In this chapter, you will learn about the following:
- The principles of defense in depth and breadth, and how these principles apply to IP network traffic plane security
- IP network element interface concepts and how these apply to IP network traffic plane security
- IP network edge and core security concepts, how these differ for enterprise and SP environments, and how these apply to IP network traffic plane security
IP traffic plane concepts provide the mechanisms from which comprehensive IP network security strategies can be implemented. Before discussing detailed security techniques and implementations for each of the four IP network traffic planes, which occur in Chapters 4 through 7, it is useful to look at how cohesive, integrated security policies based on IP network traffic plane concepts can be developed. The first important concept is that of defense in depth and breadth, and specifically, how the principles of defense in depth and breadth apply to IP traffic plane security. The next concept involves the special relationships between the network edge and core and the ability to classify packets and enforce security policies.
Principles of Defense in Depth and Breadth
The concepts of "defense in depth" or, more appropriately, "defense in depth and breadth" are often used by network security professionals to operationalize "layered defense" techniques for protecting network assets. Defense in depth became popularized in the late 1990s under research conducted by military and intelligence organizations as well as by various universities. Knowing that the concepts of defense in depth were formalized in a military environment aids in the understanding of how these techniques arose. Military strategies are typically defined to counter specific adversaries, weapons, and objectives. In the networking world, these concepts were adopted for cyber adversaries under certain attack scenarios and led to the development of various defensive strategies.
Initially, defense in depth applied multiple layers of defense technologies—including network-based techniques such as access lists and encryption, security appliances such as firewalls and intrusion detection systems (IDS), and software programs such as antivirus, host-based intrusion detection, and personal firewalls—throughout an enterprise network to protect sensitive information and business-critical resources. In theory, greater security is provided by forcing the attacker to penetrate these multiple layers, devices, or software elements, often of different implementations (for example, a hardware-based firewall and then a software-based personal firewall), such that if one layer is compromised, secondary layers are available to mitigate the attack. This approach is predicated on the expectation that adding multiple layers increases the difficulty and skills required to successfully attack the target. Defense in depth was later expanded to encompass more than hardware and software systems by incorporating personnel and operational requirements as well.
Defense in depth is often illustrated through the use of analogies taken from the physical world and then (oftentimes inappropriately) extended to the cyber world. One of the most popular examples describes a high-security facility with fences (perhaps multiple, separated by some distance), locked doors, guards inside the doors, and video surveillance cameras. Although this seems appealing as an analogy, these physical concepts do not necessarily translate well in the cyber world. Most obvious of course is the physical aspect of the analogy. IP reachability and connectivity to the Internet means that anyone with a networked personal computer (PC) located anywhere in the world can target any other Internet-connected device. Conversely, in the real world, you must be physically proximate to the target to attack it. Less obvious, perhaps, is the "asymmetry" afforded attackers in the cyber world. A single PC or a single person who has organized a "zombie army" of compromised PCs (that is, a roBOT NETwork or botnet as it is commonly referred to) may cause great damage with little or no active involvement of others or expenditures of funds. In the real world, a single person is limited in destructive capability and generally requires the active cooperation of others to launch a large-scale attack.
Perhaps least translatable is the notion of spectrum. In the physical world, visible, thermal, acoustic, and seismic sensors, all guarding the same valuable object, provide the ability to measure parameters in different spectra, which improves the protection capabilities over a single spectrum sensor. In the networking world, most security revolves around scrutinizing and controlling IP packets. It is often difficult to find a measurable analog to spectrum in the cyber world. Monitoring parameters such as CPU and memory utilization of devices and enforcing application behaviors may be useful for detecting (and preventing) some types of attack. Finally, it is not often that a protection mechanism in the physical world actually becomes a liability to defense, but this happens often in the cyber world, specifically with respect to DoS attacks. (This concept is discussed in more detail in the "What Are Defensive Layers" section.)
