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DMZ
DMZ Setting
DMZ means "Demilitarized Zone." If an application has trouble working from behind the router, you
can expose one computer to the Internet and run the application on that computer.
When a LAN host is configured as a DMZ host, it becomes the destination for all incoming packets
that do not match some other incoming session or rule. If any other ingress rule is in place, that will be
used instead of sending packets to the DMZ host; so, an active session, virtual server, active port
trigger, or port forwarding rule will take priority over sending a packet to the DMZ host. (The DMZ
policy resembles a default port forwarding rule that forwards every port that is not specifically sent
anywhere else.)
The router provides only limited firewall protection for the DMZ host. The router does not forward a
TCP packet that does not match an active DMZ session, unless it is a connection establishment
packet (SYN). Except for this limited protection, the DMZ host is effectively "outside the firewall".
Anyone considering using a DMZ host should also consider running a firewall on that DMZ host
system to provide additional protection.
Packets received by the DMZ host have their IP addresses translated from the WAN-side IP address
of the router to the LAN-side IP address of the DMZ host. However, port numbers are not translated;
so applications on the DMZ host can depend on specific port numbers.
The DMZ capability is just one of several means for allowing incoming requests that might appear
unsolicited to the NAT. In general, the DMZ host should be used only if there are no other alternatives,
because it is much more exposed to cyberattacks than any other system on the LAN. Thought should
be given to using other configurations instead: a virtual server, a port forwarding rule, or a port trigger.
Virtual servers open one port for incoming sessions bound for a specific application (and also allow
port redirection and the use of ALGs).
Port forwarding is rather like a selective DMZ, where incoming traffic targeted at one or more ports is
forwarded to a specific LAN host (thereby not exposing as many ports as a DMZ host). Port triggering
is a special form of port forwarding, which is activated by outgoing traffic, and for which ports are only
forwarded while the trigger is active.
Few applications truly require the use of the DMZ host. Following are examples of when a
DMZ host might be required:
‧ A host needs to support several applications that might use overlapping ingress ports such that two
port forwarding rules cannot be used because they would potentially be in conflict.
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