Developing Network Security Strategies

This chapter is from the book 

Modularizing Security Design

Security experts promote the security defense in depth principle. This principle states that network security should be multilayered, with many different techniques used to protect the network. No security mechanism can be guaranteed to withstand every attack. Therefore, each mechanism should have a backup mechanism. This is sometimes called the belt-and-suspenders approach. Both a belt and suspenders ensure that trousers stay up. A networking example is to use a dedicated firewall to limit access to resources and a packet-filtering router that adds another line of defense.

As part of implementing security defense in depth, security design should be modular. Multiple methods should be designed and applied to different parts of the network, whether it be the Internet connection, the wireless infrastructure, or the remote-access component. Cisco provides a modular approach with its SAFE security reference architecture (described in Chapter 5).

In general, using a modular approach to security design is a good way to gain an understanding of the types of solutions that must be selected to implement security defense in depth. The next few sections cover security for the following modules or components of an enterprise network:

• Internet connections
• Remote-access and virtual private networks (VPN)
• Network services and management
• Server farms
• User services
• Wireless networks

Securing Internet Connections

Internet connections should be secured with a set of overlapping security mechanisms, including firewalls, packet filters, physical security, audit logs, authentication, and authorization. Internet routers should be equipped with packet filters to prevent DoS and other attacks. These filters should be backed up with additional filters placed on firewall devices. The Internet connection should be carefully monitored. Network and host IDS devices should monitor subnets, routers, and Internet-accessible servers to detect signs of attack or malicious network activity and identify successful breaches into the protected network.

A good rule for enterprise networks is that the network should have well-defined exit and entry points. An organization that has only one Internet connection can manage Internet security problems more easily than an organization that has many Internet connections. Some large organizations require more than one Internet connection for performance and redundancy reasons, however. This is fine as long as the connections are managed and monitored. Departments or users who add Internet connections without coordination from corporate network engineers should not be tolerated.

A common risk associated with the Internet connection is reconnaissance threats from the Internet, whereby an attacker attempts to probe the network and its hosts to discover reachable networks, hosts, and services running on exposed hosts, and to develop a network map. To manage the risk of reconnaissance attempts, routers and first-line firewall devices should block all incoming connections, except those necessary to reach specific services on public servers or to complete a transaction started by a trusted client. The routers and firewalls should also block packets typically used for reconnaissance threats, such as pings.

When selecting routing protocols for the Internet connection and for routers that inject Internet routes into the interior network, you should select a protocol that offers route authentication such as Routing Information Protocol version 2 (RIPv2), Open Shortest Path First (OSPF), Enhanced Interior Gateway Routing Protocol (EIGRP), or Border Gateway Protocol, version 4 (BGP4). Static and default routing is also a good option because with static and default routing there are no routing updates that could be compromised.

When securing the Internet connection, Network Address Translation (NAT) can be used to protect internal network addressing schemes. As discussed in Chapter 6, "Designing Models for Addressing and Naming," NAT hides internal network numbers from outside networks. NAT translates internal network numbers when outside access is required.

Securing Public Servers

Most companies have a need for public servers that are accessible from the Internet. These include World Wide Web, File Transfer Protocol (FTP), Domain Name System (DNS), email, and e-commerce servers. Public servers should be placed on a demilitarized zone (DMZ) network that is protected from other networks via firewalls. DMZ networks were discussed in more detail in Chapter 5.

To protect public servers from DoS attacks, server administrators should use reliable operating systems and applications that have been patched with the most recent security fixes. Adding Common Gateway Interface (CGI) or other types of scripts to servers should be done with great care. Scripts should be thoroughly tested for security leaks.

Public servers should run firewall software and be configured for DoS protection. For example, the server should be configured to limit the number of connection establishments that can occur in a particular timeframe. Servers should also run software that can examine the content carried by application protocols so that the software can scan, and possibly eliminate, dangerous content such as viruses or mobile code. (Mobile code is software that can be transmitted across a network and executed on another device.)

If a customer can afford two separate servers, security experts recommend that FTP services not run on the same server as web services. FTP users have more opportunities for reading and possibly changing files than web users do. A hacker could use FTP to damage a company's web pages, thus damaging the company's image and possibly compromising web-based electronic commerce and other applications. Security experts recommend never allowing Internet access to Trivial File Transfer Protocol (TFTP) servers, because TFTP offers no authentication features.

