Cisco ASA Services Module CLI Configuration Guide, 8.5
Information About NAT
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Information About NAT

Table Of Contents

Information About NAT

Why Use NAT?

NAT Terminology

NAT Types

NAT Types Overview

Static NAT

Information About Static NAT

Information About Static NAT with Port Translation

Information About One-to-Many Static NAT

Information About Other Mapping Scenarios (Not Recommended)

Dynamic NAT

Information About Dynamic NAT

Dynamic NAT Disadvantages and Advantages

Dynamic PAT

Information About Dynamic PAT

Dynamic PAT Disadvantages and Advantages

Identity NAT

NAT in Routed and Transparent Mode

NAT in Routed Mode

NAT in Transparent Mode

How NAT is Implemented

Main Differences Between Network Object NAT and Twice NAT

Information About Network Object NAT

Information About Twice NAT

NAT Rule Order

NAT Interfaces

Routing NAT Packets

Mapped Addresses and Routing

Transparent Mode Routing Requirements for Remote Networks

Determining the Egress Interface

DNS and NAT

Where to Go Next


Information About NAT


This chapter provides an overview of how Network Address Translation (NAT) works on the ASASM. This chapter includes the following sections:

Why Use NAT?

NAT Terminology

NAT Types

NAT in Routed and Transparent Mode

How NAT is Implemented

NAT Rule Order

Routing NAT Packets

DNS and NAT

Where to Go Next


Note To start configuring NAT, see Chapter 28 "Configuring Network Object NAT," or Chapter 29 "Configuring Twice NAT."


Why Use NAT?

Each computer and device within an IP network is assigned a unique IP address that identifies the host. Because of a shortage of public IPv4 addresses, most of these IP addresses are private, not routable anywhere outside of the private company network. RFC 1918 defines the private IP addresses you can use internally that should not be advertised:

10.0.0.0 through 10.255.255.255

172.16.0.0 through 172.31.255.255

192.168.0.0 through 192.168.255.255

One of the main functions of NAT is to enable private IP networks to connect to the Internet. NAT replaces a private IP address with a public IP address, translating the private addresses in the internal private network into legal, routable addresses that can be used on the public Internet. In this way, NAT conserves public addresses because it can be configured to advertise at a minimum only one public address for the entire network to the outside world.

Other functions of NAT include:

Security—Keeping internal IP addresses hidden discourages direct attacks.

IP routing solutions—Overlapping IP addresses are not a problem when you use NAT.

Flexibility—You can change internal IP addressing schemes without affecting the public addresses available externally; for example, for a server accessible to the Internet, you can maintain a fixed IP address for Internet use, but internally, you can change the server address.


Note NAT is not required. If you do not configure NAT for a given set of traffic, that traffic will not be translated, but will have all of the security policies applied as normal.


NAT Terminology

This document uses the following terminology:

Real address/host/network/interface—The real address is the address that is defined on the host, before it is translated. In a typical NAT scenario where you want to translate the inside network when it accesses the outside, the inside network would be the "real" network. Note that you can translate any network connected to the ASASM, not just an inside network, Therefore if you configure NAT to translate outside addresses, "real" can refer to the outside network when it accesses the inside network.

Mapped address/host/network/interface—The mapped address is the address that the real address is translated to. In a typical NAT scenario where you want to translate the inside network when it accesses the outside, the outside network would be the "mapped" network.

Bidirectional initiation—Static NAT allows connections to be initiated bidirectionally, meaning both to the host and from the host.

Source and destination NAT—For any given packet, both the source and destination IP addresses are compared to the NAT rules, and one or both can be translated/untranslated. For static NAT, the rule is bidirectional, so be aware that "source" and "destination" are used in commands and descriptions throughout this guide even though a given connection might originate at the "destination" address.

NAT Types

NAT Types Overview

Static NAT

Dynamic NAT

Dynamic PAT

Identity NAT

NAT Types Overview

You can implement NAT using the following methods:

Static NAT—A consistent mapping between a real and mapped IP address. Allows bidirectional traffic initiation. See the "Static NAT" section.

