Overview of ISA and TMG Networking and ISA Networking Case Study (Part 1)

by [Published on 16 Dec. 2008 / Last Updated on 20 May 2013]

What ISA/TMG firewall Networks are about and how the firewall uses these networks to perform several key functions.

If you to read the other parts in this article series please go to:

Last week I did a blog post asking our ISAserver.org members what kind of content they would like to see on the site. I expected the typical stuff, such as “more articles on integrating with other networking equipment vendors” and “more information on how NLB works” and “more articles on how to make ISA and TMG work with Exchange 2007, SharePoint and OCS” and maybe even “more stuff about ISA and TMG add-ons”. I was not disappointed. I did get requests for all of that kind of content.

There was also another comment that I thought was interesting. Someone wrote to me and said that what he would like is some information on the basics. For example, the basics of ISA networking. This fellow said that many Microsoft admins who use ISA have a basic understanding of TCP/IP networking but do not have a good grip on how the ISA firewall see the networked world and any information that would help along those lines would be very helpful.

The comment was a timely one for me, as it dovetailed with some other experiences I was having last week. Therefore, in the spirit of this request for some return to the basics and my experiences last week, we will go over some of the basics of ISA/TMG firewall networking.

ISA/TMG Firewall Networks

NOTE:
Pay close attention to the capitalization I use in this article. Network with a capital “N” refers to an ISA/TMG Firewall Network – which is a network objects that the firewall uses to define collections of IP addresses directly accessible from a specific network interface. In contrast, when a lower case “n” is used for network, I am referring to a generic network or network segment.

ISA and TMG firewalls see the networked world based on the concept of the Network network object. The Network network object defines traffic that moves through the firewall. All traffic that moves to or through the firewall must source from one Network and have a destination to another Network. If the source and destination traffic are on the same Network, then the traffic doesn’t move through the firewall. However, there are times when traffic with the same source and destination Network can bounce off the firewall. We will take a look at this example later.

What is an ISA Firewall Network? An ISA/TMG Firewall Network is a collection of IP addresses that can directly reach a NIC on the firewall without having to traverse the firewall. For example, consider a simple scenario where the ISA firewall has two NICs: an internal interface with an IP address of 10.0.0.1 and an external interface with a public IP address. There is a host connected to the same network as the firewall’s internal interface and that client has an IP address of 10.0.0.2. In this example, the internal interface and the client at 10.0.0.2 are part of the same network, since the client can directly reach that interface without crossing the firewall. In addition, the client can’t be on the same network as the external interface of the firewall, since it would have to cross the firewall to reach that interface.

The figure below depicts this example. The internal interface has the IP address 10.0.0.1 and the client behind that interface has IP address 10.0.0.2. The client behind the internal interface can reach the internal interface directly. The client behind the internal interface cannot reach the external interface directly. Therefore, the client could never be a member of the ISA Firewall Network that the external interface belongs to.


Figure 1

As I mentioned earlier, an ISA Firewall Network is defined as a collection of IP addresses that can be reached directly through one of the interfaces on the ISA or TMG firewall. However, this does not mean that all of those IP addresses have to be on the same network ID as the interface on the ISA firewall.

For example, in the figure above, the internal interface of the ISA firewall was on network ID 10.0.0.0/24 and the client was an “on subnet” client that was also on network ID 10.0.0.0/24. The ISA Firewall Network defined for that interface was 10.0.0.0-10.0.0.255.

What if there is a router behind the ISA firewall’s internal interface and there are remote network IDs that need to connect to the Internet through the ISA Firewall’s internal interface? For example, in the figure below you see that I have added a router and a remote network ID behind that router, which in this case is 192.168.1.0/24. Will the ISA Firewall need to see connections from the 192.168.1.0/24 network ID as being on the same ISA Firewall Network as connections from the 10.0.0.0/24 network ID?

