Security Blueprint for Enterprise Networks

"It is impossible for ideas to compete in the marketplace if no forum for
their presentation is provided or available." Thomas Mann, 1896

A Security Blueprint for Enterprise Networks

Authors: Sean Convery and Bernie Trudel are the original authors of this White Paper
The second version was authored by Greg Abelar and Jason Halpern of the SAFE Architecture Team.

Contributed by Cisco Systems, Inc.

Introduction

The SAFE Blueprint from Cisco Systems® is a secure blueprint for enterprise networks. Its principle goal is to provide best practices information on designing and implementing secure networks. SAFE takes a defense-in-depth approach to network security design, serving as a guide to network designers considering the security requirements of their networks. This type of design focuses on expected threats and their methods of mitigation, resulting in a layered approach to security where the failure of one security system is not likely to lead to the compromise of the rest of the network. Although this white paper is a product-agnostic document, the SAFE proof-of-concept lab is based on products from Cisco and its partners.

This document begins with an overview of the blueprint’s architecture, and then details the specific modules that make up the actual network design. When discussing each module, the first three sections describe the traffic flows, primary devices, and expected threats, with basic mitigation diagrams. Detailed technical analysis of the design follows, along with more detailed threat mitigation techniques and migration strategies.

Appendix A details the validation lab for SAFE and includes configuration snapshots.

Appendix B is a primer on network security. Readers unfamiliar with basic network security concepts are encouraged to read Appendix B before the rest of the document.

Appendix C contains definitions of the technical terms used in this document, and a legend for the included figures.

This document focuses on threats encountered in enterprise environments. Network designers who understand these threats can better decide where and how to deploy mitigation technologies. Without this understanding, deployments tend to be incorrectly configured, too focused on security devices, or lacking in threat response options. By taking the threat mitigation approach, this document provides network designers with information for making sound network security choices.

In addition to this enterprise document, Cisco has published several companion papers that address security issues for specific technologies and smaller-scaled networks (small, midsized, and remote).

These detailed documents can be found at the SAFE library at: www.cisco.com/go/safe and include:

SAFE: Extending the Security Blueprint to Small, Midsize, and Remote-User Networks
SAFE: IPSec Virtual Private Networks in Depth
SAFE: Wireless LAN Security in Depth — Version 2
SAFE: IP Telephony Security in Depth
SAFE: IDS Deployment, Tuning, and Logging in Depth

In addition, the SAFE library contains documents that provide a step-by-step analysis for combating specific high-profile network attacks. These are also located at www.cisco.com/go/safe and include:

SAFE: Worm Mitigation
SAFE: Layer 2 Best Practices

Audience

Though this document is technical in nature, it can be read at different levels of detail, depending on the reader. A network manager, for example, can read the introductory sections in each area to obtain an overview of network security design strategies and considerations. A network engineer or designer can read this document in its entirety and gain design information and threat analysis details, which are supported by configuration snapshots for the devices involved.

Caveats

This document presumes that you already have a security policy in place. Cisco does not recommend deploying security technologies without an associated policy. For further information about security policies and their use, consult the SANS Security Policy Project at: http://www.cisco.com/go/safe

This document directly addresses the needs of large enterprise customers. Readers interested in security best practices for smaller networks should read “SAFE: Extending the Security Blueprint to Small, Midsize, and Remote-User Networks” mentioned above.

Following the guidelines in this document does not guarantee a secure environment, or that you will prevent all intrusions. However, you can achieve reasonable security by establishing a good security policy, following the guidelines in this document, staying up to date on the latest developments in the hacker and security communities, and maintaining and monitoring all systems with sound system administration practices. This includes awareness of application security issue that are not comprehensively addressed in this paper.

Although VPNs are part of this architecture, they are not described in great detail in this document. Scaling details, resilience strategies, and other topics related to VPNs are covered in more detail in “SAFE VPN: IPSec Virtual Private Networks in Depth.” Like VPNs, identity strategies (including certificate authorities) are not discussed at any level of detail in this paper.

Wireless and IP telephony are also part of this architecture, but are not described in great detail in this document. More information is available in the “SAFE: Wireless LAN Security in Depth — Version 2” and “SAFE: IP Telephony Security in Depth” papers.

The SAFE blueprint uses products from Cisco and its partners. In this document, however, components are referred to by functional purpose rather than model number or name. During the validation of SAFE, real products were configured in the exact network implementations described in this document. Specific configuration snapshots from the lab are included in Appendix A.

Architecture Overview

Design Fundamentals

SAFE emulates as closely as possible the functional requirements of today’s enterprise networks. Implementation decisions varied depending on the network capabilities required. However, the following design objectives, listed in order of priority, guided the decision-making process.

Security and attack mitigation based on policy
Security implementation throughout the infrastructure (not just on specialized security devices)
Secure management and reporting
Authentication and authorization of devices, users, and administrators to critical network resources
Intrusion detection and prevention for critical resources and subnets
Support for emerging networked applications

First and foremost, SAFE is a security architecture. It must prevent most attacks from successfully affecting valuable network resources. The attacks that succeed in penetrating the first line of defense, or that originate from inside the network, must be accurately detected and quickly contained to minimize their effect on the rest of the network.

However, in being secure, the network must continue to provide critical services that users expect. Proper network security and good network functioning can be provided at the same time — the SAFE architecture is not a revolutionary way of designing networks, but merely a blueprint for making networks secure.

SAFE is also resilient and scalable. Resilience in networks includes physical redundancy to protect against device failure, whether from misconfiguration, physical failure, or network attack. Although simpler designs are possible, particularly if a network’s performance needs are not great, this document uses a complex design as an example because designing security in a complex environment is more involved than in simpler environments. Options to limit the complexity of the design are discussed throughout this document as well as in the other SAFE documents listed earlier. At many points in the network design process, an enterprise will need to choose between a network device with integrated functions and a specialized functional appliance. Integrated functioning is attractive because you can implement it on exiting equipment, the features can interoperate with the rest of the device to provide a better functional solution, or the features can be deployed incrementally to facilitate increased bandwidth requirements.