Dynamic NAT—A group of real IP addresses are mapped to a (usually smaller) group of mapped IP addresses, on a first come, first served basis. Only the real host can initiate traffic. See the "Dynamic NAT" section.

Dynamic Port Address Translation (PAT)—A group of real IP addresses are mapped to a single IP address using a unique source port of that IP address. See the "Dynamic PAT" section.

Identity NAT—A real address is statically translated to itself, essentially bypassing NAT. You might want to configure NAT this way when you want to translate a large group of addresses, but then want to exempt a smaller subset of addresses. See the "Identity NAT" section.

Static NAT

This section describes static NAT and includes the following topics:

Information About Static NAT

Information About Static NAT with Port Translation

Information About One-to-Many Static NAT

Information About Other Mapping Scenarios (Not Recommended)

Information About Static NAT

Static NAT creates a fixed translation of a real address to a mapped address. Because the mapped address is the same for each consecutive connection, static NAT allows bidirectional connection initiation, both to and from the host (if an access rule exists that allows it). With dynamic NAT and PAT, on the other hand, each host uses a different address or port for each subsequent translation, so bidirectional initiation is not supported.

Figure 27-1 shows a typical static NAT scenario. The translation is always active so both real and remote hosts can initiate connections.

Figure 27-1 Static NAT

Information About Static NAT with Port Translation

Static NAT with port translation lets you specify a real and mapped protocol (TCP or UDP) and port.

This section includes the following topics:

Information About Static NAT with Port Address Translation

Static NAT with Identity Port Translation

Static NAT with Port Translation for Non-Standard Ports

Static Interface NAT with Port Translation

Information About Static NAT with Port Address Translation

When you specify the port with static NAT, you can choose to map the port and/or the IP address to the same value or to a different value.

Figure 27-2 shows a typical static NAT with port translation scenario showing both a port that is mapped to itself and a port that is mapped to a different value; the IP address is mapped to a different value in both cases. The translation is always active so both translated and remote hosts can initiate connections.

Figure 27-2 Typical Static NAT with Port Translation Scenario


Note For applications that require application inspection for secondary channels (for example, FTP and VoIP), the ASASM automatically translates the secondary ports.


Static NAT with Identity Port Translation

The following static NAT with port translation example provides a single address for remote users to access FTP, HTTP, and SMTP. These servers are actually different devices on the real network, but for each server, you can specify static NAT with port translation rules that use the same mapped IP address, but different ports. (See Figure 27-3. See the "Single Address for FTP, HTTP, and SMTP (Static NAT-with-Port-Translation)" section for details on how to configure this example.)

Figure 27-3 Static NAT with Port Translation

Static NAT with Port Translation for Non-Standard Ports

You can also use static NAT with port translation to translate a well-known port to a non-standard port or vice versa. For example, if inside web servers use port 8080, you can allow outside users to connect to port 80, and then undo translation to the original port 8080. Similarly, to provide extra security, you can tell web users to connect to non-standard port 6785, and then undo translation to port 80.

Static Interface NAT with Port Translation

You can configure static NAT to map a real address to an interface address/port combination. For example, if you want to redirect Telnet access for the ASASM outside interface to an inside host, then you can map the inside host IP address/port 23 to the ASASM interface address/port 23. (Note that although Telnet to the ASASM is not allowed to the lowest security interface, static NAT with interface port translation redirects the Telnet session instead of denying it).

Information About One-to-Many Static NAT

Typically, you configure static NAT with a one-to-one mapping. However, in some cases, you might want to configure a single real address to several mapped addresses (one-to-many). When you configure one-to-many static NAT, when the real host initiates traffic, it always uses the first mapped address. However, for traffic initiated to the host, you can initiate traffic to any of the mapped addresses, and they will be untranslated to the single real address.

Figure 27-4 shows a typical one-to-many static NAT scenario. Because initiation by the real host always uses the first mapped address, the translation of real host IP/1st mapped IP is technically the only bidirectional translation.

