CISSP - Certified Information Systems Security Professional Study Guide, 2nd Edition (2004)

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A two-tier deployment architecture uses a firewall with three or more interfaces. This allows for a DMZ or a publicly accessible extranet. The DMZ is used to host information server systems to which external users should have access. The firewall routes traffic to the DMZ or the trusted network according to its strict filtering rules. This architecture introduces a moderate level of routing and filtering complexity.

A three-tier deployment architecture is the deployment of multiple subnets between the private network and the Internet separated by firewalls. Each subsequent firewall has more stringent filtering rules to restrict traffic to only trusted sources. The outermost subnet is usually a DMZ. A middle subnet can serve as a transaction subnet where systems needed to support complex web applications in the DMZ reside. The third or back-end subnet can support the private network. This architecture is the most secure; however, it is also the most complex to design, implement, and manage.

Other Network Devices

There are several other ways to protect a network. Repeaters, concentrators, and amplifiers are used to strengthen the communication signal over a cable segment. These devices can be used to extend the maximum length of a specific cable type by deploying one or more repeaters along a lengthy cable run. Repeaters, concentrators, and amplifiers operate at OSI layer 1.

Hubs Hubs are used to connect multiple systems in a star topology. They repeat inbound traffic over all outbound ports. This ensures that the traffic will reach its intended host. A hub is a multiport repeater. Hubs operate at OSI layer 1.

Bridges A bridge is used to connect two networks together. A bridge forwards traffic from one network to another. Bridges that connect networks using different transmission speeds may have a buffer to store packets until they can be forwarded on to the slower network. This is known as a store-and-forward device. Bridges operate at OSI layer 2.

Routers Routers are used to control traffic flow on networks and are often used to connect similar networks and control traffic flow between the two. They can function using statically defined routing tables or they can employ a dynamic routing system. There are numerous dynamic routing protocols, such as RIP, OSPF, and BGP. Routers operate at OSI layer 3.

Switches Switches can be used instead of hubs. A switch is often called an intelligent hub because it knows the addresses of the systems connected on each outbound port. Instead of repeating traffic on every outbound port, a switch repeats only traffic out of the port on which the destination is known to exist. Switches offer greater efficiency for traffic delivery, create separate collision domains, and improve the overall throughput of data. Switches can also create separate broadcast domains when used to create VLANs. In such configurations, broadcasts are allowed within a single VLAN but not allowed to cross from one VLAN to another. Switches operate primarily at OSI layer 2. When switches have additional features, such as routing, they can operate at OSI layer 3 as well.

Gateways A gateway connects networks that are using different network protocols. A gateway is responsible for transferring traffic from one network to another by transforming the format of that traffic into a form compatible with the protocol or transport method used by each network. There are many types of gateways, including data, mail, application, secure, and Internet. Gateways typically operate at OSI layer 7.

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Proxies A proxy is any system that performs a function or requests a service on behalf of another system. Proxies are most often used in the context of providing clients on a private network with Internet access while protecting the identity of the clients. A proxy accepts requests from clients, alters the source address of the requester, maintains a mapping of requests to clients, and sends the altered request packets out. Once a reply is received, the proxy server determines which client it is destined for by reviewing its mappings and then sends the packets on to the client.

Remote Access Security Management

Telecommuting, or remote connectivity, has become a common feature of business computing. Remote access is the ability of a distant client to establish a communication session with a network. This can take the form of using a modem to dial up directly to a remote access server, connecting to a network over the Internet through a VPN, or even connecting to a terminal server system through a thin-client connection. The first two examples use fully capable clients. They establish connections just as if they were directly connected to the LAN. The last example, with terminal server, establishes a connection from a thin client. In such a situation, all computing activities occur on the terminal server system rather than on the distant client.

