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Pearson Education
Cisco Secure Virtual Private Networks

Cisco Secure Virtual Private Networks

by Andrew G. Mason, Rick Stiffler


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Plan, implement, and manage Cisco VPNs with the official CSVPN Coursebook

  • Reduce network cost, enable network scalability, and increase remote access efficiency by deploying Cisco-based virtual private networks
  • Secure remote access connections to corporate networks with IPSec
  • Based on officially developed course materials from Cisco Systems
  • Recommended training materials for the Cisco security specialization certification
  • Apply your knowledge immediately by following the task-based configuration techniques

Plan, implement, and manage Cisco VPNs with the official CSVPN coursebook. Cisco Secure Virtual Private Networks is based on the official training course of the same name, and gives you the knowledge to plan, administer, and maintain a Virtual Private Network (VPN). Learn how to reduce network cost, enable network scalability, and increase remote access efficiency by deploying Cisco-based VPNs. You will also learn to identify the features, functions, and benefits of Cisco Secure VPN products; identify the component technologies implemented in Cisco Secure VPN products; use commands required to configure and test IPSec in Cisco IOS software and PIX Firewalls; secure remote access connections to corporate networks with IPSec; install and configure the Cisco VPN Client to create a secure tunnel to a Cisco VPN Concentrator and PIX Firewall; and configure the Cisco VPN Concentrator, Cisco router, and PIX Firewall for interoperability.

Product Details

ISBN-13: 9781587050336
Publisher: Pearson Education
Publication date: 07/28/2002
Series: Cisco Qualified Specialist Training Series
Pages: 388
Product dimensions: 7.54(w) x 9.42(h) x 1.22(d)

About the Author

Andrew Mason is the CEO of Mason Technologies Limited, a Cisco Premier Partner in the U.K. whose main business is delivered through Cisco consultancy focusing on Internet security. Andrew has hands-on experience of the Cisco Secure product family with numerous clients ranging from ISPs to large financial organizations. Currently, Andrew is leading a project to design and implement the most secure ISP network in Europe. Andrew holds the Cisco CCNP and CCDP certifications.

Read an Excerpt

Chapter 1: VPNs and VPN Technologies

This chapter defines virtual private networks (VPNs) and explores fundamental Internet Protocol Security (IPSec) technologies. This chapter covers the following topics:
  • Overview of VPNs and VPN technologies
  • Internet Protocol Security (IPSec)
  • IPSec crypto components
  • IKE overview
  • How IPSec works
  • IPSec security associations
  • CA support overview

Overview of VPNs and VPN Technologies

Cisco products support the latest in VPN technology. A VPN is a service that offers secure, reliable connectivity over a shared public network infrastructure such as the Internet.

Figure 1-1 shows various VPNs between a main site and branch offices and small office, home office (SOHO) workers.

VPNs maintain the same security and management policies as a private network. They are the most cost effective method of establishing a virtual point-to-point connection between remote users and an enterprise customer's network. There are three main types of VPNs.

  • Access VPNs—Provide remote access to an enterprise customer's intranet or extranet over a shared infrastructure. Access VPNs use analog, dial, ISDN, digital subscriber line (DSL), mobile IP, and cable technologies to securely connect mobile users, telecommuters, and branch offices.

  • Intranet VPNs—Link enterprise customer headquarters, remote offices, and branch offices to an internal network over a shared infrastructure using dedicated connections. Intranet VPNs differ from extranet VPNs in that they allow access only to the enterprise customer's employees.

  • Extranet VPNs—Link outside customers, suppliers, partners, or communities of interest to an enterprise customer's network over a shared infrastructure using dedicated connections. Extranet VPNs differ from intranet VPNs in that they allow access to users outside the enterprise.
The following main components make up Cisco's VPN offerings:
  • Cisco VPN routers—Use Cisco IOS software IPSec support to enable a secure VPN. VPN-optimized routers leverage existing Cisco investment, perfect for the hybrid WAN.

  • Cisco Secure PIX Firewall—Offers a VPN gateway alternative when the security group "owns" the VPN.

  • Cisco VPN Concentrator series—Offers powerful remote access and site-to-site VPN capability, easy-to-use management interface, and a VPN client.

  • Cisco Secure VPN Client—Enables secure remote access to Cisco router and PIX Firewalls and runs on the Windows operating system.

