IT Infrastructure Security

IT security is a multi-discipline subject requiring a number of different skills sets and knowledge areas. A key area of knowledge which is vital for any security specialist is a clear understanding of IT infrastructure and how it relates to the creation of a comprehensive security strategy.

In this chapter of Security+ Essentials we will look at the various components that comprise the IT infrastructure of an organization. Some of these components are specifically designed to provide security (such as firewalls), whilst others a simply necessary to meet the needs of an IT operation (such as routers and switches).

Much like a firewall in real life protects parts of a buildings from a spreading fire, an IT firewall protects computer systems from the dangers posed by an internet connection. A firewall is essentially a component located between a computer or a network of computers and the internet. The specific purpose of a firewall is to prevent unauthorized access to the computer systems it is configured to protect. Firewalls take the form of software, hardware or a combination of both and are not limited to use by large companies. Any one who owns a computer (including home users) that is connected to the internet for even short periods of time should have a firewall configured.

A good security strategy should consist of multiple layers of protection and in such a scenario, the firewall is typically the first line of defense.

Firewalls fall into three main categories - Packet-filtering, Proxy-service and Stateful-inspection firewalls, each of which will be covered in detail here.

Packet-filtering firewalls operate at the Network layer (layer 3) of OSI model are the most basic of firewall types. The concept of packet filtering involves defining which data packets are permitted to pass through the firewall based on a number of criteria. Essentially, filtering can be specified based IP addresses, ports and protocols. It is possible, for example to block any packets originating from a particular IP address (or IP address range), or to block all traffic attempting to enter on port 23 (the Telnet port).

Packet-filtering firewalls are typically built into routers and provide either a command-line to graphical interface for specifying the filtering rules. The defenses provided by such firewalls are considered to be weak since they can only block what IP addresses that an administrator knows in advance are likely to be malicious. They are, however, extremely fast and easy to configure, and provide good first line of defense.

A proxy service firewall is placed between the internet and an internal network of computers and acts as a go-between for the two environments. With a proxy service in place, internal client computers do not connect directly to outside resources. Instead they connect to the proxy server which in turn connects with the external resource on behalf of the client, thereby masking the internal IP address of the client. Any response from the external resource is handled by the proxy service and passes them along to the client that originally requested the data.

Under such a scenario no internal system are ever in direct contact with a remote server or service and all internal IP addresses are masked by the proxy server. Proxy servers can also provide caching functions, where web pages which are frequently accessed by internal clients are stored by the server such that they can quickly be supplied when requested by the client leading to faster response times. Proxy service firewalls are available in two basic forms, Circuit-level gateway which works at the Session layer of the OSI model to verify that all sessions are legitimate and Application level-gateway which works at the OSI Application layer to control traffic of particular types (such as HTTP, FTP and SNMP).

Stateful-inspection firewalls (also known as dynamic packet filtering firewalls) operate at the OSI Network layer and combine some features of both packet-filtering and proxy server firewalls. A stateful-inspection firewalls not only examines the header information of packets, but also monitors sessions to ensure that they are legitimate and maintains state tables for each connections. Using these state tables, every packet received by the firewall can be viewed within the context of preceding traffic, allowing malicious data to be intercepted and blocked.

Routers are devices used to connect different network segments and operate at the OSI Network layer. Routers operate by examining each received packet and using algorithms together with routing tables to determine the optimal path for the data to reach its ultimate destination. Routers essentially form the backbone of the internet. Routing tables are either updated manually by an administrator, for configured automatically using a variety of different protocols including Routing Information Protocol (RIP), Interior Gateway Routing Protocol (IGRP), Enhanced Interor Gateway Routing Protocol (EIGRP) and Open Shortest Path First (OSPF).

Routers also include some security in the form of Access Control Lists (ACLs) which drop packets based on pre-defined rules, stateful-inspection and packet filtering.

Perhaps the biggest potential security risk for routers involves remote access to internal functions and configuration options. Due to their distributed nature all routers provide remote administration features. It is essential, therefore, that strict password conventions are used and that encrypted communications are used when logging into a remote router.

Most switches operate at the Data Link layer (layer 2) of the OSI model (although newer models are now moving up to the Network Layer) and are the basis of most Ethernet based local networks. Each port on a switch is a separate collision domain making switches much more efficient than Hubs (where all ports are on the same collision domain). Routing is based on the MAC addresses of devices connect to the switch.

As with routers, administrative access to switch devices must be carefully controlled using strict passwords and secure communications protocols during remote access.