Understanding Defense in Depth and Breadth Concepts
When properly understood and implemented, defense in depth and breadth techniques are very useful for constructing and deploying network security policies from an IP network traffic plane perspective. This requires a clear understanding of the most important defense in depth and breadth concepts. This can be accomplished by addressing the following questions in the context of IP network traffic planes:
- What needs to be protected?
- What are defensive layers?
- What is the operational envelope of the network?
- What is your organization's operational model?
Let's look at these important questions separately.
What Needs to Be Protected?
Determining what needs to be protected is not necessarily as straightforward as it seems. Some organizations may need to protect assets such as trade (or military) secrets and other intellectual property. Others need to protect e-commerce site access (which could be bandwidth or server resources or both), credit card or customer databases, and health care records. Service providers (SP), on the other hand, often have very different needs because their value is in the network and services they provide. Ensuring network and service availability is paramount for SPs, so they need to protect network assets, including IP routers, switches, VoIP gateways, security appliances, and other network assets such as DNS servers, Internet peering links, and billing servers.
As is most often the case, you will need to expend some effort to deploy security measures, and when they are deployed, you will incur a level of administrative overhead and operational inconvenience, and may also find that there is an impact to network performance. Not everything can be protected equally, and you will need to make trade-offs that fully consider the risk and the cost of applying the security measures needed to mitigate the risk to acceptable levels.
In addition, orthogonal linkages between high-value assets and peripheral or relatively obscure services or devices may expose vulnerabilities that enable indirect attacks. These indirect attacks can cause substantially the same kind of impacts against a target that has only been protected against direct attack. DNS is a classic example from the e-commerce world. You may expend significant resources and money protecting your web servers but give little consideration to the DNS servers, leaving them vulnerable to any number of malicious attacks. Without DNS, the availability of the web site that itself was the primary focus of your security efforts will be severely impacted. ARP tables and routing tables are good examples of control plane elements that are often attacked not for the direct impact but for the indirect, collateral damage effects that these attacks cause on surrounding systems.
In summary, the key concepts when determining what needs to be protected are:
- Understand where the value is in the network and how this translates to the primary services and devices that must be protected.
- Understand the interrelationships between various network services and devices and how each may be leveraged to indirectly target the high-value resource.
What Are Defensive Layers?
Defense in depth and breadth describes the use of multiple layers, which are often implemented as distinct devices such as routers, firewalls, and intrusion protection systems (IPS), or as software such as antivirus or personal firewall applications. In most cases, this granularity is too coarse, because within each of these devices or applications themselves, multiple operations may be considered as providing some layer of protection. When considering a router, for example, packets ingressing an interface are affected by a number of hard-coded and configurable processes both before and after the routing function occurs. Figure 3-1 illustrates the typical packet processing "order of operation" that Cisco IOS routers employ. (Some variations in feature ordering may occur in specific router platforms and IOS software releases.)
Figure 3-1 Cisco IOS Feature Order of Operations
Each of these features, when implemented, must be considered as a layer because each may potentially impact the forwarding of the packet (permit, deny, rate-limit, mark/color), and in fact each operation may impact the performance of the router (CPU and memory, throughput, and so on). It is also important to note that each upstream layer may also have an impact on the effectiveness and performance of other downstream layers in the overall system.
Layers are selected to protect against specific attack vectors. By considering each feature as an individual layer rather than considering the entire device as a layer, you can clearly distinguish the purpose that each layer fulfils. This enables you to develop a security architecture that addresses both depth and breadth aspects, as required. But what are these concepts of depth and breadth? Depth and breadth can be described as follows:
- Depth—When considering a single service, if one layer is added to protect against a particular attack vector, and then a second layer is added to protect against the same attack vector, the second layer provides depth against that specific attack vector. Depth is generally used to provide redundant layers such that if one is compromised, the target remains protected by the secondary layers. An example of depth principles would be using a router-based ACL to permit traffic only to TCP port 80 of a web server, and then deploying a host-based ACL on the web server that also restricts inbound traffic to only TCP port 80.
