how does nat-t work with ipsec

8/13/2019 How Does NAT-T Work With IPSec

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How Does NAT-T work with IPSec?

Introduction:

This document describes details on how NAT-T works.

Background:

ESP encrypts all critical information, encapsulating the entire inner TCP/UDP datagram

within an ESP header. ESP is an IP protocol in the same sense that TCP and UDP are IPprotocols (OSI Network Layer 3), but it does not have any port information like TCP/UDP

(OSI Transport Layer 4). This is a difference from ISAKMP which uses UDP port 500 as its

transport layer.

PAT (Port Address Translation) is used to provide many hosts access to the internet through

the same publically routable ip address. PAT works by building a database that binds each

local host's ip address to the publically routable ip address using a specific port number. In

this manner, any packet sourced from an inside host will have its IP header modified by the

PAT devcie such that the source address and port number are changed from the RFC 1918address/port to the publically routable ip address and a new unique port. Referencing this

binding database, any return traffic can be untranslated in the same manner.

Q1: Why can't an ESP packet pass through a PAT device?

It is precisely because ESP is a protocol without ports that prevents it from passing through

PAT devices. Because there is no port to change in the ESP packet, the binding database

can't assign a unique port to the packet at the time it changes its RFC 1918 address to thepublically routable address. If the packet can't be assigned a unique port then the database

binding won't complete and there is no way to tell which inside host sourced this packet. As

a result there is no way for the return traffic to be untranslated successfully.

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Q2: How does NAT-T work with ISAKMP/IPsec?

NAT Traversal performs two tasks:

1. Detects if both ends support NAT-T2. Detects NAT devices along the transmission path (NAT-Discovery)

Step one occurs in ISAKMP Main Mode messages one and two. If both devices support

NAT-T, then NAT-Discovery is performed in ISKAMP Main Mode messages (packets) three

and four. THe NAT-D payload sent is a hash of the original IP address and port. Devices

exchange two NAT-D packets, one with source IP and port, and another with destination

IP and port. The receiving device recalculates the hash and compares it with the hash it

received; if they don't match a NAT device exists.

If a NAT device has been determined to exist, NAT-T will change the ISAKMP transport with

ISAKMP Main Mode messages five and six, at which point all ISAKMP packets change from

UDP port 500 to UDP port 4500. NAT-T encapsulates the Quick Mode (IPsec Phase 2)

exchange inside UDP 4500 as well. After Quick Mode completes data that gets encrypted

on the IPsec Security Association is encapsulated inside UDP port 4500 as well, thus

providing a port to be used in the PAT device for translation.

To visualize how this works and how the IP packet is encapsulated:

1. Clear text packet will be encrypted/encapsulated inside an ESP packet2. ESP packet will be encapsulated inside a UDP/4500 packet.

NAT-T encapsulates ESP packets inside UDP and assigns both the Source and Destination

ports as 4500. After this encapsulation there is enough information for the PAT database

binding to build successfully. Now ESP packets can be translated through a PAT device.

When a packet with source and destination port of 4500 is sent through a PAT device (from

inside to outside), the PAT device will change the source port from 4500 to a random high

port, while keeping the destination port of 4500. When a different NAT-T session passes

through the PAT device, it will change the source port from 4500 to a different random high

port, and so on. This way each local host has a unique database entry in the PAT devices

mapping its RFC1918 ip address/port4500 to the public ip address/high-port.






Q3: What is the difference between NAT-T and IPSec-over-UDP ?

Although both these protocols work similiar, there are two main differences.

• When NAT-T is enabled, it encapsulates the ESP packet with UDP only when itencounters a NAT device. Otherwise, no UDP encapsulation is done. But, IPSecOver UDP, always encapsulates the packet with UDP.

• NAT-T always use the standard port, UDP-4500. It is not configurable. IPSec overUDP normally uses UDP-10000 but this could be any other port based on the

configuration on the VPN server.

Example:

In above diagram, how does the device with PAT make unique identifiers in thePAT Table for both users if NAT-T sets the source and destination UDP ports4500 ?

If client A sends a packet, the packet will have the form:






src: 192.168.1.5:4500 dst: 205.151.255.10:4500 - > src: 205.151.254.10:600 dst:

205.151.255.10:4500

If client B sends a packet, the packet will have the form:

src: 192.168.1.6:4500 dst: 205.151.255.10:4500 - > src: 205.151.254.10:601 dst:

205.151.255.10:4500

the response from the server will have the form to each Client:

src: 10.0.1.5:80 dst: 205.151.254.10:600 - > src: 205.151.255.10:4500

dst: 205.151.254.10:600src: 10.0.1.5:80 dst: 205.151.254.10:601 - > src:205.151.255.10:4500 dst:

205.151.254.10:601

References-----

Here is the RFC for the IPSec aware NAT (NAT-Traversal) for your reference:

http://datatracker.ietf.org/doc/rfc3947/

(It includes the full explaination of the negotiation for your reference)

Document was create from the following discussion thread----

https://supportforums.cisco.com/thread/2049410?tstart=0

how does nat-t work with ipsec

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