Email servers have long been a source for intruder break-ins, probably because email protocols and implementations have been around a long time and hackers can easily understand them. Also, by its very nature, an email server must allow outsider access. To secure email servers, network administrators should keep current on well-known bugs and security leaks by subscribing to mailing lists dedicated to security information.

DNS servers should be carefully controlled and monitored. Name-to-address resolution is critical to the operation of any network. An attacker who can successfully control or impersonate a DNS server can wreak havoc on a network. DNS servers should be protected from security attacks by packet filters on routers and versions of DNS software that incorporate security features.

Traditionally, DNS had no security capabilities. In particular, there was no way to verify information returned in a DNS response to a query. A hacker could hijack the query and return a counterfeit name-to-address mapping. Digital signatures and other security features are being added to the protocol to address this issue and other security concerns. Refer to RFC 4033, "DNS Security Introduction and Requirements," and its companion documents, RFC 4034 and RFC 4035, for more information.

Securing E-Commerce Servers

E-commerce servers are vulnerable to the same attacks that threaten all public servers, but a compromise of an e-commerce server results in more substantial loss because these servers hold highly confidential and sensitive customer and financial data. E-commerce servers are often targets of DoS attacks, directed at their operating systems or applications. E-commerce servers must be protected from DoS attacks with packet-filtering rules and rules that deny successive connection attempts in a short period of time. They should also be protected from attackers who want to compromise them to launch an attack on other servers, including other e-commerce servers.

In some network designs, e-commerce applications run on multiple servers. For example, an e-commerce application front-end web server accepts encrypted sessions from Internet clients, processes the requests, and queries a database server, which holds sensitive customer and financial data. For optimum protection of sensitive data, and to avoid a compromised server attacking another server, you can separate the servers into their own DMZ networks. For example, design the topology so that there is a firewall that protects the database server from the front-end web server, in case the web server is compromised. Servers on the same segment can also be separated by LAN switch access control mechanisms (such as private VLANs). Network and host IDS devices should monitor subnets and individual servers to detect signs of attacks and confirm successful breaches.

Securing Remote-Access and VPNs

To support mobile users, many enterprise networks include remote-access technologies, VPN concentrators, and site-to-site VPN gateways. The users' data is sent over public networks, such as the Public Switched Telephone Network (PSTN) and the Internet, so protecting the data from eavesdropping is important. Protecting from identity spoofing of remote clients or sites is also important, to avoid an attacker impersonating a legitimate client and logging in to the network. This can happen if an attacker steals a legitimate user's credentials (such as a username and password pair) or learns the authentication keys used on a VPN connection.

Securing Remote-Access Technologies

Security is critical for remote-access technologies and should consist of firewall technologies, physical security, authentication and authorization mechanisms, auditing, and possibly encryption. Authentication and authorization are the most important features and can be implemented with the Challenge Handshake Authentication Protocol (CHAP) and the Remote Authentication Dial-In User Service (RADIUS) protocol.

Remote users and remote routers that use the Point-to-Point Protocol (PPP) should be authenticated with CHAP. The Password Authentication Protocol (PAP), which offers less security than CHAP, is not recommended. The "Remote-Access Technologies" section of Chapter 11, "Selecting Technologies and Devices for Enterprise Networks," covers PPP, CHAP, and PAP in more detail.

Another option for authentication, authorization, and accounting is RADIUS. Livingston, Inc., developed RADIUS, which has become an industry standard and is documented in RFC 2865. RADIUS gives an administrator the option of having a centralized database of user information. The database includes authentication and configuration information and specifies the type of service permitted by a user (for example, PPP, Telnet, rlogin, and so on). RADIUS is a client/server protocol. An access server acts as a client of a RADIUS server.

Dialup services should be strictly controlled. Users should not be allowed to attach modems and analog lines to their own workstations or servers. (Some companies actually fire employees who do this.) If some remote users still need to dial in to the network using a modem and analog telephone line, it's helpful to have a single dial-in point (for example, a single modem pool or access server) so that all users are authenticated in the same way. A different set of modems should be used for any dial-out services. Both dial-in and dial-out services should be authenticated.

There are many operational security considerations with dialup networks, and if possible, dialup networks should be eliminated from modern networks. If that is not possible, modems and access servers should be carefully configured and protected from hackers reconfiguring them. Modems should be programmed to reset to the standard configuration at the start and end of each call, and modems and access servers should terminate calls cleanly. Servers should force a logout if the user hangs up unexpectedly.