Figure 27-4 One-to-Many Static NAT

For example, you have a load balancer at 10.1.2.27. Depending on the URL requested, it redirects traffic to the correct web server (see Figure 27-5). (See the "Inside Load Balancer with Multiple Mapped Addresses (Static NAT, One-to-Many)" section for details on how to configure this example.)

Figure 27-5 One-to-Many Static NAT

Information About Other Mapping Scenarios (Not Recommended)

The ASASM has the flexibility to allow any kind of static mapping scenario: one-to-one, one-to-many, but also few-to-many, many-to-few, and many-to-one mappings. We recommend using only one-to-one or one-to-many mappings. These other mapping options might result in unintended consequences.

Functionally, few-to-many is the same as one-to-many; but because the configuration is more complicated and the actual mappings may not be obvious at a glance, we recommend creating a one-to-many configuration for each real address that requires it. For example, for a few-to-many scenario, the few real addresses are mapped to the many mapped addresses in order (A to 1, B to 2, C to 3). When all real addresses are mapped, the next mapped address is mapped to the first real address, and so on until all mapped addresses are mapped (A to 4, B to 5, C to 6). This results in multiple mapped addresses for each real address. Just like a one-to-many configuration, only the first mappings are bidirectional; subsequent mappings allow traffic to be initiated to the real host, but all traffic from the real host uses only the first mapped address for the source.

Figure 27-6 shows a typical few-to-many static NAT scenario.

Figure 27-6 Few-to-Many Static NAT

For a many-to-few or many-to-one configuration, where you have more real addresses than mapped addresses, you run out of mapped addresses before you run out of real addresses. Only the mappings between the lowest real IP addresses and the mapped pool result in bidirectional initiation. The remaining higher real addresses can initiate traffic, but traffic cannot be initiated to them (returning traffic for a connection is directed to the correct real address because of the unique 5-tuple (source IP, destination IP, source port, destination port, protocol) for the connection).


Note Many-to-few or many-to-one NAT is not PAT. If two real hosts use the same source port number and go to the same outside server and the same TCP destination port, and both hosts are translated to the same IP address, then both connections will be reset because of an address conflict (the 5-tuple is not unique).


Figure 27-7 shows a typical many-to-few static NAT scenario.

Figure 27-7 Many-to-Few Static NAT

Instead of using a static rule this way, we suggest that you create a one-to-one rule for the traffic that needs bidirectional initiation, and then create a dynamic rule for the rest of your addresses.

Dynamic NAT

This section describes dynamic NAT and includes the following topics:

Information About Dynamic NAT

Dynamic NAT Disadvantages and Advantages

Information About Dynamic NAT

Dynamic NAT translates a group of real addresses to a pool of mapped addresses that are routable on the destination network. The mapped pool typically includes fewer addresses than the real group. When a host you want to translate accesses the destination network, the ASASM assigns the host an IP address from the mapped pool. The translation is created only when the real host initiates the connection. The translation is in place only for the duration of the connection, and a given user does not keep the same IP address after the translation times out. Users on the destination network, therefore, cannot initiate a reliable connection to a host that uses dynamic NAT, even if the connection is allowed by an access rule.

Figure 27-8 shows a typical dynamic NAT scenario. Only real hosts can create a NAT session, and responding traffic is allowed back.

Figure 27-8 Dynamic NAT

Figure 27-9 shows a remote host attempting to initiate a connection to a mapped address. This address is not currently in the translation table; therefore, the ASASM drops the packet.

Figure 27-9 Remote Host Attempts to Initiate a Connection to a Mapped Address


Note For the duration of the translation, a remote host can initiate a connection to the translated host if an access rule allows it. Because the address is unpredictable, a connection to the host is unlikely. Nevertheless, in this case you can rely on the security of the access rule.


Dynamic NAT Disadvantages and Advantages

Dynamic NAT has these disadvantages:

If the mapped pool has fewer addresses than the real group, you could run out of addresses if the amount of traffic is more than expected.

