What is a DMZ? DMZ or demilitarized zone in Computing terms
DMZ or demilitarized zone in Computing Parlance , Image Credit : WiseGeek

What is a DMZ? DMZ or demilitarized zone in Computing terms

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 an untrusted network, usually a larger network such as 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 network node can access only what is exposed in the DMZ, while the rest of the organization's network is firewalled. The DMZ functions as a small, isolated network positioned between the Internet and the private network.

A Demilitarized Zone (DMZ) is a network segment that is separated from other networks. Many organizations use them to separate their Local Area Networks (LAN) from the Internet. This puts additional security between their corporate network and the public Internet. It can also be used to separate one particular machine from the rest of a network, moving it outside of the protection of a firewall.

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.

Frequent Uses

Common items that are placed in a DMZ are public-facing servers. For example, if an organization maintains its website on a server, that web server could be placed in a computer "Demilitarized Zone." In this way, if a malicious attack ever compromises the machine, the remainder of the company's network remains safe from danger. Someone can also place a computer on a DMZ outside of a network to test for connectivity issues being created by a firewall protecting the rest of the system.

Router Setup and Functionality

When connecting a LAN to the Internet, a router provides a physical connection to the public Internet, and firewall software offers a gateway to prevent malicious data from entering the network. One port on the firewall often connects to the network using an internal address, allowing traffic being sent out by individuals to reach the Internet. Another port is usually configured with a public address, which allows Internet traffic to reach the system. These two ports allow inbound and outbound data to communicate between the network and the Internet.

Purpose of a Demilitarized Zone

In creating a DMZ, an organization adds another network segment or subnet that is still part of the system, but not connected directly to the network. Adding a DMZ makes use of a third interface port on the firewall. This configuration allows the firewall to exchange data with both the general network and the isolated machine using Network Address Translation (NAT). The firewall does not usually protect the isolated system, allowing it to connect more directly to the 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 subnetwork in order to protect the rest of the network should any of them become compromised.

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 a separate Classified Militarized Zone (CMZ), a highly monitored militarized zone comprising mostly 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 database servers). In such architecture, the DMZ usually has the application firewall and the FTP while the CMZ hosts the Web servers. (The database servers could be in the CMZ, 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
  • Mail servers
  • FTP servers
  • VoIP servers

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:

  • Obliges internal users (usually employees) to use the proxy server for Internet access.
  • Reduced Internet access bandwidth requirements since some web content may be cached by the proxy server.
  • Simplifies recording and monitoring of user activities.
  • Centralized web content filtering.

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.

Architecture

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.

Single firewall

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).

Dual firewall

The most secure approach, according to Colton Fralick, 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[dubious – discuss] 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. One of the drawbacks of this architecture is that it's 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" security strategy.

Diagram of a typical three-legged network model employing a DMZ using a single firewall.

Diagram of a typical network employing DMZ using dual firewalls.

DMZ host

Some home routers refer to a DMZ host. 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. 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. This tactic (establishing a DMZ host) is also used with systems which do not interact properly with normal firewalling rules or NAT. This can be because no forwarding rule can be formulated ahead of time (varying TCP or UDP port numbers for example, as opposed to a fixed number or fixed range). This is also used for network protocols for which the router has no programming to handle (6in4 or GRE tunnels are prototypical examples).

NAT Functionality

Network Address Translation allows data received on a certain port or interface to be routed to a specified network. For example, when someone visits an organization's web site, the browser is sent to the server hosting the site. If this organization keeps its web server in a DMZ, the firewall knows that all traffic sent to the address associated with their web site should be passed to the server sitting in the DMZ, rather than directly into the organization's internal network.

Drawbacks and Other Methods

Since the DMZ computer lies outside of the firewall's protection, it may be vulnerable to attacks from malicious programs or hackers. Companies and individuals should not store sensitive data on this type of system, and know that such a machine can potentially become corrupted and "attack" the rest of the network. Many networking professionals recommend "port-forwarding" for people experiencing networking or connection issues. This provides specific, targeted access to certain network ports, without opening up a system entirely.

Further Reading: 

How to Approach Cyber Security for Industrial Control Systems

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