Cisco ASA 5500 Series Configuration Guide using the CLI, 8.3
Configuring Twice NAT
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Configuring Twice NAT

Table Of Contents

Configuring Twice NAT

Information About Twice NAT

Licensing Requirements for Twice NAT

Prerequisites for Twice NAT

Guidelines and Limitations

Configuring Twice NAT

Configuring Dynamic NAT

Configuring Dynamic PAT (Hide)

Configuring Static NAT or Static NAT with Port Translation

Configuring Identity NAT

Monitoring Twice NAT

Configuration Examples for Twice NAT

Different Translation Depending on the Destination (Dynamic PAT)

Different Translation Depending on the Destination Address and Port (Dynamic PAT)

Feature History for Twice NAT


Configuring Twice NAT


Twice NAT lets you identify both the source and destination address in a single rule. This chapter shows you how to configure twice NAT and includes the following sections:

Information About Twice NAT

Licensing Requirements for Twice NAT

Prerequisites for Twice NAT

Guidelines and Limitations

Configuring Twice NAT

Monitoring Twice NAT

Configuration Examples for Twice NAT

Feature History for Twice NAT


Note For detailed information about how NAT works, see Chapter 27 "Information About 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 a source address should be translated to A when going to destination X, but be translated to B when going to destination Y, for example.


Note 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. For example, if you configure static NAT with port address translation, and specify the source address as a Telnet server, and you want all traffic going to that Telnet server to have the port translated from 2323 to 23, then in the command, you must specify the source ports to be translated (real: 23, mapped: 2323). You specify the source ports because you specified the Telnet server address as the source address.


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.

For detailed information about the differences between twice NAT and network object NAT, see the "How NAT is Implemented" section.

Twice NAT rules are added to section 1 of the NAT rules table, or if specified, section 3. For more information about NAT ordering, see the "NAT Rule Order" section.

Licensing Requirements for Twice NAT

Model
License Requirement

All models

Base License.


Prerequisites for Twice NAT

For both the real and mapped addresses, configure network objects or network object groups (the object network or object-group network command). Network object groups are particularly useful for creating a mapped address pool with discontinuous IP address ranges or multiple hosts or subnets. To create a network object or group, see the "Configuring Objects and Groups" section.

For static NAT with port translation, configure TCP or UDP service objects (the object service command). To create a service object, see the "Configuring a Service Object" section.

For specific guidelines for objects and groups, see the configuration section for the NAT type you want to configure. See also the "Guidelines and Limitations" section.

Guidelines and Limitations

This section includes the guidelines and limitations for this feature.

Context Mode Guidelines

Supported in single and multiple context mode.

Firewall Mode Guidelines

Supported in routed and transparent firewall mode.

In transparent mode, you must specify the real and mapped interfaces; you cannot use any.

In transparent mode, you cannot configure interface PAT, because the transparent mode interfaces do not have IP addresses. You also cannot use the management IP address as a mapped address.

IPv6 Guidelines

Does not support IPv6.

Additional Guidelines

If you change the NAT configuration, and you do not want to wait for existing translations to time out before the new NAT information is used, you can clear the translation table using the clear xlate command. However, clearing the translation table disconnects all current connections that use translations.


Note If you remove a dynamic NAT or PAT rule, and then add a new rule with mapped addresses that overlap the addresses in the removed rule, then the new rule will not be used until all connections associated with the removed rule time out or are cleared using the clear xlate command. This safeguard ensures that the same address is not assigned to multiple hosts.


Objects and object groups used in NAT cannot be undefined; they must include IP addresses.

The mapped IP address pool cannot include:

The mapped interface IP address. If you specify any interface for the rule, then all interface IP addresses are disallowed. For interface PAT (routed mode only), use the interface keyword instead of the IP address.

(Transparent mode) The management IP address.

(Dynamic NAT) The standby interface IP address when VPN is enabled.

Existing VPN pool addresses.

Configuring Twice NAT

This section describes how to configure twice NAT to create rules for dynamic NAT, dynamic PAT, static NAT, static NAT with port translation, and identity NAT. This section includes the following topics:

Configuring Dynamic NAT

Configuring Dynamic PAT (Hide)

Configuring Static NAT or Static NAT with Port Translation

Configuring Identity NAT

Configuring Dynamic NAT

This section describes how to configure a dynamic NAT rule using twice NAT. For more information about dynamic NAT, see the "Dynamic NAT" section.

