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Developing Network Security Strategies

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Oct 4, 2010.

Chapter Description

To help you handle the difficulties inherent in designing network security for complex networks, this chapter teaches a systematic, top-down approach that focuses on planning and policy development before the selection of security products.

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Security Mechanisms

This section describes some typical ingredients of secure network designs. You can select from these ingredients when designing solutions for common security challenges, which are described in the "Modularizing Security Design" section later in this chapter.

Physical Security

Physical security refers to limiting access to key network resources by keeping the resources behind a locked door and protected from natural and human-made disasters. Physical security can protect a network from inadvertent misuses of network equipment by untrained employees and contractors. It can also protect the network from hackers, competitors, and terrorists walking in off the street and changing equipment configurations.

Depending on the level of protection, physical security can protect a network from terrorist and biohazard events, including bombs, radioactive spills, and so on. Physical security can also protect resources from natural disasters such as floods, fires, storms, and earthquakes.

Depending on your particular network design customer, physical security should be installed to protect core routers, demarcation points, cabling, modems, servers, hosts, backup storage, and so on. Work with your customer during the early stages of the network design project to make sure equipment will be placed in computer rooms that have card key access and/or security guards. Computer rooms should also be equipped with uninterruptible power supplies, fire alarms, fire-abatement mechanisms, and water-removal systems. To protect equipment from earthquakes and high winds during storms, equipment should be installed in racks that attach to the floor or wall.

Because physical security is such an obvious requirement, it is easy to forget to plan for it, but it should never be overlooked or considered less important than other security mechanisms. As mentioned in the "Secure Network Design Topologies" section of Chapter 5, you should start working with your design customer at the beginning of the design project to make sure that critical equipment will be protected. Planning for physical security should start during the early phases of the top-down design process in case there are lead times to build or install security mechanisms.


Authentication identifies who is requesting network services. The term authentication usually refers to authenticating users but can also refer to authenticating devices or software processes. For example, some routing protocols support route authentication, whereby a router must pass some criteria before another router accepts its routing updates.

Most security policies state that to access a network and its services, a user must enter a login ID and password that are authenticated by a security server. To maximize security, one-time (dynamic) passwords can be used. With one-time password systems, a user's password always changes. This is often accomplished with a security card, also called a Smartcard. A security card is a physical device about the size of a credit card. The user types a personal identification number (PIN) into the card. The PIN is an initial level of security that simply gives the user permission to use the card. The card provides a one-time password that is used to access the corporate network for a limited time. The password is synchronized with a central security card server that resides on the network. Security cards are commonly used by telecommuters and mobile users. They are not usually used for LAN access.

Authentication is traditionally based on one of three proofs:

  • Something the user knows: This usually involves knowledge of a unique secret that is shared by the authenticating parties. To a user, this secret appears as a classic password, a PIN, or a private cryptographic key.
  • Something the user has: This usually involves physical possession of an item that is unique to the user. Examples include password token cards, security cards, and hardware keys.
  • Something the user is: This involves verification of a unique physical characteristic of the user, such as a fingerprint, retina pattern, voice, or face.

Many systems use two-factor authentication, which requires a user to have two proofs of identity. An example is an access control system that requires a security card and a password. With two-factor authentication, a compromise of one factor does not lead to a compromise of the system. An attacker could learn a password, but the password is useless without the security card. Conversely, if the security card is stolen, it cannot be used without the password.


Whereas authentication controls who can access network resources, authorization says what they can do after they have accessed the resources. Authorization grants privileges to processes and users. Authorization lets a security administrator control parts of a network (for example, directories and files on servers).

Authorization varies from user to user, partly depending on a user's department or job function. For example, a policy might state that only Human Resources employees should see salary records for people they don't manage.

Security experts recommend use of the principle of least privilege in the implementation of authorization. This principle is based on the idea that each user should be given only the minimal necessary rights to perform a certain task. Therefore, an authorization mechanism should give a user only the minimum access permissions that are necessary. Explicitly listing the authorized activities of each user with respect to every resource is difficult, so techniques are used to simplify the process. For example, a network manager can create user groups for users with the same privileges.

Accounting (Auditing)

To effectively analyze the security of a network and to respond to security incidents, procedures should be established for collecting network activity data. Collecting data is called accounting or auditing.

For networks with strict security policies, audit data should include all attempts to achieve authentication and authorization by any person. It is especially important to log "anonymous" or "guest" access to public servers. The data should also log all attempts by users to change their access rights.

The collected data should include user- and hostnames for login and logout attempts, and previous and new access rights for a change of access rights. Each entry in the audit log should be timestamped.

