Home > Articles > Cisco Network Technology > Security > General Design Considerations for Secure Networks

General Design Considerations for Secure Networks

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Jun 18, 2004.

Chapter Description

At the beginning of any secure network design project, many best practices apply more or less uniformly to all areas of the design. This article by Sean Convery presents these practices in a single location.

Routing Considerations

As we continue to slowly work our way up the OSI model with these best practices, it is now useful to develop some design considerations in the realm of routing. The most important is the security of the routing protocol.

Routing Protocol Security

Routing security has received varying levels of attention over the past several years and has recently begun to attract more attention specifically around BGP on the public Internet. Despite this new attention, however, the area most open to attack is often not the Internet's BGP tables but the routing systems within your own enterprise network. Because of some of the sniffing-based attacks discussed in Chapter 3 and earlier in this chapter, an enterprise routing infrastructure can easily be attacked with MITM and other attacks designed to corrupt or change the routing tables with the following results:

  • Traffic redirection—In this attack, the adversary is able to redirect traffic, enabling the attacker to modify traffic in transit or simply sniff packets.

  • Traffic sent to a routing black hole—Here the attacker is able to send specific routes to null0, effectively kicking IP addresses off of the network.

  • Router DoS—Attacking the routing process can result in a crash of the router or a severe degradation of service.

  • Routing protocol DoS—Similar to the attack previously described against a whole router, a routing protocol attack could be launched to stop the routing process from functioning properly.

  • Unauthorized route prefix origination—This attack aims to introduce a new prefix into the route table that shouldn't be there. The attacker might do this to get a covert attack network to be routable throughout the victim network.

There are four primary attack methods for these attacks:

  • Configuration modification of existing routers

  • Introduction of a rogue router that participates in routing with legitimate routers

  • Spoofing a valid routing protocol message or modifying a valid message in transit

  • Sending of malformed or excess packets to a routing protocol process

These four attack methods can be mitigated in the following ways:

  • To counter configuration modification of existing routers, you must secure the routers. This includes not only the configuration of the router but also the supporting systems it makes use of, such as TFTP servers. See Chapter 5, "Device Hardening," for more information.

  • Anyone can attempt to introduce a rogue router, but to cause damage, the attacker needs the other routing devices to believe the information that is sent. This can most easily be blocked by adding message authentication to your routing protocol. More on this subject can be found in the next section. Additionally, the routing protocol message types can be blocked by ACLs from networks with no need to originate them.

  • Message authentication can also help prevent the spoofing or modification of a valid routing protocol message. In addition, the transport layer protocol (such as TCP for BGP) can further complicate message spoofing because of the difficulty in guessing pseudorandom initial sequence numbers (assuming a remote attacker).

  • Excess packets can be stopped through the use of traditional DoS mitigation techniques, which are discussed later in the chapter. Malformed packets, however, are nearly impossible to stop without the participation of the router vendor. Only through exhaustive testing and years of field use do routing protocol implementations correctly deal with most malformed messages. This is an area of computer security that needs increased attention, not just in routing protocols but in all network applications.

As you can see, stopping all these attacks is not a matter of flipping on the secure option in your routing protocols. As stated in Chapter 2, you must decide for your own network what threats need to be stopped. In addition to the specific threats mentioned here, it is also very useful to follow the network design best practices of not running routing protocols on interfaces with no reason to route and of using distribution lists to limit the routing prefixes that are sent or received by a specific routing instance. Details on distribution lists can be found in your favorite Internet routing book.

Routing Protocol Message Authentication

Although they vary in the strength of the authentication they offer, nearly all routing protocols support some form of message authentication. There are two principal types of authentication used in routing protocols today: plaintext password and MD5 digest.

Plaintext Password Authentication

Plaintext password authentication is just what it sounds like. A password is attached to the routing update and is sent in the clear along with the routing update. The passwords have specific length requirements as defined by the routing protocol in use. Plaintext password authentication should be considered specious security because anyone who sees a single routing update on the wire sees the authentication information if it is in use. From this point on, the attacker can appear to be a member of the trusted routing domain. The plaintext password does offer some benefit in that it prevents routing protocol changes when an invalid router is accidentally introduced into a production routing environment.

