1.5.2 FTD & ASA NAT – Terminology, ACL Logic


Welcome to part 2 of this series of blog posts covering Network Address Translation on the ASA and Firepower threat Defense.  In this installment, we’re going to cover some terminology.

Notes on (confusing) terminology

Something that has always made NAT somewhat confusing to work with is overlapping terminology.  And it’s not just between vendors, or even vendor platforms, but also within the product documentation and the implementation of the same product.

Here’s the most relevant example.  The first screenshot is from the ASA 9.6 documentation. CLI book 2, NAT section.  The second screenshot is show output from an ASA that has the items referenced in the documentation snippet configured.

If you are brand new to the ASA, you’d never know from looking at these images that they are referencing the exact same thing.  When you’re trying to learn and discern what’s important, this terminology discrepancy presents an impediment.  This section is intended to help with that as best as I can.

Overall there are two categories where the terminology can be a bit confusing.  The first is items referred to with one term in the documentation, and a different one in the device .  We’ll call these synonymous terms:

The second is terms for what I would call use cases in the documentation, that don’t have an analog in the device configuration.  It’s just a term for specific use.  We’ll call these use case terms

When discussing configuring the technology where terms are synonymous, I’ll do my best to always use the term that’s referenced in the device configuration as it’s the most relevant.

When discussing use cases, I’ll point out where there’s a specific term defined for that use, but it’s not used anywhere in the device itself.

Synonymous terms

Auto NAT = Object NAT

They are the same thing,  the documentation calls it Object NAT, the command line interface calls it Auto NAT.  It’s called Object NAT in the documentation because the translation is always the property of an object.

!——-auto nat example—–!
object network foo
nat (i,o) static

Manual NAT = Twice NAT

They are exactly the same thing.  It’s called Twice NAT in the documentation to signify that both source and destination fields in the packet can be matched and altered.  NOTE: This can also be done with object NAT on a limited basis by using multiple statements, one in each direction.

!——-manual nat example—–!
object network foo
object network MAP-foo
object puppy
object MAP-puppy
nat (i,o) source static foo MAP-foo destination static puppy MAP-puppy

Use Case Terms

Identity NAT

Identity NAT is a use of Manual NAT.  The firewall is instructed  not to translate any of the fields in the flow.  This is used in situations where you would be performing NAT for most traffic flows, but you want to exempt speicifc destinations from translations.

The most common case for Identity NAT is VPN traffic..  As NAT is performed prior to encryption, most of the time you’ll want to make sure that the firewall doesn’t translate the source IP of the return packets, which prevent it from being sent over the tunnel as it would no longer match the crypto ACL.  Even if for the sake of argument the packet did get sent over the tunnel, because it’s source address has been altered, the receiving host would not recognize the packet as part of a flow and it would be dropped.

Here’s an illustrated example:

Policy NAT

Policy NAT is a use of manual NAT.  The term policy means setting the condition based on a specific destination.  Identity NAT does the same thing, but the difference is in the case of policy NAT a translation will be performed.

A common use for Policy NAT is Extranets and Software as a Service (SaaS) providers.  These types of connectivity typically call for the customer server to be seen as coming from a specific Mapped address.

For example, let’s say Server-web runs a web app that’s accessed from the Internet with Mapped address of  A  SaaS provider needs to get information from this server, but the server-web must appear to be at the IP address, which has been designated by the SaaS provider.

Here’s a graphical representation:


Destination NAT

A strict definition of Destination NAT is Manual Nat where destination address and/or port number is being translated.  On the surface this implies the use of manual NAT, as it’s the only form of NAT that allows us to map the destination address or port to a different value.

Considered this for a moment:

Object network Server-A-In
nat (i,o) static
Object network Server-A-Out
host host
nat (o,i)

These two statements produce the exact same end result.  The distinction being with the second object, we’re making the public facing address the real address and the internal address the mapped address.   So the term destination NAT is really a matter of perspective.

Access Control Rule Processing and NAT

Access control rules in ASA NAT are always applied to the real address.  This is really very straightforward, but it can be a bit counter-intuitive at first.  Let’s illustrate with a simple example:

Let’s say we have a web server in a DMZ, and we want to allow access to it from the internet.