The answer is YES. The reason for this is that both 10.0.0.0/24 and 192.168.1.0/24 in this example have to connect to and through the ISA firewall using the same NIC. Since the ISA Firewall see each NIC as the root of an ISA Firewall Network, all connections made directly to and through the firewall on that interface are part of the same ISA Firewall Network.


Figure 2

However, in order to make this work, you need to add those addresses to the definition of the ISA Firewall Network. In this example, the definition of the default Internal Network would include the addresses 10.0.0.0-10.0.0.255 and 192.168.1.0-192.168.1.255. All of these IP addresses are part of the default Internal Network and reach the ISA firewall through the same network interface card.


Figure 3

The reason we need to include all the addresses that are behind a specific NIC on the firewall is that if there is a host that tries to connect through the ISA firewall on that NIC from a source IP address that is not part of that ISA Firewall Network, the connection request will be dropped as a spoof attempt. The ISA or TMG firewall sees the connection attempt as a spoof because the IP address is not part of the definition of that ISA Firewall Network.

For example, check out the figure below. We have defined the default Internal Network in this example as all IP addresses in the 10.0.0.0/24 and 192.168.1.0/24 ranges (note that I have included all the addresses in each network ID – that is not a requirement. I could have included only a subset of those IP addresses if I wanted to). What if a host with the IP address 172.16.0.2 tried to connect to the ISA Firewall through the NIC that represents the “root” of the default Internal Network?

The connection attempt would fail. The reason why it would fail is that 172.16.0.2 is not part of the definition of the default Internal Network in this example. Since the ISA Firewall does not recognize this source IP address as part of the default Internal Network, it will not allow the connection through the NIC that defines the “root” of the default Internal Network. It will call out this connection as a spoof attempt. All spoof attempts are blocked by the firewall.


Figure 4

What if you wanted to allow connections from that host at 172.16.0.2? It is a simple matter of adding that IP address to the definition of the ISA Firewall Network that this host uses to connect to and through the ISA firewall. In this case, you could add just that IP address, or if you have other hosts on that network ID, you could add the IP addresses of those hosts, or you could add all the addresses in that network ID.

You define that addresses that belong to a specific ISA Firewall network in the Properties dialog box for that Network. In the figure below, you can see the addresses tab for the default Internal Network. This default Internal ISA Firewall Network includes all addresses on the network ID 192.168.1.0/24.


Figure 5

You can create multiple ISA Firewall Networks on a single ISA Firewall. For example, suppose you wanted to create an ISA Firewall Network for wireless guest computers to connect to the Internet. In this case, you would add a third NIC to the ISA firewall (the other two interfaces are for the external interface and the internal interface). The third NIC would become the “root” of a new ISA Firewall Network. You would then assign addresses to that ISA Firewall Network. Each NIC on the ISA firewall needs to be on a different network ID, so after installing the third NIC, we assign it an IP address on a network ID that is different than the other two NICs. Then we assign IP addresses for the new ISA Firewall Network. In the figure below, you can see that all addresses on network ID 192.168.0.0/24 are part of the Guest ISA Firewall Network.


Figure 6

It is important to remember that an IP address can participate on a single ISA Firewall Network. You can not assign the same IP address to two different ISA Firewall Networks. If you do, you will receive an error message.

Out of the box, the ISA or TMG firewall will have the following Networks defined:

  • The default External Network – the default External Network is defined by all IP addresses that are used by any other ISA Firewall Network. Any address that is not used by any other ISA Firewall Network will automatically be included as part of the default External Network. The NIC that defines the default External Network is usually the NIC with the default gateway bound to it. ISA and TMG MBE firewalls support a single default gateway
  • The default Internal Network – this is the network you define during setup that represents your primary internal network. You can have multiple internal networks if you like, but there is only one default Internal Network which you set up during installation of the ISA firewall. The default Internal Network typically contains your key infrastructure services, such as DNS, DHCP and Active Directory domain services. The default Internal Network is important because much of the ISA and TMG firewall’s System Policy is configured to access resources on the default Internal Network
  • The Local Host Network – The Local Host Network is defined by the IP addresses bound to all NICs on the ISA or TMG firewall. For example, if the firewall had two interfaces, one with IP address 2.2.2.2 bound to it and the other with 10.0.0.1 bound to it, then IP addresses 2.2.2.2 and 10.0.0.1 are members of the Local Host Network. Note that this breaks one of the rules of ISA/TMG Networks – in that these IP addresses are also members of the Networks to which those NICs are connected. The 2.2.2.2 is likely a member of the default External Network and the 10.0.0.1 is a member of the default Internet Network.
  • VPN Clients Network – The VPN Clients Network contains the IP addresses of connected VPN clients. There are two ways to assign IP addresses to VPN clients: using a static address pool and using DHCP. If you assign IP addresses to VPN clients using a static address pool, then you must remove those IP addresses from any other Network that might contain them. For example, if you want to assign on-subnet addresses to VPN clients (such as 192.168.1.200-192.168.1.225/24 when the internal interface is on 192.168.1.1/24), you must remove those addresses from the definition of the on-subnet network.
    In contrast, if you want to use DHCP to assign IP addresses to VPN clients, then you do not have to remove those addresses from the definition of any other Network that might also be using those addresses. It makes sense, since when you use DHCP to assign these addresses; you know that no other host should be able to use the same IP address on any other Network. In contrast, if you assign static addresses to VPN clients, you do not know for sure that there might be an error that would lead you to use the same addresses on another Network. Addresses are automatically added and removed from the VPN clients Network when they are used and released by the VPN clients. Note that this represents a second exception to our rule that an IP address can belong to a single Network – since you use DHCP to assign IP addresses to VPN clients, those addresses can belong to another ISA/TMG Firewall Network.
  • Quarantined VPN Clients Network – The Quarantined VPN Clients Network contains the IP addresses of VPN clients that have not yet passed VPN quarantine control. This is configured as a separate Network from the VPN Clients Network because you might want to create Firewall Rules that allow quarantined VPN clients access to resources on a Protected Network (a Protected Network is any ISA/TMG Network that isn’t the default External Network) or even on the Internet so that they can remediate themselves. IP addresses are automatically moved from the Quarantined VPN Clients Network to the VPN Clients Network when the VPN client passes quarantine control checks.


Figure 7

Summing up what we know at this point:

  • ISA/TMG Firewall Networks are used for spoof detection. If a source IP address arrives at an interface that is a root of an ISA Firewall Network that isn’t an IP address defined for that Network, then the connection attempt is dropped as a spoofed connection attempt
  • An IP address can be assigned to a single ISA/TMG Firewall Network. The only exceptions to this rule are seen with the Local Host Network and the VPN Clients and Quarantined VPN Clients Networks when you use DHCP to assign addresses to VPN clients.
  • An ISA/TMG Firewall Network can contain IP addresses from multiple network IDs. What all these IP addresses have in common is that if they need to connect to and through the ISA or TMG firewall through the same NIC

ISA/TMG Firewall Networks also are used to do one more important task: define whether connections are routed or NATed from the systems on a particular Network to another Network. In order to hosts on a Network to communicate with hosts on another Network, the two Networks must be connected using a Network Rule. The Network Rule accomplishes two things:

  • Enables communications between the two ISA/TMG Firewall Networks
  • Sets a routing relationship between the two Networks

I’ll go into more details on Network Rules and connecting Networks to one another in the second part of this series on ISA/TMG firewall networking.

Summary

In this article, we went over what ISA/TMG firewall Networks are about and how the firewall uses these networks to perform several key functions. We saw that an IP address can belong to only a single Network, with the exception of the Local Host Network and the VPN Clients and Quarantined VPN Clients Networks. We then finished off with a brief overview of the default ISA/TMG Firewall Networks. Next week I will continue the story by showing you how ISA/TMG Networks are used to connect hosts on one Network to another, and how Networks are used to define a route relationship between source and destination. See you then! –Tom.

If you to read the other parts in this article series please go to:

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