Thursday, April 05, 2007

Interested in Cellular? Setup a DMZ Lab

Summary: Last post I suggested people interested in cellular data start by learning how to use their home cable router. In this post I suggest the next step, of how to make your life easy at work once you're ready to start testing real cellular or satellite access by IP.

Your Second Step should be to set up a simple, isolated low-speed broadband link at work ... create your own DMZ lab.

Sigh - I waste so much time listening to customers complain about how difficult it is to get the IT department to give them custom firewall permissions. Since modern "Security" wisdom is to block everything until proven safe, I waste more time asking customers complaining that their Modbus/TCP or Rockwell access not working to first talk to their IT group to make sure they aren't blocking unknown binary traffic by default. I waste yet more time when customers struggle for days and finally have to formally get someone in IT to help study the corporate firewall logs to see if any traffic is getting through or not. An interesting epiphany occurs when I suggest they just look into paying roughly $50 per month for a private connection for this. It is surprisingly cheap to do this and makes a lot of people's jobs 200% easier.

So far the feedback from customers has been quite positive, with IT departments over-joyed at the idea (slight exaggeration :-] lessor-of-two-evils may be a better term). This really makes sense; IT is charged with keeping the corporate system working and secure, so when you ask for yet another odd, unknown firewall hole to be opened, you ask them to risk their jobs. Plus trying to keep custom firewall settings updated for 50 different projects is an ongoing headache and ongoing risk for mistakes. I know that Digi's IT group is very satisfied with their policy of not offering custom firewall rules on the corporate LAN but instead helping teams set up such private connections in a safe, isolated manner.