- Breadth—When considering a single service, if one layer is added to protect against one specific attack vector that could compromise the service, and then a second layer is added to protect against a completely different attack vector against that same service, these layers are considered as providing breadth for attacks against that service. For example, consider the BGP service. One layer might configure MD5 authentication on each BGP peer to mitigate the risk of router advertisement spoofing. Adding an edge ACL to permit only valid BGP peers from communicating protects the BGP service from the separate and distinct attack vector by preventing non-BGP peers from reaching the service. (For more information on ACLs and MD5 authentication, refer to Chapter 4, "Data Plane Security," and Chapter 5, "Control Plane Security," respectively.)
When combined, defense in depth and breadth aim to mitigate as many potential attack vectors as practical, while at the same time providing backup protection if any one defensive layer is compromised.
A single layer may also provide protection against multiple attack vectors. When viewed from an IP network traffic plane perspective, a single layer may be effective in protecting (or have an impact on) multiple traffic planes. In IOS, for example, features such as interface ACLs and Unicast Reverse Path Forwarding (uRPF) affect every packet ingressing an interface and therefore have an impact on all four traffic planes. Other features such as Control Plane Policing (CoPP) or Receive ACLs (rACL) apply to punted traffic only and therefore affect only control plane and management plane traffic. (For more information on ACLs and uRPF, refer to Chapter 4. For more information on CoPP and rACL, refer to Chapter 5.)
It is critical to note that simply adding more layers is not always beneficial. Each layer, although intended to provide protection against a specific attack vector, may also enable additional attack vectors that previously did not exist without that layer having been deployed. That is, adding a protection layer against an attack vector in one domain may also create a new attack vector that may be exploitable in another domain. Stateful security devices such as firewalls and IPS systems often have this effect when improperly sized for different attack conditions, potentially enabling a DoS attack vector where one previously did not exist. The entire system must be considered when developing a layered strategy.
In addition, adding one type of security layer may negate the effectiveness of another type of security layer. For example, encryption is often added to provide confidentiality and integrity protection for data traversing unsecured networks. However, this same encryption layer negates the effectiveness (against certain attack vectors) of intrusion detection and protection systems (IDS/IPS) by making payload inspection impossible.
In summary, the key concepts regarding defensive layers are as follows:
- Understand which layers are available per device.
- Understand what attack vectors each layer is effective against.
- Understand how adding layers impacts each IP network traffic plane.
- Understand how layers can be combined to provide depth and breadth as a system.
- Understand the implications and interactions each layer has on other layers and the system as a whole.
Chapters 4 through 7 provide details on how different techniques may provide distinct layers of protection for each of the IP traffic planes.
What Is the Operational Envelope of the Network?
All network devices have certain performance characteristics that can be measured in terms of parameters such as bits per second of throughput, packets per second of forwarding, transactions per second of application processing, and so on as might be relevant to a particular device. For most network devices, performance characteristics are impacted not only by the type and number of features that are enabled, but also by the type and quantity of network traffic being processed. These performance characteristics then define the operational envelope of the device. The combination of devices within a network topology in aggregate implies that the overall system also has an operational envelope. Whereas it is necessary to understand the operational envelope for your devices and the overall network under ideal or normal operating conditions, knowing these operational envelopes is especially crucial under attack conditions.
In Chapter 1, "Internet Protocol Operations Fundamentals," you learned that the forwarding functions of a router may be implemented in hardware (fast path) or software (slow path). This is also true of the security features. All devices, security and otherwise, have performance limits. Each feature enabled on a device may potentially have some impact on its performance. Depending on the feature and its implementation method (hardware or software), this impact may be negligible or significant to the operational envelop of the device. This is one reason why the previous section stressed that enabling a feature (layer) for protection may actually produce adverse effects or enable a new attack surface that makes the overall system more susceptible to attack. In addition, enabling a particular security feature on one type of device (or router platform) may have a far different impact than enabling the same type of feature on a different type of device (or router platform).