Detailed Steps

 
Command
Purpose

Step 1 

Network object:

object network obj_name

{host ip_address | subnet 
subnet_address netmask | range 
ip_address_1 ip_address_2}

Network object group:

object-group network grp_name

{network-object {object net_obj_name | 
subnet_address netmask | 
host ip_address} | 
group-object grp_obj_name}
 
        
Example:

hostname(config)# object network MyInsNet

hostname(config-network-object)# subnet 10.1.1.0 255.255.255.0

Configure the real source addresses.

You can configure either a network object or a network object group. For more information, see the "Configuring Objects" section.

If you want to translate all traffic, you can specify the any keyword instead of creating an object or group; skip this step.

Step 2 

Network object:

object network obj_name

range ip_address_1 ip_address_2

Network object group:

object-group network grp_name

{network-object {object net_obj_name | 
host ip_address} | 
group-object grp_obj_name}
 
        
Example:

hostname(config)# object network NAT_POOL

hostname(config-network-object)# range 209.165.201.10 209.165.201.20

Configure the mapped source addresses.

You can configure either a network object or a network object group.

For dynamic NAT, you typically configure a larger group of addresses to be mapped to a smaller group.

Note The mapped object or group cannot contain a subnet.

You can share this mapped IP address across different dynamic NAT rules, if desired.

See the "Guidelines and Limitations" section for information about disallowed mapped IP addresses.

Step 3 

(Optional)

Network object:

object network obj_name

{host ip_address | subnet 
subnet_address netmask | range 
ip_address_1 ip_address_2}

Network object group:

object-group network grp_name

{network-object {object net_obj_name | 
subnet_address netmask | 
host ip_address} | 
group-object grp_obj_name}
 
        
Example:

hostname(config)# object network Server1

hostname(config-network-object)# host 209.165.201.8

Configure the real destination addresses.

You can configure either a network object or a network object group.

Although the main feature of twice NAT is the inclusion of the destination IP address, the destination address is optional. If you do specify the destination address, you can configure static translation for that address or just use identity NAT for it. You might want to configure twice NAT without a destination address to take advantage of some of the other qualities of twice NAT, including the use of network object groups for real addresses, or manually ordering of rules. For more information, see the "Main Differences Between Network Object NAT and Twice NAT" section.

Step 4 

(Optional)

Network object:

object network obj_name

{host ip_address | subnet 
subnet_address netmask | range 
ip_address_1 ip_address_2}

Network object group:

object-group network grp_name

{network-object {object net_obj_name | 
subnet_address netmask | 
host ip_address} | 
group-object grp_obj_name}
 
        
Example:

hostname(config)# object network Server1_mapped

hostname(config-network-object)# host 10.1.1.67

Configure the mapped destination addresses.

You can configure either a network object or a network object group.

The destination translation is always static. For identity NAT, simply use the same object or group for both the real and mapped addresses, and skip this step.

If you want to translate the destination address, then the static mapping is typically one-to-one, so the real addresses have the same quantity as the mapped addresses. You can, however, have different quantities if desired. For more information, see the "Static NAT" section.

For static interface NAT with port translation, you can specify the interface keyword instead of a network object/group for the mapped address; you can skip this step. For more information, see the "Static Interface NAT with Port Translation" section.

See the "Guidelines and Limitations" section for information about disallowed mapped IP addresses.

Step 5 

(Optional)

object service obj_name

service {tcp | udp} destination 
operator port
 
        
Example:

hostname(config)# object service REAL_SVC

hostname(config-service-object)# service tcp destination eq 80

hostname(config)# object service MAPPED_SVC

hostname(config-service-object)# service tcp destination eq 8080

Configure service objects for:

Destination real TCP or UDP port

Destination mapped TCP or UDP port

Dynamic NAT does not support port translation. However, because the destination translation is always static, you can perform port translation for the destination port. A service object can contain both a source and destination port, but only the destination port is used in this case. If you specify the source port, it will be ignored. NAT only supports TCP or UDP. When translating a port, be sure the protocols in the real and mapped service objects are identical (both TCP or both UDP). For identity NAT, you can use the same service object for both the real and mapped ports. The "not equal" (neq) operator is not supported.

Step 6 

nat [(real_ifc,mapped_ifc)] [line | {after-auto [line]}] source dynamic {real_obj | any} {mapped_obj [interface]} [destination static {mapped_obj | interface} real_obj] [service mapped_dest_svc_obj real_dest_svc_obj] [dns] [inactive] [description desc]

Example:

hostname(config)# nat (inside,outside) source dynamic MyInsNet NAT_POOL destination static Server1_mapped Server1 service MAPPED_SVC REAL_SVC

Configures dynamic NAT. See the following guidelines:

Interfaces—If you do not specify the real and mapped interfaces, all interfaces are used. You can also specify the keyword any for one or both of the interfaces.