If the modems and access servers support callback (which most do), callback should be used. With callback, when a user dials in and is authenticated, the system disconnects the call and calls back on a specified number. Callback is useful because the system calls back the actual user, not a hacker who might be masquerading as the user. Callback can easily be compromised, however, and should not be the only security mechanism used.

Securing VPNs

Organizations that use VPNs to connect private sites and end users via a public network such as the Internet should use NAT, firewalls, strong authentication, and data encryption. The client operating systems that connect via the VPN should use personal firewall and virus protection software. It is important to protect against a compromise of a client or remote site that allows an attacker to successfully attack the enterprise network over the VPN. An example is a VPN client that has been compromised by a Trojan horse that turns the client system into a relay. Such an attack could mean that when the client is connected to the enterprise network via an Internet remote-access VPN, the attacker can connect to the client over the Internet, and then from the client connect to the protected enterprise network.

In VPN topologies, private data travels across a public network, so encryption is a must. The most common solution for encryption is to use the IP Security Protocol (IPsec), which is an Internet Engineering Task Force (IETF) standard that provides data confidentiality, data integrity, and authentication between participating peers at the IP layer. IPsec provides a secure path between remote users and a VPN concentrator, and between remote sites and a VPN site-to-site gateway.

Numerous RFCs deal with Ipsec, and many Internet drafts. To learn IPsec better, the main RFCs you should read are as follows:

• RFC 4301, "Security Architecture for the Internet Protocol"
• RFC 4302, "IP Authentication Header"
• RFC 4303, "IP Encapsulating Security Payload (ESP)"
• RFC 4306, "Internet Security Association and Key Management Protocol (ISAKMP)"

IPsec enables a system to select security protocols and algorithms, and establish cryptographic keys. The Internet Key Exchange (IKE) protocol provides authentication of IPsec peers. It also negotiates IPsec keys and security associations. IKE uses the following technologies:

• DES: Encrypts packet data.
• Diffie-Hellman: Establishes a shared, secret, session key.
• Message Digest 5 (MD5): A hash algorithm that authenticates packet data.
• Secure Hash Algorithm (SHA): A hash algorithm that authenticates packet data.
• RSA encrypted nonces: Provides repudiation.
• RSA signatures: Provides nonrepudiation.

Securing Network Services and Network Management

To protect internal network services, it is important to protect internal internetworking devices, such as routers and switches. You should treat each network device as a high-value host and harden (strengthen) it against possible intrusions. This involves common practices such as running only the minimal necessary services and establishing trust only with authentic partners. For example, a router should not accept routing updates from a router that has not been authenticated. Routing protocols that support authentication should be selected, including RIPv2, OSPF, EIGRP, and BGP4. Static and default routes are also a good choice because they eliminate the need to accept routing updates.

Login IDs and passwords should be required for accessing routers and switches, whether the user accesses the device via a console port or via the network. A first-level password can be used for administrators that simply need to check the status of the devices. A second-level password should be used for administrators who have permission to view or change configurations. Avoid using a nonsecure protocol such as Telnet to access routers and switches over a network. A better choice is Secure Shell (SSH).

When administrators (or hackers posing as administrators) connect to a router or switch, they should not see the typical connect message, which often says something simple, such as Welcome to This Router. Instead, a router or switch should display warnings about authorized usage and the monitoring of all activity on the device. Many security experts recommend getting help from a lawyer when writing the connect message.

If modem access to the console ports of internetworking devices is allowed, the modems must be secured just as standard dial-in user modems are, and the phone numbers should be unlisted and unrelated to the organization's main number(s). The phone numbers should also be changed when there is staff turnover.

For customers with numerous routers and switches, a protocol such as the Terminal Access Controller Access Control System (TACACS) can be used to manage large numbers of router and switch user IDs and passwords in a centralized database. TACACS also offers auditing features, which can be helpful when an inexperienced network administrator tries to avoid responsibility for a misconfiguration that led to a security incident.

To protect against the misconfiguration of devices by hackers (or inexperienced network administrators), you can enforce authorization on specific configuration commands. TACACS and other authorization methods can be configured to permit only specific administrators to enter risky commands, such as commands to change IP addresses or ACLs. The use of a well-managed, centralized change-control process is also recommended.

Limiting use of the Simple Network Management Protocol (SNMP) should be considered on enterprise networks for which security goals outweigh manageability goals. One of the main issues with SNMP is the set operation, which allows a remote station to change management and configuration data. If SNMPv3 is used, this is not as big a concern, because SNMPv3 supports authentication for use with the set operation and other SNMP operations.