  • Cisco Secure Intrusion Detection System (CSIDS) and Cisco Secure Scanner— Can be used to monitor and audit the security of the VPN.

  • Cisco Secure Policy Manager and Cisco Works 2000—Provide VPN-wide system management.
These components can all be seen in Figure 1-2....

The main Cisco VPN product offerings are discussed in more detail in Chapter 2, "Cisco VPN Family of Products."

Internet Protocol Security (IPSec)

Cisco IOS uses the industry-standard IPSec protocol suite to enable advanced VPN features. The PIX IPSec implementation is based on the Cisco IOS IPSec that runs in Cisco routers.

IPSec acts at the network layer, protecting and authenticating IP packets between a PIX Firewall and other participating IPSec devices (peers), such as other PIX Firewalls, Cisco routers, the Cisco Secure VPN Client, the VPN 3000 Concentrator series, and other IPSec-compliant products.

IPSec enables the following Cisco IOS VPN features:

  • Data confidentiality—The IPSec sender can encrypt packets before transmitting them across a network.
  • Data integrity—The IPSec receiver can authenticate packets sent by the IPSec sender to ensure that the data has not been altered during transmission.
  • Data origin authentication—The IPSec receiver can authenticate the source of the IPSec packets sent. This service is dependent upon the data integrity service.
  • Antireplay—The IPSec receiver can detect and reject replayed packets.

IPSec Overview

IPSec is a framework of open standards that provides data confidentiality, data integrity, and data authentication between participating peers at the IP layer. IPSec can be used to protect one or more data flows between IPSec peers. IPSec is documented in a series of Internet RFCs, all available at The overall IPSec implementation is guided by "Security Architecture for the Internet Protocol," RFC 2401. IPSec consists of the following two main protocols:
  • Authentication Header (AH)
  • Encapsulating Security Payload (ESP)
IPSec also uses other existing encryption standards to make up a protocol suite, which are explained in the next sections. IPSec has several standards that are supported by Cisco IOS and the PIX Firewall.
  • IP Security Protocol
    • Authentication Header (AH)
    • Encapsulating Security Payload (ESP)
  • Data Encryption Standard (DES)
  • Triple DES (3DES)
  • Diffie-Hellman (D-H)
  • Message Digest 5 (MD5)
  • Secure Hash Algorithm-1 (SHA-1)
  • Rivest, Shamir, and Adelman (RSA) Signatures
  • Internet Key Exchange (IKE)
  • Certificate Authorities (CAs)
IP Security Protocol—Authentication Header (AH)
Authentication Header (AH) provides authentication and integrity to the datagrams passed between two systems.

It achieves this by applying a keyed one-way hash function to the datagram to create a message digest. If any part of the datagram is changed during transit, it will be detected by the receiver when it performs the same one-way hash function on the datagram and compares the value of the message digest that the sender has supplied. The one-way hash also involves the use of a secret shared between the two systems, which means that authenticity can be guaranteed.

AH can also enforce antireplay protection by requiring that a receiving host sets the replay bit in the header to indicate that the packet has been seen. Without this protection, an attacker might be able to resend the same packet many times: for example, to send a packet that contains "withdraw $100 from account X." Figure 1-3 shows two routers and confirms that the data between them is sent in clear text.

The AH function is applied to the entire datagram except for any mutable IP header fields that change in transit: for example, Time to Live (TTL) fields that are modified by the routers along the transmission path. AH works as follows:

Step 1 The IP header and data payload is hashed.

Step 2 The hash is used to build a new AH header, which is appended to the original packet.

Step 3 The new packet is transmitted to the IPSec peer router.

Step 4 The peer router hashes the IP header and data payload, extracts the transmitted hash from the AH header, and compares the two hashes. The hashes must match exactly. Even if one bit is changed in the transmitted packet, the hash output on the received packet will change and the AH header will not match.

This process can be seen in Figure 1-4....

IP Security Protocol—Encapsulating Security Payload (ESP)

Encapsulating Security Payload (ESP) is a security protocol used to provide confidentiality (encryption), data origin authentication, integrity, optional antireplay service, and limited traffic flow confidentiality by defeating traffic flow analysis. Figure 1-5 shows that the data payload is encrypted with ESP.

