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A firewall can be hardware, software, or a combination whose purpose is to enforce a set of network security policies across network connections It is much like a wall with a window: the wall serves to keep things out, except those permitted through the window (see Figure 8-3) Network security policies act like the glass in the window; they permit some things to pass, such as light, while blocking others, such as air The heart of a firewall is the set of security policies that it enforces Management determines what is allowed in the form of network traffic between devices, and these policies are used to build rule sets for the firewall devices used to filter network traffic across the network Security policies are rules that define what traffic is permissible and what traffic is to be blocked or denied These are not universal rules, and many different sets of rules are created for a single company with multiple connections A web server connected to the
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Figure 8-2 A small home office router for cable modem/DSL use
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Figure 8-3 How a firewall works
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Internet may be configured to allow traffic only on port 80 for HTTP and have all other ports blocked, for example An e-mail server may have only necessary ports for e-mail open, with others blocked The network firewall can be programmed to block all traffic to the web server except for port 80 traffic, and to block all traffic bound to the mail server except for port 25 In this fashion, the firewall acts as a security filter, enabling control over network traffic, by machine, by port, and in some cases based on application level detail A key to setting security policies for firewalls is the same as has been seen for other security policies the principle of least access Allow only the necessary access for a function; block or deny all unneeded functionality How a firm deploys its firewalls determines what is needed for security policies for each firewall As will be discussed later, the security topology will determine what network devices are employed at what points in a network At a minimum, the corporate connection to the Internet should pass through a firewall This firewall should block all network traffic except that specifically authorized by the firm This is actually easy to do: Blocking communications on a port is simple just tell the firewall to close the port The issue comes in deciding what services are needed and by whom, and thus which ports should be open and which should be closed This is what makes a security policy useful The perfect set of network security policies, for a firewall, is one that the end user never sees and that never allows even a single unauthorized packet to enter the network As with any other perfect item, it will be rare to find the perfect set of security policies for firewalls in an enterprise To develop a complete and comprehensive security policy, it is first necessary to have a complete and comprehensive understanding of your network resources and their uses Once you know how the network will be used, you will have an idea of what to permit In addition, once you understand what you need to protect, you will have an idea of what to block Firewalls are designed to block attacks before they reach a target
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machine Common targets are web servers, e-mail servers, DNS servers, FTP services, and databases Each of these has separate functionality, and each has unique vulnerabilities Once you have decided who should receive what type of traffic and what types should be blocked, you can administer this through the firewall
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Firewalls enforce the established security policies through a variety of mechanisms, including the following: Network Address Translation (NAT) Basic packet filtering Stateful packet filtering PART III ACLs Application layer proxies One of the most basic security functions provided by a firewall is NAT, which allows you to mask significant amounts of information from outside of the network This allows an outside entity to communicate with an entity inside the firewall without truly knowing its address NAT is a technique used in IPv4 to link private IP addresses to public ones Private IP addresses are sets of IP addresses that can be used by anyone and by definition are not routable across the Internet NAT can assist in security by preventing direct access to devices from outside the firm, without first having the address changed at a NAT device The benefit is less public IP addresses are needed, and from a security point of view the internal address structure is not known to the outside world If a hacker attacks the source address, he is simply attacking the NAT device, not the actual sender of the packet NAT is described in detail in the Security Topologies section later in this chapter NAT was conceived to resolve an address shortage associated with IPv4 and is considered by many to be unnecessary for IPv6 The added security features of enforcing traffic translation and hiding internal network details from direct outside connections will give NAT life well into the IPv6 timeframe Basic packet filtering, the next most common firewall technique, involves looking at packets, their ports, protocols, source and destination addresses, and checking that information against the rules configured on the firewall Telnet and FTP connections may be prohibited from being established to a mail or database server, but they may be allowed for the respective service servers This is a fairly simple method of filtering based on information in each packet header, such as IP addresses and TCP/UDP ports Packet filtering will not detect and catch all undesired packets, but it is fast and efficient To look at all packets and determine the need for each and its data requires stateful packet filtering Stateful means that the firewall maintains, or knows, the context of a conversation In many cases, rules depend on the context of a specific communication connection For instance, traffic from an outside server to an inside server may be allowed if it is requested but blocked if it is not A common example is a request for a web page This request is actually a series of requests to multiple servers, each of which can
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