Network management systems should be especially protected because they host extremely sensitive data about network and security device configuration. Moreover, network management systems are sometimes connected to other devices over a separate (out-of-band) management network, which, without careful design, could provide a path around security mechanisms such as firewalls.

To minimize risk, network management systems should be placed in their own DMZ behind a firewall. They should run a hardened operating system that has been patched with the latest security fixes. All unnecessary services should be disabled.

As is the case with routers and switches, network management systems must be protected from the impersonation of administrators, where an attacker steals the credentials (usernames or passwords) of an administrator. To manage the risk of administrator impersonation, provide the administrator with strong authentication mechanisms. A good example is a two-factor, one-time password system based on security cards.

Securing Server Farms

Server farms host file, print, database, and application servers inside campus networks and branch offices. These servers often contain an enterprise's most sensitive information, so they must be protected. Because servers are accessed by a large number of users, network performance is usually a critical issue, which can limit the choice of protection mechanisms. Nonetheless, methods should be deployed to protect against the compromise of exposed applications and unauthorized access to data. Network and host IDS devices should be deployed to monitor subnets and individual servers to detect signs of attacks and confirm successful breaches.

When servers in a server farm are compromised, attackers can use those servers to attack other servers. To manage this risk, configure network filters that limit connectivity from the server. In many cases, a server has no need to initiate connections. Connection establishments generally come from the client. There are numerous exceptions, however, which can be programmed into filters. For example, with active (port-mode) FTP, the server initiates a connection. Also, various network management, naming, resource-location, and authentication and authorization protocols might require the server to initiate a connection. As part of the top-down network design process, you should have analyzed the protocols present in server farm locations (see Chapter 3, "Characterizing the Existing Internetwork" and Chapter 4, "Characterizing Network Traffic" for more information). The data you gathered can help you determine which protocols a server will need to allow.

To maximize security, both server and end-user software should be carefully selected and maintained. Server and desktop administrators should be required to keep current as to the latest hacker tricks and viruses. Known security bugs in operating systems should be identified and fixed. In addition, application software should be selected based partly on its adherence to modern, secure programming practices. With the creation of safer high-level programming languages and increasing programmer awareness of security issues, many applications are available that are reasonably secure. Most stock software, which is still used by many businesses, is vulnerable to simple attacks to defeat its security, however.

For customers with stringent security requirements, server applications might incorporate encryption. This is in addition to any client/server encryption used to protect data traveling across a network. To protect against the unauthorized use of data, cryptographic methods can protect data on a disk drive. For example, the data on disk drives can be encrypted so that it can be read only by the proper application.

File and other servers should provide authentication and authorization features. Security policies and procedures should specify accepted practices regarding passwords: when they should be used, how they should be formatted, and how they can be changed. In general, passwords should include both letters and numbers, be at least six characters, not be a common word, and be changed often.

On servers, root password knowledge (or the non-UNIX equivalent) should be limited to a few people. Guest accounts should be avoided if possible. Protocols that support the concept of trust in other hosts should be used with caution (examples include rlogin and rsh on UNIX systems). Hosts that permit guest accounts and support trusted hosts should be isolated from other hosts if possible.

Kerberos is an authentication system that provides user-to-host security for application-level protocols such as FTP and Telnet. If requested by the application, Kerberos can also provide encryption. Kerberos relies on a symmetric key database that uses a key distribution center (KDC) on a Kerberos server.

Securing User Services

A security policy should specify which applications are allowed to run on networked PCs and restrict the downloading of unknown applications from the Internet or other sites. The security policy should also require that PCs have personal firewall and antivirus software installed. Security procedures should specify how this software is installed and kept current.

Users should be encouraged to log out of their sessions with servers when leaving their desks for long periods of time and to turn off their machines when leaving work, to protect against unauthorized people walking up to a system and accessing services and applications. Automatic logouts can also be deployed to automatically log out a session that has had no activity for a period of time.

One other aspect of securing the end-user part of a network is ensuring that users connect only permitted computers or other devices to the LAN interfaces in their offices. In particular, one area of concern is users who connect wireless access points that are not properly secured. These unauthorized access points are sometimes called rogue access points. Security for wireless networks, which is discussed in more detail in the next section, should not be left to end users. It should be carefully planned and implemented and not compromised by users installing their own wireless access points.