Appliances are often used when the depth of capability required is advanced or when performance needs require using specialized hardware (see Appendix D for information regarding integrated security blades for Layer 3 switches versus appliances). Decisions should be based on the capacity and capability of the appliance, not the integration advantage of the device. For example, sometimes you can choose an integrated higher-capacity router operating Cisco IOS® Software with the firewall feature, as opposed to a smaller Cisco IOS Software-based router with a separate firewall device. Throughout this architecture, both types of systems are used. Historically, most critical security functions have migrated toward dedicated appliances because of the performance requirements of large enterprise networks. Recently, however, integrated equipment has become much more attractive because of performance and capability enhancements. A security specialist now has more viable options when choosing between security appliances and integrated devices. If flexibility for expansion is a high priority, then line cards that plug into Layer 3 switches, routers, or PN concentrators may be attractive options.

Because a security architecture is an end-to-end concern, this paper will address security issues ranging from the network perimeter to the host application, and all elements in between.

SAFEv2 Updates

This document contains updates to the original SAFE document, published in the summer of 2000. Changes reflect new technologies in the security market between summer of 2000 and September 2003. These changes include the way enterprises are using the network to do business, and the way that hackers have chosen to exploit networks.

Enterprises are now extending the perimeter of the network, allowing partner connectivity, telecommuting, and application service provider (ASP) connectivity. Hackers are using more sophisticated techniques to sniff passwords, exploit Layer 2 switches, exploit routing protocols, and propagate worms that install malicious code on hosts. These and many more issues have implications that need to be addressed by modern security techniques. The technology and best practices added to this document to address these changes include:

Layer 2 attack mitigation schemes
Router hardening
Integrated security modules that plug into Layer 3 switches, including firewall, network intrusion detection system (IDS), Secure Sockets Layer (SSL) termination, and VPN termination
IDS deployment best practices
Design and best practices for a three-tier data center
Design and best practices for building a secure lab within an enterprise
Best practices for using host intrusion prevention software (HIPS) in the enterprise
Content-aware devices that can filter for viruses, proxy Web connections, and authenticate users
802.1x best practices
Recommendations for Simple Network Management Protocol Version 3 (SNMPv3), Secure Shell (SSH) Protocol, and SSL (encrypted management methods)

Module Concept

Although most enterprise networks have evolved with growing IT requirements, the SAFE architecture uses a modular approach, which has two main advantages. First, it allows the architecture to address the security relationship between the various functional blocks of the network. Second, it permits designers to evaluate and implement s curity on a module-by-module basis, instead of attempting the complete architecture i.e. a single phase.

Figure 1 illustrates the first layer of modularity in SAFE. Each block represents a functional area. The Internet service provider (ISP) module is not implemented by the enterprise, but is included to the extent that specific security features should be requested of an ISP in order to mitigate against certain attacks.

The second layer of modularity represents a view of the modules within each functional area (Figure 2). These modules perform specific roles in the network and have specific security requirements, but their sizes are not meant to reflect their scale in a real network. For example, the building module, which represents the end-user devices, may include 80 percent of the network devices. The security design of each module is described separately, but is validated as part of the complete enterprise design.

While most existing enterprise networks cannot be easily dissected into clear-cut modules, this approach provides a guide for implementing different security functions throughout the network. The authors do not expect network engineers to design networks identical to the SAFE implementation, but rather to use a combination of the modules described and integrate them into the existing network.

SAFE Axioms

This section outlines general best practices that apply to the entire SAFE blueprint. They are addressed here to avoid duplication throughout the individual modules.

Routers Are Targets

Router security is a critical element in any security deployment. Routers control access from every network to every network. They advertise networks and filter who can use them, and are potentially an attacker’s best friend. They should be secured to reduce the likelihood that they can be directly compromised. When securing routers, the primary areas of focus are as follows:

Locking down Telnet access to a router or using more secure methods such as SSH
Locking down SNMP access to a router
Controlling access to a router through the use of TACACS+
Turning off unneeded services
Logging at appropriate levels
Authentication of routing updates
Enabling switching functions, such as Cisco Express Forwarding, on routers that will use a fast switching path for new packet flows

Many software tools are now available that audit router configurations. One of these tools, the Router Audit Tool (RAT), is a freeware utility that compares existing router configurations to a baseline and suggests methods to increase security. Cisco IOS routers also support an integrated hardening feature known as AutoSecure that performs automatic lockdown of a router, following Cisco router hardening best practices. The following link provides more information on these topics:

Switches Are Targets

Like routers, both Layer 2 and Layer 3 switches have their own sets of network security requirements. Unlike routers, however, there is not much public information available that discusses the network security risks in switches and what can be done to mitigate those risks.

Switches use VLANs to provide Layer 2 traffic segmentation. Private VLANs provide additional traffic segmentation and a small measure of additional security within the VLAN. Private VLANs work by limiting which ports within a VLAN can communicate with other ports in the same VLAN. There are three categories of ports within a VLAN — isolated ports, community ports, and promiscuous ports. Isolated ports within a VLAN can communicate only with promiscuous ports; community ports can communicate only with other ports within the same community or promiscuous ports; and promiscuous ports can communicate with any other port. This is an effective way to mitigate the effects of a single compromised host on a network segment.

In the following example, there is a standard public services segment with three hosts — an FTP service, a Web, and a Domain Name System (DNS) server. If the DNS server is compromised, an attacker can pursue the other two hosts without passing back through the firewall. With private VLANs deployed, if one system is compromised, it cannot communicate with the other two systems on the public services segment. The only targets the attacker can pursue are the hosts on the other side of the firewall. As a second layer of defense, Dynamic Address Resolution Protocol (ARP) Inspection, IP spoofing protection, and Dynamic Host Control Protocol (DHCP) snooping protection should be considered.