Oftentimes, network security architectures are developed where certain features are enabled full-time to create a security baseline, and then additional features are enabled dynamically, under attack conditions. For example, an SP may enable on-the-fly (in reaction to an attack) an ACL on the interface serving the customer under attack. In this scenario, two conditions are occurring simultaneously, both of which may have an impact on the operational envelop of the device or network. First, an attack condition is underway. Thus, the packet rate, packet size, or packet characteristics (for example fragments, IP header options, and so on) may be much different from what they are under normal conditions. Second, the addition of the ACL may change the device performance. This is why it is critical to understand the operational envelop of your devices and networks when specific features are enabled, and under normal and attack conditions. At some point, under certain conditions, every device can reach some resource exhaustion state. It is critical to understand how each device behaves when certain features are enabled under adverse conditions. This is why it is critical to understand the operational envelop of your devices and networks when specific features are enabled, and under both normal and attack conditions. For DoS attacks in particular, the most destructive approach possible is often used.
In summary, the key concepts in determining the operational envelope of the network are as follows:
- Understand the base operational envelop of the device.
- Understand how enabling each defensive layer impacts the operational envelope, especially under adverse conditions.
What Is Your Organization's Operational Model?
An organization's operational model can help or hinder network security efforts. In many enterprise organizations, for example, the network staff and the security staff belong to separate groups. The network staff typically focuses on the routers and switches and has a good understanding of routing protocols such as OSPF, EIGRP, and BGP. Conversely, the security staff typically focuses on things such as firewalls and IDS/IPS devices, mail filters, and antivirus software. The security staff typically has limited hands-on knowledge of router operations and routing protocols (especially BGP), but rather is more familiar with end-station operating systems, servers, and some applications and the configuration and monitoring of their security systems.
When these operational impediments occur, the potential synergy that must exist between routing and security is often lost. For example, a good IP addressing plan and routing scheme can greatly enhance the ability of the security staff to efficiently configure firewall rules. Avoiding the use of default routes also enhances security. Many other examples exist.
In summary, the key concept here is to understand that networking and security operations must be coordinated and that a team approach will maximize the effectiveness of both groups. After all, both groups have a vested interest in network availability, which is directly linked with network security.
IP Network Traffic Planes: Defense in Depth and Breadth
From a defense in depth and breadth perspective, many features are available to protect each IP traffic plane and its protocols. Which specific features you select will depend on many aspects. Defense in depth and breadth should be considered when selecting these mechanisms to ensure that the important attack vectors are adequately covered (breadth), redundant mechanisms are applied where appropriate (depth), and interdependencies between components are considered to mitigate the risk of one attack vector leveraging some component to indirectly target another component (depth and breadth). In addition, the mechanisms selected must be supportable from an architectural standpoint and an operational standpoint. Chapters 4 through 7 provide detailed descriptions of many protection mechanisms available for each IP traffic plane. In order to provide some context for the mechanisms detailed in those chapters, each IP traffic plane is briefly described in turn from a defense in depth and breadth perspective.
As you learned in Chapter 1, the data plane contains customer application traffic generated by hosts, clients, servers, and applications that use the network as transport. Thus, data plane traffic should never have source or destination IP addresses that belong to any network elements such as routers and switches, but rather should be sourced from and destined to end devices such as PCs and servers. Network elements are optimized to forward data plane traffic as quickly as possible. As you learned in Chapter 2, many types of attacks attempt to use data plane traffic to indirectly influence other IP traffic planes (most often the control plane) to disrupt network operations. Data plane packets with IP header options, low TTL values, or spoofed source IP addresses belonging to the control plane are examples of where this may occur.
From a defense in depth and breadth perspective, the primary role of selecting protection mechanisms is to ensure that these data plane packets stay within the data plane and, further, are forwarded downstream only if authorized. Chapter 4 provides detailed descriptions of many mechanisms that may be used to protect the data plane, each with its own benefits and drawbacks.
The control plane is described in Chapter 1 as the logical entity associated with router processes and functions used to create and maintain the necessary intelligence about the state of the network and a router's interfaces. The control plane includes network protocols, such as routing, signaling, and link-state protocols that are used to build and maintain the operational state of the network, and provide IP connectivity between IP hosts.