Line—By default, the NAT rule is added to the end of section 1 of the NAT table (see the "NAT Rule Order" section). If you want to add the rule into section 3 instead (after the network object NAT rules), then use the after-auto keyword. You can insert a rule anywhere in the applicable section using the line argument.

Source addresses:

Real—Specify a network object, group, or the any keyword (see Step 1). Use the any keyword if you want to translate all traffic from the real interface to the mapped interface.

Mapped—Specify a different network object or group (see Step 2). You can share this mapped object across different dynamic NAT rules, if desired. If you specify a mapped object or group followed by the interface keyword (routed mode only), then the IP address of the mapped interface is only used if all other mapped addresses are already allocated. For this option, you must configure a specific interface for the mapped_ifc.

Destination addresses:

Mapped—Specify a network object or group, or for static interface NAT with port translation only, specify the interface keyword (see Step 4). If you specify interface, be sure to also configure the service keyword. For this option, you must configure a specific interface for the real_ifc. See the "Static Interface NAT with Port Translation" section for more information.

Real—Specify a network object or group (see Step 3). For identity NAT, simply use the same object or group for both the real and mapped addresses.

Destination port—Specify the service keyword along with the mapped and real service objects (see Step 5). For identity port translation, simply use the same service object for both the real and mapped ports.

(For a source-only rule) DNS—The dns keyword translates DNS replies. Be sure DNS inspection is enabled (it is enabled by default). You cannot configure the dns keyword if you configure a destination address. See the "DNS and NAT" section for more information.

Inactive—To make this rule inactive without having to remove the command, use the inactive keyword. To reactivate it, reenter the whole command without the inactive keyword.

Description—Provide a description up to 200 characters using the description keyword.

Configuring Dynamic PAT (Hide)

This section describes how to configure a dynamic PAT (hide) rule using twice NAT. For more information about dynamic PAT, see the "Dynamic PAT" section.

Detailed Steps

 
Command
Purpose

Step 1 

Network object:

object network obj_name

{host ip_address | subnet 
subnet_address netmask | range 
ip_address_1 ip_address_2}

Network object group:

object-group network grp_name

{network-object {object net_obj_name | 
subnet_address netmask | 
host ip_address} | 
group-object grp_obj_name}
 
        
Example:

hostname(config)# object network MyInsNet

hostname(config-network-object)# subnet 10.1.1.0 255.255.255.0

Configure the real source addresses.

You can configure either a network object or a network object group. For more information, see the "Configuring Objects" section.

If you want to translate all traffic, you can specify the any keyword instead of creating an object or group; skip this step.

Step 2 

object network obj_name

host ip_address

Example:

hostname(config)# object network PAT_IP

hostname(config-network-object)# host 209.165.201.10

Configure the mapped source address.

You can configure a network object.

For dynamic PAT, configure a group of addresses to be mapped to a single address. You can either translate the real addresses to a single mapped address of your choosing, or you can translate them to the mapped interface address. If you want to use the interface address, do not configure a network object for the mapped address; instead use the interface keyword.

Note You can share this mapped object across different dynamic PAT rules, if desired.

See the "Guidelines and Limitations" section for information about disallowed mapped IP addresses.

Step 3 

(Optional)

Network object:

object network obj_name

{host ip_address | subnet 
subnet_address netmask | range 
ip_address_1 ip_address_2}

Network object group:

object-group network grp_name

{network-object {object net_obj_name | 
subnet_address netmask | 
host ip_address} | 
group-object grp_obj_name}
 
        
Example:

hostname(config)# object network Server1

hostname(config-network-object)# host 209.165.201.8

Configure the real destination addresses.

You can configure either a network object or a network object group.

Although the main feature of twice NAT is the inclusion of the destination IP address, the destination address is optional. If you do specify the destination address, you can configure static translation for that address or just use identity NAT for it. You might want to configure twice NAT without a destination address to take advantage of some of the other qualities of twice NAT, including the use of network object groups for real addresses, or manually ordering of rules. For more information, see the "Main Differences Between Network Object NAT and Twice NAT" section.

Step 4 

(Optional)

Network object:

object network obj_name

{host ip_address | subnet 
subnet_address netmask | range 
ip_address_1 ip_address_2}

Network object group:

object-group network grp_name

{network-object {object net_obj_name | 
subnet_address netmask | 
host ip_address} | 
group-object grp_obj_name}
 
        
Example:

hostname(config)# object network Server1_mapped

hostname(config-network-object)# host 10.1.1.67

Configure the mapped destination addresses.