Cisco and other vendors support an IEEE standard called 802.1X, which provides port-based security on switch ports. With 802.1X enabled on a switch port, no device can connect to the network without first using 802.1X to authenticate. This is one method for ensuring that a user doesn't install an unknown device, such as a wireless access point. With this use of 802.1X, it is the access point that is authenticated. Another use of 802.1X is to authenticate wireless client devices, such as laptops. When a legitimate wireless infrastructure is in place, 802.1X is no longer needed on the ports that connect known access points, but it can be used to authenticate wireless users, as discussed later in the "802.1X with Extensible Authentication Protocol" section of this chapter.

Securing Wireless Networks

Wireless networks are gaining widespread popularity in enterprise campus networks, at branch offices, and in home offices. Most organizations support the increases in productivity and employee satisfaction that wireless networking offers but at the same time are concerned about the security risks, as they should be. In recent years, glaring holes have been discovered in the typical methods used for wireless security, resulting in the development of new methods and models for providing security on wireless networks. This section covers some overall design guidelines first and then includes information on the following two topics:

• Authentication in wireless networks
• Data privacy in wireless networks

As mentioned in Chapter 5, it is best to place wireless LANs (WLAN) in their own subnet and their own VLAN. This simplifies addressing for stations that roam and also improves management and security. Keeping all wireless clients in their own subnet makes it easier to set up traffic filters to protect wired clients from an attack launched on the wireless network. To maximize roaming flexibility, all WLANs can be a single VLAN and IP subnet, so that there is no need to retrieve a new IP address when moving from one area to another. To maximize security, however, it might be wiser to subdivide the WLAN into multiple VLANs and IP subnets.

Keep in mind that security requirements for wireless users vary with the type of user. Guests who visit an enterprise might need easy access to the Internet but should be prevented from accessing the enterprise network. These guests cannot be expected to know an encryption key or to have VPN software installed. This is different from the employees who want wireless access while having lunch in the cafeteria or while meeting in private conference rooms. Those users could be expected to know a key or to have the corporate-approved VPN software installed. The use of VLANs comes in handy here. When you understand the different user types and where they might roam, you can divide the WLAN into multiple VLANs and apply security policies separately for each VLAN.

You should implement ACLs on wireless access points and on wired switches and routers that carry traffic that originated on a wireless network. The ACLs should allow only specific protocols, in accordance with security policies.

All wireless (and wired) laptop computers should be required to run antivirus and personal firewall software. They should also be regularly updated with the most recent operating system security patches. Depending on security requirements, you might also want to require corporate wireless laptop users to use VPN software to access the enterprise network. The final section in this chapter, "Using VPN Software on Wireless Clients," discusses using IPsec VPN software as a security option for wireless networks.

Authentication in Wireless Networks

In a wired Ethernet LAN, a device must physically plug into the network to communicate. This fundamental feature of a wired Ethernet is not present in the realm of wireless networking, however. There is nothing to plug in. The IEEE 802.11 standard provides a method for devices to authenticate to a wireless access point, thus emulating the basic security provided by a wired network where a user must have physical access to a port to communicate.

Authentication takes place after a wireless client has located an access point with a sufficiently strong signal and selected a channel. The 802.11 client initialization process consists of the following steps:

• Step 1. The client broadcasts a Probe Request frame on every channel.
• Step 2. Access points within range respond with a Probe Response frame.
• Step 3. The client decides which access point is the best for access and sends an Authentication Request frame.
• Step 4. The access point sends an Authentication Response frame.
• Step 5. Upon successful authentication, the client sends an Association Request frame to the access point.
• Step 6. The access point replies with an Association Response frame. The client can now pass traffic to the access point.

IEEE 802.11 specifies two forms of authentication: open and shared key. With open authentication, the client is always authenticated as long as the access point has been configured to allow open authentication. This is the default mode for most systems. Open authentication can be thought of as null authentication. The client asks to be authenticated and the access point permits the authentication. It might sound pointless to use such an algorithm, but open authentication has its place in 802.11 networks. Open authentication is often used for guest access, where it would be impractical to provide users with a key. Also, many 802.11-compliant devices are handheld data-acquisition units, such as bar-code readers. They do not have the CPU capabilities required for complex authentication algorithms.

With shared key authentication, a Wired Equivalent Privacy (WEP) static key must be properly configured in both the client and the access point. The steps for shared key authentication are as follows:

• Step 1. The client sends an Authentication Request to the access point requesting shared key authentication.
• Step 2. The access point responds with an Authentication Response containing challenge text.
• Step 3. The client uses its locally configured WEP key to encrypt the challenge text and replies with another Authentication Request.
• Step 4. If the access point can decrypt the Authentication Request and retrieve the original challenge text, the client must be using the correct WEP key, so the access point responds with an Authentication Response that grants the client access.