The audit process should not collect passwords. Collecting passwords creates a potential for a security breach if the audit records are improperly accessed. Neither correct nor incorrect passwords should be collected. An incorrect password often differs from the valid password by only a single character or transposition of characters.

A further extension of auditing is the concept of security assessment. With security assessment, the network is examined from within by professionals, trained in the vulnerabilities exploited by network invaders. Part of any security policy and audit procedure should be periodic assessments of the vulnerabilities in a network. The result should be a specific plan for correcting deficiencies, which might be as simple as retraining staff.

Data Encryption

Encryption is a process that scrambles data to protect it from being read by anyone but the intended receiver. An encryption device encrypts data before placing it on a network. A decryption device decrypts the data before passing it to an application. A router, server, end system, or dedicated device can act as an encryption or decryption device. Data that is encrypted is called ciphered data (or simply encrypted data). Data that is not encrypted is called plain text or clear text.

Encryption is a useful security feature for providing data confidentiality. It can also be used to identify the sender of data. Although authentication and authorization should also protect the confidentiality of data and identify senders, encryption is a good security feature to implement in case the other types of security fail.

There are performance tradeoffs associated with encryption, however, as mentioned in the "Analyzing Security Tradeoffs" section earlier in the chapter. Encryption should be used when a customer has analyzed security risks and identified severe consequences if data is not kept confidential and the identity of senders of data is not guaranteed. On internal networks and networks that use the Internet simply for web browsing, email, and file transfer, encryption is usually not necessary. For organizations that connect private sites via the Internet, using virtual private networking (VPN), encryption is recommended to protect the confidentiality of the organization's data.

Encryption has two parts:

  • An encryption algorithm is a set of instructions to scramble and unscramble data.
  • An encryption key is a code used by an algorithm to scramble and unscramble data.

Children sometimes play with encryption by using a simple algorithm such as "find the letter on the top row and use the letter on the bottom row instead," and a key that might look something like the following table:

In this example, LISA is encrypted as WTFI. The key shows only uppercase letters, but there are many other possibilities also, including lowercase letters, digits, and so on. Most algorithms are more complex than the one in the children's example to avoid having to maintain a key that includes a value for each possible character.

The goal of encryption is that even if the algorithm is known, without the appropriate key, an intruder cannot interpret the message. This type of key is called a secret key. When both the sender and receiver use the same secret key, it is called a symmetric key. The Data Encryption Standard (DES) is the best known example of a symmetric key system. DES encryption is available for most routers and many server implementations.

Although secret keys are reasonably simple to implement between two devices, as the number of devices increases, the number of secret keys increases, which can be hard to manage. For example, a session between Station A and Station B uses a different key than a session between Station A and Station C, or a session between Station B and Station C, and so on. Asymmetric keys can solve this problem.

Public/Private Key Encryption

Public/private key encryption is the best known example of an asymmetric key system. With public/private key systems, each secure station on a network has a public key that is openly published or easily determined. All devices can use a station's public key to encrypt data to send to the station.

The receiving station decrypts the data using its own private key. Because no other device has the station's private key, no other device can decrypt the data, so data confidentiality is maintained. Mathematicians and computer scientists have written computer programs that identify special numbers to use for the keys so that the same algorithm can be used by both the sender and receiver, even though different keys are used. Figure 8-1 shows a public/private key system for data confidentiality.

Figure 8-1

Figure 8-1 Public/Private Key System for Ensuring Data Confidentiality

Public/private key systems provide both confidentiality and authentication features. Using asymmetric keys, a recipient can verify that a document really came from the user or host that it appears to have come from. For example, suppose you are sending your tax returns to the Internal Revenue Service (IRS). The IRS needs to know that the returns came from you and not from a hostile third party that wants to make it look like you owe more than you do.

You can encrypt your document or a part of your document with your private key, resulting in what is known as a digital signature. The IRS can decrypt the document, using your public key, as shown in Figure 8-2. If the decryption is successful, the document came from you because nobody else should have your private key.

Figure 8-2

Figure 8-2 Public/Private Key System for Sending a Digital Signature

The digital signature feature of asymmetric keys can be used with the feature for data confidentiality. After encrypting your document with your private key, you can also encrypt the document with the IRS's public key. The IRS decrypts the document twice. If the result is plain-text data, the IRS knows that the document came from you and that you meant for the document to go to the IRS and not anyone else.

