How do I implement firewall security? 

How do I implement firewall security?

It is necessary for us to approach the task of implementing a Firewall by going through the following steps:
a. Determine the access denial methodology to use.
It is recommended you begin with the methodology that denies all access by default. In other words, start with a gateway that routes no traffic and is effectively a brick wall with no doors in it.
b. Determine inbound access policy.
If all of your Internet traffic originates on the LAN this may be quite simple. A straightforward NAT router will block all inbound traffic that is not in response to requests originating from within the LAN. The true IP addresses of hosts behind the firewall are never revealed to the outside world, making intrusion extremely difficult. Indeed, local host IP addresses in this type of configuration are usually non-public addresses, making it impossible to route traffic to them from the Internet. Packets coming in from the Internet in response to requests from local hosts are addressed to dynamically allocated port numbers on the public side of the NAT router. These change rapidly making it difficult or impossible for an intruder to make assumptions about which port numbers to use.
If your requirements involve secure access to LAN based services from Internet based hosts, then you will need to determine the criteria to be used in deciding when a packet originating from the Internet may be allowed into the LAN. The stricter the criteria, the more secure your network will be. Ideally you will know which public IP addresses on the Internet may originate inbound traffic. By limiting inbound traffic to packets originating from these hosts, you decrease the likelihood of hostile intrusion. You may also want to limit inbound traffic to certain protocol sets such as ftp or http. All of these techniques can be achieved with packet filtering on a NAT router. If you cannot know the IP addresses that may originate inbound traffic, and you cannot use protocol filtering then you will need more a more complex rule-based model and this will involve a stateful multilayer inspection firewall.
c. Determine outbound access policy.
If your users only need access to the web, a proxy server may give a high level of security with access granted selectively to appropriate users. As mentioned, however, this type of firewall requires manual configuration of each web browser on each machine. Outbound protocol filtering can also be transparently achieved with packet filtering and no sacrifice in security. If you are using a NAT router with no inbound mapping of traffic originating from the Internet, then you may allow LAN users to freely access all services on the Internet with no security compromise. Naturally, the risk of employees behaving irresponsibly with email or with external hosts is a management issue and must be dealt with as such.
d. Determine if dial-in or dial-out access is required.
Dial-in requires a secure remote access PPP server that should be placed outside the firewall. If dial-out access is required by certain users, individual dial-out computers must be made secure in such a way that hostile access to the LAN through the dial-out connection becomes impossible. The surest way to do this is to physically isolate the computer from the LAN. Alternatively, personal firewall software may be used to isolate the LAN network interface from the remote access interface.
e. Decide whether to buy a complete firewall product, have one implemented by a systems integrator or implement one yourself.
Once the above questions have been answered, it may be decided whether to buy a complete firewall product or to configure one from multipurpose routing or proxy software. This decision will depend as much on the availability of in-house expertise as on the complexity of the need. A satisfactory firewall may be built with little expertise if the requirements are straightforward. However, complex requirements will not necessarily entail recourse to external resources if the system administrator has sufficient grasp of the elements. Indeed, as the complexity of the security model increases, so does the need for in-house expertise and autonomy.

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