By restricting Layer 2 connectivity, private VLANs may make troubleshooting networks more difficult, albeit more secure. Most of the network security techniques detailed in the “Routers Are Targets” section also apply to switches, which are subject to network attacks in unique ways. The following precautions and best practices should be used with respect to switches:

Disable all unused ports and put them in an unused VLAN. This helps prevent attackers from plugging into unused ports and communicating with the rest of the network.
Always use a dedicated VLAN ID for all trunk ports.
Use Dynamic ARP Inspection to ensure that Layer 2 attacks cannot compromise the ARP table of a switch and open the door, allowing hackers to sniff data off of the switch. If Dynamic ARP Inspection is not available, then private VLANs should be used to prevent hosts from capturing data from the local network segment.
Place all user ports in non-trunking mode to mitigate the possibility that an attacker will plug into the switch and spoof the system as another switch in trunking mode.
Avoid using VLAN 1 for management purposes and eliminate native VLANs from 802.1q trunks.
Deploy port security where possible for user ports.
Consider using Layer 2 port authentication such as 802.1X to authenticate clients attempting connectivity to a network.
Have a plan for possible ARP security issues in the network. This includes the use of DHCP snooping and IP source guard to protect against DHCP starvation, as well as Dynamic ARP Inspection to guard against MAC address spoofing.
Enable Spanning Tree Protocol attack mitigation (bridge protocol data unit [BPDU] Guard, Root Guard) to help mitigate the possibility of an attacker spoofing a root bridge in the network topology and successfully executing a man-in-the-middle attack.
Use private VLANs where appropriate to further divide Layer 2 networks.
Use Cisco Discovery Protocol only where appropriate. Attackers can use it to gain information about the devices on a network, including device model information and the version of software it is running.
Implement secure change control by use of VLAN Trunking Protocol (VTP) passwords to authenticate VTP advertisements.
Use procedures for change control and configuration analysis to ensure that changes result in a secure configuration. This is especially valuable in cases where several organizational groups may control the same switch, and even more valuable in network security deployments requiring even greater care.

Refer to the “SAFE Layer 2 Application Note” in the SAFE library for a more rigorous analysis of various attacks against Layer 2 devices, as well as how to mitigate those attacks.

Hosts Are Targets

The most likely target during an attack is the host, which presents some of the most difficult challenges from a security perspective. There are numerous hardware platforms, operating systems, and applications, all of which have updates, patches, and fixes available at different times. Because hosts provide the application services to other hosts that request them, they are extremely visible within the network. For example, many people have visited www.whitehouse.gov which is a host, but few have attempted to access s2-0.whitehouseisp.net, which is a router.

Because of this visibility and the fact that hosts usually contain critical data such as e-mail, they are the most frequently attacked devices in any network intrusion attempt.

In part because of the security challenges mentioned above, hosts are also the most successfully compromised devices. For example, a given Web server on the Internet might run a hardware platform, a network card, an operating system, and a Web server — all from different vendors. That same Web server might run applications that are freely distributed via the Internet, and might communicate with a database server that starts the variations all over again.

This is not to say that security vulnerabilities are specifically caused by multiple vendors or sources, but rather that as the complexity of a system increases, so does the likelihood of a failure.

To secure hosts, pay careful attention to each component within the systems. Keep any systems up to date with the latest patches and fixes. In particular, pay attention to how these patches affect the operation of other system components. Evaluate all updates on test systems before you implement them in a production environment. Failure to do so might result in the patch itself causing a denial of service (DoS) attack. Operating systems should be locked down. Tasks that should be done by an enterprise to secure its hosts include strong password enforcement, correcting file permissions set on shares, turning off unnecessary network services, and turning off all networking protocols that are not being used. For a description of lock-down techniques for specific operating systems, refer to the following links on SAN’s Website:

Other excellent Web sites are available as well. In aWeb search, use the word “hardening” and include the operating system name; many URLs will be returned.

Also, ensure that you have current virus and host intrusion prevention software. In previous versions of SAFE, this software was referred to exclusively as host intrusion detection, but that only described a small subset of the capabilities of the software. To reflect a the broader functionality, the name has been changed to host IPS, or HIPS.

HIPSs improve the security of hosts and servers by using rules that control operating system and network stack behavior. Processor control limits activity such as buffer overflows, registry updates, writes to the system directory, and the launching of installation programs. Regulation of network traffic can help to ensure that the host does not participate in accepting or initiating FTP sessions, can rate-limit when a DoS attack is detected, or can keep the network stack from participating in a DoS attack.

Because of this type of policy enforcement, IPSs are effective in mitigating what are known as “zero-day” attacks. In a zero-day attack, a worm or virus generally overflows a buffer, writes to the registry, or writes to the system directory. Mitigating zero-day attacks means that the first day a new attack hits the Internet, hosts and servers are protected because IPS software stops the behavior that infects the host or server. If a worm does not use common exploitations such as buffer overflows and system writes, an IPS may not effectively mitigate this type of attack.

Appendix B provides an in-depth discussion of zero-day attacks.

Networks Are Targets

Network attacks are among the most difficult attacks to deal with, because they typically take advantage of intrinsic characteristics in the way your network operates. These attacks include ARP- and MAC-based Layer 2 attacks, sniffers, and distributed denial-of-service (DDoS) attacks. Some of the ARP- and MAC-based Layer 2 attacks can be mitigated through best practices on switches and routers, and sniffers are discussed in Appendix B. DDoS, however, is a unique attack that deserves special attention.

The worst attack is the one that you cannot stop. When performed properly, DDoS is just such an attack. DDoS works by causing tens or hundreds of machines to simultaneously send spurious data to an IP address. The goal of such an attack is generally not to shut down a particular host, but rather to make the entire network unresponsive.

Consider an organization with a DS-1 (1.5 Mbps) connection to the Internet that provides e-commerce services to its Website users. The site is very security-conscious and has intrusion detection, firewalls, logging, and active monitoring. Unfortunately, none of these security devices help when a hacker launches a successful DDoS attack.

Consider 100 devices around the world, each with DSL (500 Kbps) connections to the Internet. If these systems are remotely told to flood the serial interface of the Internet router, they can easily flood the DS-1 with erroneous data.

Even if each host is able to generate only 100 Kbps of traffic (lab tests indicate that a stock PC can easily generate more than 50 Mbps with a popular DDoS tool), that amount is still almost ten times the amount of traffic that a site can handle. As a result, legitimate Web requests are lost, and the site appears to be down for most users. The local firewall drops all of the erroneous data, but the damage is done. The traffic has crossed the WAN connection and filled up the link.