MD5 Digest Authentication

MD5 digest works by creating a 16-byte hash of the routing message combined with a secret key. The 16-byte value is, therefore, message-specific, and modification of the message by an attacker invalidates the 16-byte digest appended to the message. Without the secret key, which is never sent over the wire by the routing protocol, the attacker is unable to reconstruct a valid message. It is worth noting that the MD5 option provides authentication and packet integrity, not confidentiality. Figure 6-16 shows how the hash function operates.

Figure 16Figure 6-16 MD5 Digest for Routing Authentication

WARNING

MD5 passwords should have the same properties as other critical passwords in your network. They should follow the password creation guidelines in your security policy. If you choose a weak password, it is possible for an attacker to use brute-force guessing to determine your digest password, thereby allowing the attacker to become a trusted member of the routing domain.

Specific Routing Protocol Security Options

This section details the security options available in the most widely used routing protocols.

Routing Information Protocol

Routing Information Protocol (RIP) version 1 (RFC 1058) has no mechanism whatsoever to authenticate routing messages. As such, it should never be used in security-sensitive environments.

RIP v2

RIP v2 (RFC 1723) supports a 16-byte plaintext password that can be attached to routing updates. RFC 2082 specifies a proposed standard for adding MD5 authentication to RIP v2. Whenever possible, use the MD5 digest instead of the basic password.

RIP v2 plaintext messages have the format shown in Figure 6-17.

Figure 17Figure 6-17 RIP v2 Plaintext Authentication

RIP v2 MD5 authenticated messages have the format shown in Figure 6-18.

Figure 18Figure 6-18 RIP v2 MD5 Authentication

The configuration for RIPv2 authentication is as follows:

!Enable RIP authentication
Router(config-if)# ip rip authentication key-chain name-of-chain
!Specify authentication type
Router(config-if)# ip rip authentication mode {text | md5}
!Identify key chain
Router(config)# key chain name-of-chain
!Specify key number
Router(config-keychain)# key number
!Specify actual key
Router(config-keychain-key)# key-string text

Open Shortest Path First

Open Shortest Path First (OSPF) (RFC 2328) is one of the most widely used interior gateway protocols today. It supports nearly every bell and whistle you could ask of your routing protocol. On the security side, it offers both plaintext authentication (with basic message checksum) and the much more secure MD5 digest.

OSPF MD5 authenticated messages have the format shown in Figure 6-19.

Figure 19Figure 6-19 OSPF Packet Header

Note that there is no special format for OSPF when you use authentication. Authentication is assumed, even though it defaults to null authentication. In Figure 6-19, AuType specifies the authentication type.

The configuration for OSPF MD5 authentication is as follows:

!The MD5 key is always defined per interface but enabling MD5 can be
!done either on the interface as shown in the first command
!or at the area as in the second command. The
!third command is required for both options.
!Specify OSPF authentication type
Router(config-if)# ip ospf authentication message-digest
!Enable MD5 for an area
Router(config-router)# area area-id authentication message-digest
!Specify MD5 key
Router(config-if)# ip ospf message-digest-key key-id md5 key

BGP

BGP is most widely used in routing between two different routed domains, such as between you and your ISP or your ISP and the upstream ISP. BGP supports MD5 authentication. Note that because BGP uses TCP as a transport protocol, the MD5 authentication is done as a TCP option. More details on this can be found in RFC 2385. TCP Option 19 is specified for this authentication and takes the format specified in Figure 6-20.

Figure 20Figure 6-20 TCP Option 19 for BGP MD5 Authentication

The configuration for BGP MD5 authentication is as follows:

! Enable TCP MD5 authentication for a specific neighbor
Router(config-router)# neighbor neighbor_ip_addr password text

NOTE

As of this writing, BGP security is receiving a fair amount of attention in the industry. Several extensions are being proposed to allow the BGP messages to be authenticated, as well as to check that an advertiser of a particular prefix is authorized to do so. Most of these mechanisms make at least partial use of a Public Key Infrastructure (PKI). These options will take some time to be agreed upon; in the meantime, best practices are the best line of defense.

