Using NAT with PIX/ASA Devices v1.11 – Aaron Balchunas
* * *
All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
1
- Using NAT with PIX/ASA Devices -
NAT (Network Address Translation)
The rapid growth of the Internet resulted in a shortage of IPv4 addresses. In
response, the powers that be designated a specific subset of the IPv4 address
space to be private, to temporarily alleviate this problem.
A public address can be routed on the Internet. Thus, devices that should be
Internet accessible (such web or email servers) must be configured with
public addresses.
A private address is only intended for use within an organization, and can
never be routed on the internet. Three private addressing ranges were
allocated, one for each IPv4 class:
• Class A - 10.x.x.x
• Class B - 172.16-31.x.x
• Class C - 192.168.x.x
NAT (Network Address Translation) is used to translate between private
addresses and public addresses. NAT allows devices configured with a
private address to be stamped with a public address, thus allowing those
devices to communicate across the Internet.
NAT is not restricted to just public-to-private address translations, though
this is the most common application of NAT. NAT can perform a public-topublic
address translation, or a private-to-private address translation as well.
NAT provides an additional benefit – hiding the specific addresses and
addressing structure of the internal network.
Both Cisco IOS devices and PIX/ASA firewalls support NAT.
(Reference: http://www.cisco.com/en/US/tech/tk648/tk361/technologies_white_paper09186a0080194af8.shtml)
Using NAT with PIX/ASA Devices v1.11 – Aaron Balchunas
* * *
All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
2
Types of NAT
NAT can be implemented using one of three methods:
Static NAT – performs a static one-to-one translation between two
addresses, or between a port on one address to a port on another address.
Static NAT is most often used to assign a public address to a device behind a
NAT-enabled firewall/router.
Dynamic NAT – utilizes a pool of global addresses to dynamically translate
the outbound traffic of clients behind a NAT-enabled device.
NAT Overload or Port Address Translation (PAT) – translates the
outbound traffic of clients to unique port numbers off of a single global
address. PAT is necessary when the number of internal clients exceeds the
available global addresses.
NAT Terminology
Specific terms are used to identify the various NAT addresses:
• Inside Local – the specific IP address assigned to an inside host
behind a NAT-enabled device (usually a private address).
• Inside Global – the address that identifies an inside host to the
outside world (usually a public address). Essentially, this is the
dynamically or statically-assigned public address assigned to a private
host.
• Outside Global – the address assigned to an outside host (usually a
public address).
• Outside Local – the address that identifies an outside host to the
inside network. Often, this is the same address as the Outside Global.
However, it is occasionally necessary to translate an outside (usually
public) address to an inside (usually private) address.
For simplicity sake, it is generally acceptable to associate global addresses
with public addresses, and local addresses with private addresses.
However, remember that public-to-public and private-to-private translation
is still possible. Inside hosts are within the local network, while outside
hosts are external to the local network.
Using NAT with PIX/ASA Devices v1.11 – Aaron Balchunas
* * *
All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
3
NAT Terminology Example
Consider the above example. For a connection from HostA to HostB, the
NAT addresses are identified as follows:
• Inside Local Address - 10.1.1.10
• Inside Global Address - 55.1.1.1
• Outside Global Address – 99.1.1.2
• Outside Local Address – 99.1.1.2
HostA’s configured address is 10.1.1.10, and is identified as its Inside Local
address. When HostA communicates with the Internet, it is stamped with
RouterA’s public address, using PAT. Thus, HostA’s Inside Global address
will become 55.1.1.1.
When HostA communicates with HostB, it will access HostB’s Outside
Global address of 99.1.1.2. In this instance, the Outside Local address is also
99.1.1.2. HostA is never aware of HostB’s configured address.
It is possible to map an address from the local network (such as 10.1.1.5) to
the global address of the remote device (in this case, 99.1.1.2). This may be
required if a legacy device exists that will only communicate with the local
subnet. In this instance, the Outside Local address would be 10.1.1.5.
HostA
10.1.1.10
HostB
192.168.1.5
RouterA
NAT-Enabled
10.1.1.1 55.1.1.1
Internet
RouterA
NAT-Enabled
99.1.1.1 192.168.1.1
Static NAT Translation
99.1.1.2 = 192.168.1.5
SRC Address = 10.1.1.10
DST Address = 99.1.1.2
SRC Address = 55.1.1.1:31092
DST Address = 99.1.1.2
SRC Address = 55.1.1.1:31092
DST Address = 192.168.1.5
The above example demonstrates how the source (SRC) and destination
(DST) IP addresses within the Network-Layer header are translated by NAT.
