Domains of Trust

Within all networks, there are devices with differing levels of value and differing levels of attack susceptibility. This concept is discussed in Chapter 2, "Security Policy and Operations Life Cycle." By combining these factors, you can start to define the relative attention needed for a given information asset. In a flat network, as shown in Figure 12-7, the security of these assets is left completely up to the applications.

As you can see, there is no segmentation except where necessary (WAN connections). All users and servers share the same network, and any required access control is expected to be done by each host. Some network security functions are possible, such as NIDS or VLAN ACLs, but this is not commonly done in a design with this topology. Although some networks are still designed this way, the vast majority opt for some basic segmentation.

Segmentation can be done for several reasons. When one of those reasons is security, you are creating domains of trust. If you are segmenting the network for performance or scalability reasons only, you are just creating more segments but still have one domain of trust. Figure 12-8 shows the topology from Figure 12-7 mapped into a possible set of trust domains.

Figure 12-8. Domains of Trust

Figure 12-8. Domains of Trust

Depending on the size of the network, your security policy, and the decisions you make regarding your security system, you might have dozens of security domains or only a few. A domain of trust in network security is created to segment information assets with certain trust, value, and attack risk from one another. This allows the topology of the network (physical or logical) to help enforce your security policy. Typically, a domain of trust contains more than one system but not always. Often a domain of trust contains both clients and servers at a particular trust level, but there are exceptions. For example, in Figure 12-8, the external server is considered at a different trust level than its users (the Internet at large). The following are several common domains of trust, all from the perspective of the primary location for an organization:

• Internet

• Public servers

• Business partners (connected by WAN or VPN)

• Basic internal network (users and servers)

• Department-specific network (users and servers)

• Management network

• E-commerce network (with subdomains based on application trust)

Each of these domains can overlap with one another. For example, although members of the finance group clearly need special access to the finance servers, they also need the same level of rights afforded to other users for basic system access (e-mail, Internet access, and so on).

Continue reading here: Domains of Trust and Network Design

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