When remote access capabilities are deployed in any environment, security must be considered and implemented to provide protection for your private network against remote access complications. Remote access users should be strongly authenticated before being granted access. Only those users who specifically need remote access for their assigned work tasks should be granted permission to establish remote connections. All remote communications should be protected from interception and eavesdropping. This usually requires an encryption solution that provides strong protection for both the authentication traffic as well as all data transmission.

When outlining your remote access security management strategy, be sure to address the following issues:

Remote connectivity technology Each type of connection has its own unique security issues. Fully examine every aspect of your connection options. This can include modems, DSL, ISDN, wireless networking, and cable modems.

Transmission protection There are several forms of encrypted protocols, encrypted connection systems, and encrypted network services or applications. Use the appropriate combination of secured services for your remote connectivity needs. This can include VPNs, SSL, TLS, Secure Shell (SSH), IPSec, and L2TP.

Authentication protection In addition to protecting data traffic, you must also ensure that all logon credentials are properly secured. This requires the use of an authentication protocol and may mandate the use of a centralized remote access authentication system. This can include Password Authentication Protocol (PAP), Challenge Handshake Authentication Protocol (CHAP), Extensible Authentication Protocol (EAP), Remote Authentication Dial In User Service (RADIUS), and Terminal Access Controller Access Control System (TACACS).

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Remote user assistance Remote access users may periodically require technical assistance. You must have a means established to provide this as efficiently as possible. This can include addressing software and hardware issues, user training issues, and so on.

The ability to use remote access or establish a remote connection should be tightly controlled. As mentioned earlier, only those users who require remote access for their work tasks should be granted such access. You can control and restrict use of remote connectivity by using filters, rules, or access controls based on user identity, workstation identity, protocol, application, content, and time of day. To provide protection and restriction of remote access only to authorized users, you can use callback and caller ID. Callback is a mechanism that disconnects a remote user upon initial contact and then immediately attempts to reconnect to them using a predefined phone number (i.e., the number defined in the user account's security database). Callback does have a user-defined mode. However, this mode is not used for security; it is used to reverse toll charges to the company rather than charging the remote client. Caller ID verification can be used for the same purpose callback is used by verifying the physical location (via phone number) of the authorized user.

It should be a standard element in your security policy that no unauthorized modems be present on any system connected to the private network. You may need to further specify this policy by indicating that portable systems must either remove their modems before connecting to the network or boot with a hardware profile that disables the modem’s device driver.

Network and Protocol

Security Mechanisms

TCP/IP is the primary protocol used on most networks and on the Internet. It is a robust protocol, but it has numerous security deficiencies. In an effort to improve the security of TCP/IP, many subprotocols, mechanisms, or applications have been developed to protect the confidentiality, integrity, and availability of transmitted data. It is important to remember that even with the single foundational protocol of TCP/IP, there are literally hundreds, if not thousands, of individual protocols, mechanisms, and applications in use across the Internet. Some of them are designed to provide security services. Some protect integrity, others confidentiality, and others provide authentication and access control. In the next sections, some of the more common network and protocol security mechanisms are discussed.

VPN Protocols

A virtual private network (VPN) protocol is used to establish a secured tunnel for communications across an untrusted network. That network can be the Internet or a private network. The VPN can link two networks or two individual systems. VPNs can link clients, servers, routers, firewalls, and switches.

Point-to-Point Tunneling Protocol (PPTP) is an enhancement of PPP that creates encrypted tunnels between communication endpoints. PPTP is used on VPNs, but it is often replaced by the Layer 2 Tunneling Protocol (L2TP), which uses IPSec to provide traffic encryption for VPNs.

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IP Security (IPSec) is a standards-based mechanism for providing encryption for point-to- point TCP/IP traffic. IPSec has two primary components or functions: Authentication Header (AH) and Encapsulating Security Payload (ESP). AH provides authentication, integrity, and nonrepudiation. ESP provides encryption to protect the confidentiality of transmitted data, but it can also perform limited authentication. IPSec is often used in a VPN in either transport or tunnel mode. In transport mode, the IP packet data is encrypted but the header of the packet is not. In tunnel mode, the entire IP packet is encrypted and a new header is added to the packet to govern transmission through the tunnel. IPSec functions at layer 3 of the OSI model.