Simple DMZ Lab Design
The simplest lab design is little more than a copy of what you have at home: a computer or two, an 8 or 24-port Ethernet switch, and a simple NAT router to "share" the internet connection with a dozen devices. This allows you to freely set up a few OPC servers and Master PLC to test access to remote cellular and other wide-area-network based systems.
  • Locate an empty office or lab room for your new network. Perhaps your IT people should pull out or disable the corporate Ethernet in this room. You are going to create a small "DMZ"; a small isolated network that has limited security consequences if you goof up and let a hacker inside. You'll want good security tool installed on your Windows and Linux computer used in here.
  • Arrange for a low-speed business broadband link with one fixed IP address. 256Kbps is more than enough for general PLC/SCADA testing and should cost in the range of $35 to $50 per month. Yes, just $35-50 per month! I had one customer forced to pay his IT department $100 per month to open one TCP/IP hole in the corporate firewall!! Gee, he could install 2 DSL links for that. Now, be patent when you talk to your carrier, as they are geared to sell the expensive primary access lines used for all corporate traffic including servers. Keep stressing that you want a low-speed secondary line for use with some network testing and eventually you'll locate the low-cost plans you want.
  • Set up a DNS name for your DMZ lab. Online dynamic DNS providers support user-selected DNS names for static IP addresses. I use for both my dynamic and static IP but there are many out there. You won't need any form of DDNS update client since your IP never changes and you must enter the name manually anyway.
  • Unless you plan to implement large VPN systems, just buy a nice consumer-grade DSL/Cable Router ... the same kind you use at home is fine. If you plan to set up a serious VPN infra-structure, then you'll just need to bite-the-bullet (& suffer the learning curve) of buying a commercial IT-grade router with VPN server capability built in. Be warned that while many consumer-grade routers mention "VPN Support", they are in fact sub-optimized and documented only for home-office users who connect into a corporate Windows or Cisco VPN server. Normal human beings will find them nearly impossible to set up for anything else!
  • Do you want more than one public IP address? You need to pay a monthly surcharge which varies greatly per carrier, but could be in the range of $25 per month for 8 IP addresses instead of just 1. Plus you will need a larger IT-grade router since the consumer-grade routers won't support more than 1 public IP address. Most users won't need more than 1 IP address. However, having more than one IP address is helpful if more than 1 team shares the lab; this prevents them from trying to setup conflicting router configurations. Also, a few extra IP are helpful if you want to place a PLC "online" for your customers or sales force to access during customer-site demonstrations in the field.
  • If you need to access your corporate network from your DMZ lab, then you need to arrange some rules with your IT people. Perhaps the rule is using a notebook computer with 802.11 wireless to the corporate network is Ok as long as the notebook is NEVER connected to the lab's Ethernet. Remember, since your lab has it's own public IP address you can even use FTP or a VPN client to connect "legally" out your corporate network and back into your lab via the Internet.
Fancier DMZ Lab Design
Since Digi is basically a "communication company", the lab I get to use is much fancier. It has 32 public IP addresses and even limited secure access from the corporate LAN. Of course I have to share this lab with other teams, so I'm not owner of 32 IP. As an example, here is how my lab is setup:
  • Digi's IT group maintains a Cisco PIX router (a mid-range $800 model) that manages the 32 public IP addresses. Actually, this is NOT a complication since this router does not by default firewall any traffic; it merely distributes raw traffic based on public IP to one of many to internal IP addresses in an organized manner. I was lucky enough to get in the lab early and to be assigned 2 of the 32 public IP addresses.
  • My first IP address receives 100% raw internet traffic at an internal static IP I selected; so an external IP such as 70.x.x.140 forwards to my internal IP of Since the goal of the lab is to avoid burdening IT with TCP/UDP port forwarding chores, one could place a consumer-grade DSL/Cable router at this IP. Placing 2 routes in series is NOT a problem - do a net-trace of how you access and you'll see a dozen or more routers in series. Instead of a pure hardware box I have a Ubuntu Linux machine running firewall and router tools at this IP. This is where I forward Modbus/TCP to one PLC and Rockwell Ethernet/IP to another PLC. I prefer the Linux box to the $39 hardware box because it gives me a richer view of traffic in and out, plus I can run an Ethernet sniffer such as WireShark to see a complete trace of the 2-way conversation taking place.
  • For my second IP address I had Digi IT setup the PIX router to just forward a simple, safe list of Modbus, Rockwell, Digi, and other industrial protocol TCP and UDP ports. I normally have a Digi One IAP (an industrial-protocol aware Ethernet-to-serial device) at this IP address, but I can safely swap in a Windows machine when a test requires use of Windows tools.
Just as your home router does, the main Digi IT-supplied PIX router does out-going NAT to the internet and internal DHCP address assignment. So our DMZ lab has its own internal subnet of 192.168.20.x addresses. Any device in the DMZ lab can access the Internet - with or without going through my Linux firewall/router. Of course the other teams in the lab don't go through my router, but direct to the PIX router.

Since i study cellular usage, I have have a Digi Connect WAN providing an Ethernet-based cellular router in the DMZ lab. So I really have 3 potential routers to use - while it takes a bit of IP experience to not get confused, this allows me to have devices configured to selectively treat any 1 of the 3 routers as "the default gateway/router".
  • For example, I can have a Master/Client device connect out to a cellular-based Slave/Server using a route such as Master => out PIX+DSL => in Cellular => Slave.
  • For example, I can have a Master/Client device connect via cellular to a DSL-based Slave/Server using a route such as Master => out Cellular => in DSL+PIX => in Linux-Box => Slave.
  • In both of this situations I can see BOTH ends of the conversation, which is a huge help in testing, timing, or troubleshooting new applications.
The bonus for having Digi's IT team involved is the PIX router also allows controlled, secure access into the DMZ lab from our corporate LAN. Technically, the PIX runs a set of rules similar to those used by the main corporate firewall out to the Internet and it just treats the 192.168.20.x subnet as a miniature internet. So I can sit at my desk and safely check on equipment running in the DMZ lab. Of course this is limited to equipment treating the PIX as the default router - if you understand IP routing you'll understand why, but at least it lets me log into my Ubuntu Linux router from desk.

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