Control plane traffic is generated and processed by network elements such as switches and routers. Thus, the source and destination IP addresses (for Layer 3 control plane packets) should correspond to the addresses of the network elements themselves. As described in Chapter 1, control plane packets are ultimately processed as receive-adjacency traffic by participating network elements and thus are processed by slow path mechanisms (for example, the IOS process level). Under normal operating conditions, the load placed on the network element by control plane traffic is relatively small. However, as you learned in Chapter 2, attacks may target the control plane, either directly or indirectly, to disrupt network element operations. If the network element CPU is busy processing bogus packets, resources may be unavailable for processing legitimate control plane traffic. Control plane failures may then prevent IP reachability within the data, management, and services planes.
From a defense in depth and breadth perspective, the primary goal for selecting protection mechanisms for the control plane is to ensure that the IOS process level resources, as well as slow path and receive-adjacency resources, are available for use by legitimate control plane functions. This is accomplished by doing the following:
- Ensuring the integrity of the control plane such that only legitimate control plane traffic is processed by the network element
- Ensuring that other IP traffic plane packets that may use the slow path (such as exception data plane packets, as described in the preceding section) do not overwhelm the IOS process level resources
Chapter 5 provides detailed descriptions of many different security techniques available to protect the control plane.
The control plane is unique in that it is at the same time both something that must be itself protected and something that facilitates protection of other IP traffic planes. That is, from a defense in depth and breadth perspective, there are control plane–based security techniques that are quite important for protecting the data plane, management plane, and services plane. Full details of these and many other control plane security techniques are described in detail in Chapters 4 and 5.
The management plane is the logical entity that describes the traffic used to access, manage, and monitor all of the network elements. The management plane supports all required provisioning, maintenance, and monitoring functions for the network. Like all other IP traffic planes, management plane traffic can be handled in-band with all other IP traffic. But, unlike other IP traffic planes, the management plane also has the capability to be carried via a separate out-of-band (OOB) management network to provide alternate reachability in the event that the primary in-band IP management path is not available. OOB management access is typically available through a console port or auxiliary port, or, depending on the device, a separate management Ethernet port. Each of these OOB access methods has its own security requirements, and defense in depth and breadth can be applied here as well.
Management plane traffic is both generated and consumed by network elements such as switches and routers and by servers running provisioning and monitoring applications, billing systems, security alerting systems, and other management applications. Thus, the source and destination IP addresses should correspond to the addresses of the network elements themselves, and a select range of trusted management devices. As described in Chapter 1, management plane packets ultimately are processed as receive-adjacency traffic by destination network elements, similar to control plane packets. Thus, management plane traffic is processed at the IOS process level, like control plane traffic, when these packets arrive at the network element itself. As you learned in Chapter 2, attacks may target the management plane for reconnaissance purposes, to gain unauthorized access to a device, or to disrupt network element operations. If the network element CPU is busy processing bogus packets, resources may be unavailable for processing legitimate management plane traffic.
From a defense in depth and breadth perspective, protection mechanisms selected for the management plane must prevent unauthorized access and ensure that the IOS process level, as well as slow path and receive-adjacency resources are available for use by legitimate management plane functions. Some of the same mechanisms that are useful for the data plane and control plane are also useful for the management plane. Additional features are available to provide depth and breadth to the overall protection scheme that are specific to the management plane. Chapter 6, "Management Plane Security," provides detailed descriptions of many security techniques available to protect the management plane.
Network convergence has led to multiple services of differing characteristics, running over a common IP network core. The services plane is the logical entity that enables network-based services and includes all traffic requiring dedicated network-based services, such as IP VPNs (for example, MPLS, IPsec), private-to-public interfacing (NAT, firewall, and IDS/IPS), QoS (voice and video), and many others. Services plane traffic generally requires high-touch traffic handling and as a result often introduces greater network complexity.