More sophisticated attacks use port 80 (HTTP) traffic with the ACK bit set so that the traffic appears to be legitimate Web transactions. It is unlikely that an administrator could properly categorize such an attack, because acknowledged TCP communications are exactly the sort that you want to allow into your network. SYN floods appear to be multiple simultaneous incoming requests, but are intended to tie up resources blocking any new legitimate connections. Although stateful firewalls and other content inspection devices may mitigate these older attacks, more recent DDoS attacks initiate sessions that are perfectly protocol-legitimate. These are easily traced, but the sources of this traffic may be compromised systems or unsuspecting hosts serving as reflectors. For example, consider the sessions destined to a Website. If all requests adhere to the HTTP specification, it will not be possible to differentiate valid from illegitimate requests.

Only through cooperation with its ISP can an enterprise business hope to thwart such an attack. An ISP can configure rate limiting on the outbound interface to the company’s site. This rate limiting can drop most undesired traffic when it exceeds a pre-specified amount of the available bandwidth. The key is to correctly flag traffic as undesired. Another option that can be implemented by the ISP is black-hole routing. This process uses a combination of Border Gateway Protocol (BGP), DNS, and a static route to direct the malicious DDoS traffic to a null interface on a router, effectively keeping the attack from reaching its intended destination and moving the target to another address or network. For more information on black-hole and sink-hole routing, visit:

www.cisco.com/public/cons/isp/security/

Common forms of DDoS attacks are Internet Control Message Protocol (ICMP) floods, TCP SYN floods, or User Datagram Protocol (UDP) floods. In an e-commerce environment, this type of traffic is fairly easy to categorize. Only when limiting a TCP SYN attack on port 80 (HTTP) does an administrator run the risk of locking out legitimate users during an attack. Even then, it is better to temporarily lock out new legitimate users and retain routing and management connections than to have the router overrun and lose all connectivity.

Another approach to limiting this sort of attack is to follow filtering guidelines for networks outlined in RFC 1918, RFC 2827, and bogon filtering. RFC 1918 specifies the networks that are reserved for private use and that should never be seen across the public Internet. RFC 2827 filtering is discussed in the “IP Spoofing” section of Appendix B.

For inbound traffic on a router that is connected to the Internet, you employ RFC 1918 and 2827 filtering to prevent unauthorized traffic from reaching the corporate network. Bogon filtering is the process of filtering addresses that have not yet been distributed for use on the Internet. When these filters are implemented, they prevent DDoS attack packets that use these addresses as sources from traversing the WAN link, potentially saving bandwidth during the attack. If ISPs worldwide were to implement the guidelines in RFC 2827, source address spoofing would be greatly diminished. Although this strategy does not directly prevent DDoS attacks, it does prevent such attacks from masking their sources, making trace back to the attacking networks much easier. Ask your ISP about which DDoS mitigation options they make available to their customers.

Unicast Reverse Path Forwarding (uRPF) can also be used to help mitigate network attacks that use IP address spoofing. uRPF uses a combination of the routed interface and network adjacencies to determine if the packet is valid before forwarding it on to the next hop.

Applications Are Targets

Applications are mostly coded by human beings and, as such, are subject to error. These errors can be benign (an error that causes your document to print incorrectly) or malignant (an error that makes the credit card numbers on your database server available via anonymous FTP). It is the malignant problems, as well as other general security vulnerabilities, that need careful attention. Care needs to be taken to ensure that commercial and public domain applications have the latest security fixes. Public domain applications, as well as custom-developed applications, also require code review to ensure that the applications are not introducing any security risks caused by poor programming. This programming includes scenarios such as how an application makes calls to other applications (or the operating system itself), the privilege level at which the application runs, the degree of trust that the application has for the surrounding systems, or the method the application uses to transport data across the network.

Methods to help protect against application attacks are network IDSs (NIDSs) and HIPSs. IDSs act like an alarm system in the physical world. When an IDS detects something that it considers an attack, it can either take corrective action itself or notify a management system for actions by the administrator. Some systems are equipped to respond to and prevent certain attacks. HIPSs intercept OS and application calls on an individual host and stop the application or host that is running the malicious software. For an in-depth discussion regarding IDS and IPS best practices, refer to “SAFE: IDS Deployment, Tuning, and Logging in Depth,” located in the SAFE library.

Secure Management and Reporting

“If you’re going to log it, read it.” Almost everyone familiar with network security has said this, yet logging and reading information from hundreds of devices can prove to be a challenging proposition. Which logs are most important? How do I separate important messages from mere notifications? How do I ensure that logs are not tampered with in transit? How do I ensure that my time stamps match each other when multiple devices report the same alarm? What information is needed if log data is required for a criminal investigation? How do I deal with the volume of messages that can be generated by a large network? Effective log file management requires addressing all of these questions.

From a management standpoint, a different set of questions needs to be asked. How do I securely manage a device? How can I push content out to public servers and ensure that it is not tampered with in transit? How can I track changes on devices to troubleshoot when attacks or network failures occur?

From an architectural point of view, providing Out-Of-Band (OOB) management of network systems is the best first step in any management and reporting strategy. No production traffic resides on an out-of-band network. Devices should have a direct local connection to such a network where possible, and where impossible due to geographic or system-related issues, the device should connect via a private encrypted tunnel over the production network. Such a tunnel should be preconfigured to communicate only across the specific ports required for management and reporting. The tunnel should also be locked down so that only appropriate hosts can initiate and terminate tunnels.

Be sure that the OOB network does not itself create security issues. See the “Management Module” section of this document for more details.

After implementing an OOB management network, logging and reporting becomes simpler. Most networking devices can send syslog data, which can be invaluable when troubleshooting network problems or security threats. Send this data to one or more syslog analysis hosts on the management network. Depending on the device involved, you can choose various logging levels to ensure that the correct amount of data is sent to the logging devices. You also need to flag device log data within the analysis software to permit detailed viewing and reporting. For example, during an attack, the log data provided by Layer 2 switches might not be as interesting as the data provided by the IDS.