Some examples of asymmetric key systems include the Rivest, Shamir, and Adleman (RSA) standard, the Diffie-Hellman public key algorithm, and the Digital Signature Standard (DSS). Cisco uses the DSS standard to authenticate peer routers during the setup of an encrypted session. The peer routers use the Diffie-Hellman algorithm to send information on a secret key to use to encrypt data. The actual data is encrypted using the DES algorithm and the secret key.

Packet Filters

Packet filters can be set up on routers, firewalls, and servers to accept or deny packets from particular addresses or services. Packet filters augment authentication and authorization mechanisms. They help protect network resources from unauthorized use, theft, destruction, and DoS attacks.

A security policy should state whether packet filters implement one or the other of the following policies:

  • Deny specific types of packets and accept all else
  • Accept specific types of packets and deny all else

The first policy requires a thorough understanding of specific security threats and can be hard to implement. The second policy is easier to implement and more secure because the security administrator does not have to predict future attacks for which packets should be denied. The second policy is also easier to test because there is a finite set of accepted uses of the network. To do a good job implementing the second policy requires a good understanding of network requirements. The network designer should work with the security administrator to determine what types of packets should be accepted.

Cisco implements the second policy in its packet filters, which Cisco calls access control lists (ACL). An ACL on a router or switch running Cisco IOS Software always has an implicit deny-all statement at the end. Specific accept statements are processed before the implicit deny-all statement. (The statement is implicit because the administrator does not have to actually enter it, although it is a good idea to enter it to make the behavior of the list more obvious.)

ACLs let you control whether network traffic is forwarded or blocked at interfaces on a router or switch. ACL definitions provide criteria that are applied to packets that enter or exit an interface. Typical criteria are the packet source address, the packet destination address, or the upper-layer protocol in the packet.

Because Cisco IOS Software tests a packet against each criteria statement in the list until a match is found, ACLs should be designed with care to provide good performance. By studying traffic flow, you can design the list so that most packets match the earliest conditions. Fewer conditions to check per packet means better throughput. Good advice for designing ACLs is to order the list with the most general statements at the top and the most specific statements at the bottom, with the last statement being the general, implicit deny-all statement.


As discussed in Chapter 5, a firewall is a device that enforces security policies at the boundary between two or more networks. A firewall can be a router with ACLs, a dedicated hardware appliance, or software running on a PC or UNIX system. Firewalls are especially important at the boundary between the enterprise network and the Internet.

A firewall has a set of rules that specifies which traffic should be allowed or denied. A static stateless packet-filter firewall looks at individual packets and is optimized for speed and configuration simplicity. A stateful firewall can track communication sessions and more intelligently allow or deny traffic. For example, a stateful firewall can remember that a protected client initiated a request to download data from an Internet server and allow data back in for that connection. A stateful firewall can also work with protocols, such as active (port-mode) FTP, that require the server to also open a connection to the client.

Another type of firewall is a proxy firewall. Proxy firewalls are the most advanced type of firewall but also the least common. A proxy firewall acts as an intermediary between hosts, intercepting some or all application traffic between local clients and outside servers. Proxy firewalls examine packets and support stateful tracking of sessions. These types of firewalls can block malicious traffic and content that is deemed unacceptable.

Intrusion Detection and Prevention Systems

An intrusion detection system (IDS) detects malicious events and notifies an administrator, using email, paging, or logging of the occurrence. An IDS can also perform statistical and anomaly analysis. Some IDS devices can report to a central database that correlates information from multiple sensors to give an administrator an overall view of the real-time security of a network. An intrusion prevention system (IPS) can dynamically block traffic by adding rules to a firewall or by being configured to inspect (and deny or allow) traffic as it enters a firewall. An IPS is an IDS that can detect and prevent attacks.

There are two types of IDS devices:

  • Host IDS: Resides on an individual host and monitors that host.
  • Network IDS: Monitors all network traffic that it can see, watching for predefined signatures of malicious events. A network IDS is often placed on a subnet that is directly connected to a firewall so that it can monitor the traffic that has been allowed and look for suspicious activity.

In the past a major concern with both IDS and IPS devices was the volume of false alarms that they tended to generate. A false alarm occurs when an IDS or IPS reports a network event as a serious problem when it actually isn't a problem. This false-alarm problem has been ameliorated by sophisticated software and services on modern IPS devices. Cisco IPS solutions, for example, include anomaly detection that learns about typical actual network traffic on a customer's network and alarms only upon deviation from that traffic. Cisco also supports reputation filtering and global correlation services so that an IPS can keep up-to-date on global security trends and more accurately deny traffic from networks known to be currently associated with botnets, spam, and other malware.

3. Modularizing Security Design | Next Section Previous Section

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