisco Secure ACS instructs the audit server on which hosts to scan by using the Generic Authorization Message Exchange (GAME) protocol. When the scan is complete, the audit server returns the results to Cisco Secure ACS through the GAME protocol, and Cisco Secure ACS uses these results to apply a security posture and overall policy to the end host.

NAC Program Participants

Cisco Systems leads the NAC program, but is open to any vendor that wants to participate. To ensure interoperability, Cisco requires all vendors shipping NAC-enabled code to have it tested either by an independent third-party testing center or by Cisco Systems. At the time of publication, more than 75 vendors were enrolled in the NAC program. A current list of program participants is maintained by Cisco at http://www.cisco.com/web/partners/pr46/nac/partners.html.

The following sections examine the individual components that make up the NAC Framework solution. Although only an overview is provided here, each component is covered in detail in its associated chapter in this book.

Cisco Trust Agent

Cisco Trust Agent (CTA) is a small software application (approximately 3MB) that is installed locally on a PC and that allows Cisco Secure ACS to communicate directly with the PC to query it for posture credentials. Some common posture credentials are the OS name, the service pack installed, and specific hotfixes applied. Table 1-1 lists the posture credentials that CTA supports or for which CTA is a broker.CTA is a core component of NAC and is the only communications interface between the NAD and the applications that reside on the PC. It receives posture credential queries from Cisco Secure ACS, brokers them to the correct application, and then forwards the application responses to Cisco Secure ACS. CTA has three key responsibilities (see Figure 1-4):

CTA also includes an 802.1X supplicant bundled with it that supports EAP-FAST when running NAC. The 802.1X supplicant is needed to implement NAC-L2-802.1X. However, the 802.1X supplicant is limited to wired interfaces only—no wireless interfaces.

Chapter 2, "Cisco Trust Agent," fully covers the installation, configuration, operation, and troubleshooting of CTA.

Cisco Security Agent (CSA) is the Cisco award-winning host-based intrusion-prevention system (HIPS) installed on a desktop or server PC that protects it from known and unknown threats. CSA adds a shim into the network layer and into the kernel layer (to watch both network traffic and API calls to kernel). This allows CSA to not only be a personal firewall, but also to protect against buffer-overflow attacks and spyware/adware. In addition, it provides file protection, malicious application protection, and operating-system integrity protection. CSA is one of the few HIPS products that provide true protection against "Day Zero" attacks.

Starting with version 4.5, CSA integrates seamlessly with NAC through CTA. CTA queries CSA to establish the presence of the agent and determine whether it is in protect mode. This information is part of the posture credentials returned to Cisco Secure ACS and is used to determine the end host's overall security posture.

Based on this posture, Cisco Secure ACS can apply a policy that alters the state of CSA. CSA's state change dynamically activates additional rules within CSA, thereby providing another level of protection to the host.Cisco Security Agent Management Center (CSA MC) provides a powerful, scalable application used to manage all agents. When an agent is installed on a host, it first registers with CSA MC and downloads any updates to its rule set. Thereafter, the agent periodically polls CSA MC to check for any new software or rule updates. Besides the configuration and software update function, CSA MC receives real-time security events from the agents and immediately displays them in the Event Monitor for the network administrator to see. In addition, CSA MC correlates the events, received from all agents in the network, to detect suspicious activity across several hosts. If similar threats are detected across several agents, CSA MC creates and deploys dynamic rules to all the agents to provide an additional layer of protection against this newly spreading threat.

Chapter 9, "Cisco Security Agent," covers the installation, configuration, and operation of CSA.

See Chapter 9 for installation and configuration information about CSA and CSA MC.

NADs query the CTA installed on the endpoint. In NAC Phase I, the NAD could be only an IOS router. In Phase II, any of the following devices can be a NAD:

Cisco IOS Router

Cisco IOS routers first supported NAC in Cisco IOS Release 12.3(8)T, in the Advanced Security, Advanced IP Services, or Advanced Enterprise Services feature sets. Table 1-2 lists Cisco IOS routers by platform and current NAC capability.

Table 1-2NAC Support in IOS Routers

When NAC is implemented on a router, this is called NAC-L3-IP. That is, the security enforcement point becomes the Layer 3 gateway instead of the physical port into which the end host is plugged.

Posture validation is triggered by defining an intercept ACL on the router's interface. Any traffic arriving on the interface from a nonpostured source that matches the intercept ACL triggers the posture-validation process, as illustrated in Figure 1-1. When the overall security posture of the host is determined, Cisco Secure ACS sends a host-based downloadable ACL to the router to restrict, prohibit, or permit that client's access to the network. Thus, policy enforcement takes place at Layer 3 with an ACL on the router's interface.