Secure Communications Protocols

Protocols that provide security services for application-specific communication channels are called secure communication protocols. Simple Key Management for IP (SKIP) is an encryption tool used to protect sessionless datagram protocols. SKIP was designed to integrate with IPSec and functions at layer 3. SKIP is able to encrypt any subprotocol of the TCP/IP suite.

Software IP encryption (SWIPE) is another layer 3 security protocol for IP. It provides authentication, integrity, and confidentiality using an encapsulation protocol.

Secure Remote Procedure Call (S-RPC) is an authentication service and is simply a means to prevent unauthorized execution of code on remote systems.

Secure Sockets Layer (SSL) is an encryption protocol developed by Netscape to protect the communications between a web server and a web browser. SSL can be used to secure Web, e-mail, FTP, or even Telnet traffic. It is a session-oriented protocol that provides confidentiality and integrity. SSL is deployed using a 40-bit key or a 128-bit key.

E-Mail Security Solutions

E-mail is inherently insecure. Internet e-mail relies primarily upon Simple Mail Transfer Protocol (SMTP). SMTP provides no security services. In fact, all e-mail transmitted over the Internet is transmitted in cleartext. Thus, messages that are intercepted or subjected to eavesdropping attacks can be easily read. The only means to provide protection for e-mail is to add encryption to the client applications used. The following paragraphs describe four common e-mail security solutions.

Secure Multipurpose Internet Mail Extensions (S/MIME) secures the transmission of e-mail and attachments. S/MIME provides protection through public key encryption and digital signatures. Two types of messages can be formed using S/MIME—signed messages and enveloped messages. A signed message provides integrity and sender authentication. An enveloped message provides integrity, sender authentication, and confidentiality.

Secure Electronic Transaction (SET) is a security protocol for the transmission of transactions over the Internet. SET is based on Rivest, Shamir, and Adelman (RSA) encryption and Data Encryption Standard (DES). It has the support of major credit card companies, such as Visa and MasterCard.

Privacy Enhanced Mail (PEM) is an e-mail encryption mechanism that provides authentication, integrity, confidentiality, and nonrepudiation. PEM is a layer 7 protocol and uses RSA, DES, and X.509.

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Pretty Good Privacy (PGP) is a public-private key system that uses the IDEA algorithm to encrypt files and e-mail messages. PGP is not a standard, but rather an independently developed product that has wide Internet grassroots support.

Dial-Up Protocols

When a remote connection link is established, some protocol must be used to govern how the link is actually created and to establish a common communication foundation for other protocols to work over. Dial-up protocols provide this function not only for true dial-up links but also for some VPN links.

Point-to-Point Protocol (PPP) is a full-duplex protocol used for the transmission of TCP/IP packets over various non-LAN connections, such as modems, ISDN, VPNs, Frame Relay, and so on. PPP is widely supported and is the transport protocol of choice for dial-up Internet connections. PPP authentication is protected through the use of various protocols, such as CHAP or PAP. PPP is a replacement for SLIP and can support any LAN protocol, not just TCP/IP.

Serial Line Internet Protocol (SLIP) is an older technology developed to support TCP/IP communications over asynchronous serial connections, such as serial cables or modem dial-up. SLIP is rarely used but is still supported on many systems. SLIP can support only IP, requires static IP addresses, offers no error detection or correction, and does not support compression.

Authentication Protocols

After a connection is initially established between a remote system and a server or a network, the first activity that should take place is to verify the identity of the remote user. This activity is known as authentication. There are several authentication protocols that control how the logon credentials are exchanged and whether or not those credentials are encrypted during transport.