Object network server-web
nat (d,o) static
access-list outside-in permit tcp any host eq www
access-group outside-in in interface outside

The intuitive thing would be to consider that an internet user would use the address to access the web server, therefore that would be the destination IP you would use in the access control list.  so this is very confusing.  A good way to think of this is that we’re really applying the ACL to the object.  Since the object can only have one real address, but potentially many mapped addresses, it makes sense for the ACL logic work this way.

In fact, using objects and object groups in Access-control lists instead of bare ip addresses is quite handy.  If you get in the mindset of working with objects instead of addresses (which are the properties of an object), the operational logic of the ASA then becomes more intuitive.

Let’s re-write our ACL to use the object instead:

Object network server-web
nat (d,o) static
access-list outside-in permit tcp any host object server-web eq www
access-group outside-in in interface outside

Makes more sense now right?


Wrap up

My goodness, where does the time go.  In our next installment we’ll we’ll talk about static and dynamic NAT and PAT, services, objects and object groups.

1.5.1 FTD & ASA NAT – Introduction


In this series of posts, I’m going to discuss NAT on Firepower and the ASA in the way that I comprehend it. For this post I don’t plan on to getting too far into the configuration and verification aspects, we’ll dive into that later.

It’s not as complex as it looks.  Really.

In my opinion this is one of those topics where the difficulty level can be measured by the quality of your foundation knowledge. Put plainly – if you can clearly visualize what the traffic flow should look like, then it’s easy.   The actual configuration syntax is easy to learn and work with.  And although the configuration interface may look different, there is in actuality zero difference between FTD and ASA NAT.  So you only have to learn it once for both platforms, nice right?!?

Armed with these insights, let’s do a level set on traffic flows.  This baseline will give NAT syntax and semantics their context, so please at least skim it.  You can always come back to it as needed.

The 5 tuple flow (and stateful firewalls).

A unique IP traffic flow is defined by 5 fields.

  • Source IP address
  • Destination IP address
  • Transport protocol (Ex: TCP/UDP)
  • Source port
  • Destination port

For the purpose of working with NAT, I find it helpful to visualize this in a left to right fashion like this:

[| | TCP | 50922 | 80]

In this example a device at is communicating with a web server at

A conversation between two hosts can be seen as two unidirectional Flows were the IP addresses and Ports are a mirror image in the reverse direction.

Taking our above example, that is what the two flows in the conversation look like:

[ | | TCP | 50922 | 80]
[ | | TCP | 80 | 50922]

And that’s pretty much what it looks like in a packet capture:

firewall# capture example int inside
firewall# sh capture example

5 packets captured

1: 00:39:03.453925 > […] 1520352048:1520352048(0) win 4128 <mss 536>
2: 00:39:03.459921 > […]


A stateful firewall recognizes these mirror image flows and identifies them as related.  This simplifies usage – we only have to define our traffic rules in one direction, and the firewall can imply how the return traffic should be processed.

This logic also applies to NAT.  If you define the flow in one direction, the NAT engine in the firewall processes the mirror image packets to look for a match.

Note how the firewall sees the two flows as a single connection:

firewall# sh conn
1 in use, 1 most used

TCP outside inside, idle 0:00:07, bytes 0, flags UB

Nat rule types

FTD and the ASA have two types of NAT rules:  Auto and Manual.

Auto Nat

Auto NAT is the simplest and easiest form of NAT to configure.  I think of it as the microwave popcorn button of NAT.    We define a network object, then attach a NAT statement to the object that tells that firewall what translation we want to perform based on the source and destination interface.

Here is a basic example:

Object network Server-A
nat (inside,outside) static

This is how the fields relate to the flow:

Original packet:
[ | | TCP | 80 | 50922]
Translated packet:
[ | | TCP | 80 | 50922]

The individual components:.

Host – The firewall will evaluate this against the first interface in the NAT statement.  With object NAT this is always going to be the source IP address

Nat (inside,outside) – The source ip address is coming from the inside interface of the router, and the destination ip address is on the outside interface of the router.  the destination interface is determined by the routing table.

