Linux Demilitarized Zone (DMZ) Ethernet Interface Requirements and Configuration

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Q. Can you tell me more about Linux Demilitarized Zone and Ethernet Interface Card Requirements for typical DMZ implementation? How can a rule be set to route traffic to certain machines on a DMZ for HTTP or SMTP?

A. Demilitarized zone, used to secure an internal network from external access. You can use Linux firewall to create DMZ easily. There are many different ways to design a network with a DMZ. The basic method is to use a single Linux firewall with 3 Ethernet cards. The following simple example discusses DMZ setup and forwarding public traffic to internal servers.

Sample Example DMZ Setup

Consider the following DMZ host with 3 NIC:
[a] eth0 with 192.168.1.1 private IP address – Internal LAN ~ Desktop system
[b] eth1 with 202.54.1.1 public IP address – WAN connected to ISP router
[c] eth2 with 192.168.2.1 private IP address – DMZ connected to Mail / Web / DNS and other private servers

Linux Demilitarized Zone (DMZ) Ethernet Single Firewall Design
(Fig 01: A typical Linux based DMZ setup [ Image modified from Wikipedia article] )

Routing traffic between public and DMZ server

To set a rule for routing all incoming SMTP requests to a dedicated Mail server at IP address 192.168.2.2 and port 25, network address translation (NAT) calls a PREROUTING table to forward the packets to the proper destination.

This can be done with appropriate IPTABLES firewall rule to route traffic between LAN to DMZ and public interface to DMZ. For example, all incoming mail traffic from internet (202.54.1.1) can be send to DMZ mail server (192.168.2.2) with the following iptables prerouting rule (assuming default DROP all firewall policy):

### end init firewall .. Start DMZ stuff ####
# forward traffic between DMZ and LAN
iptables -A FORWARD -i eth0 -o eth2 -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD -i eth2 -o eth0 -m state --state ESTABLISHED,RELATED -j ACCEPT

# forward traffic between DMZ and WAN servers SMTP, Mail etc
iptables -A FORWARD -i eth2 -o eth1 -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD -i eth1 -o eth2 -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT

# Route incoming SMTP (port 25 ) traffic to DMZ server 192.168.2.2
iptables -t nat -A PREROUTING -p tcp -i eth1 -d 202.54.1.1 --dport 25 -j DNAT --to-destination 192.168.2.2

# Route incoming HTTP (port 80 ) traffic to DMZ server load balancer IP 192.168.2.3
iptables -t nat -A PREROUTING -p tcp -i eth1 -d 202.54.1.1 --dport 80 -j DNAT --to-destination 192.168.2.3

# Route incoming HTTPS (port 443 ) traffic to DMZ server reverse load balancer IP 192.168.2.4
iptables -t nat -A PREROUTING -p tcp -i eth1 -d 202.54.1.1 --dport 443 -j DNAT --to-destination 192.168.2.4
### End DMZ .. Add other rules ###

Where,

  • -i eth1 : Wan network interface
  • -d 202.54.1.1 : Wan public IP address
  • –dport 25 : SMTP Traffic
  • -j DNAT : DNAT target used set the destination address of the packet with –to-destination
  • –to-destination 192.168.2.2: Mail server ip address (private IP)

Multi port redirection

You can also use multiport iptables module to matches a set of source or destination ports. Up to 15 ports can be specified. For example, route incoming HTTP (port 80 ) and HTTPS ( port 443) traffic to WAN server load balancer IP 192.168.2.3:

iptables -t nat -A PREROUTING -p tcp -i eth1 -d 202.54.1.1 -m multiport --dport 80,443 -j DNAT --to-destination 192.168.2.3

Pitfalls

Above design has few pitfalls:

  1. Single point of failure – The firewall becomes a single point of failure for the network.
  2. Hardware – The firewall Host must be able to handle all of the traffic going to the DMZ as well as the internal network.

Linux / BSD Firewall Distros

If you find above discussion little hard to digest, I suggest getting a Linux / BSD distribution which aims to provide a simple-to-manage firewall appliance based on PC hardware to setup DMZ and gateways:

Further readings:

Updated for accuracy.

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin and a trainer for the Linux operating system/Unix shell scripting. He has worked with global clients and in various industries, including IT, education, defense and space research, and the nonprofit sector. Follow him on Twitter, Facebook, Google+.

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28 comment

  1. I want to add that there’s a mistake in the iptables rules: iptables -t nat -A PREROUTING -p tcp -i eth2 -d 202.54.1.1 –dport 25 -j DNAT –to-destination 192.168.2.2. The problem is coming from -i eth2, the correct way is -i eth1 because we want packets coming from internet to be redirect to the DMZ.

  3. Hi,

    I have configured the DMZ mentioned in the above article. Routing concept is working fine, where as if I click on my LAN/Other networks by accessing the web-page [ex :www.xyz.com or http://xyz.com], it showing Apache test page, where as domain related page not working, what may be the problem.