In August 2001, cryptanalysts Fluhrer, Mantin, and Shamir determined that a WEP key can be derived by passively collecting particular frames from a wireless LAN. Researchers at AT&T and Rice University and the developers of the AirSnort application implemented the vulnerability and verified that either 64- or 128-bit WEP keys can be derived after as few as 4 million frames. For high-usage wireless LANs, this translates to roughly 4 hours until a 128-bit WEP key can be derived.

In addition to WEP's vulnerability to passive attacks, WEP is also vulnerable to inductive key derivation, which is the process of deriving a key by coercing information from the wireless LAN. Man-in-the-middle attacks, a form of inductive key derivation, are effective in 802.11 networks because of the lack of effective message integrity. The receiver of a frame cannot verify that the frame was not tampered with during its transmission.

The shared WEP key, as specified by IEEE 802.11, is a static key. If the key is discovered by an unauthorized user, it must be changed on access points and every individual client. Attackers can discover the key in many ways, including eavesdropping on numerous packets, but also by using simpler methods, such as asking naive users for the key or stealing users' laptop computers where the key is configured.

The 802.11 specification stipulates only the two mechanisms for authenticating wireless devices that have already been discussed: open authentication and shared key authentication. Other mechanisms that are also commonly used include setting an unpublished Service Set Identifier (SSID), authenticating devices by their client Media Access Control (MAC) address, and using 802.1X with the Extensible Authentication Protocol (EAP). These are described in the next three sections.

Using an Unpublished Service Set Identifier

Every WLAN has an SSID that identifies it. To gain access to a wireless LAN, a client must know the correct SSID. Some network administrators rely on this as a method for security even though it doesn't truly authenticate the client and doesn't provide any data privacy. Also, an eavesdropper can easily determine the SSID with the use of a wireless protocol analyzer. The SSID is advertised in plain text in beacon messages that the access point sends.

Some access point vendors, including Cisco, offer the option to disable SSID broadcasts in beacon messages, but this does not offer much protection. The SSID can still be determined by analyzing Probe Response frames from an access point. Also, disabling SSID broadcasts might have an adverse effect on wireless interoperability for mixed-vendor deployments. Therefore, most experts do not recommend using the SSID as a mode of security.

MAC Address Authentication

MAC address authentication verifies a client's MAC address against a configured list of allowed addresses. MAC address authentication is used to augment the open and shared key authentications provided by 802.11, further reducing the likelihood of unauthorized devices accessing the network. Depending on the access point, the list of MAC addresses might be locally configured on the access point, or the access point might use an authentication protocol such as RADIUS and an external authentication server. A server is helpful for large installations where configuring individual access points would be difficult. If a server is used, redundancy must be considered so that the server does not become a single point of failure.

MAC addresses are sent as clear text per the 802.11 specification. As a result, in wireless LANs that use MAC address authentication, a network attacker might be able to subvert the MAC authentication process by spoofing a valid MAC address. Network attackers can use a protocol analyzer to determine valid MAC addresses that are being used in the network and change their own wireless NICs to use that address (on NICs that support changing the MAC address).

MAC address authentication is labor-intensive. A network administrator must know the address of every allowed NIC and configure this into the access point or server. Also, as mentioned, hackers can get around MAC address authentication by changing their own address to match an allowed address. Therefore, most experts do not recommend relying on MAC address authentication as the only mode of security.

802.1X with Extensible Authentication Protocol

IEEE 802.1X specifies a method for authenticating and authorizing a device attached to a LAN port. It is used on both wired switches and on wireless access points (where the "attachment" is not physical). 802.1X provides optional support for use of an authentication server, such as a RADIUS server, which is recommended for larger installations.

802.1X is extensible and supports a variety of authentication algorithms. The most common are varieties of EAP, which is an IETF standard, documented in RFC 2284. With 802.1X and EAP, devices take on one of three roles:

• The supplicant resides on the wireless LAN client.
• The authenticator resides on the access point.
• An authentication server resides on a RADIUS server.

When 802.1X and EAP are implemented, a client that associates with an access point cannot use the network until the user is authenticated. After association, the client and the network (access point or RADIUS server) exchange EAP messages to perform authentication. An EAP supplicant on the client obtains credentials from the user, which could be a user ID and password, a user ID and one-time password, or a digital certificate. The credentials are passed to the authenticator or server and a session key is developed.