See Chapter 5, "Configuring Layer 3 NAC on Network-Access Devices," for more information on configuring and troubleshooting NAC on a Cisco IOS router.

Catalyst switches first supported NAC in the summer of 2005 across various platforms and release trains. One benefit of adding NAC on the switch is enhanced posture-enforcement capabilities through containment. On Cisco IOS routers, policy enforcement was applied with a downloadable ACL on the router's interface. This enabled the administrator to restrict (or even deny) the endpoint's access through the router. However, the endpoint could not be restricted from sending packets to Layer 2–adjacent devices (because those packets did not traverse the router and, therefore, would not be subject to the downloadable ACL). However, on access switches, the endpoints are typically directly connected to a physical port on the switch. This allows for policy enforcement (through VLAN or ACL) as well as containment (because the endpoint is typically the only device connected to that port).

Catalyst switches can implement NAC on a per-port basis at Layer 2 or Layer 3. As mentioned previously in this chapter, when NAC is implemented at Layer 2, it is known as NAC-L2-802.1X because 802.1X is used as the underlying Layer 2 transport protocol. When NAC is implemented on a switch at Layer 3, it is known as NAC-L2-IP.NAC-L2-802.1X and NAC-L2-IP have several administrative and operational differences that you should fully consider before selecting which one to deploy.

The following are attributes of NAC-L2-802.1X:

The following are attributes of NAC-L2-IP:

No "right" or "wrong" choice exists between the two. But there is a best choice for your network. If you don't know what that choice is, read Cisco Network Admission Control, Volume I: NAC Architecture and Design, which walks through several design scenarios, discusses the options available, and provides the rationale for the choices made.

An additional consideration (and probably the most important one) is which one will run on your existing switch hardware. Table 1-3 should come in handy in making that determination; it lists the various models of Catalyst switches and their NAC capabilities based on the OS.

Catalyst switches are an integral part of the NAC solution, providing protection and containment of hosts that do not meet corporate security policies at the access layer. As such, Cisco is committed to providing NAC support on all new switch hardware.

See Chapter 4 for more information on configuring and troubleshooting NAC on a Catalyst switch.

NAC support for the VPN 3000 series concentrators was first added in Release 4.7. The concentrator is a Layer 3 NAD and postures remote-access IPSec (or Layer 2 Tunneling Protocol [L2TP] over IPSec) clients. The posturing process is almost identical to that of NAC-L3-IP, described previously in the section "NAC: Phase I" (refer to Figure 1-1). The only difference is that the router is replaced with a VPN 3000 concentrator, and an IPSec tunnel is first established to the concentrator before the EAPoUDP session starts.

When the EAPoUDP session starts, a PEAP session is established between the client and the Cisco Secure ACS so posture validation can take place. Cisco Secure ACS then notifies the concentrator (through RADIUS) of the client's posture and passes down a filter list to be applied to the client. The filter list is the 3000's equivalent to a downloadable ACL.

One unique option that the concentrator provides is that clients can be excluded from posture validation based solely on OS type. This is because the Cisco VPN client sends its OS information during IPSec tunnel establishment, which occurs before NAC posture validation. Host exemption, along with all other NAC configuration, is specified under the group policy settings on the 3000. NAC configuration on the VPN 3000 concentrator is covered in detail in Chapter 6, "Configuring NAC on Cisco VPN 3000 Series Concentrators."

Cisco ASA 5500 Series Adaptive Security Appliance and PIX 500 Series Security Appliance

The NAC implementation on the Cisco 5500 series Adaptive Security Appliances (ASA) and PIX 500 series security appliances is identical to the implementation on the VPN 3000 concentrators. NAC-L3-IP is supported starting with Version 7.2(1) on all IPSec and L2TP over IPSec remote-access tunnels. Posture enforcement is provided by way of a downloadable ACL from Cisco Secure ACS. Additionally, just as with the VPN 3000, remote-access clients can be exempted from NAC posture validation based on OS type.

The ASA and PIX also support clientless authentication. Those hosts connecting through a remote-access tunnel that do not have CTA installed are marked as clientless. Cisco Secure ACS can then apply the clientless policy to those hosts, to limit (or remove entirely) their access to the network. Chapter 7, "Configuring NAC on Cisco ASA and PIX Security Appliances," contains the complete configuration of NAC on the ASA 5500 series appliances and PIX 500 series security appliances.

Cisco Wireless Devices

NAC Framework support for wireless devices is available on autonomous Access Points (AP), lightweight access points running the Lightweight Access Point Protocol (LWAPP), and the Wireless LAN Services Module (WLSM) for the Catalyst 6500. Table 1-4 lists the wireless devices and minimum supported software.