Static – The source and destination address will be linked together in a fixed 1:1 relationship.  This is most commonly used for servers that require a fixed public (mapped) ip address.  Examples would be a web or email server.  – The mapped (public) ip address.  If source address, source interface, and destination interfaces all match, then the firewall will perform the translation.

Here’s what our NAT table looks like with the Auto Nat Rule Configured:

firewall# sh nat

Auto NAT Policies (Section 2)
1 (inside) to (outside) source static Server-A
translate_hits = 0, untranslate_hits = 0

Manual NAT

Manual NAT is useful for more advanced requirements, such as translating multiple fields in both directions, and conditional translation.

Happily, the way the NAT configuration syntax is structured makes it very easy to work with  once one can relate the fields in the flow to how that NAT statements are laid out.

In Manual Nat, the full five tuple flow can be matched and transformed. Be aware that unlike object Nat where the mapped address can be given directly, Manual Nat requires that all addresses/ranges/subnets in the statement be predefined as objects.

Manual NAT basic example:

Object network Server-A
Object network MAP-Server-A
nat (inside,outside) source static Server-A MAP-Server-A

As you can see, when you’re doing simple source translation Manual NAT requires more lines of configuration to accomplish the same result as auto nat.

Here’s what our NAT rule looks like for the above manual NAT example:

firewall# sh nat
Manual NAT Policies (Section 1)
1 (inside) to (outside) source static Server-A MAP-Server-A
translate_hits = 0, untranslate_hits = 0

Let’s look at common use for Manual NAT.  We’ll dive deeper in a subsequent post, This is just to give you a starting example.

Let’s say you’ve configured the Auto NAT translation shown earlier.  You are then asked to create a site to site VPN with a branch office.  In that case, when users from the branch office attempt to connect to Server-A, the auto NAT rule will kick in, translating the source address of the server on the return leg, and traffic will not return over the VPN tunnel.——————————————————————–>
*source IP in return traffic is translated, breaking the flow

So how do we fix this problem?  We use manual NAT to tell the firewall not to translate the address of Server-A when the destination is Branch-PC.  Like This:

Object network Srv-A
Object network Br-PC
nat (inside,outside)  source static Srv-A Srv-A destination static Br-PC Br-PC

Here is the generalized form is what this statement is doing:
(incoming, exit) static source real mapped destination static real mapped

The important thing to grasp is that for both source and destination, we’re setting a condition.  Match this . If all conditions match, then change to this.

We’re telling the firewall, if you have this source and destination pair, Don’t change anything.  This overrides the Auto NAT and allows it and our VPN connection to co-exist.  This a use called Identity NAT.


Nat rule table structure

The Nat rule Table has Three sections.

  1. Manual before auto

  2. Auto

  3. Manual After Auto

The user can set the order of the Manual NAT rules.  The Auto Nat rule order is set by the firewall automatically from most to least specific traffic match.  i.e. a host object would be ordered before a subnet object.

NAT table with Auto NAT rule, plus the identity nat override

firewall# sh nat
Manual NAT Policies (Section 1)
1 (inside) to (outside) source static Srv-A Srv-A destination static Br-PC Br-PC
translate_hits = 0, untranslate_hits = 0

Auto NAT Policies (Section 2)
1 (inside) to (outside) source static Server-A
translate_hits = 0, untranslate_hits = 0

Now a reason to order Manual NAT ahead of Auto NAT makes some sense right?

Wrap up

Well that’s quite a few words for an introduction, so let’s stop here.  In our next post we’ll go deeper into the terminology and usage examples .

3.11 FlexVPN – Flex Server w/Next Generation Encryption. Routing Design, Device enrollment


This is a kickoff post for a series demonstrating the capabilities of FlexVPN server.

Since we’re building up this sample network from a clean sheet of paper, we’re going all in.  We’re going to build ourselves a solid foundation, and then up the ante with high availability and integration with Identity Services Engine down the road.

The base build is going to use Next Generation Encryption (NGE), Elliptic curve certificates, and overlay routing design.  We’ll also demonstrate how we can support a site with an older design (firewall w/crypto maps) with the exact same head end.