(Reference: http://www.cisco.com/warp/public/556/8.html)
Using NAT with PIX/ASA Devices v1.11 – Aaron Balchunas
* * *
All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
4
Configuring Dynamic Inside NAT Translations
To configure the PIX/ASA to dynamically translate inside addresses to one
or more outside addresses, the addresses to be NATed (or translated) must be
identified. The nat command will accomplish this:
pixfirewall(config)# nat (inside) 1 172.16.0.0 255.255.0.0
The above command states that any addresses on the 172.16.0.0 255.255.0.0
subnet, located off of the inside interface, are eligible for translation. The 1
is the NAT ID, which will be referenced later.
To specify that all addresses off the inside interfaces should be translated:
pixfirewall(config)# nat (inside) 1 0.0.0.0 0.0.0.0
Once traffic to be translated has been identified, the address (or addresses)
this traffic will be translated to must be identified. The global command will
accomplish this:
pixfirewall(config)# global (outside) 1 66.1.1.9 netmask 255.255.255.255
The above global command will translate all addresses from the nat
statement to the 66.1.1.9 address on the outside interface. Since only one
global address was specified, Port Address Translation (PAT) or NAT
overload will be employed.
Please NOTE: The NAT ID must match between nat and global statements.
This connects them together, as multiple nat/global pairings can exist.
To specify a range of global addresses:
pixfirewall(config)# global (outside) 1 66.1.1.9-66.1.1.14 netmask 255.255.255.248
The global command can alternatively reference the IP address currently
active on an interface. This is useful when the public interface is obtaining
an address dynamically through DHCP:
pixfirewall(config)# global (outside) 1 interface
The above command will translate addresses specified in the nat statement
to the currently active IP address on the outside interface.
Using NAT with PIX/ASA Devices v1.11 – Aaron Balchunas
* * *
All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
5
Configuring Dynamic Inside NAT Translations (continued)
Recall that the NAT ID must match between nat and global statements. This
connects them together, as multiple nat/global pairings can exist:
pixfirewall(config)# nat (inside) 1 192.168.1.0 255.255.255.0
pixfirewall(config)# global (outside) 1 66.1.1.9-66.1.1.14 netmask 255.255.255.248
pixfirewall(config)# nat (inside) 2 192.168.2.0 255.255.255.0
pixfirewall(config)# global (outside) 2 66.1.1.17-66.1.1.22 netmask 255.255.255.248
Inside hosts on the 192.168.1.0/24 subnet will be translated to an outside
address on the 66.1.1.8/29 subnet, while inside hosts on the 192.168.2.0/24
subnet will be translated to an outside address on the 66.1.1.16/29 subnet.
Note that the NAT IDs match between nat/global pairs.
Preventing Addresses from Being Translated
It is possible (and sometimes required) to prevent NAT from translating
specific addresses. Situations when this might be necessary include:
• If a ‘trusted’ interface contains hosts with public addresses, that need
to be Internet-accessible.
• If traffic is traversing a VPN, from one private network to another.
To specify addresses that should not be NATed:
pixfirewall(config)# nat (inside) 0 77.1.1.1 255.255.255.255
The NAT ID of 0 has been reserved for disabling NAT for specific
addresses. If NAT must be disabled for a large number of addresses/devices,
an access-list can be used in conjunction with the nat statement.
pixfirewall(config)# access-list NONAT permit ip any 172.16.1.0 255.255.255.0
pixfirewall(config)# access-list NONAT permit ip any 172.16.2.0 255.255.255.0
pixfirewall(config)# nat (inside) 0 access-list NONAT
Using NAT with PIX/ASA Devices v1.11 – Aaron Balchunas
* * *
All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
6
Configuring Static NAT Translations
Static NAT translations perform a static one-to-one translation between two
addresses, or between a port on one address to a port on another address.
The most common application for static translations is to allow outside (less
trusted) hosts access an inside (more trusted) host.
Please Note: Simply creating a Static NAT translation is not sufficient to
provide outside hosts access to an inside host. An access-list must be used in
conjunction with the static NAT to actually permit the access.
Static translations take precedence over dynamic translations.
The static command is used to configure a static NAT translation. Consider
the following scenario:
• A web server exists on the inside interface of the PIX/ASA firewall,
and is configured with a private address of 10.5.1.5.
• This web server should be publicly accessible from the outside
interface using a public address of 66.1.1.9.
The proper syntax to create this static NAT translation would be as follows:
pixfirewall(config)# static (inside,outside) 66.1.1.9 10.5.1.5
Confusing, right?
The interfaces in the parenthesis indicate the prenat interface and postnat
interface. From the PIX/ASA’s perspective, an outside address is not being
translated to an inside address. Instead, an inside address is being translated
to an outside address. Thus, the static commands logic is:
(prenat,postnat)
However, following the prenat/postnat interface parameter, the outside
address (66.1.1.9) is specified first, and then the inside address (10.5.1.5).
Why, oh why, do you torture us so, Cisco?