    Please help me out.

    Thanks,
    Sathish

  7. Hi,
    I have 8 public Ips and want to configure FreeBSD as router and firewall and also want to use all 8 public IPs for my servers so pls can you suggest me how to do this. I am very confused….

    Pls help….

  9. I tried with 2 network card to set as gateway server on Ubuntu 8.10 lts, its not working. Is thr any tips to troubleshoot

  11. Hey,
    Great Stuff !

    But i have a little different case with me and wondering if you could help me.
    I want to put a server ( Mail and proxy) in same machine and instead of assigning Private IP in the server in DMZ, I want to assign a public Ip.

    So can you please help me out with the iptables and routing in the linux server having 3 Nics.

    Thanks in advance.

  13. I am having 3 network card with
    1..public Ip
    2. 192.168.0.0—-servers
    3. 192.168.1.0—-Lan
    I tried setting as router to allow internet access on lan , it din’t work, can u help me out

  14. How to use iptables on a Debian or Ubuntu systems? You the ufw utility as a firewall and so how can one use that to forward or deny the ip packets?

    Please help me

  15. hey, great article. Very informative and helped me a lot. But in my case, i found it risky and don’t want to use 3 interfaces on the same machine. Instead, I want to configure two firewall machines — one sits in front of DMZ and other sits in front of Local LAN. Could you please explore in little in depth the configuration and setup required in this ? If you could give a diagram of it would be of great help to me. Thanks a lot.

  16. Does this iptables rules share internet to the local LAN users?

    I want to implement like this:

    INTERNET—————–(pub ip)LINUX ROUTER(pvt ip) ————PROXY/MAIL SVR
    |
    LAN
    how will be the iptable rules change if i want to direct the LAN internet access through the proxy server?

    Thanks in advance.

    Prakash

    What will be the rules if i need to direct

  18. My Network setup :-

    I have 3 network card in CentOS firewall machine connected to ADSL router
    1)Public ip –> 59.181.x.x which is nat on router to 192.168.1.1
    2) eth0 (External interface) which had IP ADDR 192.168.1.5 and Gateway 192.168.1.1
    In ifcfg-eth0 I have entry GATEWAY=192.168.1.1
    3) eth1 (Lan network) which has IP ADDR 192.168.2.1 and connected to switch1
    In ifcfg-eth1 I have not mentioned any GAETWAY
    4) eth2 (DMZ) which has IP ADDR 192.168.0.50 and connected to switch2
    In ifcfg-eth2 I have not mentioned any GATEWAY
    5) Webserver is connected in DMZ network and has IP ADDR 192.16.0.51 (other Centos machine)

    My Problem :-

    I am able to ssh from firewall machine to 192.168.0.51 and vice versa.ALso I am able to ping 192.168.1.5 from 192.168.0.51, BUT I am not able to ping 192.168.1.1 which is GATEWAY to 192.1681.5
    I want my machine 192.168.0.51 to access outside network (internet) i.e it should ping 59.181.x.x
    Can someone suggest solution for this problem?

  20. Why would you want to include these rules:
    iptables -A FORWARD -i eth0 -o eth2 -m state –state NEW,ESTABLISHED,RELATED -j ACCEPT
    iptables -A FORWARD -i eth2 -o eth0 -m state –state ESTABLISHED,RELATED -j ACCEPT
    Since eth2 is on DMZ wouldnt you want to completely separate it from the lan ?

  23. Hi there,

    My schematic is a little different

    I’ve got a ubuntu server 10.10, and

    router -> Firewall -> Lan
    router -> DMZ

    So What I’ve got it’s the wan interface conncetion to firewall then on the other interface of router connects to DMZ so at the end we have a different situation that you got

  24. Someone please help me I’m struggling to get port forwarding working. I have 2 machines
    system 1. with 2 ethernet ports
    eth1 public (ip 192.168.56.2)
    eth0 connected to system 2 (192.168.0.240)
    system 2. with 1 ethernet port
    eth0 connected to system1 (192.168.0.201) running a web server at 80

    On system 1 I’ve set the following rule
    iptables -t nat -A PREROUTING -p tcp -i eth1 -d 192.168.56.2 –dport 80 -j DNAT –to-destination 192.168.0.201
    but it doesn’t work at all when I access http://192.168.56.2/
    But http://192.168.0.201/ works indicating that port 80 is open on system 2

  25. Hi,
    nice article, like every reading from this blog.
    I just want to suggest another distro: Zeroshell.
    Ok I am a little bit patriot (it’s an italian dostro) but I am using zeroshell since a year ago now and I find it is really simple and effective.

  26. I have single ethernet card with eth1:1 (LAN) and eth1:2 (DMZ) virtually configured. Another card (eth0) supports WAN. Is it possible to use port forwarding from eth0 to eth1:2 and eth1:1 to eth1:2. Thanks

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