With 802.1X and EAP, session timeouts force a client to reauthenticate to maintain network connectivity. Although reauthentication is transparent to the client, the process of reauthentication generates new WEP keys at every reauthentication interval. This is important for mitigating statistical key derivation attacks and is a critical WEP enhancement. One disadvantage of 802.1X with EAP, however, is that reauthentication can cause some delay, when compared to using a static WEP key. This might cause a problem for users that roam with delay-sensitive devices, such as 802.11 phones.

Note that EAP authenticates users. Whereas 802.11 authentication is device-based, EAP is based on authenticating a user rather than a wireless LAN device. This avoids the problems caused by theft of a laptop computer using a static WEP key, which would allow the thief access to the network and would probably result in a network administrator needing to change the WEP key on the affected access points and all clients. EAP generates unique keying material for each user. This relieves network administrators from the burden of managing static keys. EAP also supports mutual authentication, which allows a client to be certain that it is communicating with the intended authentication server.

Selecting the right EAP implementation can be a challenging process due to the large number of options. The funny-sounding names, such as LEAP and PEAP, don't help matters. You must get this right though. The supplicant, authenticator, and authentication server must all support the same variety of EAP, which is mostly likely one of the following:

• Lightweight EAP (LEAP): Developed by Cisco and is sometimes called EAP-Cisco. Cisco licenses LEAP to other vendors, including Apple Computer and Intel. LEAP supports user-based authentication and dynamic WEP keys that are generated after authentication and when session timeouts occur. User authentication is based on a user's Windows logon, which means the user does not have to supply additional logon information to access the wireless network, which makes LEAP easy to use. LEAP supports mutual authentication, which means that the client authenticates the server and the server authenticates the client. This is important for ensuring that the client talks to an authorized server and not a hacker posing as a server.
• EAP-Transport Layer Security (EAP-TLS): Developed by Microsoft and is documented in RFC 2716. Microsoft supports EAP-TLS in all versions of Windows XP and has released a free Windows 2000 client. Like LEAP, EAP-TLS supports mutual authentication, dynamic keys, and session timeouts. EAP-TLS requires certificates for clients and servers. Because of this, some users consider Microsoft's EAP to be more secure than other EAPs. However, the certificate requirement also means EAP-TLS needs certificate management, such as the use of a trusted certificate authority and the ability to quickly revoke certificates.
• EAP-Tunneled TLS (EAP-TTLS): Developed by Funk and Certicom and then turned over to the IETF, where it is currently (as of this writing) a standards-based Internet draft. EAP-TTLS is an enhancement of EAP-TLS, with support for advanced authentication methods such as token cards. A variety of vendors have signed on to support EAP-TTLS.
• Protected EAP (PEAP): Supported by Cisco, Microsoft, and RSA Security. Like LEAP and EAP-TLS, PEAP supports mutual authentication, dynamic keys, and session timeouts. PEAP uses a certificate for the client to authenticate the RADIUS server. The server uses a one-time password or a username and password to authenticate the client. When the client validates the server's certificate, it builds an encrypted tunnel and then uses EAP in the tunnel to authenticate. PEAP is more manageable and scalable than EAP-TLS. Organizations can avoid installing digital certificates on every client machine, as required by EAP-TLS, and select the method of client authentication that best suits them.
• EAP-MD5: Has no key management features or dynamic key generation. Although EAP-MD5 is supported on many platforms, it will probably be phased out of most wireless networks because it has few benefits over WEP.

Data Privacy in Wireless Networks

Previous sections discussed methods for authenticating a wireless device (or the user of a wireless device). Another important requirement for wireless networks is data privacy. The original IEEE 802.11 standard specified WEP as the method for encrypting data to meet privacy requirements. Unfortunately, WEP has been shown to be ineffective as a data-privacy mechanism because of the many ways of compromising it. Cisco and other vendors implemented many enhancements to WEP, which IEEE standardized as part of its IEEE 802.11i standard.

One set of enhancements that addresses the shortcomings with WEP is known as the Temporal Key Integrity Protocol (TKIP). TKIP provides the following:

• A message integrity check (MIC), which provides effective frame authenticity to mitigate man-in-the-middle vulnerabilities
• Per-packet keying, which provides every frame with a new and unique WEP key that mitigates WEP key derivation attacks

In addition to TKIP, the IEEE recognized the need for stronger encryption mechanisms and adopted the use of the Advanced Encryption Standard (AES) for the data-privacy section of the 802.11i standard. The development of AES was facilitated by the National Institute of Standards and Technology (NIST), which solicited the cryptography community for new encryption algorithms. The algorithms had to be fully disclosed and available royalty free. The NIST judged candidates on cryptographic strength as well as practical implementation. The finalist, and adopted method, is known as the Rijndael algorithm. The Rijndael algorithm provides for a variety of key sizes, including 128, 192, and 256 bits.