Use PAT or a PAT fallback method if this event occurs often because PAT provides over 64,000 translations using ports of a single address.

You have to use a large number of routable addresses in the mapped pool, and routable addresses may not be available in large quantities.

The advantage of dynamic NAT is that some protocols cannot use PAT. PAT does not work with the following:

IP protocols that do not have a port to overload, such as GRE version 0.

Some multimedia applications that have a data stream on one port, the control path on another port, and are not open standard.

See the "Default Settings" section for more information about NAT and PAT support.

Dynamic PAT

This section describes dynamic PAT and includes the following topics:

Information About Dynamic PAT

Dynamic PAT Disadvantages and Advantages

Information About Dynamic PAT

Dynamic PAT translates multiple real addresses to a single mapped IP address by translating the real address and source port to the mapped address and a unique port. If available, the real source port number is used for the mapped port. However, if the real port is not available, by default the mapped ports are chosen from the same range of ports as the real port number: 0 to 511, 512 to 1023, and 1024 to 65535. Therefore, ports below 1024 have only a small PAT pool that can be used.

Each connection requires a separate translation session because the source port differs for each connection. For example, 10.1.1.1:1025 requires a separate translation from 10.1.1.1:1026.

Figure 27-10 shows a typical dynamic PAT scenario. Only real hosts can create a NAT session, and responding traffic is allowed back. The mapped address is the same for each translation, but the port is dynamically assigned.

Figure 27-10 Dynamic PAT

After the connection expires, the port translation also expires after 30 seconds of inactivity. The timeout is not configurable. Users on the destination network cannot reliably initiate a connection to a host that uses PAT (even if the connection is allowed by an access rule).


Note For the duration of the translation, a remote host can initiate a connection to the translated host if an access rule allows it. Because the port address (both real and mapped) is unpredictable, a connection to the host is unlikely. Nevertheless, in this case you can rely on the security of the access rule.


Dynamic PAT Disadvantages and Advantages

Dynamic PAT lets you use a single mapped address, thus conserving routable addresses. You can even use the ASASM interface IP address as the PAT address.

Dynamic PAT does not work with some multimedia applications that have a data stream that is different from the control path. See the "Default Settings" section for more information about NAT and PAT support.

Dynamic PAT may also create a large number of connections appearing to come from a single IP address, and servers might interpret the traffic as a DoS attack. (8.4(2)/8.5(1) and later) You can configure a PAT pool of addresses and use a round-robin assignment of PAT addresses to mitigate this situation.

Identity NAT

You might have a NAT configuration in which you need to translate an IP address to itself. For example, if you create a broad rule that applies NAT to every network, but want to exclude one network from NAT, you can create a static NAT rule to translate an address to itself.

Figure 27-11 shows a typical identity NAT scenario.

Figure 27-11 Identity NAT

NAT in Routed and Transparent Mode

You can configure NAT in both routed and transparent firewall mode. This section describes typical usage for each firewall mode and includes the following topics:

NAT in Routed Mode

NAT in Transparent Mode

NAT in Routed Mode

Figure 27-12 shows a typical NAT example in routed mode, with a private network on the inside.

Figure 27-12 NAT Example: Routed Mode

1. When the inside host at 10.1.2.27 sends a packet to a web server, the real source address of the packet, 10.1.2.27, is changed to a mapped address, 209.165.201.10.

2. When the server responds, it sends the response to the mapped address, 209.165.201.10, and the ASASM receives the packet because the ASASM performs proxy ARP to claim the packet.

3. The ASASM then changes the translation of the mapped address, 209.165.201.10, back to the real address, 10.1.2.27, before sending it to the host.

NAT in Transparent Mode

Using NAT in transparent mode eliminates the need for the upstream or downstream routers to perform NAT for their networks.

NAT in transparent mode has the following requirements and limitations:

Because the transparent firewall does not have any interface IP addresses, you cannot use interface PAT.