Specialized applications such as IDSs often use their own logging protocols to transmit alarm information. Usually this data should be logged to separate management hosts that are better equipped to deal with attack alarms. When combined, alarm data from many different sources can provide information about the overall health of the network.

To ensure that log messages are time-synchronized, clocks on hosts and network devices must be synchronized. For devices that support it, Network Time Protocol (NTP) provides a way to ensure that accurate time is kept on all devices. When dealing with attacks, seconds matter because it is important to identify the order in which a specified attack took place.

When a network device is compromised, the longer it takes to find out about the compromise, the greater the financial impact to the corporation. Time is at a premium. The primary function of logging devices and software is to notify a security specialist as soon as possible regarding possible network attacks. To do this effectively, a security monitoring device or software should have the ability to:

Consolidate both syslog and IDS alarm data
Classify the data based on user-provided rules
Automatically notify security specialists of critical alarms in real time
Automatically investigate critical alarms (for a description of threat response software, see the axiom section in this paper titled “Applications are Targets” in the “Intrusion Detection and Prevention” section)
Provide fast, flexible reporting for a large amount of syslog and IDS alarm data
Graph alarm data for easy and quick analysis of alarm types, attack sources, and destinations

OOB management is not always desirable. It often depends on the type of management application you are running and the protocols that are required. Consider a management tool whose goal is to determine the reachability of all of the devices on the production network. If a critical link failed between two core switches, you would want this management console to alert an administrator. If this management application was configured to use an OOB network, it might never determine that the link had failed — the OOB network makes all devices appear to be attached to a single network. With management applications such as these, it is preferred to run the management application in-band. This in-band management needs to be configured as securely as possible. Often this in-band and OOB management can be configured from the same management network, provided there is a firewall between the management hosts and the devices needing management. Please see the “Management Module” section for more details.

When in-band management of a device is required, you should consider several factors. First, what management protocols does the device support? IP Security (IPSec) devices should be managed by simply creating a tunnel from the management network to the device. This setup allows many insecure management protocols to flow over a single encrypted tunnel. When IPSec is not possible because it is not supported on a device, less-secure alternatives must be chosen. For configuration of the device, SSH or SSL can often be used instead of Telnet to encrypt any configuration modifications made to a device. These same protocols can sometimes also be used to push and pull data to a device, instead of insecure protocols such as FTP and Trivial FTP (TFTP). However, TFTP is often required on Cisco equipment to back up configurations or to update software versions. Newer versions of network devices support Secure Copy Protocol (SCP), a file transfer utility with all of the benefits of SSH.

The second factor to consider is whether the management channel needs to be active at all times. If not, temporary access can be enabled in a firewall while the management functions are performed and then later removed. This process does not scale with large numbers of devices, however, and should be used sparingly (if at all) in enterprise deployments. If the channel needs to be active at all times, such as with SNMP, a third factor should be considered — do you really need this management tool? SNMP managers are often used inside a network to ease troubleshooting and configuration. However, SNMP should be treated with the utmost care, because the underlying protocol has its own set of security vulnerabilities. If required, consider providing read-only access to devices via SNMP and treat the SNMP community string with the same care you might treat a root password on a critical UNIX host. Know that by introducing SNMP into your production network, you are introducing a potential vulnerability into your environment. If your network has the capability to use SNMPv3 with encryption, this can be used on either the in-band or out-of-band management network. Keep in mind that although SNMPv3 is more secure than previous versions, it is protected by 56-bit Data Encryption Standard (DES), which has been compromised by brute-force attacks.

Configuration change management is another issue related to secure management. When a network is under attack, it is important to know the state of critical network devices and when the last known modifications took place.

Creating a plan for change management should be a part of your comprehensive security policy, but at minimum, record changes using authentication systems on the devices, and archive configurations via FTP, TFTP, or SCP.

Enterprise Module

The enterprise comprises two functional areas — the campus and the edge. This document further divides these areas into modules that detail the functions of each area. Following the discussion of the modules in the “Enterprise Campus” and “Enterprise Edge” sections, the “Enterprise Options” section of this document describes design options.

Expected Threats

From a threat perspective, the enterprise network is like most networks connected to the Internet. There are internal users who need access out, and external users who need access in. Several common threats can generate the initial compromise that a hacker needs to penetrate the network with secondary exploits.

As reported by the Computer Security Institute and the FBI, most attacks originate from the internal network. Disgruntled employees, corporate spies, visiting guests, malfunctioning test software, hosts that have been infected with viruses or worms, and inadvertent users are all potential sources of such attacks. When designing network security, it is important to be aware of the potential for internal threats.

Publicly addressable hosts that are connected to the Internet or extranet will likely be attacked via application-layer vulnerabilities that may provide privileged access, or by DoS attacks that limit system availability.

A hacker might try to gain access to the network by using a “war-dialer” to determine your data phone numbers. War-dialers are software or hardware designed to dial many phone numbers and to determine the type of system on the other end of the connection. Personal systems with remote-control software installed by the user are the most vulnerable, because they typically are not very secure. Because these devices are behind the firewall, once hackers have access via the host they dialed in to, they can impersonate users on the network.

With the advent of wireless networking technology (WLANs), new threats have emerged. “War-driving” has become very popular with hackers. With the simple addition of a wireless card to a laptop and sniffer software, hackers can drive by an enterprise and easily steal credentials required to access a network. See the “SAFE:Wireless LAN Security in Depth: Version 2” in the SAFE library for a description on how to mitigate these and other WLAN attacks. With the advent of DSL and other high-bandwidth, “always-on” networks, the enterprise network now extends into employees’ remote work places, such as homes or telecommuter offices. Devices in these locations are subject to the same threats as those considered inside the enterprise — and from a security standpoint, they should be treated accordingly. Enterprises may want to encrypt all data transversing remote sites. See the “SAFE VPN: IPSec Virtual Private Networks in Depth” and “SAFE: Extending the Security Blueprint to Small, Midsize, and Remote-User Networks” documents in the SAFE library for an in-depth discussion on issues related to remote networks.
For a complete discussion of threat details, refer to Appendix B.

Enterprise Campus

Following is a detailed analysis of all of the modules contained within the enterprise campus. Figure 3 shows this campus.