Challenge Handshake Authentication Protocol (CHAP) is one of the authentication protocols used over PPP links. CHAP encrypts usernames and passwords. It performs authentication using a challenge-response dialog that cannot be replayed. CHAP also periodically reauthenticates the remote system throughout an established communication session to verify persistent identity of the remote client. This activity is transparent to the user.

Password Authentication Protocol (PAP) is a standardized authentication protocol for PPP. PAP transmits usernames and passwords in the clear. It offers no form of encryption; it simply provides a means to transport the logon credentials from the client to the authentication server.

Extensible Authentication Protocol (EAP) is a framework for authentication instead of an actual protocol. EAP allows customized authentication security solutions, such as supporting smart cards, tokens, and biometrics.

Centralized Remote Authentication Services

As remote access becomes a key element in an organization's business functions, it is often important to add additional layers of security between remote clients and the private network.

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Centralized remote authentication services, such as RADIUS and TACACS, provide this extra layer of protection. These mechanisms provide a separation of the authentication and authorization processes for remote clients from that performed for LAN or local clients. If the RADIUS or TACACS servers are ever compromised, then only remote connectivity is affected, not the rest of the network.

Remote Authentication Dial-In User Service (RADIUS) is used to centralize the authentication of remote dial-up connections. A network that employs a RADIUS server is configured so the remote access server passes dial-up user logon credentials to the RADIUS server for authentication. This process is similar to the process used by domain clients sending logon credentials to a domain controller for authentication.

Terminal Access Controller Access Control System (TACACS) is an alternative to RADIUS. TACACS is available in three versions: original TACACS, Extended TACACS (XTACACS), and TACACS+. TACACS integrates the authentication and authorization processes. XTACACS keeps the authentication, authorization, and accounting processes separate. TACACS+ improves XTACACS by adding two-factor authentication. TACACS operates similarly to RADIUS and provides the same functionality as RADIUS.

Network and Protocol Services

Another aspect of networking is the protocol services used to connect a LAN to WAN communication technologies. A basic knowledge of these services is important for anyone working in a security field or serving as a network manager. The following sections introduce some key issues about several WAN communication technologies.

Remote Access and Telecommuting Techniques

There are three main types of remote access techniques: service specific, remote control, and remote node operation. Service-specific remote access gives users the ability to remotely connect to and manipulate or interact with a single service, such as e-mail. Remote control remote access grants a remote user the ability to fully control another system that is physically distant from them. The monitor and keyboard act as if they are directly connected to the remote system. Remote node operation is just another name for dial-up connectivity. A remote system connects to a remote access server. That server provides the remote client with network services and possible Internet access.

Telecommuting is performing work at a location other than the primary office. In fact, there is a good chance that you perform some form of telecommuting as part of your current job. Telecommuting clients can use any or all of these remote access techniques to establish connectivity to the central office LAN.

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Frame Relay

Frame Relay is a layer 2 connection mechanism that uses packet-switching technology to establish virtual circuits between communication endpoints. Unlike dedicated or leased lines, for which cost is based primarily on the distance between endpoints, Frame Relay’s cost is primarily based on the amount of data transferred. The Frame Relay network is a shared medium across which virtual circuits are created to provide point-to-point communications. All virtual circuits are independent of and invisible to each other. Companies using Frame Relay establish a Committed Information Rate (CIR) contract that guarantees a minimum bandwidth for their communications at all times. However, if additional bandwidth is required and the Frame Relay network can support additional traffic, the virtual circuit can automatically expand to allow a higher throughput rate. Frame Relay is a connection-oriented service.

Frame Relay requires the use of data terminal equipment (DTE) and data circuit-terminating equipment (DCE) at each connection point. The customer owns the DTE, which acts like a router or a switch and provides the customer’s network with access to the Frame Relay network. The Frame Relay service provider owns the DCE, which performs the actual transmission of data over the Frame Relay as well as establishes and maintains the virtual circuit for the customer.