Wi-Fi Protected Access

Another development that is related to 802.11i and wireless security is Wi-Fi Protected Access (WPA). WPA is a subset of the 802.11i standard that was adopted by the Wi-Fi Alliance. The Wi-Fi Alliance is a nonprofit international association formed in 1999 to certify interoperability of wireless products based on IEEE 802.11 specifications.

The Wi-Fi Alliance introduced WPA because 802.11i was not ratified yet and also because 802.11i was expected to include specifications that would eventually require hardware upgrades for some devices. The Wi-Fi Alliance decided to introduce a subset of the specification that was stable and could be achieved via software upgrades.

WPA uses 802.1X with EAP for authentication and TKIP for data encryption. For enterprise networks, WPA should be used with an authentication server, such as a RADIUS server, to provide centralized access control and management. In small and home offices, WPA allows the use of manually entered keys or passwords. The small or home office user enters a password (also called a master key or a preshared key) in the access point and each client, and WPA takes over from there. The password ensures that only devices with a matching password can join the network. Entering a correct password also starts the TKIP encryption process.

In some configurations, a WPA1 option is distinguished from a WPA2 option. WPA1 uses TKIP and predates 802.11i. WPA2 uses AES and is compatible with 802.11i. Another distinction is WPA Personal versus WPA Enterprise. WPA Personal uses preshared keys and is appropriate for home or small office networks. WPA Enterprise uses a RADIUS server and is appropriate for larger business networks.

Using VPN Software on Wireless Clients

Although EAP and WPA solve many of the problems with WEP, they can be difficult to implement, especially in multivendor environments. Another option for customers with a strong need to protect data confidentiality is to use VPN software on the wireless clients. With this solution, the clients reach the campus network by connecting to a VPN concentrator. VPN software that uses IPsec has many advantages, including Triple Data Encryption Standard (3DES) encryption, one-time password support, and support for per-user policies.

When VPN software is installed, WLAN clients still associate with a wireless access point to establish connectivity at Layer 2. The clients then establish connectivity at Layer 3 using DHCP and DNS. The clients establish a VPN tunnel to a VPN concentrator to securely communicate with the campus network.

The wireless network should be considered an untrusted network, suitable only as a transit network for IPsec traffic once a VPN tunnel has been established. To isolate the untrusted wireless network, administrators should place the WLAN users in their own VLAN. The wireless clients should also run personal firewall software to protect them while they are connected to the untrusted WLAN network without the protection of IPsec.

Another protection mechanism is a feature called Publicly Secure Packet Forwarding (PSPF), which prevents WLAN clients on the same access point from communicating with each other (or attacking each other). PSPF provides Internet access to clients without providing other typical LAN services, such as the capability to share files.

To minimize security threats, you should configure the wireless access point to allow only the protocols required for establishing a secure tunnel to a VPN concentrator. These protocols include DHCP for initial client configuration, DNS for name resolution, and IPsec VPN-specific protocols—IP protocol 50 for ESP and UDP port 500 for IKE. (The DNS traffic is necessary only if the VPN client accesses the VPN gateway by name.)

Despite the assurance of data privacy that IPsec provides, an IPsec VPN solution for wireless security has some disadvantages. For example, some VPN software requires the user to start the software and provide additional logon information before accessing the campus network. Roaming from one area to another might require the acquisition of a new IP address or a mobile IP solution. Also, when 3DES encryption is provided in software, users may notice a performance degradation.

In general, IPsec VPN in a wireless network has the same disadvantages that it has in wired networks, including diminished ease of use and performance, configuration complexity, the need for local software to be installed on the client computers, interoperability problems with various applications, the lack of support for multicast applications, and the fact that IPsec is an IP-only solution. Also, handheld devices, including 802.11 phones, might not support IPsec.

Understanding the needs of the various user communities and their applications will help you decide whether IPsec should be required instead of (or in addition to) the wireless security measures discussed in the previous sections. Determining the size of wireless user communities and the traffic volume they will generate is also important. Many VPN solutions were designed to handle a small number of remote users rather than a large number of transient wireless users. An analysis of traffic flow and volume may be necessary to determine if a VPN solution will scale to support your wireless users.