A home router DMZ host is a single address (e.g., IP address) on the internal network that has all traffic sent to it which is not otherwise forwarded to other LAN hosts.

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The purpose of a DMZ is to add an additional layer of security to an organizations local area network (LAN): an external network node can access only what is exposed in the DMZ, while the rest of the organizations network is firewalled.

The DMZ functions as a small, isolated network positioned between the Internet and the private network.

This metaphor applies to the computing use as the DMZ acts as a gateway to the public Internet.

It is neither as secure as the internal network, nor as insecure as the public internet.

Because of the increased potential of these hosts suffering an attack, they are placed into this specific subnetwork in order to protect the rest of the network should any of them become compromised.

Similarly, communication between hosts in the DMZ and to the external network is also restricted to make the DMZ more secure than the Internet and suitable for housing these special purpose services.

This allows hosts in the DMZ to communicate with both the internal and external network, while an intervening firewall controls the traffic between the DMZ servers and the internal network clients, and another firewall would perform some level of control to protect the DMZ from the external network.

The database servers could be in the CMZ, in the LAN, or in a separate VLAN altogether.).

The web servers can communicate with database servers either directly or through an application firewall for security reasons.

Instead of providing a service to internal users wanting to access an external network, it provides indirect access for an external network (usually the Internet) to internal resources.

For example, a back office application access, such as an email system, could be provided to external users (to read emails while outside the company) but the remote user would not have direct access to their email server (only the reverse proxy server can physically access the internal email server).

This is an extra layer of security particularly recommended when internal resources need to be accessed from the outside, but its worth noting this design still allows remote (and potentially malicious) users to talk to the internal resources with the help of the proxy.

Since the proxy functions as a relay between the non-trusted network and the internal resource: it may also forward malicious traffic (e.g.

Usually such a reverse proxy mechanism is provided by using an application layer firewall that focuses on the specific shape and contents of the traffic rather than just controlling access to specific TCP and UDP ports (as a packet filter firewall would do), but a reverse proxy is usually not a good substitute for a well thought out DMZ design as it has to rely on continuous signature updates for updated attack vectors.

Two of the most basic methods are with a single firewall, also known as the three legged model, and with dual firewalls, also known as back to back.

These architectures can be expanded to create very complex architectures depending on the network requirements.

The external network is formed from the ISP to the firewall on the first network interface, the internal network is formed from the second network interface, and the DMZ is formed from the third network interface.

The firewall becomes a single point of failure for the network and must be able to handle all of the traffic going to the DMZ as well as the internal network.

The zones are usually marked with colors -for example, purple for LAN, green for DMZ, red for Internet (with often another color used for wireless zones).

The first firewall (also called the front-end or perimeter 5 firewall) must be configured to allow traffic destined to the DMZ only.

The second firewall (also called back-end or internal firewall) only allows traffic to the DMZ from the internal network.

There is even more protection if the two firewalls are provided by two different vendors, because it makes it less likely that both devices suffer from the same security vulnerabilities.

For example a security hole found to exist in one vendors system is less likely to occur in the other one.

One of the drawbacks of this architecture is that its more costly, both to purchase, and to manage.

The practice of using different firewalls from different vendors is sometimes described as a component of a defense in depth 7 security strategy.

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