You can configure either a network object or a network object group.

The destination translation is always static. For identity NAT, simply use the same object or group for both the real and mapped addresses, and skip this step.

If you want to translate the destination address, then the static mapping is typically one-to-one, so the real addresses have the same quantity as the mapped addresses. You can, however, have different quantities if desired. For more information, see the "Static NAT" section.

For static interface NAT with port translation, you can specify the interface keyword instead of a network object/group for the mapped address; you can skip this step. For more information, see the "Static Interface NAT with Port Translation" section.

See the "Guidelines and Limitations" section for information about disallowed mapped IP addresses.

Step 5 

(Optional)

object service obj_name

service {tcp | udp} destination 
operator port
 
        
Example:

hostname(config)# object service REAL_SVC

hostname(config-service-object)# service tcp destination eq 80

hostname(config)# object service MAPPED_SVC

hostname(config-service-object)# service tcp destination eq 8080

Configure service objects for:

Destination real TCP or UDP port

Destination mapped TCP or UDP port

Dynamic PAT does not support additional port translation. However, because the destination translation is always static, you can perform port translation for the destination port. A service object can contain both a source and destination port, but only the destination port is used in this case. If you specify the source port, it will be ignored. NAT only supports TCP or UDP. When translating a port, be sure the protocols in the real and mapped service objects are identical (both TCP or both UDP). For identity NAT, you can use the same service object for both the real and mapped ports. The "not equal" (neq) operator is not supported.

Step 6 

nat [(real_ifc,mapped_ifc)] [line | {after-auto [line]}] source dynamic {real-obj | any} {mapped_obj | interface} [destination static {mapped_obj | interface} real_obj] [service mapped_dest_svc_obj real_dest_svc_obj] [dns] [inactive] [description desc]

Example:

hostname(config)# nat (inside,outside) source dynamic MyInsNet interface destination static Server1 Server1 description Interface PAT for inside addresses when going to server 1

Configures dynamic PAT (hide). See the following guidelines:

Interfaces—If you do not specify the real and mapped interfaces, all interfaces are used. You can also specify the keyword any for one or both of the interfaces.

Line—By default, the NAT rule is added to the end of section 1 of the NAT table (see the "NAT Rule Order" section). If you want to add the rule into section 3 instead (after the network object NAT rules), then use the after-auto keyword. You can insert a rule anywhere in the applicable section using the line argument.

Source addresses:

Real—Specify a network object, group, or the any keyword (see Step 1). Use the any keyword if you want to translate all traffic between the from the real interface to the mapped interface.

Mapped—Specify a network object that contains a host address (see Step 2). You can share this mapped object across different dynamic PAT rules, if desired. To use the mapped interface IP address, specify the interface keyword (routed mode only). For this option, you must configure a specific interface for the mapped_ifc.

Destination addresses:

Mapped—Specify a network object or group, or for static interface NAT with port translation only, specify the interface keyword (see Step 4). If you specify interface, be sure to also configure the service keyword. For this option, you must configure a specific interface for the real_ifc. See the "Static Interface NAT with Port Translation" section for more information.

Real—Specify a network object or group (see Step 3). For identity NAT, simply use the same object or group for both the real and mapped addresses.

Destination port—Specify the service keyword along with the real and mapped service objects (see Step 5). For identity port translation, simply use the same service object for both the real and mapped ports.

(For a source-only rule) DNS—The dns keyword translates DNS replies. Be sure DNS inspection is enabled (it is enabled by default). You cannot configure the dns keyword if you configure a destination address. See the "DNS and NAT" section for more information.

Inactive—To make this rule inactive without having to remove the command, use the inactive keyword. To reactivate it, reenter the whole command without the inactive keyword.

Description—Provide a description up to 200 characters using the description keyword.

Configuring Static NAT or Static NAT with Port Translation

This section describes how to configure a static NAT rule using twice NAT. For more information about static NAT, see the "Static NAT" section.

Detailed Steps

 
Command
Purpose

Step 1 

Network object:

object network obj_name

{host ip_address | subnet 
subnet_address netmask | range 
ip_address_1 ip_address_2}

Network object group:

object-group network grp_name

{network-object {object net_obj_name | 
subnet_address netmask | 
host ip_address} | 
group-object grp_obj_name}
 
        
Example:

hostname(config)# object network MyInsNet

hostname(config-network-object)# subnet 10.1.1.0 255.255.255.0

Configure the real source addresses.

You can configure either a network object or a network object group. For more information, see the "Configuring Objects" section.