Services plane traffic is generally created by customer-based clients, servers, and applications that use the network as transport and thus would normally appear as transit traffic to the routers. Because of the specialized services being applied, however, routers and other forwarding devices typically use dedicated hardware or forwarding mechanisms to handle services plane traffic. That is, services plane traffic may be processed in a very different manner from regular data plane traffic, or even control or management plane traffic. For example, IPsec VPNs require high-speed encryption and decryption services, which are usually performed in dedicated hardware optimized for this purpose.
From a defense in depth and breadth perspective, then, the primary goal for selecting protection mechanisms for services plane traffic is to ensure that the specialized resources are available for use by legitimate services plane traffic. This is accomplished by doing the following:
- Ensuring the integrity of the services plane such that only legitimate traffic is allowed within specific service types
- Ensuring that one service type does not impact any other service type
- Ensuring that other IP traffic planes do not impact services plane traffic
Chapter 7, "Services Plane Security," provides detailed descriptions of security techniques available to protect the services plane.
The services plane also can have unique requirements. When services are delivered (for example, MPLS VPN services), potential attack vectors may exist against the traffic within the service itself as well as against the delivery of the service. Hence, security techniques both within the services plane and in protection of the services plane are required to fully mitigate the risk of attacks against the service. These types of considerations are among those discussed in Chapter 7.
Network Interface Types
In a perfect world, network elements would operate in ideal conditions and simply be required to forward well-behaved data and services plane packets through a network built and managed by optimized control and management planes. Unfortunately, this is not a perfect world and network elements must operate in more hostile and unpredictable environments where network attacks (intentional), misconfigurations (unintentional), and software and hardware failures stress the real-world operational environment. From a security perspective, this means that you must take proactive steps to make the network elements themselves more resilient to these events. In total, network elements include devices such as routers, LAN switches, wireless access points, firewalls, IDS/IPS components, load balancers, deep packet inspection components, web servers, clients, and anything else that forwards, inspects, generates, or processes IP packets within any one of the IP traffic planes. This book focuses on routers as an example of the type of considerations that are necessary from a defense in depth and breadth perspective to properly secure an IP network and the individual network elements.
A router must be able to forward well-behaved packets and gracefully handle harmful packets. Cisco routers and IOS software have both evolved over time to include more built-in and configurable security functions that allow these devices to be protected in the operational environment. Some of these capabilities are platform dependent, while others are generic across all IOS routers. Further, some of the platform-dependent capabilities are designed for particular router architectures (central versus distributed processing, for example). From a defense in depth and breadth standpoint, it is essential to understand both the performance envelop of the platform and the operating environment. Both of these are critical for developing appropriate security strategies.
For routers, externally sourced packets can physically enter a router only through physical network interfaces. Physical interfaces are those that include a data link layer with an associated link-layer encapsulation. However, other types of interfaces exist on routers as well. These, of course, are the logical interface types. Although logical interfaces do not have a data link layer, they are real in the sense that they are IP reachable, keep track of associated packet statistics, may have certain features that can be applied to them, including security features, and packets that logically use these interfaces can be impacted by these features.
From a defense in depth and breadth perspective, all interface types, both physical and logical, must be considered in order to develop an overall security strategy. With this in mind, it makes sense to fully categorize these interfaces. For physical interfaces, three types exist: external, internal, and OOB interfaces. For logical interfaces, four types exist: loopback, null0, services, and receive interfaces. Each of these interface types is illustrated in Figure 3-2.
Figure 3-2 External, Internal, Out-of-Band, Loopback, Null0, Service, and Receive Interfaces
Not all of these types of interfaces need be present or configured in every router. However, recognizing which types do exist and understanding how each differs from the other allows for the most appropriate security strategies to be developed. Each of these interface types are described next in turn.
Physical interfaces include the types external, internal, and out-of-band. Each of these is described next. Note that physical interfaces include those with any number of IP subinterfaces such as FR DLCIs, ATM VCs and Ethernet VLANs encapsulations as well as when multiple physical interfaces are bonded into a single IP interface (for example, MLPPP link bundling). In all cases, defense in depth and breadth concepts must be applied to each distinct IP interface.
