Firewalls make it possible to filter incoming and outgoing traffic that flows through your system. A firewall can use one or more sets of “rules” to inspect the network packets as they come in or go out of your network connections and either allows the traffic through or blocks it. The rules of a firewall can inspect one or more characteristics of the packets, including but not limited to the protocol type, the source or destination host address, and the source or destination port.
Firewalls can greatly enhance the security of a host or a network. They can be used to do one or more of the following things:
To protect and insulate the applications, services and machines of your internal network from unwanted traffic coming in from the public Internet.
To limit or disable access from hosts of the internal network to services of the public Internet.
To support network address translation (NAT), which allows your internal network to use private IP addresses and share a single connection to the public Internet (either with a single IP address or by a shared pool of automatically assigned public addresses).
Types of Firewalls
There are two basic ways to create firewall rulesets: “inclusive” or “exclusive”. An exclusive firewall allows all traffic through except for the traffic matching the ruleset. An inclusive firewall does the reverse. It only allows traffic matching the rules through and blocks everything else.
Inclusive firewalls are generally safer than exclusive firewalls because they significantly reduce the risk of allowing unwanted traffic to pass through the firewall.
Security can be tightened further using a “stateful firewall”. With a stateful firewall the firewall keeps track of which connections are opened through the firewall and will only allow traffic through which either matches an existing connection or opens a new one. The disadvantage of a stateful firewall is that it can be vulnerable to Denial of Service (DoS) attacks if a lot of new connections are opened very fast. With most firewalls it is possible to use a combination of stateful and non-stateful behavior to make an optimal firewall for the site.
Network layer and packet filtersNetwork layer firewalls, also called packet filters, operate at a relatively low level of the TCP/IP protocol stack, not allowing packets to pass through the firewall unless they match the established rule set. The firewall administrator may define the rules; or default rules may apply. The term "packet filter" originated in the context of BSD operating systems.
Application layer firewallAn application layer firewall is a firewall operating at the application layer of a protocol stack.[1] Generally it is a host using various forms of proxy servers to proxy traffic instead of routing it. As it works on the application layer, it may inspect the contents of the traffic, blocking what the firewall administrator views as inappropriate content, such as certain websites, viruses, attempts to exploit known logical flaws in client software, and so forth.
An application layer firewall does not route traffic on the network layer. All traffic stops at the firewall which may initiate its own connections if the traffic satisfies the rules.
Proxy serverIn computer networks, a proxy server is a server (a computer system or an application program) which services the requests of its clients by forwarding requests to other servers. A client connects to the proxy server, requesting some service, such as a file, connection, web page, or other resource, available from a different server. The proxy server provides the resource by connecting to the specified server and requesting the service on behalf of the client. A proxy server may optionally alter the client's request or the server's response, and sometimes it may serve the request without contacting the specified server. In this case, it would 'cache' the first request to the remote server, so it could save the information for later, and make everything as fast as possible.
A proxy server that passes all requests and replies unmodified is usually called a gateway or sometimes tunneling proxy.
A proxy server can be placed in the user's local computer or at specific key points between the user and the destination servers or the Internet.
Network address translationIn computer networking, network address translation (NAT) is the process of modifying network address information in datagram packet headers while in transit across a traffic routing device for the purpose of remapping a given address space into another.
Top Firewall Security for your Computer
http://www.all-internet-security.com/top_10_firewall_software.html
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