Step 2 

Network object:

object network obj_name

{host ip_address | subnet 
subnet_address netmask | range 
ip_address_1 ip_address_2}

Network object group:

object-group network grp_name

{network-object {object net_obj_name | 
subnet_address netmask | 
host ip_address} | 
group-object grp_obj_name}
 
        
Example:

hostname(config)# object network MyInsNet_mapped

hostname(config-network-object)# subnet 192.168.1.0 255.255.255.0

Configure the mapped source addresses.

You can configure either a network object or a network object group.

For static NAT, the mapping is typically one-to-one, so the real addresses have the same quantity as the mapped addresses. You can, however, have different quantities if desired. For more information, see the "Static NAT" section.

For static interface NAT with port translation, you can specify the interface keyword instead of a network object/group for the mapped address; you can skip this step. For more information, see the "Static Interface NAT with Port Translation" section.

See the "Guidelines and Limitations" section for information about disallowed mapped IP addresses.

Step 3 

(Optional)

Network object:

object network obj_name

{host ip_address | subnet 
subnet_address netmask | range 
ip_address_1 ip_address_2}

Network object group:

object-group network grp_name

{network-object {object net_obj_name | 
subnet_address netmask | 
host ip_address} | 
group-object grp_obj_name}
 
        
Example:

hostname(config)# object network Server1

hostname(config-network-object)# host 209.165.201.8

Configure the real destination addresses.

You can configure either a network object or a network object group. For more information, see the "Configuring Objects" section.

Although the main feature of twice NAT is the inclusion of the destination IP address, the destination address is optional. If you do specify the destination address, you can configure static translation for that address or just use identity NAT for it. You might want to configure twice NAT without a destination address to take advantage of some of the other qualities of twice NAT, including the use of network object groups for real addresses, or manually ordering of rules. For more information, see the "Main Differences Between Network Object NAT and Twice NAT" section.

Step 4 

(Optional)

Network object:

object network obj_name

{host ip_address | subnet 
subnet_address netmask | range 
ip_address_1 ip_address_2}

Network object group:

object-group network grp_name

{network-object {object net_obj_name | 
subnet_address netmask | 
host ip_address} | 
group-object grp_obj_name}
 
        
Example:

hostname(config)# object network Server1_mapped

hostname(config-network-object)# host 10.1.1.67

Configure the mapped destination addresses.

You can configure either a network object or a network object group.

The destination translation is always static. For identity NAT, simply use the same object or group for both the real and mapped addresses, and skip this step.

If you want to translate the destination address, then the static mapping is typically one-to-one, so the real addresses have the same quantity as the mapped addresses. You can, however, have different quantities if desired. For more information, see the "Static NAT" section.S

For static interface NAT with port translation, you can specify the interface keyword instead of a network object/group for the mapped address; you can skip this step. For more information, see the "Static Interface NAT with Port Translation" section.

See the "Guidelines and Limitations" section for information about disallowed mapped IP addresses.

Step 5 

(Optional)

object service obj_name

service {tcp | udp} [source operator 
port] [destination operator port]
 
        
Example:

hostname(config)# object service REAL_SRC_SVC

hostname(config-service-object)# service tcp source eq 80

hostname(config)# object service MAPPED_SRC_SVC

hostname(config-service-object)# service tcp source eq 8080

Configure service objects for:

Source or destination real TCP or UDP port

Source or destination mapped TCP or UDP port

A service object can contain both a source and destination port. You should specify either the source or the destination port for both service objects. You should only specify both the source and destination ports if your application uses a fixed source port (such as some DNS servers); but fixed source ports are rare. NAT only supports TCP or UDP. When translating a port, be sure the protocols in the real and mapped service objects are identical (both TCP or both UDP). For identity NAT, you can use the same service object for both the real and mapped ports. The "not equal" (neq) operator is not supported.

For example, if you want to translate the port for the source host, then configure the source service.

Step 6 

nat [(real_ifc,mapped_ifc)] [line | {after-object [line]}]
source static real_obj [mapped_obj | interface]
[destination static {mapped_obj | interface} real_obj]
[service real_src_mapped_dest_svc_obj mapped_src_real_dest_svc_obj] [dns] [unidirectional] [inactive] [description desc]

Example:

hostname(config)# nat (inside,dmz) source static MyInsNet MyInsNet_mapped destination static Server1 Server1 service REAL_SRC_SVC MAPPED_SRC_SVC

Configures static NAT. See the following guidelines:

Interfaces—If you do not specify the real and mapped interfaces, all interfaces are used. You can also specify the keyword any for one or both of the interfaces.