Management Module

The primary goal of the management module is to facilitate the secure management of all devices and hosts within the enterprise SAFE architecture (Figures 4-6). This includes logging and reporting the information flow from the enterprise network devices through to the management hosts, as well as content, configurations, and new software flow to the devices from the management hosts.

Management Traffic Flow

Primary Devices

SNMP management server — Provides SNMP management for devices
Alarm reporting server — Provides alarm aggregation for all NIDS, IPS, and threat response alarms and messages
Syslog host(s) — Aggregates log information for firewall and NIDS hosts
Access control server — Delivers one-time, two-factor authentication services to network devices
One-time password (OTP) server — Authorizes OTP information relayed from the access control server
System administration host — Provides configuration, software, and content changes on devices
NIDS appliance — Provides Layers 4-7 monitoring of primary network segments in the module
Firewall — Allows granular control for traffic flows between the management hosts and the managed devices
Layer 2 switch (with private VLAN support) — Helps ensure that data from managed devices can only cross directly to the firewall

Threats Mitigated

Unauthorized access — Filtering at the firewall stops most unauthorized traffic in both directions
Man-in-the-middle attacks — Management data is crossing a private network, making man-in-the-middle attacks difficult
Network reconnaissance — Because management traffic does not cross the production network where it could be intercepted, IPS software mitigates hosts responding to some scanning
Password attacks — The access control server allows for strong two-factor authentication at each device
IP spoofing — Spoofed traffic is stopped in both directions at the firewall
Packet sniffers — A properly configured switched infrastructure, SNMPv3, SCP and SSH limits the effectiveness of sniffing
Trust exploitation — Private VLANs prevent a compromised device from masquerading as a management host
Worm and virus execution — Intrusion prevention and antivirus software help to mitigate worms and viruses, and provide zero-day protection for critical hosts and servers

Attack Mitigation Roles for Management Module Design Guidelines

As can be seen in Figure 6, the SAFE enterprise management network has two network segments that are separated by a firewall with VPN termination. The segment outside the firewall connects to all of the devices that require management. The segment inside the firewall contains the management hosts themselves, and the Cisco IOS routers that act as terminal servers. The remaining interface connects to the production network, but only for selective Internet access, limited in-band management traffic, and IPSec-protected management traffic from predetermined hosts.

As discussed in the “Axioms” section, in-band management only occurs when the application itself cannot function OOB, or if the Cisco device being managed does not physically have enough interfaces to support the normal management connection, which would require IPSec tunnels. The firewall is configured to allow syslog information into the management segment, as well as Telnet, SSH, SSL, and SNMP, if these are first initiated by the inside network.

Both management subnets operate under an address space that is completely separate from the rest of the production network. This ensures that the management network will not be advertised by any routing protocols. This also enables the production network devices to block any traffic from the management subnets that appears on the production network links. Any in-band management or Internet access occurs through a Network Address Translation (NAT) process on the Cisco IOS router that translates the non-routable management IP addresses to pre-specified production IP ranges.

The management module provides configuration management for nearly all devices in the network through the use of two primary technologies — Cisco IOS routers acting as terminal servers and a dedicated management network segment. The routers provide a reverse-Telnet function to the console ports on the Cisco devices throughout the enterprise. More extensive management features (software changes, content updates, log and alarm aggregation, and SNMP management) are provided through the dedicated management network segment (with caveats as noted above).

Because the management network has administrative access to nearly every area of the network, it can be a very attractive target to hackers. The management module has been built with several technologies designed to mitigate those risks. The first primary threat is a hacker attempting to gain access to the management network itself. This threat can only be mitigated through effective deployment of security features in the remaining modules in the enterprise. All of the remaining threats assume that the primary line of defense has been breached. To mitigate the threat of a compromised device, access control is implemented at the firewall and at every other possible device, to prevent exploitation of the management channel. A compromised device cannot even communicate with other hosts on the same subnet, because private VLANs on the management segment switches force all traffic from the managed devices directly to the firewall where filtering takes place. See the “Switches are Targets” section for a more detailed discussion. Password sniffing is of limited use in an OTP environment. HIPSs and NIDSs are also implemented on the management subnet and are configured in a restrictive stance. Because the types of traffic on this network should be very limited, any signature match on this segment should be met with an immediate response.

SNMP management has its own set of security needs. Keeping SNMP traffic on the management segment allows it to traverse an isolated segment when pulling management information from devices. With SAFE, SNMP management only pulls information from devices, and cannot push changes. To ensure this, each device is configured with a “read-only” string. Proper aggregation and analysis of the syslog information is critical to the proper management of a network. From a security perspective, syslog provides important information regarding security violations and configuration changes. Depending on the device in question, different levels of syslog information might be required.

Having full logging with all messages sent might provide too much information for an individual or syslog analysis algorithm to sort. SNMP “read-write” may be configured when using an OOB network, but be aware of the increased security risk due to a clear text string allowing modification of device configurations.

Threat-response software should be run on a server in this module. In conjunction with an IDS, it will perform automated forensics and help to reduce false-positive alarms. This will result in fewer false positives and much faster response time for identifying compromised hosts.

For the SAFE validation lab, all configurations were done using standalone management applications and the command-line interface (CLI). Nothing in SAFE, however, precludes using more advanced integrated management systems for configuration. Establishing this management module makes deployments of such technology completely viable. CLI and standalone management applications were chosen because the majority of current network deployments use this configuration method.

Alternatives

As mentioned in the “Axioms” section, complete OOB management is not always possible. When in-band management is required, more emphasis needs to be placed on securing the transport of the management protocols.

This can be accomplished through the use of IPSec, SSH, SSL, or any other encrypted and authenticated transport that allows management information to traverse it. When management happens on the same interface that a device uses for user data, importance needs to be placed on passwords, community strings, cryptographic keys, and the access lists that control communications to the management services.

SNMPv3 encryption should be considered as an option for using SNMP tools with in-band management. SNMPv3 allows the addition function of granting access to users to only a subset of device information, giving network managers more control over who has access to critical configuration data.

