Guide to SSL VPNs - NIST

1 Special Publication 800-113 Guide to SSL VPNs Recommendations of the National Institute of Standards and Technology Sheila Frankel Paul Hoffman Angela orebaugh Richard Park Guide to SSL VPNs Recommendations of the National Institute of Standards and Technology Sheila Frankel Paul Hoffman Angela orebaugh Richard Park NIST Special Publication 800-113 C O M P U T E R S E C U R I T YComputer Security Division Information Technology Laboratory National Institute of Standards and Technology Gaithersburg, MD 20899-8930 July 2008 Department of Commerce Carlos M. Gutierrez, Secretary National Institute of Standards and Technology James M. Turner, Deputy Director Guide TO SSL VPNS Reports on Computer Systems Technology The Information Technology Laboratory (ITL) at the National Institute of Standards and Technology (NIST) promotes the economy and public welfare by providing technical leadership for the nation s measurement and standards infrastructure.

2 ITL develops tests, test methods, reference data, proof of concept implementations, and technical analysis to advance the development and productive use of information technology. ITL s responsibilities include the development of technical, physical, administrative, and management standards and guidelines for the cost-effective security and privacy of sensitive unclassified information in Federal computer systems. This Special Publication 800-series reports on ITL s research, guidance, and outreach efforts in computer security and its collaborative activities with industry, government, and academic organizations. Certain commercial entities, equipment, or materials may be identified in this document in order to describe an experimental procedure or concept adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the entities, materials, or equipment are necessarily the best available for the purpose.

3 National Institute of Standards and Technology Special Publication 800-113 Natl. Inst. Stand. Technol. Spec. Publ. 800-113, 87 pages (July 2008) iiGUIDE TO SSL VPNS Acknowledgements The authors, Sheila Frankel of the National Institute of Standards and Technology (NIST), Paul Hoffman of the Virtual Private Network Consortium (VPNC), and Angela orebaugh and Richard Park of Booz Allen Hamilton, wish to thank their colleagues who reviewed drafts of this document and contributed to its technical content, especially Elaine Barker, Jim St. Pierre and Tim Polk of NIST. The authors would like to acknowledge Tim Grance and Karen Scarfone of NIST for their keen and insightful assistance throughout the development of the document. The authors particularly want to thank Guy Snyder, Darren Hartman and Thang Phan of ICSA Labs for their careful review and valuable contributions to improving the quality of this publication.

4 The authors would also like to express their thanks to Mike Hillhouse of Juniper Networks and Mahesh Jethanandani of Cisco for their worthwhile comments and suggestions. iiiGUIDE TO SSL VPNS ivTable of Contents Executive ES-1 1. Authority ..1-1 Purpose and Document 2. Network and Transport Layer The Need for Network and Transport Layer Virtual Private Networking (VPN)..2-4 SSL Portal SSL Tunnel Administering SSL 3. SSL VPN SSL VPN SSL VPN SSL VPN Features and Security High Availability and Portal Encryption and Integrity Access Endpoint Security Controls ..3-8 Intrusion SSL Protocol Versions of SSL and Cryptography Used in SSL Authentication Used for Identifying SSL SSL VPN 4. SSL VPN Planning and Identify SSL VPNs and FIPS 140-2 Versions of SSL ..4-3 Key Establishment Used by Hash Functions Used by SSL Certificates Used During SSL Design the Design the Access Control Design the Endpoint Security Select the Authentication Design the Guide TO SSL VPNS Cryptography Policy and FIPS Other Design Summary of Design Implement and Test Application and Client Deploy the Solution.

5 4-28 Manage the 5. SSL VPN Recommended Practices ..5-1 6. Alternatives to SSL Data Link Layer VPN Network Layer VPN Protocols ..6-2 Application Layer VPNs ..6-3 7. Case Identifying Needs and Evaluating Options ..7-1 Designing the Access Control Endpoint Security Authentication Architecture Selection of Hardware Device Placement and Firewall Routing High Client Software Portal Encryption Implementing a Example configuration steps ..7-6 Deploying and Managing the List of Appendices Appendix A Glossary .. A-1 Appendix B B-1 Guide TO SSL VPNS List of Figures Figure 2-1. TCP/IP Layers ..2-1 Figure 3-1. SSL VPN Architecture ..3-2 Figure 4-1. Firewall with SSL VPN Functionality ..4-14 Figure 4-2. SSL VPN Device in Internal Network ..4-15 Figure 4-3. SSL VPN Device in DMZ Figure 4-4. SSL VPN Device with Two Interfaces.