There are two types of virtual circuits: permanent virtual circuit (PVC) and switched virtual circuit (SVC). A PVC is a predefined virtual circuit that is always available. The virtual circuit may be closed down when not in use, but it can be instantly reopened whenever needed. An SVC is more like a dial-up connection. Each time the customer needs to transmit data over Frame Relay, a new virtual circuit is established using the best paths currently available. A PVC is like a two-way radio or walkie-talkie. Whenever communication is needed, you press the button and start talking; the radio reopens the predefined frequency automatically (i.e., the virtual circuit). A SVC is more like a shortwave or ham radio. You must tune the transmitter and receiver to a new frequency every time you want to communicate with someone.

Other WAN Technologies

Switched Multimegabit Data Services (SMDS) is a connectionless network communication service. It provides bandwidth on demand and is a preferred connection mechanism for linking remote LANs that communicate infrequently. SMDS is often a competitor of Frame Relay.

X.25 is an older WAN protocol that uses a carrier switch to provide end-to-end connections over a shared network medium. It is the predecessor to Frame Relay and operates in much the same fashion. However, X.25 use is declining due to its lower performance and throughput rates when compared to Frame Relay or ATM.

Asynchronous transfer mode (ATM) is a cell-switching technology, as opposed to a packetswitching technology like Frame Relay. ATM uses virtual circuits much like Frame Relay, but because it uses fixed-size frames or cells, it can guarantee throughput. This makes ATM an excellent WAN technology for voice and video conferencing.

High Speed Serial Interface (HSSI) is a layer 1 protocol used to connect routers and multiplexers to ATM or Frame Relay connection devices.

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Synchronous Data Link Control (SDLC) is a layer 2 protocol employed by networks with dedicated or leased lines. SDLC was developed by IBM for remote communications with SNA systems. SDLC is a bit-oriented synchronous protocol.

High-Level Data Link Control (HDLC) is a layer 2 protocol used to transmit data over synchronous communication lines. HDLC is an ISO standard based on IBM’s SDLC. HDLC supports full-duplex communications and both point-to-point and multipoint connections, offers flow control, and includes error detection and correction.

Integrated Services Digital Network (ISDN) is a digital end-to-end communications mechanism. ISDN was developed by telephone companies to support high-speed digital communications over the same equipment and infrastructure that is used to carry voice communications. ISDN is available in two primary varieties: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). BRI provides two B channels for data and one D channel for call management. Each B channel offers 64Kbps and the D channel is 16Kbps. PRI provides up to 23 B channels and one D channel (at 64Kbps). Thus, a full PRI ISDN connection offers 1.544Mbps throughput, the same as a T1 line.

Avoiding Single Points of Failure

Any element in your IT infrastructure, physical environment, or staff can be a single point of failure. A single point of failure is simply any element that would cause total or significant downtime if compromised, violated, or destroyed, affecting the ability of members of your organization to perform essential work tasks. Avoiding single points of failure means designing your networks and your physical environment with redundancy and backups. The use of systems, devices, and solutions with fault-tolerant capabilities is a means to improve resistance to single point of failure vulnerabilities. Taking steps to establish a means to provide alternate processing, failover capabilities, and quick recovery will also aid in avoiding single points of failure.

Redundant Servers

Using redundant servers is one fault-tolerant deployment option. Redundant servers can take numerous forms. Server mirroring is the deployment of a backup system along with the primary system. Every change made to the primary system is immediately duplicated to the secondary system. Electronic vaulting is the collection of changes on a primary system into a transaction or change document. Periodically, the change document is sent to an offsite duplicate server where the changes are applied. This is also known as batch processing because changes are duplicated over intervals rather than in real time. Remote journaling is the same as electronic vaulting except that changes are sent immediately to the offsite duplicate server rather than in batches. This provides a more real-time server backup. Database shadowing is remote journaling to more than one destination duplicate server. There may be one or more local duplicates and one or more offsite duplicates.