ESP provides confidentiality by performing encryption at the IP packet layer. It supports a variety of symmetric encryption algorithms. The default algorithm for IPSec is 56-bit DES. This cipher must be implemented to guarantee interoperability among IPSec products. Cisco products also support use of 3DES for strong encryption. Confidentiality can be selected independent of all other services.

NOTE Deciding whether to use AH or ESP in a given situation might seem complex, but it can be simplified to a few rules, as follows. When you want to make sure that data from an authenticated source gets transferred with integrity and does not need confidentiality, use the AH protocol. If you need to keep data private (confidentiality), then you must use ESP. ESP will encrypt the upper-layer protocols in transport mode and the entire original IP datagram in tunnel mode so that neither are readable from the wire. However, ESP can now also provide authentication for the packets. This situation is covered later in this chapter in the "ESP Tunnel Versus Transport Mode" section.

DES Algorithm
DES uses a 56-bit key, ensuring high-performance encryption. DES is used to encrypt and decrypt packet data. DES turns clear text into ciphertext with an encryption algorithm. The decryption algorithm on the remote end restores clear text from ciphertext. Shared secret keys enable the encryption and decryption.

Triple DES Algorithm (3DES)
Triple DES (3DES) is also a supported encryption protocol for use in IPSec on Cisco products. The 3DES algorithm is a variant of the 56-bit DES. 3DES operates similarly to DES in that data is broken into 64-bit blocks. 3DES then processes each block three times, each time with an independent 56-bit key. 3DES effectively doubles encryption strength over 56-bit DES.

Diffie-Hellman (D-H)
Diffie-Hellman (D-H) is a public-key cryptography protocol. It allows two parties to establish a shared secret key used by encryption algorithms (DES or MD5, for example) over an insecure communications channel. D-H is used within IKE to establish session keys. 768-bit and 1024-bit D-H groups are supported in the Cisco routers and PIX Firewall. The 1024-bit group is more secure because of the larger key size.

Message Digest 5 (MD5)
Message Digest 5 (MD5) is a hash algorithm used to authenticate packet data. Cisco routers and the PIX Firewall use the MD5 hashed message authentication code (HMAC) variant that provides an additional level of hashing. A hash is a one-way encryption algorithm that takes an input message of arbitrary length and produces a fixed length output message. IKE, AH, and ESP use MD5 for authentication.

Secure Hash Algorithm-1 (SHA-1)
Secure Hash Algorithm-1 (SHA-1) is a hash algorithm used to authenticate packet data. Cisco routers and the PIX Firewall use the SHA-1 HMAC variant, which provides an additional level of hashing. IKE, AH, and ESP use SHA-1 for authentication.

Rivest, Shamir, and Adelman (RSA) Signatures
Rivest, Shamir, and Adelman (RSA) is a public-key cryptographic system used for authentication. IKE on the Cisco router or PIX Firewall uses a D-H exchange to determine secret keys on each IPSec peer used by encryption algorithms. The D-H exchange can be authenticated with RSA signatures or preshared keys.

Internet Key Exchange (IKE)
Internet Key Exchange (IKE) is a hybrid protocol that provides utility services for IPSec: authentication of the IPSec peers, negotiation of IKE and IPSec security associations, and establishment of keys for encryption algorithms used by IPSec....

Table of Contents

I: Virtual Private Network Fundamentals
1: VPNs and VPN Technologies

II: Cisco VPN Family of Products
2: Cisco VPN Family of Products

3: Configuring Cisco IOS Routers for Preshared Keys Site-to-Site
4: Configuring Cisco IOS Routers for CA Site-to-Site
5: Troubleshooting Cisco IOS VPNs

IV: Cisco PIX Firewall VPNs
6: Configuring the Cisco PIX Firewall for Preshared Keys Site-to-Site
7: Configuring the Cisco PIX Firewall for CA Site-to-Site
8: Troubleshooting Cisco PIX Firewall VPNs

V: Cisco VPN Concentrator VPNs
9: Configuring the Cisco VPN 3000 for Remote Access Using Preshared Keys
10: Configuring the Cisco VPN 3000 for Remote Access Using Digital Certificates
11: Monitoring and Administration of Cisco VPN 3000 Remote Access Networks

VI: Scaling Cisco VPN Solutions
12: Scaling Cisco IPSec Virtual Private Networks

Appendix A: Answers to Review Questions

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