Because BGP is unicast as opposed to broadcast or multicast, IPsec can be used with it to provide even greater security. As of this writing, some networks were in the testing phase of their deployments. I would recommend waiting until IPsec in combination with BGP receives more testing before deploying on your own network. Even then, the complexity of the configuration and troubleshooting difficulty might prevent this from being a viable option.

Interior Gateway Routing Protocol

Interior Gateway Routing Protocol (IGRP) is a proprietary Cisco routing protocol meant to address some of the limitations of RIP. The initial version did not address any of its security limitations, however, because IGRP supports no form of authentication. Like RIP, IGRP should be avoided in security-sensitive environments.

Enhanced Interior Gateway Routing Protocol

Enhanced Interior Gateway Routing Protocol (EIGRP) is an extension to IGRP that is also Cisco proprietary. It supports MD5 message authentication.

The configuration for EIGRP authentication is as follows:

!Specify EIGRP MD5 authentication
Router(config-if)# ip authentication mode eigrp autonomous-system md5
!Specify authentication key
Router(config-if)# ip authentication key-chain eigrp autonomous-system 
 name-of-chain
!Identify key chain
Router(config)# key chain name-of-chain
!Specify key number
Router(config-keychain)# key number
!Specify actual key
Router(config-keychain-key)# key-string text

Asymmetric Routing and State-Aware Security Technology

As networks increase in size, so do the chances that they have asymmetric traffic somewhere within them. Asymmetric traffic is traffic that uses a different path for its return than the original path of the request. The topology in Figure 6-21 shows a representative network with several places where asymmetric traffic can occur.

Figure 21Figure 6-21 Asymmetric Traffic

Traffic between the user PC and either the finance server or the WWW server can flow in an asymmetric manner at several points along the network. Between the PC and the finance server, switches S1 and S3 are the main location it can occur. Between the PC and the WWW server, traffic could take an asymmetric route at S1 and S2 or at the Internet when returning through ISP A or ISP B.

So far, this is network design 101. Most network designers don't have any problem with asymmetric traffic because IP networks are asymmetric by nature. At each point in the transmission, an IP router makes a forwarding decision based on its view of the network.

This becomes problematic when security devices are introduced that rely on state information to make forwarding decisions. Consider the revised diagram in Figure 6-22, where two stateful firewalls are introduced between campus A and the two Internet connections.

Figure 22Figure 6-22 Asymmetric Traffic with Security Devices

Now asymmetric flows really start to cause problems! Again, consider the PC communicating with server HTTP://WWW. A perfectly reasonable packet flow might have the outgoing connection flow through S4, S1, FW1, Inet_RTR_1, ISP A, and then to server HTTP://WWW. Along the way, FW1 learns that the PC is trying to communicate with server WWW, and so it adds an entry in its state table to enable the return traffic to flow when it comes back from server HTTP://WWW. Unfortunately, the return path for the packet from server WWW to the user PC happens to be ISP B, Inet_RTR_2, FW2, S2, S4, user PC. The packet never reaches the PC, though, because FW2 doesn't have any state information for the communication. As far as it is concerned, server WWW is initiating new communications to the user PC that are blocked based on the configured security policy.

This problem can be further complicated by intrusion detection systems (IDS) deployed within the campus or near the firewalls. If traffic flows by an IDS in an asymmetric manner, it won't see all of the data. Consequently, it might alarm on traffic that is benign (false positive), or it might miss an attack altogether (false negative).

I wish there were an easy answer to this problem, but unfortunately there isn't. This section is included as much to bring the problem to your attention as it is to offer possible solutions. You do have some options, however:

  • Make your routing symmetric.

  • Load balance per flow rather than per packet.