If the throughput requirements in the management module are low consider the use of a Cisco IOS firewall instead of a dedicated firewall appliance. The router could be chosen because of its flexibility in IPSec configuration and its routing options.

Core Module

The core module in the SAFE architecture is nearly identical to the core module of any other network architecture. It merely routes and switches traffic as fast as possible from one network to another (Fig 7).

Primary Devices

Layer 3 switching — Route and switch production network data from one module to another

Core Module: Detail Threats Mitigated

Packet sniffers — A properly configured switched infrastructure limits the effectiveness of sniffing
DDoS attacks — Cisco Express Forwarding and uRPF can be used to reduce the impact on a Layer 3 switch CPU during a packet flooding attack

Design Guidelines

Standard implementation guidelines were followed in accordance with the “core, distribution, and access layer” deployments commonly seen in well-designed networks based on Cisco technology.

Though no unique requirements are defined by the SAFE architecture for the core of enterprise networks, the core switches follow the switch security axiom in the “Switches Are Targets” section, to ensure that they are well-protected against direct attacks.

Building Distribution Module

This module provides distribution layer services to the building switches (Figures 8 and 9). These include routing, quality of service (QoS), and access control. Requests for data flow into these switches and onto the core, and responses follow the identical path in reverse.

Primary Devices

Layer 3 switches — Aggregate Layer 2 switches in building module and provide advanced services

Threats Mitigated

Unauthorized access — Attacks against server module resources are limited by Layer 3 filtering of specific subnets
IP spoofing — RFC 2827 filtering stops most spoofing attempts
Packet sniffers — A properly configured switched infrastructure limits the effectiveness of sniffing

Attack Mitigation Roles for Building Distribution Module Design Guidelines

In addition to standard network design fundamentals, the optimizations described in the “Switches Are Targets” section were implemented to provide added security within the enterprise user community. Intrusion detection is not implemented at the building distribution module — it is implemented in the modules that contain the resources that are likely to be attacked for their content (server, remote access, Internet).

The building distribution module provides the first line of defense and prevention against internally originated
attacks. It can mitigate the chance of a department accessing confidential information on another department’s server through the use of access control. For example, a network that contains marketing and research and development (R&D) might segment off the R&D server to a specific VLAN and filter access to it, ensuring that only R&D staff have access to it. For performance reasons, it is important that this access control be implemented on a hardware platform that can deliver filtered traffic at near-wire rates. This generally dictates the use of Layer 3 switching, as opposed to more traditional dedicated routing devices. This same access control can also prevent local source-address spoofing through the use of RFC 2827 filtering. Finally, subnet isolation is used to route voice-over-IP (VoIP) traffic to the call manager and to any associated gateways. This prevents VoIP traffic from crossing the same segments that all other data traffic crosses, reducing the likelihood of sniffing voice communications, and allowing a smoother QoS implementation. For a more complete description of securing IP telephony, refer to the “SAFE: IP Telephony Security in Depth” white paper in the SAFE library.

Alternatives

Depending on the size and performance requirements of the network, the distribution layer can be combined with the core layer to reduce the number of devices required in the environment.

The distribution layer is the first Layer 3 defense for internally originated attacks such as viruses or worms that could be present on workstations. A possible configuration in a high-bandwidth environment is to use IDS and firewall modules that plug directly into the backplane of the Layer 3 switches. Firewalling and IDSs in this location add a critical extra layer of security protection for the data going in and out of the user host environment. The use of integrated switch modules provides flexibility for bandwidth expansion and reduces the number of security devices required at this location. See Appendix D for a more in-depth discussion on integrated devices vs. standalone appliances, and the “Applications Are Targets” section for a more in-depth description of NIDSs.

Building Access Module

SAFE defines the building module as the extensive network portion that contains end-user workstations, phones, and their associated Layer 2 access points (Figures 10 and 11). Its primary goal is to provide services to end users.

Primary Devices

Layer 2 switch — Provides Layer 2 services to phones and user workstations
User workstation — Provides data services to authorized users on the network
IP phone — Provides IP telephony services to users on the network

Threats Mitigated

Packet sniffers — A properly configured switched infrastructure and private VLAN services limit the effectiveness of sniffing
ARP cache poisoning — Layer 2 ARP control commands mitigate the ARP attacks necessary for
man-in-the-middle attacks and DHCP flooding
IP spoofing — Layer 2 switches’ IP source guard prevents IP snooping
Virus and Trojan horse applications — Host-based virus scanning and IPSs prevent most viruses and many Trojan horses
Root kit, worm, and zero-day attacks — HIPSs will mitigate these attacks

Attack Mitigation Roles for Building Access Module Design Guidelines

Because user devices are generally the largest single element of the network, implementing security in a concise and effective manner is challenging. From a security perspective, the building distribution module, rather than anything in the building module, provides most of the access control that is enforced at the end-user level. This is because the Layer 2 switch that the workstations and phones connect to has no capability for Layer 3 access control. In addition to the network security guidelines described in the “Switch Security” section, host-based virus scanning is implemented at the workstation level.

Hosts in the building access module do not have addresses that are exposed to the Internet, but that does not mean that these hosts are secure. It is not necessary for a system to have an address available to the Internet to be vulnerable to attack. Worms and viruses are often launched using end-user workstation e-mail accounts, or Internet downloads to end-user workstations. DDoS attacks often use end-user workstations and zombies that lay in wait for an event (time- or command-based) to launch an attack without the knowledge of the workstation user. The methods used in this type of attack are often root kits, Trojan horses, and port redirectors. All internal hosts with Internet access should be looked at as potential threats to an enterprise environment.

To Building Distribution Module VLANs

Enterprises must consider that e-mail data (and corporate files, tools, and resources) residing on an end-user host machine, in many cases, is company confidential. If your enterprise hosts store this type of data, make sure that these hosts have current operating system security patches, current antivirus software and signatures, and HIPS software to protect them from malicious access.

For a more in-depth discussion on host intrusion prevention, refer to the “Hosts are Targets” section.

802.1x can be used for two purposes in this module. It can help to ensure that hosts (or telephones) logging on to the building access module have proper authentication credentials, and it can apply per-user access control lists (ACLs) to limit the network resources that can be accessed. This can be especially helpful to give guests Internet-only access, if they need to work from inside an enterprise for a short period of time.