In this installment, we’re going to review the routing design, cryptography suite selection, and enroll our devices with shiny 384 bit elliptic curve certificates.

Included at the end of this post are links to useful documents.



Great slide deck on FlexVPN

Densemode Labbing Topology 1

Cisco Next Generation Encryption Techology Document

Tim Glen breaks down Diffie Hellman Groups

RFC 6379: Suite B Cryptographic Suites for IPsec

Elliptic Curve Cryptography

3.10 FlexVPN Smart Defaults


In this installment we’re going to take a quick look a the main configuration blocks for FlexVPN on Cisco IOS devices.  then we will take advantage of smart defaults to turn up a tunnel with just a handful of commands.

Here’s the config from the video.  As you can see it’s ridiculously easy to use. There’s 5 lines of config that relate to ipsec.  That’s it.

PKI for Network Engineers (9/?): Elliptic Curve Setup

Greetings fellow networkers,

In this installment of PKI for network engineers, we’re going to build up our two tier Elliptic Curve PKI hierarchy in one shot.  There are a lot of tasks, but only a few of them differ from our RSA setup.  I’ll highlight those below.

  1. Cryptographic service provider and Hash algorithm.
    1. I’m using ECDSA 384 and SHA384
      1. Although the Microsoft CA supports 521 bit EC Keys, Cisco IOS maxes at 384
  2. No NDES/SCEP.  NDES supports RSA only for in-band device enrollment
    1. There is a new standard called EST (enrollment over secure transport)
      1. IOS and IOS-XE support EST as clients
      2. There’s an open source project called libEST you can use to test.
      3. Cisco ISE as of version 2.2 supports EST
  3. Web enrollment doesn’t support version 3 or 4 templates
    1. When duplicating templates, be aware of this fact
  4. Mind your signatures and public key algorithms.
    1. RSA public keys can be signed by a EC CA and vice versa.  Keep this in mind when creating your templates and take care to test them and inspect your certificates to make sure you’re getting what you think you’re getting




Settings, scriptlets, helpful text blocks:


PKI For Network Engineers (8/?) Windows Certificate Autoenrollment

Greetings Programs!

In this installment of the series, we set up the Active directory plumbing needed to do Certificate Autoenrollment for users and computers, and then we test it.  In the vein of the series, rather than taking defaults which are often not the best idea in a production network, we build things up in a more realistic manner.

In addition to learning how to set up auto enrollment, if you follow along, you’ll get some practice in doing some basic windows sysadmin including creating users, groups and organizational units, creating group policy objects, and most importantly working with certificate templates.

As usual, I included some links of interest at the bottom.

Happy labbing!

configuring Certificate Autoenrollment: https://technet.microsoft.com/en-us/library/cc731522(v=ws.11).aspx

Troubleshooting Certificate Autoenrollment:  https://social.technet.microsoft.com/wiki/contents/articles/3048.troubleshooting-certificate-autoenrollment-in-active-directory-certificate-services-ad-cs.aspx



PKI For Network Engineers (7/?): Network Device Enrollment Service (SCEP)

Greetings fellow Networkers!

In today’s installment, we’re going to configure network Device Enrollment Service, Microsoft’s implementation of SCEP.  Then we’ll enroll a router in-band.

A couple of things to note:

  1. By Default, NDES requires a one time password for enrollment.  This can be disabled via registry key.
  2. SCEP does not work with Elliptic Curve Certificate Authorities.  Enrollment over Secure Transport Supports in-band EC enrollment and is considered the successor to SCEP.
  3. I attached a pcap of all SCEP activity between the router and the CA.  if you look at the Cisco doc, it’ll explain what’s happening so you can interpret the activity in the PCAP.



PCAP of SCEP activity in video:  https://www.cloudshark.org/captures/62d72e489181

SCEP RFC: https://www.ietf.org/id/draft-gutmann-scep-06.txt

Cisco TechNote on SCEP (very well written):   https://www.cisco.com/c/en/us/support/docs/security-vpn/public-key-infrastructure-pki/116167-technote-scep-00.html

Microsoft Guidance on installing and configuring NDES: https://technet.microsoft.com/en-us/library/hh831498(v=ws.11).aspx