DMZ Question

DMZ Question

[Deepak Seshadri]

Hello everybody,

 

 

                             --------------------------

                             |                               |     WAN
(x.x.x.58/28, default gateway - x.x.x.49)

                             |       FC2           e0
|-------------------------- ISP

             LAN         |                               |

-----------------------|  e1                         |     DMZ

    10.0.1.x            |                        e2
|--------------------------

                             |                               |

                             |-------------------------|

I have 3 computers that need to have public addresses and their IP addresses
are:

A - x.x.x.50/28, DG - x.x.x.49

B - x.x.x.51/28, DG - x.x.x.49

C - x.x.x.55/28, DG - x.x.x.49

Now the problem is I do not understand how I will give access to these PCs
from public without putting these PCs on a different subnet. Some firewalls
such as sonicwall do not require an IP for the DMZ port. You can add any
number of IPs behind the DMZ and it works. How is that done? Is it possible
with Linux?

If I connect them on the DMZ interface, should they all be put in a
different subnet, probably with /29 bit mask? If I do it this way, should I
use iptables & DNAT or should/can I use just the "routing" in linux?

If you have a better way to do it, please let me know. Any help will be
greatly appreciated.

Thank you,

Deepak Seshadri

 

Re: DMZ Question

[John A. Sullivan III]

On Mon, 2004-10-04 at 13:41, Deepak Seshadri wrote:
> Hello everybody,
> 
>  
> 
> 
> 
>                              --------------------------
> 
>                              |                               |     WAN
> (x.x.x.58/28, default gateway - x.x.x.49)
> 
>                              |       FC2           e0
> |-------------------------- ISP
> 
>              LAN         |                               |
> 
> -----------------------|  e1                         |     DMZ
> 
>     10.0.1.x            |                        e2
> |--------------------------
> 
>                              |                               |
> 
>                              |-------------------------|
> 
> I have 3 computers that need to have public addresses and their IP addresses
> are:
> 
> A - x.x.x.50/28, DG - x.x.x.49
> 
> B - x.x.x.51/28, DG - x.x.x.49
> 
> C - x.x.x.55/28, DG - x.x.x.49
> 
> Now the problem is I do not understand how I will give access to these PCs
> from public without putting these PCs on a different subnet. Some firewalls
> such as sonicwall do not require an IP for the DMZ port. You can add any
> number of IPs behind the DMZ and it works. How is that done? Is it possible
> with Linux?
> 
> If I connect them on the DMZ interface, should they all be put in a
> different subnet, probably with /29 bit mask? If I do it this way, should I
> use iptables & DNAT or should/can I use just the "routing" in linux?
> 
> If you have a better way to do it, please let me know. Any help will be
> greatly appreciated.
> 
> Thank you,
> 
> Deepak Seshadri
> 
>  
You have several different options.  You do not have to put the public
devices on a separate subnet but I would strongly recommend doing so. 
If the public has access, there is the chance that a public user can
crack into your publicly exposed device.  If the device sits on your
internal network, there is nothing between the intruder and the rest of
your private systems.

I would also recommend using DNAT and iptables access control.  This
way, you can restrict what services are exposed to the public and hide
the true addressing scheme.  If you really wanted to get tricky, you
could even alter the TTL so that only your publicly exposed services can
go any further than your ISP's router.

You will need to ensure that the public interface of the firewall
responds to ARP requests for the other addresses.  You do this by
binding those addresses to the physical interface, e.g., 

ip address add x.x.x.58/28 dev eth0 brd +

When it is released, ISCS (http://iscs.sourceforge.net) will do all of
this from NAT to access control to ARP to even TTL automatically.  Until
then, you'll need to set it up manually or use a rule configurator like
fwbuilder (http://www.fwbuilder.org).  Good luck - John
-- 
John A. Sullivan III
Open Source Development Corporation
Financially sustainable open source development
http://www.opensourcedevel.com

RE: DMZ Question

[Daniel Chemko]

You ASCII arts looks like death!

In order to 'bind' any number of DNAT addresses to the firewall, you
have two choices.

1. Actually bind the IP address to the physical interface, such as:
	ip addr add w.x.y.z/24 dev eth1
This will then get the DNAT from iptables.
	iptables -t nat -A PREROUTING --destination w.x.y.z -j DNAT --to
${My_NEW_ADDR}

2. ProxyARP the IP address (http://www.sjdjweis.com/linux/proxyarp/):
	echo 1 > /proc/sys/net/ipv4/conf/eth1/proxy_arp
	echo 1 > /proc/sys/net/ipv4/conf/eth0/proxy_arp
This will then get the DNAT from iptables.
	iptables -t nat -A PREROUTING --destination w.x.y.z -j DNAT --to
${My_NEW_ADDR}


There is one to catch when doing this. If you have a single DNS source
for external and internal machines, you'll have to resolve the machine's
ip's the external IP. Netfilter will not work performing what I call
reflective nat out of the box.

I consider reflective nat to be when you want to connect to a machine on
your subnet but under a different IP address.