Security practitioners who work with firewalls and other security devices have always understood the concept of external and internal interfaces (or inside and outside, as they are often called). Data-link interfaces on routers may be considered as external or internal based on the trust relationship of connected devices. Routers that provide connectivity between two (or more) different administrative domains will have (at least) one interface in each domain. From the perspective of the administrator of the router, the connection to the uncontrolled domain is considered to be an external (or outside) interface. Routers such as these are also referred to as border or edge routers. For enterprises, this is commonly found at the Internet boundary, but could just as easily be representative of a router (or switch) that connects different organizations within a single company, or an extranet connection. For SPs, this describes essentially every edge router in the network.
Interfaces designated as external provide the first and typically the best opportunity to describe the traffic that should be crossing this untrusted boundary (both ingress and egress), in such terms as expected source and destination address ranges, traffic types, rates, and others. That is, it should be possible to describe the appropriate traffic according to each IP traffic plane that should be seen at each external interface. For example, external interfaces may be expected to see only data plane traffic and a small subset of control plane traffic. Taking this approach allows you to define customized traffic policies that are most effective for your network topology, traffic behavior, and organizational mission. Figure 3-3 illustrates this concept.
Figure 3-3 IP Traffic Plane Relationships to Router Interfaces
As you can see in Figure 3-3, classifying packets within their respective IP traffic planes helps to establish the security policies that will be carried throughout the network. What traffic types should be seen in the data plane? Similarly, what protocols are used within the control plane and management plane? Should there be any control plane or management plane traffic on the external interface? Can these specific traffic types be filtered with ACLs or rate limiting, or is another technique required? What other security techniques are available to be applied to external interfaces, and do these techniques affect transit or receive traffic or both?
Referring to Figure 3-2 again, from the perspective of the administrator of a router, connections to routers within the same domain are considered to be internal (or inside) interfaces. For enterprises, the Internet boundary (or edge) router has at least one internal interface and one external interface. The internal interfaces only connect to routers within a single organization. For SPs, internal interfaces represent the backbone uplinks on every edge router in the network, plus all interfaces of core routers within the SP infrastructure that provide connectivity between border routers. Core routers are unique in that all data-link interfaces in the router are internal interfaces. Routers with all internal interfaces may also be found in enterprise networks.
When an interface is distinguished as internal, it defines the frame of reference for traffic crossing this trusted interface boundary, again in terms such as expected source and destination address ranges, traffic types, rates, and others. Thus, it should be possible to describe the appropriate traffic according to each IP traffic plane that should be seen at each internal interface. As illustrated in Figure 3-3, internal interfaces see not only data plane traffic, but also control plane and management plane traffic, and may see services plane traffic as well. Classifying packets relative to the IP traffic planes helps to establish the optimal policies and identify the appropriate security features necessary to implement a defense in depth and breadth security architecture. Note, however, that just because an interface is defined as internal does not mean traffic entering the interface is trusted. Nor is it safe to assume that routers with only internal interfaces are secure. As described in Chapter 2, many attack methods target core routers using transit attacks such as TTL expiry and reflection attacks using source address spoofing. Just because a router should not see a certain type of traffic arriving via an internal interface does not mean it will not see this traffic. Protection mechanisms are still required on internal interfaces.
Finally, routers and other network elements usually contain OOB interfaces for management purposes. Unlike the other IP traffic planes, the management plane has the capability to be carried via a separate OOB management network to provide alternate reachability in the event the primary in-band IP (management plane) path is lost. OOB access is typically available through a console port, auxiliary port, and, depending on the device, a dedicated management Ethernet port. As illustrated in Figure 3-2, these special OOB interfaces typically have direct access to the route processor. Hence, these interface types have their own special security requirements.