Line—By default, the NAT rule is added to the end of section 1 of the NAT table. See the "NAT Rule Order" section for more information about sections. If you want to add the rule into section 3 instead (after the network object NAT rules), then use the after-auto keyword. You can insert a rule anywhere in the applicable section using the line argument.

Source addresses:

Real—Specify a network object or group (see Step 1).

Mapped—Specify a different network object or group (see Step 2). For static interface NAT with port translation only, you can specify the interface keyword (routed mode only). If you specify interface, be sure to also configure the service keyword (in this case, the service objects should include only the source port). For this option, you must configure a specific interface for the mapped_ifc. See the "Static Interface NAT with Port Translation" section for more information.

Destination addresses:

Mapped—Specify a network object or group, or for static interface NAT with port translation only, specify the interface keyword (see Step 4). If you specify interface, be sure to also configure the service keyword (in this case, the service objects should include only the destination port). For this option, you must configure a specific interface for the real_ifc.

Real—Specify a network object or group (see Step 3). For identity NAT, simply use the same object or group for both the real and mapped addresses.

   

(Continued)

Port—Specify the service keyword along with the real and mapped service objects (see Step 5). For source port translation, the objects must specify the source service. The order of the service objects in the command is service real mapped. For destination port translation, the objects must specify the destination service. The order of the service objects is service mapped real. In the rare case where you specify both the source and destination ports in the object, the first service object contains the real source port/mapped destination port; the second service object contains the mapped source port/real destination port. For identity port translation, simply use the same service object for both the real and mapped ports (source and/or destination ports, depending on your configuration).

(For a source-only rule) DNS—The dns keyword translates DNS replies. Be sure DNS inspection is enabled (it is enabled by default). You cannot configure the dns keyword if you configure a destination address. See the "DNS and NAT" section for more information.

Inactive—To make this rule inactive without having to remove the command, use the inactive keyword. To reactivate it, reenter the whole command without the inactive keyword.

Description—Provide a description up to 200 characters using the description keyword.

Examples

The following example shows the use of static interface NAT with port translation. Hosts on the outside access an FTP server on the inside by connecting to the outside interface IP address with destination port 65000 through 65004. The traffic is untranslated to the internal FTP server at 192.168.10.100:6500 through :65004. Note that you specify the source port range in the service object (and not the destination port) because you want to translate the source address and port as identified in the command; the destination port is "any." Because static NAT is bidirectional, "source" and "destination" refers primarily to the command keywords; the actual source and destination address and port in a packet depends on which host sent the packet. In this example, connections are originated from outside to inside, so the "source" address and port of the FTP server is actually the destination address and port in the originating packet.

hostname(config)# object service FTP_PASV_PORT_RANGE

hostname(config-service-obvject)# service tcp source range 65000 65004

hostname(config)# object network HOST_FTP_SERVER

hostname(config-network-obvject)# host 192.168.10.100

hostname(config)# nat (inside,outside) source static HOST_FTP_SERVER interface service FTP_PASV_PORT_RANGE FTP_PASV_PORT_RANGE

Configuring Identity NAT

This section describes how to configure an identity NAT rule using twice NAT. You configure identity NAT using a static NAT rule where you map an address to itself. For more information about identity NAT, see the "Identity NAT" section.

Detailed Steps

 
Command
Purpose

Step 1 

Network object:

object network obj_name

{host ip_address | subnet 
subnet_address netmask | range 
ip_address_1 ip_address_2}

Network object group:

object-group network grp_name

{network-object {object net_obj_name | 
subnet_address netmask | 
host ip_address} | 
group-object grp_obj_name}
 
        
Example:

hostname(config)# object network MyInsNet

hostname(config-network-object)# subnet 10.1.1.0 255.255.255.0

Configure the real source addresses.

You can configure either a network object or a network object group. For more information, see the "Configuring Objects" section.

These are the addresses on which you want to perform identity NAT. If you want to perform identity NAT for all addresses, you can instead use the keywords any any; skip this step.

Step 2 

(Optional)

Network object:

object network obj_name

{host ip_address | subnet 
subnet_address netmask | range 
ip_address_1 ip_address_2}

Network object group:

object-group network grp_name

{network-object {object net_obj_name | 
subnet_address netmask | 
host ip_address} | 
group-object grp_obj_name}
 
        
Example:

hostname(config)# object network Server1

hostname(config-network-object)# host 209.165.201.8

Configure the real destination addresses.

You can configure either a network object or a network object group.