Server Module

The server module’s primary goal is to provide application services to end users and devices (Figures 12 and 13).

Traffic flows on the server module are inspected by on-board intrusion detection within the Layer 3 switches.

Primary Devices

Layer 3 switch — Provides Layer 3 services to the servers and inspects data crossing the server module with NIDS
Cisco CallManager — Performs call-routing functions for IP telephony devices in the enterprise
Corporate and department servers — Delivers file, print, and DNS services to workstations in the building module
E-mail server — rovides Simple Mail Transfer Protocol (SMTP) and POP3 services to internal users

Threats Mitigated

Unauthorized access — Mitigated through the use of host-based intrusion detection and access control
Application-layer attacks — Operating systems, devices, and applications are kept up-to-date with the latest security fixes and protected by HIPSs
IP spoofing — RFC 2827 filtering prevents source address spoofing
Packet sniffers — A properly configured switched infrastructure limits the effectiveness of sniffing
Trust exploitation — Trust arrangements are explicit; private VLANs prevent hosts on the same subnet from communicating unless necessary
Port redirection, root kit, virus, worm, and zero-day attacks — HIPSs and antivirus software will help mitigate these attacks

Attack Mitigation Roles for Server Module Design Guidelines

The server module is often overlooked from a security perspective. When examining the levels of access that most employees have to the servers they attach to, the servers can often become the primary goal of internally originated attacks. Simply relying on effective passwords does not provide for a comprehensive attack mitigation strategy.

NIDSs, HIPSs, private VLANs, access control, antivirus software, and good system administration practices (such as keeping systems up to date with the latest patches) provide a much more comprehensive response to attacks. Because the NIDS is limited in the amount of traffic it can analyze, it is important to send it attack-sensitive traffic only. This varies from network to network, but should include SMTP, Telnet, FTP, and WWW. The switch-based NIDS was chosen because of its ability to look only at “interesting” traffic across all VLANs, as defined by the security policy.

Once properly tuned, this NIDS can be set up in a restrictive manner, because required traffic streams should be well-known. Refer to the “SAFE: IDS Best Practices” and the “SAFE: IP Telephony Security in Depth” documents in the SAFE library for detailed IDS tuning guidelines and for guidelines on securing voice components, respectively.

Alternatives

Like the building distribution module, the server module can be combined with the core module if performance needs do not dictate separation. For sensitive high-performance server environments, installing more than one NIDS blade and directing policy-matched traffic to specific blades can scale the NIDS capability in the Layer 3 switch. For critical systems such as IP telephony, call manager, or an accounting database, consider separating these hosts from the rest of the module with a stateful firewall.

Using integrated blade technology in Layer 3 switches also provides the flexibility to expand bandwidth by adding more blades as required. IDS is already recommended in Layer 3 switches. For a high-security environment and to further protect the integrity of the servers, a firewall services module may also be considered. For more information on integrated blades, refer to Appendix D.

Edge Distribution Module

This module aggregates the connectivity from the various elements at the edge (Figures 14 and 15). Traffic is filtered and routed from the edge modules and routed into the core.

Primary Devices

Layer 3 switches — Aggregate edge connectivity and provide advanced services

Threats Mitigated

Unauthorized access — Filtering provides control over specific edge subnets and their ability to reach areas within the campus
IP spoofing — RFC 2827 filtering limits locally initiated spoof attacks
Network reconnaissance — Filtering limits nonessential traffic from entering the campus, limiting an attacker’s ability to perform network reconnaissance
Packet sniffers — A properly configured switched infrastructure limits the effectiveness of sniffing

Attack Mitigation Roles for Edge Distribution Module Design Guidelines

The edge distribution module is similar to the building distribution module in terms of overall function. Both modules employ access control to filter traffic, although the edge distribution module can rely somewhat on the entire edge functional area to perform additional security functions. Both modules use Layer 3 switching to achieve high performance, but the edge distribution module can include additional security functions because the performance requirements are not as great. The edge distribution module provides the last line of defense for all traffic destined to the campus module from the edge module. This includes mitigation of spoofed packets, erroneous routing updates, and provisions for network layer access control.

Alternatives

Like the server and building distribution modules, the edge distribution module can be combined with the core module if performance requirements are not as stringent as the SAFE reference implementation. A NIDS is not present in this module, but could be placed here through the use of IDS services modules in the Layer 3 switches.

This would reduce the need for NIDS appliances at the exit from the critical edge modules as they connect to the campus. However, performance reasons may dictate, as they did in SAFE’s reference design, that dedicated IDSs be placed in the various edge modules as opposed to the edge distribution module.

Enterprise Edge

Following is a detailed analysis of all of the modules contained within the enterprise edge. Figures 16 and 17 show these modules.

Enterprise Edge Detail: Part 2 Corporate Internet Module

The corporate Internet module provides internal users with connectivity to Internet services, and provides Internet users with access to information on public servers (Figures 18–20). Traffic also flows from this module to the VPN and remote-access module, where VPN termination takes place. This module is not designed to serve e-commerce-type applications. Refer to the “E-Commerce/Data Center Module” section later in this document for more details on providing Internet commerce.

Corporate Internet Traffic Flow

Primary Devices

SMTP server — Acts as a relay between the Internet and the internal mail servers; inspects content
DNS server — Serves as authoritative external DNS server for the enterprise and relays internal requests to the Internet
FTP/HTTP server — Provides public information about the organization
Firewall — Provides network-level protection of resources and stateful filtering of traffic
NIDS appliance — Provides Layers 4 through 7 monitoring of network segments in the module
URL filtering server — Filters unauthorized URL requests from the enterprise
Content-aware Web proxy — Blocks inbound URL attacks and caches Web pages to reduce traffic on the LAN; devices use Internet Content Adaptation Protocol Version 1 (ICAPv1) and antivirus servers to ensure that cached Web data is virus-free and provides outbound Web authentication if necessary
Routers — Antispoof filtering, bogon address filtering, route protocol authentication and filtering, and ACLs