6 4-17 Figure 4-5. Routing Problem with SSL VPN Traffic ..4-20 Figure 4-6. Example Portal Interface ..4-23 List of Tables Table 3-1. Access Control Examples ..3-7 Table 3-2. Access Control Examples with Endpoint Security Controls ..3-8 Table 4-1. Sample Access Control Policy ..4-9 Table 4-2. Sample Authentication Methods Table ..4-12 Table 4-3. Design Decisions Checklist ..4-24 Table 5-1. SSL VPN Life Cycle Phase Table 6-1. Comparison of SSL and Alternatives ..6-4 Table 6-2. IP Protocols and TCP/UDP Port Numbers for VPN Protocols ..6-5 Table 7-1. Organization s Access Control Policy ..7-3 Table 7-2. Organization s Authentication Methods ..7-4 viGUIDE TO SSL VPNS Executive Summary Secure Sockets Layer (SSL) virtual private networks (VPN) provide secure remote access to an organization s resources. A VPN is a virtual network, built on top of existing physical networks, that can provide a secure communications mechanism for data and other information transmitted between two endpoints.

7 Because a VPN can be used over existing networks such as the Internet, it can facilitate the secure transfer of sensitive data across public networks. An SSL VPN consists of one or more VPN devices to which users connect using their Web browsers. The traffic between the Web browser and the SSL VPN device is encrypted with the SSL protocol or its successor, the Transport Layer Security (TLS) protocol. This type of VPN may be referred to as either an SSL VPN or a TLS VPN. This Guide uses the term SSL VPN. SSL VPNs provide remote users with access to Web applications and client/server applications, and connectivity to internal networks. Despite the popularity of SSL VPNs, they are not intended to replace Internet Protocol Security (IPsec) The two VPN technologies are complementary and address separate network architectures and business needs. SSL VPNs offer versatility and ease of use because they use the SSL protocol, which is included with all standard Web browsers, so the client usually does not require configuration by the user.

8 SSL VPNs offer granular control for a range of users on a variety of computers, accessing resources from many locations. There are two primary types of SSL VPNs: SSL Portal VPNs. This type of SSL VPN allows a user to use a single standard SSL connection to a Web site to securely access multiple network services. The site accessed is typically called a portal because it is a single page that leads to many other resources. The remote user accesses the SSL VPN gateway using any modern Web browser, identifies himself or herself to the gateway using an authentication method supported by the gateway, and is then presented with a Web page that acts as the portal to the other services. SSL Tunnel VPNs. This type of SSL VPN allows a user to use a typical Web browser to securely access multiple network services, including applications and protocols that are not web-based, through a tunnel that is running under SSL.

9 SSL tunnel VPNs require that the Web browser be able to handle active content, which allows them to provide functionality that is not accessible to SSL portal VPNs. Examples of active content include Java, JavaScript, Active X, or Flash applications or plug-ins. This publication discusses the fundamental technologies and features of SSL VPNs. It describes SSL and how it fits within the context of layered network security. It presents a phased approach to SSL VPN planning and implementation that can help in achieving successful SSL VPN deployments. It also compares the SSL VPN technology with IPsec VPNs and other VPN solutions. This information is particularly valuable for helping organizations to determine how best to deploy SSL VPNs within their specific network environments. Implementing the following recommendations should assist in facilitating more efficient and effective SSL VPN use for Federal departments and agencies.

10 Federal agencies deploying SSL VPNs must configure them to only allow FIPS-compliant cryptographic algorithms, cipher suites, and versions of SSL. Some organizations, such as Federal agencies, have strict requirements for encryption and integrity protection. SSL VPNs should support the required algorithms for symmetric encryption, key exchange, and hash functions. For government agencies, traffic that requires protection must employ Federal 1 Detailed information on the IPsec components may be found in NIST Special Publication (SP) 800-77, Guide to IPsec VPNs, available from ES-1 Guide TO SSL VPNS Information Processing Standard (FIPS)-compliant cryptographic algorithms and modules. Many of the cryptographic algorithms used in some SSL cipher suites are not FIPS-approved, and therefore are not allowed for use in SSL VPNs that are to be used in applications that must conform to FIPS 140-2.