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Limitations of NAT

NAT does not work in all cases. The following sections document some of the instances where NAT will not work as expected.

Table 9-2 Network objects to create


Object Type

IP/Mask/Group Objects




Your DMZ



Mail Server in the DMZ



Translated version of smtp-dmz



Web Server in the DMZ



Translated version of web-server (for port 80)



Translated version of web-server (for port 81)




Segment shared by firewall and internal router



Segment A



Intranet Web Server



Translated version of web-intranet



Internal SMTP Server



Segment B







net-dmz + web-server-ext + web-server-ext2 + smtp-server-ext

Represents your DMZ interface's valid addresses for anti-spoofing







net-segment-a + net- segment-b + web-intranet- ext + net-router-segment

Represents your internal interface's valid addresses for anti-spoofing



Your firewall

Table 9-3 Valid address settings for firewall


Valid Address Setting


Specific: valid-dmz


Specific: valid-internal



Figure 9-4 Security policy for sample NAT configuration

Figure 9-5 NAT policy for sample NAT configuration

The main components that NAT changes are the IP addresses in the TCP/IP headers, and possibly the TCP or UDP ports. This works for some applications, but many applications embed IP addresses in the data portion of the packet (e.g., Microsoft Networking7) or expect packets to come from a particular source port (e.g., IKE negotiations for IPSec). In these cases, NAT has to act somewhat like an application proxy in that it must understand the underlying protocol and make intelligent changes to the packets so that the protocol will work despite undergoing NAT.

FireWall-1 understands certain protocols like FTP, RealAudio, and Microsoft Networking (if support is specifically enabled in FireWall-1 4.1 SP1 and later). There are plenty of applications that do not work correctly with FireWall-1's NAT, not necessarily because they are impossible to make work with NAT, but because Check Point has not added support for them. However, there are some protocols that are simply impossible to make work with NAT. Any protocol that uses IP datagram types other than TCP or UDP often fail when NAT is applied. Protocols that validate the IP packet headers between source and destination (such as the Authentication Header mechanism of IPSec) will not work with NAT. To a protocol that protects network traffic from man-in-the-middle attacks—attacks where the headers or payload changes in transit—NAT looks like a hacker. The bottom line: NAT breaks end-to-end connectivity and should only be employed in instances where you can live with the limitations.

NAT will be problematic in situations where the firewall is not between both the source and the destination. Using the example in the previous step-by-step configuration (see Figure 9-3), consider the situation where a host on Segment B ( tries to access the intranet Web server via the translated IP address ( The host tries to initiate a connection to Routing will eventually take this packet to the firewall. The packet is accepted by the firewall's security policy and is then processed by NAT. The first rule that matches the packet is Rule 3, which translates the destination of the packet from to The "source" of the packet is not changed (the rule says not to touch it). The packet will then be routed back to via

When sends its reply, it is sent to (the "source" of the connection attempt). The reply is routed to and then directly to The host expects replies from (which it tries to connect to), not, so the reply packets are ignored.

What would happen if the rule hides behind the firewall's external IP address? When tries to access, the packet gets routed to the firewall and passes through the rulebase. NAT then would rewrite the source of the packet to be The destination of the packet would still be (i.e., it does not get translated) but gets routed out the internal interface. The Internet router sees this packet and routes it back to the firewall (it is an external address, after all). The packet would ping-pong back and forth until the packet's Time To Live (TTL) value expired.

One reason you might connect to the translated IP address is because your internal client's DNS server resolves the host's name to the external address. You can resolve this problem by implementing split-horizon DNS, that is, maintaining an internal version of your DNS and an external version of your DNS, typically on separate servers. The external DNS would be accessible from the Internet and contain only a subset of names and addresses contained in the internal DNS server. An internal DNS contains all the names used internally and reflects the internal IP address for a host. The external DNS server reflects the externally resolvable IP addresses for the host.

Other than implementing split-horizon DNS, can you get around this problem? Yes, there are two tricks you can use, which are documented in the following sections. However, it is highly recommended you not place yourself in a position where you have to use these tricks.

Dual NAT (Translating Both Source and Destination)

FireWall-1 allows you to translate both the source and destination IP address at once. It is simply a matter of crafting the correct rules and placing them in the right order. In the preceding case, if you want to allow your internal network to access the internal host via its translated IP address, modify your NAT rules so they read as shown in Figure 9-6.

Figure 9-6 NAT policy with dual NAT rules

The two rules that were added are shown in Figure 9-7.

Figure 9-7 Dual NAT rules added to Figure 9–6

These rules will hide the source address behind the firewall's IP address and modify the destination IP to be the web-intranet address.

In this particular case, there is another issue to contend with: ICMP Redirects. Because the firewall will be routing a packet out the same interface from which it was received, the system sends the client an ICMP Redirect, giving it a more direct route to the host. Depending on the exact circumstances, the ICMP Redirect will either cause the connection to never take place or take a long time to establish as the client will be trying to communicate directly to a host using an IP address it knows nothing about. There are a couple of ways around this situation:

  • Bind the translated IP address to the server's loopback interface. See the next section for details.

  • Block ICMP Redirects. You can block outgoing ICMP Redirects in FireWall-1 with the FireWall-1 rule shown in Figure 9-8.

  • Figure 9-8 Rule to block ICMP Redirects

  • On some operating systems, there is an option to disable sending ICMP Redirects. On Solaris, you do this by typing:

    /usr/sbin/ndd -set /dev/ip ip_send_redirects 0

On a Nokia platform, this can be done on a per-interface basis by typing

    ipsctl -w interface:<physical-interface>:family:inet:flags:icmp_no_rdir 1

where you replace <physical-interface> with the physical interface name (e.g., eth-s1p1).


By default, ICMP Redirects are not enabled on any interface running VRRP. This is highly recommended in a VRRP configuration, as it limits the possibility that your machine's physical address is propagated.

Assuming this trick works, a side effect can occur, which makes traffic traverse your network twice: once to the firewall and once to the server. This could add to an already congested network.

Binding the NAT IP Address to the Loopback Interface

The basic idea is to bind the translated IP address to the loopback interface of the server. On Windows NT, you need to add the MS Loopback interface (a software-only network adaptor) and add the IP address to this interface with a net mask of In IPSO, you can simply add an IP address to the loop0c0 interface via Voyager. On UNIX machines, use a command such as the following:

    ifconfig lo0:0 up

If packets come into the system for the translated IP address (because, for instance, they did not come to the firewall), the system will respond to packets for this IP address. This method does require slightly more administration because you must also maintain the NAT on the individual servers.

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