  • Use state-sharing security devices.

  • Consider L2 redundancy as a workaround.

  • Manipulate flows by using routing or NAT.

  • Use stateless security features.

Make Your Routing Symmetric

This might seem easy, but in real network designs it can be a significant challenge. Even still, you would be surprised to see how many large networks use symmetric routing at certain parts of their network to enable state-aware security devices to function or to solve other networking issues. This is particularly common at Internet edges, where it is not unheard of to see an entire connection to an ISP lying dormant while the primary connection handles all of the load.

Load Balance Per Flow Rather Than Per Packet

Most L3 devices can be configured to do one of two things when equal-cost paths exist for a given network destination. In the first option, packets are simply balanced in round-robin format, with each successive packet going to the next available upstream router. This option causes the most heartache with internal security systems such as IDS. The second, more preferred, option is to load balance based on a given flow. This means traffic with a particular source and destination IP address and port (often called a four tuple) is always sent by a specific upstream router. This allows IDS systems and other state-aware devices to at least see half of the communication in a consistent manner. Unfortunately, this does nothing to the return traffic, which still might flow over a different link.

Use State-Sharing Security Devices

As the problem of asymmetric traffic manifests itself more and more in networks, network security vendors are starting to offer options allowing the state information within one security device to be shared with another. In Figure 6-22, FWs 1 and 2 could exchange their state table information to ensure that if the other device sees part of a given flow, it will know to permit the traffic. Often, the amount of information exchanged is significant and requires that dedicated links be configured between the firewalls to exchange the state information.

Consider L2 Redundancy as a Workaround

With the careful introduction of L2 redundancy as opposed to L3, technologies such as Virtual Router Redundancy Protocol (VRRP) or Hot Standby Router Protocol (HSRP) can allow traffic to flow through a single location while still providing redundancy. This option works best on high-speed connections where the use of only one path instead of two or more does not affect network performance.

The result is that normally asymmetric flows can be made symmetric for short distances in the network, such as while traffic passes through a firewall. Again, in Figure 6-22, if FWs 1 and 2 were connected on both sides to the same L2 network, they could use something like VRRP to appear as a single firewall to the upstream and downstream routers. This means that traffic can flow in an asymmetric manner out to the Internet and to the internal network but in a symmetric manner when passing through the firewall. This is generally impossible when the two devices are not in close geographic proximity to one another. For example, if FW 1 is in Brussels, Belgium, and FW 2 is in Hackensack, New Jersey, you are out of luck.

Manipulate Flows by Using Routing or NAT

Because this is a book on security, the ins and outs of BGP path preference have no place within the text. It is worth noting, however, that there are a number of things that can be done with routing protocols to affect the paths that packets take. To some degree, you can also influence which path outside networks take when they must communicate with you. Although not very elegant, some other workarounds involve using different NAT pools based on which security device a packet passes through. Return packets can then be forced to a specific security device based on the unique NAT pool they allocate from.

Use Stateless Security Features

Even though firewalls have been around for many years, a number of companies still use basic ACLs instead of stateful firewalls for, among other things, this asymmetric issue. Some security functionality is clearly lost. Basic ACLs don't track state information, but if your traffic flows are fairly easy to categorize, you can still achieve some security without needing symmetric traffic flows. Remember that if you have properly implemented a true security system as defined in Chapter 1, the access control function of a firewall is only one part of the overall security story.

With IDS, the signatures that work improperly in asymmetric environments can be turned off to prevent false positives. Again, this will reduce the security such systems provide but will still allow a number of signatures to fire properly.

6. Transport Protocol Design Considerations | Next Section Previous Section

Cisco Press Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from Cisco Press and its family of brands. I can unsubscribe at any time.

Overview

Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about Cisco Press products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites; develop new products and services; conduct educational research; and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@ciscopress.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security

Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children

This site is not directed to children under the age of 13.

Marketing

Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out

Users can always make an informed choice as to whether they should proceed with certain services offered by Cisco Press. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.ciscopress.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links

This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020