# You should have something like this line already
iptables -t nat -A PREROUTING --destination w.x.y.z -j DNAT --to
${My_NEW_ADDR}

# Allow traffic to bounce off the interface
iptables -A FORWARD -o ${IF_DMZ} -o ${IF_DMZ} -j ACCEPT
# Force the firewall to rewrite the source IP of the packet since
conntrack refuses to allow SRC->FW->DST->SRC flows
iptables -t nat -A POSTROUTING --destination ${My_NEW_ADDR} --source
${DMZ_NET}/${DMZ_MSK} -j SNAT --to ${FW_DMZ_IP}

Re: DMZ Question

[Deepak Seshadri]

Hello Daniel & John,

Thank you very much for your replies. Kudos to both of you for somehow interpreting the garbled ASCII art. It is the result of composing an "HTML format" email in Outlook '03.

I created a small subnet of /29 bits subnet mask for the DMZ and added the IPs of the machines in the DMZ to the outside interface of the firewall. I  used DNAT & restricted access to only the intended services on those machines. Everything worked well. Thank you for the prompt replies.

Best Regards,
Deepak Seshadri

  ----- Original Message ----- 
  From: Daniel Chemko 
  To: Deepak Seshadri ; netfilter@lists.netfilter.org 
  Cc: Imran Bashir ; Veena Rao ; Syed Amjad Ali ; Zia Ullah 
  Sent: Monday, October 04, 2004 2:09 PM
  Subject: RE: DMZ Question


  You ASCII arts looks like death!

  In order to 'bind' any number of DNAT addresses to the firewall, you
  have two choices.

  1. Actually bind the IP address to the physical interface, such as:
  ip addr add w.x.y.z/24 dev eth1
  This will then get the DNAT from iptables.
  iptables -t nat -A PREROUTING --destination w.x.y.z -j DNAT --to
  ${My_NEW_ADDR}

  2. ProxyARP the IP address (http://www.sjdjweis.com/linux/proxyarp/):
  echo 1 > /proc/sys/net/ipv4/conf/eth1/proxy_arp
  echo 1 > /proc/sys/net/ipv4/conf/eth0/proxy_arp
  This will then get the DNAT from iptables.
  iptables -t nat -A PREROUTING --destination w.x.y.z -j DNAT --to
  ${My_NEW_ADDR}


  There is one to catch when doing this. If you have a single DNS source
  for external and internal machines, you'll have to resolve the machine's
  ip's the external IP. Netfilter will not work performing what I call
  reflective nat out of the box.

  I consider reflective nat to be when you want to connect to a machine on
  your subnet but under a different IP address.

  # You should have something like this line already
  iptables -t nat -A PREROUTING --destination w.x.y.z -j DNAT --to
  ${My_NEW_ADDR}

  # Allow traffic to bounce off the interface
  iptables -A FORWARD -o ${IF_DMZ} -o ${IF_DMZ} -j ACCEPT
  # Force the firewall to rewrite the source IP of the packet since
  conntrack refuses to allow SRC->FW->DST->SRC flows
  iptables -t nat -A POSTROUTING --destination ${My_NEW_ADDR} --source
  ${DMZ_NET}/${DMZ_MSK} -j SNAT --to ${FW_DMZ_IP}

Re: DMZ Question

[John Black]

 > In order to 'bind' any number of DNAT addresses to the firewall, you
  >have two choices.

  >1. Actually bind the IP address to the physical interface, such as:
  >ip addr add w.x.y.z/24 dev eth1
would w.x.y.z be the private address or public address?

  >This will then get the DNAT from iptables.
  >iptables -t nat -A PREROUTING --destination w.x.y.z -j DNAT --to
  >${My_NEW_ADDR}
would w.x.y.z be the private address or public address?

thanks
John

Re: DMZ Question

[Jason Opperisano]

On Wed, 2004-10-06 at 08:02, John Black wrote:
>  > In order to 'bind' any number of DNAT addresses to the firewall, you
>   >have two choices.
> 
>   >1. Actually bind the IP address to the physical interface, such as:
>   >ip addr add w.x.y.z/24 dev eth1
> would w.x.y.z be the private address or public address?
> 
>   >This will then get the DNAT from iptables.
>   >iptables -t nat -A PREROUTING --destination w.x.y.z -j DNAT --to
>   >${My_NEW_ADDR}
> would w.x.y.z be the private address or public address?

public.

-j

-- 
Jason Opperisano <opie@817west.com>

RE: DMZ Question

[Daniel Chemko]

> 2. ProxyARP the IP address (http://www.sjdjweis.com/linux/proxyarp/):
> 	echo 1 > /proc/sys/net/ipv4/conf/eth1/proxy_arp
> 	echo 1 > /proc/sys/net/ipv4/conf/eth0/proxy_arp
> This will then get the DNAT from iptables.
> 	iptables -t nat -A PREROUTING --destination w.x.y.z -j DNAT --to
> ${My_NEW_ADDR}

Scratch this: It only applies to bridged configurations where NAT is NOT
used...