Table 1-4NAC Support in Wireless Devices

Wireless devices are Layer 2 termination devices and, as such, support NAC-L2-802.1x as the posturing method. The process that a wireless client connecting to a wireless device goes through for posture validation is the same as for a wired client. Figure 1-3 depicts this posture. Note that wireless devices provide posture enforcement through VLAN only. This means that, to support NAC, the wireless devices must be configured for multiple VLANs per service set identifier (SSID).

Configuration and troubleshooting of NAC on Cisco wireless access points is covered along with other Layer 2 network-access devices in Chapter 4.

The Cisco Secure Access Control Server (ACS) for Windows is another core required component of NAC. Cisco Secure ACS first supported NAC in Version 3.3, which was launched concurrently with Phase I in the summer of 2004. Cisco Secure ACS 4.0, released in the fall of 2005, added support for NAC Phase II, including all the NADs listed in the previous section.

Cisco Secure ACS is the central controller for all NAC policy decisions. It receives posture credentials from all agents and either processes them locally or forwards them on to partner validation servers for processing. If the posture credentials are forwarded on, Cisco Secure ACS waits to receive the application posture token (APT) back from the external validation server. It then combines this APT with the local APTs it created based on the defined policy; the result is an overall system posture token (SPT).

The SPT has one of the following values: Healthy, Checkup, Quarantine, Infected, or Unknown, which are mapped to a network access policy. The network-access policy and SPT are then transmitted to the NAD as part of policy enforcement. Optionally, Cisco Secure ACS can send a user-notification message that CTA displays on the end host. This message usually indicates the posture of the system along with some instructions (for the un-Healthy hosts). Cisco Secure ACS can also send a URL redirect to the end host via the NAD if either NAC-L3-IP or NAC-L2-IP is being used.

Chapter 8 covers installation, configuration, and troubleshooting of Cisco Secure ACS.

Event Monitoring, Analysis, and Reporting

Protecting the network from threats is the first step toward securing it. However, event monitoring, analysis, and reporting are also vital pieces in understanding the network's security posture:

Monitoring, analysis, and reporting are powerful tools that show the network administrator the state of the network at any given point in time. These tools are very important in networks where NAC is enabled because the volume of events that each network device generates for each postured host is huge. Monitoring the network devices individually for problems or anomalies is neither practical nor efficient. This is why Cisco has enhanced its Cisco Security Monitoring, Analysis, and Reporting System (CS-MARS) to support NAC.

The CS-MARS appliance is a topologically aware, high-performance event-correlation system. Syslogs, NetFlow data, Simple Network Management Protocol (SNMP) traps, and other network logging information can be sent to it from a variety of network sources. This includes routers, switches, firewalls, intrusion-prevention devices, Cisco Secure ACS, and even end hosts. All this information is then correlated within CS-MARS to detect network attacks and other types of security threats. When an attack is detected, an incident is fired and the attacker, victim, and path from attacker to victim are displayed in the CS-MARS interface. Additionally, based on the attack vector, CS-MARS can inform the user of the best way to mitigate the attack.

In support of NAC, CS-MARS parses, normalizes, correlates, and reports on posture-validation events for NAC-L3-IP, NAC-L2-IP, and NAC-L2-802.1X. Predefined reports enable network administrators to view the number of hosts in Healthy, Quarantined, Clientless, or other states throughout the entire network. Administrators can further drill down to determine the posture status on a per-device basis. They may also choose to receive daily reports (via e-mail) of the number and location of nonhealthy hosts in their network.Help-desk support teams can use CS-MARS to identify problems reported from end users. CS-MARS can display IP addresses, machine/usernames, and the logical switch port number the user is connected to, along with the posture information or authentication information of end hosts. This information can be displayed in real time and allows the help-desk teams to quickly and easily identify problems end users are having.

Chapter 17, "Monitoring the NAC Solution Using the Cisco Security Monitoring, Analysis, and Response System," covers the configuration and operation of CS-MARS in a NAC Framework solution.

Summary

This chapter answered the question, "What is network access control?" by providing a solution overview as well as taking a look at the individual components that make up NAC. The different implementations of NAC were also explained, including NAC-L3-IP, NAC-L2-IP, and NAC-L2-802.1X. Network-access devices supporting NAC were presented, along with the version of software required.Looking ahead, subsequent chapters focus on the installation, configuration, and operation of each individual NAC component. Once complete, the individual components are combined to illustrate real-world deployment scenarios in Part III, "Deployment Scenarios." Finally, Part IV, "Managing and Monitoring NAC," focuses on the overall management and monitoring of the NAC solution.