Although the main feature of twice NAT is the inclusion of the destination IP address, the destination address is optional. If you do specify the destination address, you can configure static translation for that address or just use identity NAT for it. You might want to configure twice NAT without a destination address to take advantage of some of the other qualities of twice NAT, including the use of network object groups for real addresses, or manually ordering of rules. For more information, see the "Main Differences Between Network Object NAT and Twice NAT" section.

Step 3 

(Optional)

Network object:

object network obj_name

{host ip_address | subnet 
subnet_address netmask | range 
ip_address_1 ip_address_2}

Network object group:

object-group network grp_name

{network-object {object net_obj_name | 
subnet_address netmask | 
host ip_address} | 
group-object grp_obj_name}
 
        
Example:

hostname(config)# object network Server1_mapped

hostname(config-network-object)# host 10.1.1.67

Configure the mapped destination addresses.

You can configure either a network object or a network object group.

The destination translation is always static. For identity NAT, simply use the same object or group for both the real and mapped addresses, and skip this step.

If you want to translate the destination address, then the static mapping is typically one-to-one, so the real addresses have the same quantity as the mapped addresses. You can, however, have different quantities if desired. For more information, see the "Static NAT" section.

For static interface NAT with port translation, you can specify the interface keyword instead of a network object/group for the mapped address. For more information, see the "Static Interface NAT with Port Translation" section.

See the "Guidelines and Limitations" section for information about disallowed mapped IP addresses.

Step 4 

(Optional)

object service obj_name

service {tcp | udp} [source operator 
port] [destination operator port]
 
        
Example:

hostname(config)# object service REAL_SRC_SVC

hostname(config-service-object)# service tcp source eq 80

hostname(config)# object service MAPPED_SRC_SVC

hostname(config-service-object)# service tcp source eq 8080

Configure service objects for:

Source or destination real TCP or UDP port

Source or destination mapped TCP or UDP port

A service object can contain both a source and destination port. You should specify either the source or the destination port for both service objects. You should only specify both the source and destination ports if your application uses a fixed source port (such as some DNS servers); but fixed source ports are rare. NAT only supports TCP or UDP. When translating a port, be sure the protocols in the real and mapped service objects are identical (both TCP or both UDP). For identity NAT, you can use the same service object for both the real and mapped ports. The "not equal" (neq) operator is not supported.

For example, if you want to translate the port for the source host, then configure the source service.

Step 5 

nat [(real_ifc,mapped_ifc)] [line | {after-object [line]}]
source static {nw_obj nw_obj | any any}
[destination static {mapped_obj | interface} real_obj]
[service real_src_mapped_dest_svc_obj mapped_src_real_dest_svc_obj] [inactive] [description desc]

Example:

hostname(config)# nat (inside,outside) source static MyInsNet MyInsNet destination static Server1 Server1

Configures identity NAT. See the following guidelines:

Interfaces—If you do not specify the real and mapped interfaces, all interfaces are used. You can also specify the keyword any for one or both of the interfaces.

Line—By default, the NAT rule is added to the end of section 1 of the NAT table. See the "NAT Rule Order" section for more information about sections. If you want to add the rule into section 3 instead (after the network object NAT rules), then use the after-auto keyword. You can insert a rule anywhere in the applicable section using the line argument.

Source addresses—Specify a network object, group, or the any keyword for both the real and mapped addresses (see Step 1).

Destination addresses:

Mapped—Specify a network object or group, or for static interface NAT with port translation only, specify the interface keyword (routed mode only) (see Step 3). If you specify interface, be sure to also configure the service keyword (in this case, the service objects should include only the destination port). For this option, you must configure a specific interface for the real_ifc. See the "Static Interface NAT with Port Translation" section for more information.

Real—Specify a network object or group (see Step 2). For identity NAT, simply use the same object or group for both the real and mapped addresses.

Port—Specify the service keyword along with the real and mapped service objects (see Step 4). For source port translation, the objects must specify the source service. The order of the service objects in the command is service real mapped. For destination port translation, the objects must specify the destination service. The order of the service objects is service mapped real. In the rare case where you specify both the source and destination ports in the object, the first service object contains the real source port/mapped destination port; the second service object contains the mapped source port/real destination port. For identity port translation, simply use the same service object for both the real and mapped ports (source and/or destination ports, depending on your configuration).

Inactive—To make this rule inactive without having to remove the command, use the inactive keyword. To reactivate it, reenter the whole command without the inactive keyword.

Description—Provide a description up to 200 characters using the description keyword.

