Linux Demilitarized Zone (DMZ) Ethernet Interface Requirements and Configuration

by on December 15, 2007 · 27 comments· LAST UPDATED January 2, 2008

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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.

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{ 27 comments… read them below or add one }

1 George January 2, 2008 at 4:09 pm

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.

Reply

2 nixCraft January 2, 2008 at 4:21 pm

George,

Thanks for the heads up.

Reply

3 Sathish August 1, 2008 at 8:35 am

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

Reply

4 nixCraft August 1, 2008 at 9:09 am

You need to configure Apache properly.

Reply

5 mirza August 5, 2008 at 2:38 am

after George’s correction….
is the script already updated ?

Reply

6 nixCraft August 5, 2008 at 5:41 am

mirza,

Yes, it was updated after George’s correction..

Reply

7 umesh October 7, 2008 at 6:09 pm

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….

Reply

8 Gerald Sagoonick January 19, 2009 at 8:54 pm

Nice one

Reply

9 satya February 27, 2009 at 7:32 am

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

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10 nixCraft February 27, 2009 at 7:51 am

DMZ needs 3 network card.

Reply

11 Nepguy February 27, 2009 at 9:00 am

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.

Reply

12 nixCraft February 27, 2009 at 11:09 am

Rules remains same and replace private IP with public one.

Reply

13 Nepguy March 2, 2009 at 9:48 am

Doesn’t any NATing thing required here?

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14 satya March 19, 2009 at 6:32 am

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

Reply

15 V.Balaviswanathan April 29, 2009 at 2:32 pm

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

Reply

16 yshri June 4, 2009 at 8:34 am

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.

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17 PG June 16, 2009 at 10:32 am

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

Reply

18 PG June 16, 2009 at 10:41 am

hey,

in the earlier post, the LAN actually connects to one other NIC of LINUX Router.

Reply

19 Sachin March 13, 2010 at 7:29 pm

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?

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20 sparc86 May 8, 2010 at 9:07 pm

@Sachin, could you show us your firewall script and your routing table ?

Reply

21 Mike December 1, 2010 at 4:34 pm

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 ?

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22 André Ricardo March 5, 2011 at 3:54 pm

Great!

Reply

23 Akhim March 23, 2011 at 1:43 am

great article Vivek Gite!

Reply

24 LMS April 8, 2011 at 6:08 pm

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

Reply

25 A.Jesin April 22, 2011 at 8:02 am

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

Reply

26 origama June 19, 2012 at 2:18 pm

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.

Reply

27 jayadev July 2, 2013 at 12:18 pm

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

Reply

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