ARP inspection is not supported. Moreover, if for some reason a host on one side of the ASASM sends an ARP request to a host on the other side of the ASASM, and the initiating host real address is mapped to a different address on the same subnet, then the real address remains visible in the ARP request.

Figure 27-13 shows a typical NAT scenario in transparent mode, with the same network on the inside and outside interfaces. The transparent firewall in this scenario is performing the NAT service so that the upstream router does not have to perform NAT.

Figure 27-13 NAT Example: Transparent Mode

1. When the inside host at 10.1.1.75 sends a packet to a web server, the real source address of the packet, 10.1.1.75, is changed to a mapped address, 209.165.201.15.

2. When the server responds, it sends the response to the mapped address, 209.165.201.15, and the ASASM receives the packet because the upstream router includes this mapped network in a static route directed to the ASASM management IP address. See the "Mapped Addresses and Routing" section for more information about required routes.

3. The ASASM then undoes the translation of the mapped address, 209.165.201.15, back to the real address, 10.1.1.1.75. Because the real address is directly-connected, the ASASM sends it directly to the host.

4. For host 192.168.1.2, the same process occurs, except for returning traffic, the ASASM looks up the route in its routing table and sends the packet to the downstream router at 10.1.1.3 based on the ASASM static route for 192.168.1.0/24. See the "Transparent Mode Routing Requirements for Remote Networks" section for more information about required routes.

How NAT is Implemented

The ASASM can implement address translation in two ways: network object NAT and twice NAT. This section includes the following topics:

Main Differences Between Network Object NAT and Twice NAT

Information About Network Object NAT

Information About Twice NAT

Main Differences Between Network Object NAT and Twice NAT

The main differences between these two NAT types are:

How you define the real address.

Network object NAT—You define NAT as a parameter for a network object. A network object names an IP host, range, or subnet so you can then use the object in configuration instead of the actual IP addresses. The network object IP address serves as the real address. This method lets you easily add NAT to network objects that might already be used in other parts of your configuration.

Twice NAT—You identify a network object or network object group for both the real and mapped addresses. In this case, NAT is not a parameter of the network object; the network object or group is a parameter of the NAT configuration. The ability to use a network object group for the real address means that twice NAT is more scalable.

How source and destination NAT is implemented.

Network object NAT— Each rule can apply to either the source or destination of a packet. So two rules might be used, one for the source IP address, and one for the destination IP address. These two rules cannot be tied together to enforce a specific translation for a source/destination combination.

Twice NAT—A single rule translates both the source and destination. A matching packet only matches the one rule, and further rules are not checked. Even if you do not configure the optional destination address for twice NAT, a matching packet still only matches one twice NAT rule. The source and destination are tied together, so you can enforce different translations depending on the source/destination combination. For example, sourceA/destinationA can have a different translation than sourceA/destinationB.

Order of NAT Rules.

Network object NAT—Automatically ordered in the NAT table.

Twice NAT—Manually ordered in the NAT table (before or after network object NAT rules).

See the "NAT Rule Order" section for more information.

We recommend using network object NAT unless you need the extra features that twice NAT provides. Network object NAT is easier to configure, and might be more reliable for applications such as Voice over IP (VoIP). (For VoIP, because twice NAT is applicable only between two objects, you might see a failure in the translation of indirect addresses that do not belong to either of the objects.)

Information About Network Object NAT

All NAT rules that are configured as a parameter of a network object are considered to be network object NAT rules. Network object NAT is a quick and easy way to configure NAT for a network object, which can be a single IP address, a range of addresses, or a subnet.

After you configure the network object, you can then identify the mapped address for that object, either as an inline address or as another network object or network object group.

When a packet enters the ASASM, both the source and destination IP addresses are checked against the network object NAT rules. The source and destination address in the packet can be translated by separate rules if separate matches are made. These rules are not tied to each other; different combinations of rules can be used depending on the traffic.

Because the rules are never paired, you cannot specify that sourceA/destinationA should have a different translation than sourceA/destinationB. Use twice NAT for that kind of functionality (twice NAT lets you identify the source and destination address in a single rule).