Monitoring Twice NAT

To monitor twice NAT, enter one of the following commands:

Command
Purpose

show nat

Shows NAT statistics, including hits for each NAT rule.

show nat pool

Shows NAT pool statistics, including the addresses and ports allocated, and how many times they were allocated.

show xlate

Shows current NAT session information.


Configuration Examples for Twice NAT

This section includes the following configuration examples:

Different Translation Depending on the Destination (Dynamic PAT)

Different Translation Depending on the Destination Address and Port (Dynamic PAT)

Different Translation Depending on the Destination (Dynamic PAT)

Figure 29-1 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:port. When the host accesses the server at 209.165.200.225, the real address is translated to 209.165.202.130:port.

Figure 29-1 Twice NAT with Different Destination Addresses


Step 1 Add a network object for the inside network:

hostname(config)# object network myInsideNetwork
hostname(config-network-object)# subnet 10.1.2.0 255.255.255.0
 
   

Step 2 Add a network object for the DMZ network 1:

hostname(config)# object network DMZnetwork1
hostname(config-network-object)# subnet 209.165.201.0 255.255.255.224
 
   

Step 3 Add a network object for the PAT address:

hostname(config)# object network PATaddress1
hostname(config-network-object)# host 209.165.202.129
 
   

Step 4 Configure the first twice NAT rule:

hostname(config)# nat (inside,dmz) source dynamic myInsideNetwork PATaddress1 destination static DMZnetwork1 DMZnetwork1

Because you do not want to translate the destination address, you need to configure identity NAT for it by specifying the same address for the real and mapped destination addresses.

By default, the NAT rule is added to the end of section 1 of the NAT table, See the "Configuring Dynamic PAT (Hide)" section for more information about specifying the section and line number for the NAT rule.

Step 5 Add a network object for the DMZ network 2:

hostname(config)# object network DMZnetwork2
hostname(config-network-object)# subnet 209.165.200.224 255.255.255.224
 
   

Step 6 Add a network object for the PAT address:

hostname(config)# object network PATaddress2
hostname(config-network-object)# host 209.165.202.130
 
   

Step 7 Configure the second twice NAT rule:

hostname(config)# nat (inside,dmz) source dynamic myInsideNetwork PATaddress2 destination static DMZnetwork2 DMZnetwork2


Different Translation Depending on the Destination Address and Port (Dynamic PAT)

Figure 29-2 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 Telnet services, the real address is translated to 209.165.202.129:port. When the host accesses the same server for web services, the real address is translated to 209.165.202.130:port.

Figure 29-2 Twice NAT with Different Destination Ports


Step 1 Add a network object for the inside network:

hostname(config)# object network myInsideNetwork
hostname(config-network-object)# subnet 10.1.2.0 255.255.255.0
 
   

Step 2 Add a network object for the Telnet/Web server:

hostname(config)# object network TelnetWebServer
hostname(config-network-object)# host 209.165.201.11
 
   

Step 3 Add a network object for the PAT address when using Telnet:

hostname(config)# object network PATaddress1
hostname(config-network-object)# host 209.165.202.129
 
   

Step 4 Add a service object for Telnet:

hostname(config)# object service TelnetObj
hostname(config-network-object)# service tcp destination eq telnet
 
   

Step 5 Configure the first twice NAT rule:

hostname(config)# nat (inside,outside) source dynamic myInsideNetwork PATaddress1 destination static TelnetWebServer TelnetWebServer service TelnetObj TelnetObj

Because you do not want to translate the destination address or port, you need to configure identity NAT for them by specifying the same address for the real and mapped destination addresses, and the same port for the real and mapped service.

By default, the NAT rule is added to the end of section 1 of the NAT table, See the "Configuring Dynamic PAT (Hide)" section for more information about specifying the section and line number for the NAT rule.

Step 6 Add a network object for the PAT address when using HTTP:

hostname(config)# object network PATaddress2
hostname(config-network-object)# host 209.165.202.130
 
   

Step 7 Add a service object for HTTP:

hostname(config)# object service HTTPObj
hostname(config-network-object)# service tcp destination eq http
 
   

Step 8 Configure the second twice NAT rule:

hostname(config)# nat (inside,outside) source dynamic myInsideNetwork PATaddress2 destination static TelnetWebServer TelnetWebServer service HTTPObj HTTPObj


Feature History for Twice NAT

Table 29-1 lists each feature change and the platform release in which it was implemented.

Table 29-1 Feature History for Twice NAT 

Feature Name
Platform Releases
Feature Information

Twice NAT

8.3(1)

Twice NAT lets you identify both the source and destination address in a single rule.

The following commands were modified or introduced: nat, show nat, show xlate, show nat pool.