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In computer security, a DMZ or Demilitarized Zone (sometimes referred to as a perimeter network) is a physical or logical subnetwork that contains and exposes an organization's external-facing services to a larger and untrusted network, usually the Internet. The purpose of a DMZ is to add an additional layer of security to an organization's local area network (LAN); an external attacker only has direct access to equipment in the DMZ, rather than any other part of the network. The name is derived from the term "demilitarized zone", an area between nation states in which military operation is not permitted.
In the military sense, a DMZ is not seen as belonging to either party bordering it. This concept applies to the computing use of the metaphor in that a DMZ which is, for example, acting as a gateway to the public Internet, is neither as secure as the internal network, nor as insecure as the public Internet.
In this case, the hosts most vulnerable to attack are those that provide services to users outside of the local area network, such as e-mail, web and Domain Name System (DNS) servers. Because of the increased potential of these hosts suffering an attack, they are placed into this specific sub-network in order to protect the rest of the network if an intruder were to successfully compromise any of them.
Hosts in the DMZ are permitted to have only limited connectivity to specific hosts in the internal network, as the content of DMZ is not as secure as the internal network. 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.
A DMZ configuration provides security from external attacks, but it typically has no bearing on internal attacks such as sniffing communication via a packet analyzer or spoofing such as e-mail spoofing.
It is also sometimes good practice to configure separate Classified Militarized Zone (CMZ), a highly monitored militarized zone comprising mostly of web servers (and similar servers that interface to the external world i.e. the internet) that are not in the DMZ but contain sensitive information about accessing servers within LAN (like the database servers). In such architecture, the DMZ usually has the application firewall and the FTP whilst the CMZ hosts the web servers. (The database servers could be in the CMZ or in the LAN or in a separate VLAN altogether.)
Any service that is being provided to users on the external network can be placed in the DMZ. The most common of these services are:
Web servers that communicate with an internal database require access to a database server, which may not be publicly accessible and may contain sensitive information. The web servers can communicate with database servers either directly or through an application firewall for security reasons.
E-mail messages and particularly the user database are confidential, so they are typically stored on servers that cannot be accessed from the Internet (at least not in an insecure manner), but can be accessed from email servers that are exposed to the Internet.
The mail server inside the DMZ passes incoming mail to the secured/internal mail servers. It also handles outgoing mail.
For security, compliance with legal standards such as HIPAA, and monitoring reasons, in a business environment, some enterprises install a proxy server within the DMZ. This has the following benefits:
A reverse proxy server, like a proxy server, is an intermediary, but is used the other way around. 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, which is particularly recommended when internal resources need to be accessed from the outside. Usually such a reverse proxy mechanism is provided by using an application layer firewall as they focus on the specific shape of the traffic rather than controlling access to specific TCP and UDP ports as a packet filter firewall does.
There are many different ways to design a network with a DMZ. Two of the most basic methods are with a single firewall, also known as the three legged model, and with dual firewalls. These architectures can be expanded to create very complex architectures depending on the network requirements.
A single firewall with at least 3 network interfaces can be used to create a network architecture containing a DMZ. 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).
A more secure approach is to use two firewalls to create a DMZ. The first firewall (also called the "front-end" or "perimeter"  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 from the DMZ to the internal network.
This setup is considered more secure since two devices would need to be compromised. 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, accidental misconfiguration is less likely to occur the same way across the configuration interfaces of two different vendors, and a security hole found to exist in one vendor's system is less likely to occur in the other one. This drawback of this architecture is that it's more costly. The practice of using different firewalls from different vendors is sometimes described as a component of a "defense in depth" security strategy.
Some home routers refer to a DMZ host. A home router DMZ host is a host on the internal network that has all ports exposed, except those ports otherwise forwarded. By definition this is not a true DMZ (Demilitarized Zone), since it alone does not separate the host from the internal network. That is, the DMZ host is able to connect to hosts on the internal network, whereas hosts within a real DMZ are prevented from connecting with the internal network by a firewall that separates them, unless the firewall permits the connection. A firewall may allow this if a host on the internal network first requests a connection to the host within the DMZ. The DMZ host provides none of the security advantages that a subnet provides and is often used as an easy method of forwarding all ports to another firewall / NAT device.