As illustrated in Figure 3-3, OOB interfaces should only see management plane traffic. In addition, this management plane traffic should be within a well-defined range of source addresses, protocols, and applications—for example, OOB interfaces should never receive traffic from external sources. As previously noted, because receive-adjacency management plane traffic is processed at the IOS process level, and because the management plane is critical to the proper operation of the network, from a defense in depth and breadth perspective, protection mechanisms must be applied to both in-band and OOB management plane traffic.
Whether explicitly configured or not, all network elements have certain logical interfaces. In general, four types of logical interfaces exist on IOS routers: loopback, null0, services, and receive interfaces. Depending on the device, these logical interfaces may be configurable to one degree or another. Only if configured, are some installed within the local CEF table as receive adjacencies or IP next hops. It is important to realize that these interfaces exist in network devices, and that they must be accounted for in the overall network security architecture. It is also important to realize that these interfaces have specialized security requirements. In some cases, they may also be used to enable other security mechanisms that are useful in protecting IP traffic planes. These aspects are discussed in detail in Chapters 4 through 7. Each of these logical interface types are described next.
IOS supports the configuration of loopback interfaces, which are virtual interfaces defined in software only with no associated data link layer physical interface. Because it is a logical instantiation versus a physical one, a loopback interface is always up and thus it is considered a best practice to tie control and management plane protocols such as OSPF, BGP, IS-IS, SNMP, NTP, SSH and others to loopback interfaces. This concept is illustrated in Figure 3-3. Also as illustrated in Figure 3-3, when used for control plane and management plane functions, loopback interfaces are tied to the receive path and, hence, packets destined to these interfaces are always processed at the IOS process level on the route processor.
From a defense in depth and breadth standpoint, it is appropriate to enable or disable certain features on loopback interfaces to protect the route processor. Loopback interfaces are also used as endpoints for some services plane traffic, and may be used in conjunction with tunnel interfaces for this purpose as well.
IOS also supports a null0 interface. Like the loopback interface, the null0 interface is also a virtual interface that is always up, but unlike the loopback, it can never forward or encapsulate traffic. This null0 interface is always defined and installed within the CEF table. Its purpose is to provide within the CEF (fast path) forwarding process a mechanism to discard unwanted packets. As you will see in Chapters 4 and 5, many control plane–based security mechanisms take advantage of the null0 interface in this regard. The null0 interface cannot be assigned an IP address and only one feature can be modified on the null0 interface—whether ICMP Destination Unreachable (Type 3) messages are generated for discarded packets.
Services interfaces include tunnel interfaces, dynamic virtual tunnel interfaces, and other services-oriented logical interfaces. Unlike loopback and null0 logical interfaces, however, services interfaces do provide the mechanisms to encapsulate specific packets inside of a configured transport protocol such as IP-in-IP, GRE, or IPsec. In this way, instantiations such as tunnel interfaces provide a convenient logical interface on which to configure services without being tied to any specific data link layer physical interface. This allows the creation of highly available network architectures that use routing to control data forwarding paths in the case where any physical interface may go down. When used in this manner, and as illustrated in Figure 3-3, tunnel encapsulation and decapsulation operations may or may not require slow path processing at the IOS process level within the route processor. In addition, tunneled packets may bypass other configured security mechanisms, thus potentially requiring the addition of other security features to provide defense in depth and breadth security.
In Chapter 1, you were introduced to the concepts of receive-adjacencies and receive packets. Receive-adjacencies are associated with the IP addresses that a router considers as belonging to itself. In some cases, these are the IP addresses that you configure on data link layer physical (external and internal) and logical (loopback and tunnel) interfaces. In other cases, these are packets destined to certain reserved IP addresses within broadcast and multicast ranges. Also as described in Chapter 1, exception conditions may also cause data plane packets to be punted for handling at the IOS process level (route processor) instead of by fast path forwarding mechanisms (interrupt process or ASIC hardware). In router architectures, this is often considered logically as the receive interface to the IOS process level on the route processor. Considering this as a receive interface provides a logical context within the defense in depth and breadth framework to define the appropriate protection schemes necessary to ensure that the IOS process level, as well as slow path and receive-adjacency resources are available for legitimate uses.