To start configuring network object NAT, see Chapter 28 "Configuring Network Object NAT."

Information About Twice NAT

Twice NAT lets you identify both the source and destination address in a single rule. Specifying both the source and destination addresses lets you specify that sourceA/destinationA can have a different translation than sourceA/destinationB.

The destination address is optional. If you specify the destination address, you can either map it to itself (identity NAT), or you can map it to a different address. The destination mapping is always a static mapping.

Twice NAT also lets you use service objects for static NAT with port translation; network object NAT only accepts inline definition.

To start configuring twice NAT, see Chapter 29 "Configuring Twice NAT."

Figure 27-14 shows a host on the 10.1.2.0/24 network accessing two different servers. When the host accesses the server at 209.165.201.11, the real address is translated to 209.165.202.129. When the host accesses the server at 209.165.200.225, the real address is translated to 209.165.202.130. (See the "Single Address for FTP, HTTP, and SMTP (Static NAT-with-Port-Translation)" section for details on how to configure this example.)

Figure 27-14 Twice NAT with Different Destination Addresses

Figure 27-15 shows the use of source and destination ports. The host on the 10.1.2.0/24 network accesses a single host for both web services and Telnet services. When the host accesses the server for web services, the real address is translated to 209.165.202.129. When the host accesses the same server for Telnet services, the real address is translated to 209.165.202.130.

Figure 27-15 Twice NAT with Different Destination Ports

Figure 27-16 shows a remote host connecting to a mapped host. The mapped host has a twice static NAT translation that translates the real address only for traffic to and from the 209.165.201.0/27 network. A translation does not exist for the 209.165.200.224/27 network, so the translated host cannot connect to that network, nor can a host on that network connect to the translated host.

Figure 27-16 Twice Static NAT with Destination Address Translation

NAT Rule Order

Network object NAT rules and twice NAT rules are stored in a single table that is divided into three sections. Section 1 rules are applied first, then section 2, and finally section 3. Table 27-1 shows the order of rules within each section.

Table 27-1 NAT Rule Table

Table Section
Rule Type
Order of Rules within the Section

Section 1

Twice NAT

Applied on a first match basis, in the order they appear in the configuration. By default, twice NAT rules are added to section 1.

Note

Section 2

Network object NAT

Section 2 rules are applied in the following order, as automatically determined by the ASASM:

1. Static rules.

2. Dynamic rules.

Within each rule type, the following ordering guidelines are used:

a. Quantity of real IP addresses—From smallest to largest. For example, an object with one address will be assessed before an object with 10 addresses.

b. For quantities that are the same, then the IP address number is used, from lowest to highest. For example, 10.1.1.0 is assessed before 11.1.1.0.

c. If the same IP address is used, then the name of the network object is used, in alphabetical order. For example, abracadabra is assessed before catwoman.

Section 3

Twice NAT

Section 3 rules are applied on a first match basis, in the order they appear in the configuration. You can specify whether to add a twice NAT rule to section 3 when you add the rule.


For section 2 rules, for example, you have the following IP addresses defined within network objects:

192.168.1.0/24 (static)

192.168.1.0/24 (dynamic)

10.1.1.0/24 (static)

192.168.1.1/32 (static)

172.16.1.0/24 (dynamic) (object def)

172.16.1.0/24 (dynamic) (object abc)

The resultant ordering would be:

192.168.1.1/32 (static)

10.1.1.0/24 (static)

192.168.1.0/24 (static)

172.16.1.0/24 (dynamic) (object abc)

172.16.1.0/24 (dynamic) (object def)

192.168.1.0/24 (dynamic)

NAT Interfaces

You can configure a NAT rule to apply to any interface (in other words, all interfaces), or you can identify specific real and mapped interfaces. You can also specify any interface for the real address, and a specific interface for the mapped address, or vice versa.

For example, you might want to specify any interface for the real address and specify the outside interface for the mapped address if you use the same private addresses on multiple interfaces, and you want to translate them all to the same global pool when accessing the outside (Figure 27-17).

