In ASA software version 8.3(1), Cisco completely restructured ASA NAT syntax.  Quite a bit has already been written about these changes. However, since this is often a cause of confusion, I will try to provide an explanation of three of the most commonly used forms of NAT on an ASA: dynamic PAT, static NAT, and “NONAT”.

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What is NAT?

NAT stands for Network Address Translation.  It enables private IP networks that use unregistered IP addresses to connect to the Internet. NAT operates on a router, usually connecting two networks together, and translates the private (not globally unique) addresses in the internal network into legal addresses, before packets are forwarded to another network.

As part of this capability, NAT can be configured to advertise only one address for the entire network to the outside world. This provides additional security by effectively hiding the entire internal network behind that address. NAT offers the dual functions of security and address conservation and is typically implemented in remote-access environments.

How does NAT work?

 Basically, NAT allows a single device, such as a router, to act as an agent between the Internet (or public network) and a local network (or private network), which means that only a single unique IP address is required to represent an entire group of computers to anything outside their network.

Real IP: the actual IP address of the device generating the traffic

Mapped IP: the IP address the Router/Firewall translates the real IP address to

NAT is most often used to translate private RFC 1918 IP addresses to publicly routable IP addresses.

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NAT types –
There are 3 types of NAT:

1. Static NAT –
   In this, a single private IP address is mapped with single Public IP address, i.e., a private IP address is translated to a public IP address. It is used in Web hosting.
2. Dynamic NAT –
   In this type of NAT, multiple private IP address are mapped to a pool of public IP address. It is used when we know the number of fixed users wants to access the Internet at a given point of time.
3. Port Address Translation (PAT) –
   This is also known as NAT overload. In this, many local (private) IP addresses can be translated to single public IP address. Port numbers are used to distinguish the traffic, i.e., which traffic belongs to which IP address. This is most frequently used as it is cost effective as thousands of users can be connected to the Internet by using only one real global (public) IP address.

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NAT Uses in ASA :

1. Static NAT – one to one

Static NAT translates a single real IP to a single mapped IP.  This is commonly used to NAT a device on the inside or DMZ of an ASA to a static IP on the subnet of the outside interface.

Pre-8.3:

Configuration:

asa01 (config)# static (inside,outside) 72.163.4.162 192.168.10.2 netmask 255.255.255.255

Post-8.3:

Configuration:

object network inside-host host 192.168.10.2 nat (inside,outside) static 72.163.4.162

Both of the above say the following: Traffic that is received on the inside interface from 192.168.10.2, translate to 72.163.4.162 on the outside interface, and vice versa.

In 8.2(5) and earlier, the syntax is static (inside,outside) [outside ip] [inside ip] netmask [netmask].  I find this syntax to be counter intuitive, because it reads (inside,outside) and then outside inside.  I am not aware of any logical reason for this.

I find 8.3+ syntax to be easier to read, however it does dramatically increase the number of lines per NAT statement.  As with the PAT example above, if you look at the show run output you will see the object listed twice – once to define the host and once to define the static NAT.  Therefore, what took a single line in 8.2(5) now takes four lines in 8.3(1) and above.

Both 8.3+ configuration examples so far have used network object NAT.  Object NAT is easy to use, but somewhat limited and only takes into account the source address.  If we need more control over our NAT statements, we can use twice NAT, which I will cover in greater detail in a future post.

2. Dynamic PAT – many-to-one

PAT stands for port address translation.  It is many to one NAT translation.  This is what some vendors simply refer to as NAT.  It is more accurately called PAT because in order to translate many IPs to one IP, randomly selected ephermal ports must be used on the mapped IP address.  When return traffic is received, the ASA must check the xlate table (NAT translation table) in order to determine where to send the return traffic.