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Another form of redundant servers is clustering. Clustering is deploying two or more duplicate servers in such a way as to share the workload of a mission-critical application. Users see the clustered systems as a single entity. A cluster controller manages traffic to and among the clustered systems to balance the workload across all clustered servers. As changes occur on one of the clustered systems, they are immediately duplicated to all other cluster partners.

Failover Solutions

When backup systems or redundant servers exist, there needs to be a means by which you can switch over to the backup in the event the primary system is compromised or fails. Rollover, or failover, is redirecting workload or traffic to a backup system when the primary system fails. Rollover can be automatic or manual. Manual rollover, also known as cold rollover, requires an administrator to perform some change in software or hardware configuration to switch the traffic load over from the down primary to a secondary server. With automatic rollover, also known as hot rollover, the switch from primary to secondary system is performed automatically as soon as a problem is encountered. Fail-secure, fail-safe, and fail-soft are terms related to these issues. A system that is fail-secure is able to resort to a secure state when an error or security violation is encountered. Fail-safe is a similar feature, but human safety is protected in the event of a system failure. However, these two terms are often used interchangeably to mean a system that is secure after a failure. Fail-soft describes a refinement of the fail-secure capability: only the portion of a system that encountered or experienced the failure or security breach is disabled or secured, while the rest of the system continues to function normally.

A specific implementation of a fail-secure system would be the use of TFTP servers to store network device configurations. In the event of a system failure, configuration corruption, or power outage, most network devices (such as routers and switches) can be hard-coded to pull their configuration file from a TFTP server upon reboot. In this way, essential network devices can self-restore quickly.

Power failure is always a single point of failure. If electrical power is lost, all electronic devices will cease to function. Addressing this weakness is important if 24/7 uptime is essential to your organization. Ways to combat power failure or fluctuation issues include power conditioners (i.e., surge protectors), uninterruptible power supplies, and onsite electric generators.

RAID

Within individual systems, storage devices can be a single point of failure. Redundant Array of Independent Disks (RAID) is a storage device mechanism that uses multiple hard drives in unique combinations to produce a storage solution that provides better throughput as well as resistance to device failure. The two primary storage techniques employed by RAID are mirroring and striping. Striping can be further enhanced by storing parity information. Parity information enables on-the-fly recovery or reconstruction of data lost due to the failure of one or more drives. There are several levels or forms of RAID. Some of the more common RAID levels are listed in Table 3.3.

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T A B L E 3 . 3	Common RAID levels
RAID Level	Description
0	Striping
1	Mirroring
2	Hamming code parity
3	Byte-level parity
4	Block-level parity
5	Interleave parity
6	Second parity data
10	RAID levels 1 + 0
15	RAID levels 1 + 5

RAID can be implemented in hardware or in software. Hardware-based RAID offers more reliable performance and fault tolerance protection. Hardware-based RAID performs all processing necessary for multidrive access on the drive controllers. Software-based RAID performs the processing as part of the operating system. Thus, system resources are consumed in managing and using RAID when it is deployed through software.

There are three forms of RAID drive swapping: hot, cold, and warm. Hot-swappable RAID allows for failed drives to be removed and replaced while the host server remains up and running. Cold-swappable RAID systems require the host server to be fully powered down before failed drives can be removed and replaced. Warm-swappable RAID allows for failed drives to be removed and replaced by disabling the RAID configuration via software, then replacing the drive, and then reenabling the RAID configuration. RAID is a specific technology example of Fault Resistant Disk Systems (FRDS).

No matter what fault-tolerant designs and mechanisms you employ to avoid single points of failure, no environment’s security precautions are complete without a backup solution. Backups are the only means of providing reliable insurance against minor and catastrophic losses of your data. For a backup system to provide protection, it must be configured to store all data necessary to support your organization. It must perform the backup operation as quickly and efficiently as possible. The backups must be performed on a regular basis, such as daily, weekly, or in real time. And backups must be periodically tested to verify that they are functioning and that your restore processes are adequate. An untested backup cannot be assumed to work.