Figure 27-17 Specifying Any Interface


Note For transparent mode, you must choose specific source and destination interfaces.


Routing NAT Packets

The ASASM needs to be the destination for any packets sent to the mapped address. The ASA also needs to determine the egress interface for translated packets. This section describes how the ASASM handles accepting and delivering packets with NAT, and includes the following topics:

Mapped Addresses and Routing

Transparent Mode Routing Requirements for Remote Networks

Determining the Egress Interface

Mapped Addresses and Routing

When you translate the real address to a mapped address, the mapped address you choose determines how to configure routing, if necessary, for the mapped address.

See additional guidelines about mapped IP addresses in Chapter 28 "Configuring Network Object NAT," and Chapter 29 "Configuring Twice NAT."

See the following mapped address types:

Addresses on the same network as the mapped interface.

If you use addresses on the same network as the mapped interface, the ASASM uses proxy ARP to answer any ARP requests for the mapped addresses, thus intercepting traffic destined for a mapped address. This solution simplifies routing because the ASASM does not have to be the gateway for any additional networks. This solution is ideal if the outside network contains an adequate number of free addresses, a consideration if you are using a 1:1 translation like dynamic NAT or static NAT. Dynamic PAT greatly extends the number of translations you can use with a small number of addresses, so even if the available addresses on the outside network is small, this method can be used. For PAT, you can even use the IP address of the mapped interface.


Note If you configure the mapped interface to be any interface, and you specify a mapped address on the same network as one of the mapped interfaces, then if an ARP request for that mapped address comes in on a different interface, then you need to manually configure an ARP entry for that network on the ingress interface, specifying its MAC address (see the arp command). Typically, if you specify any interface for the mapped interface, then you use a unique network for the mapped addresses, so this situation would not occur.


Addresses on a unique network.

If you need more addresses than are available on the mapped interface network, you can identify addresses on a different subnet. The upstream router needs a static route for the mapped addresses that points to the ASASM. Alternatively for routed mode, you can configure a static route on the ASASM for the mapped addresses, and then redistribute the route using your routing protocol. For transparent mode, if the real host is directly-connected, configure the static route on the upstream router to point to the ASASM: in 8.3, specify the global management IP address; in 8.4(1) and later, specify the bridge group IP address. For remote hosts in transparent mode, in the static route on the upstream router, you can alternatively specify the downstream router IP address.

The same address as the real address (identity NAT).

The default behavior for identity NAT has proxy ARP enabled, matching other static NAT rules. You can disable proxy ARP if desired. Note: You can also disable proxy ARP for regular static NAT if desired, in which case you need to be sure to have proper routes on the upstream router.

Normally for identity NAT, proxy ARP is not required, and in some cases can cause connectivity issues. For example, if you configure a broad identity NAT rule for "any" IP address, then leaving proxy ARP enabled can cause problems for hosts on the network directly-connected to the mapped interface. In this case, when a host on the mapped network wants to communicate with another host on the same network, then the address in the ARP request matches the NAT rule (which matches "any" address). The ASASM will then proxy ARP for the address, even though the packet is not actually destined for the ASASM. (Note that this problem occurs even if you have a twice NAT rule; although the NAT rule must match both the source and destination addresses, the proxy ARP decision is made only on the "source" address). If the ASASM ARP response is received before the actual host ARP response, then traffic will be mistakenly sent to the ASASM (see Figure 27-18).

Figure 27-18 Proxy ARP Problems with Identity NAT

In rare cases, you need proxy ARP for identity NAT; for example for virtual Telnet. When using AAA for network access, a host needs to authenticate with the ASASM using a service like Telnet before any other traffic can pass. You can configure a virtual Telnet server on the ASASM to provide the necessary login. When accessing the virtual Telnet address from the outside, you must configure an identity NAT rule for the address specifically for the proxy ARP functionality. Due to internal processes for virtual Telnet, proxy ARP lets the ASASM keep traffic destined for the virtual Telnet address rather than send the traffic out the source interface according to the NAT rule. (See Figure 27-19).