Here is the xlate table from the example ping above:

asa01# sh xlate

1 in use, 1 most used

Flags: D – DNS, i – dynamic, r – portmap, s – static, I – identity, T – twice

ICMP PAT from inside:192.168.10.2/1 to outside:72.163.4.161/43857 flags ri idle 0:00:09 timeout 0:00:30

asa01#

Pre-8.3:

Configuration:

asa01(config)# nat (inside) 1 192.168.10.0 255.255.255.0

asa01(config)# global (outside) 1 interface

INFO: outside interface address added to PAT pool

asa01(config)#

This configuration says: Translate any traffic received on the inside interface from 192.168.10.0/24 (NAT ID 1) to the corresponding global PAT pool.  In this case, nat (inside) 1 corresponds to global (outside) 1, so anything from 192.168.10.0/24 is translated to the outside interface IP.  Return traffic will be translated back to the real IP address by checking the xlate table for the appropriate translation.

You could also use an IP address or range of IP addresses in place of the word “interface” in the global statement.  If you were to use a range of IP addresses in the global statement, you would have a many-to-many dynamic NAT instead of many-to-one dynamic PAT.

Post-8.3:

Configuration:

asa01(config)# object network inside-network

asa01(config-network-object)# subnet 192.168.10.0 255.255.255.0

asa01(config-network-object)# nat (inside,outside) dynamic interface

asa01(config-network-object)#

This configuration says: Define a network object for subnet 192.168.10.0/24.  Any traffic received on the inside interface that matches this network object, translate to the outside interface IP address.  Return traffic will be translated back to the real IP address by checking the xlate table for the appropriate translation.

Show run output:

asa01# sh run object

object network inside-network subnet 192.168.10.0 255.255.255.0

asa01# sh run nat

!

object network inside-network

nat (inside,outside) dynamic interface

Notice when we configure object NAT in 8.3+, we configure both the subnet and the NAT under the same network object.  However, they appear separately in the show run output.  As with pre-8.3 config, we can also specify a specific IP or range of IPs for the translation.

3. No-nat (NAT exemption & identity NAT)

There are certain circumstances when traffic is being translated on an ASA, but we do not want this traffic to be translated when destined to specific subnets.  The most common example is VPN traffic.  We want the source address of the VPN traffic to have the real IP, not the mapped IP, for obvious reasons.

Pre-8.3:

Configuration:

access-list nonat extended permit ip 192.168.10.0 255.255.255.0 192.168.20.0 255.255.255.0

nat (inside) 0 access-list nonat

In ASA 8.2(5) and earlier, this is called NAT exemption.  This says: define an extended access list (in this case nonat) and specify the appropriate source and destination traffic.  Any traffic received on the inside interface that matches this access list, use NAT ID 0.  NAT ID 0 exempts the traffic from being translated.

Post-8.3:

Configuration:

asa01(config)# object network inside-network

asa01(config-network-object)# subnet 192.168.10.0 255.255.255.0

asa01(config)# object network remote-network

asa01(config-network-object)# subnet 192.168.20.0 255.255.255.0

asa01(config)# nat (inside,outside) source static inside-network inside-network destination static remote-network remote-network

In ASA 8.3(1) and above, NAT exemption no longer exists.  This can now only be accomplished by using identity NAT.  Identity NAT is a form of twice NAT, which allows us to specify both source and destination in our NAT statements.  In the above configuration example, we define two network objects: inside-network and remote-network.  We then configure an identity NAT statement that tells the ASA not to NAT the traffic.

The structure of the identity NAT statement is as follows:
nat (real_interface,mapped_interface) source static real_object mapped_object destination static mapped_object real_object

So the example above says: Any traffic received on the inside interface from inside-network destined to remote-network translate to the source to inside-network and the destination to remote-network (ie. don’t NAT it).

This is clearly less intuitive than the 8.2(5) syntax, but it’s not that bad once you get used to it.

Useful Links:

http://www.cisco.com/c/en/us/td/docs/security/asa/asa91/configuration/firewall/asa_91_firewall_config/nat_overview.html

http://www.cisco.com/c/en/us/td/docs/security/asa/asa82/configuration/guide/config/nat_overview.html