Figure 27-19 Proxy ARP and Virtual Telnet

Transparent Mode Routing Requirements for Remote Networks

If the ASASM performs NAT for a host that is not on the directly-connected network, then you need to configure a static route on the ASASM for that network. You also need to have a static route for embedded IP addresses that are at least one hop away from the ASASM (such as in VoIP or DNS traffic) when you have inspection and NAT enabled.

Determining the Egress Interface

In transparent mode, the ASASM determines the egress interface for a NAT packet by using the NAT configuration; you must specify the source and destination interfaces as part of the NAT configuration.

In routed mode, the ASASM determines the egress interface for a NAT packet in the following way:

If you specify an optional interface, then the ASASM uses the NAT configuration to determine the egress interface. For identity NAT, the default behavior is to use the NAT configuration, but you have the option to always use a route lookup instead.

If you do not specify a specific interface, then the ASASM uses a route lookup to determine the egress interface.

DNS and NAT

You might need to configure the ASASM to modify DNS replies by replacing the address in the reply with an address that matches the NAT configuration. You can configure DNS modification when you configure each translation rule.

This feature rewrites the A record, or address record, in DNS replies that match a NAT rule. For DNS replies traversing from a mapped interface to any other interface, the A record is rewritten from the mapped value to the real value. Inversely, for DNS replies traversing from any interface to a mapped interface, the A record is rewritten from the real value to the mapped value.


Note If you configure a twice NAT rule, you cannot configure DNS modification if you specify the source address as well as the destination address. These kinds of rules can potentially have a different translation for a single address when going to A vs. B. Therefore, the ASASM cannot accurately match the IP address inside the DNS reply to the correct twice NAT rule; the DNS reply does not contain information about which source/destination address combination was in the packet that prompted the DNS request.


Figure 27-20 shows a DNS server that is accessible from the outside interface. A server, ftp.cisco.com, is on the inside interface. You configure the ASASM to statically translate the ftp.cisco.com real address (10.1.3.14) to a mapped address (209.165.201.10) that is visible on the outside network. In this case, you want to enable DNS reply modification on this static rule so that inside users who have access to ftp.cisco.com using the real address receive the real address from the DNS server, and not the mapped address. When an inside host sends a DNS request for the address of ftp.cisco.com, the DNS server replies with the mapped address (209.165.201.10). The ASASM refers to the static rule for the inside server and translates the address inside the DNS reply to 10.1.3.14. If you do not enable DNS reply modification, then the inside host attempts to send traffic to 209.165.201.10 instead of accessing ftp.cisco.com directly.

Figure 27-20 DNS Reply Modification, DNS Server on Outside

Figure 27-21 shows a user on the inside network requesting the IP address for ftp.cisco.com, which is on the DMZ network, from an outside DNS server. The DNS server replies with the mapped address (209.165.201.10) according to the static rule between outside and DMZ even though the user is not on the DMZ network. The ASASM translates the address inside the DNS reply to 10.1.3.14. If the user needs to access ftp.cisco.com using the real address, then no further configuration is required. If there is also a static rule between the inside and DMZ, then you also need to enable DNS reply modification on this rule. The DNS reply will then be modified two times.In this case, the ASASM again translates the address inside the DNS reply to 192.168.1.10 according to the static rule between inside and DMZ.

Figure 27-21 DNS Reply Modification, DNS Server, Host, and Server on Separate Networks

Figure 27-22 shows a web server and DNS server on the outside. The ASASM has a static translation for the outside server. In this case, when an inside user requests the address for ftp.cisco.com from the DNS server, the DNS server responds with the real address, 209.165.20.10. Because you want inside users to use the mapped address for ftp.cisco.com (10.1.2.56) you need to configure DNS reply modification for the static translation.

Figure 27-22 DNS Reply Modification, DNS Server on Host Network

Where to Go Next

To configure network object NAT, see Chapter 28 "Configuring Network Object NAT."

To configure twice NAT, see Chapter 29 "Configuring Twice NAT."