L2TP IPsec Support for NAT and PAT Windows Clients

L2TP IPsec Support for NAT and PAT Windows Clients Feature not Enabled

The figure below shows two Windows 2000 clients that are trying to connect to an end host through a router running NAT or PAT and IPsec-enabled Cisco IOS LNS router.

Figure 1

Multiple Windows 2000 Clients, NAT Router, and Cisco IOS LNS Router

The Windows 2000 Client #1 establishes an IPsec-protected L2TP tunnel to the Cisco IOS LNS router. The Windows 2000 Client #1 and the Cisco IOS LNS router recognize that there is a NAT router located between them and the NAT router is enabled with IPsec and NAT-Traversal (NAT-T). The Windows 2000 Client #1 attempts to establish an IPsec security association (SA) and requests a transport mode (which it does by default) with proxies from 10.0.0.2, its local address, to 209.265.200.231, the Cisco IOS LNS router's address.

In transport mode, NAT, running on the router, translates all outgoing connections (including 10.0.0.2) to its outside IP address (209.265.200.232), at which the address the traffic arrives. However, NAT cannot modify the L2TP port designation (1701), which is protected by the IPsec encrypted area. So, the local address now is 209.265.200.231, the remote address the 209.265.200.232 and the remote port is 1701. The traffic that matches the tunnel 209.265.200.231, port 1701 is sent to the Windows 2000 Client #1.

Windows 2000 Client #2 establishes an IPsec-protected L2TP tunnel to the Cisco IOS LNS router and NAT translates outgoing connections to its outside IP address (209.265.200.232) again, NAT cannot modify the L2TP port designation (1701) similar to Windows Client #1. The traffic that matches tunnel 209.265.200.231, port 1701 is now sent to Windows 2000 Client #2. which ends Windows Client #1's connection with the Cisco IOS LNS router since it is no longer receiving traffic.

L2TP IPsec Support for NAT and PAT Windows Clients Feature Enabled

When the L2TP IPsec Support for NAT and PAT Windows Clients feature is enabled, IPsec can translate the L2TP ports after decryption. This feature allows IPsec to map traffic from different hosts to different source ports. L2TP can now distinguish between traffic destined for multiple Windows 2000 clients.

When an security association (SA) is created, a translated port is assigned to the SA. This port is client-specific. The same port is used for any new SA created by that client. When an encrypted request is received and decrypted, the source port is translated from the standard value 1701 to a client specific value. The request with the translated port is then forwarded to L2TP.

As shown in the above figure, with port address translation enabled, the Windows 2000 Client #1 is assigned to the translated port number 1024, and Windows 2000 Client #2 is assigned to the translated port number 1025.

When L2TP sends the reply packet, it uses the translated port number and creates a packet to that destination port. IPsec uses the destination port number to select the SA with which to encrypt the packet. Before encrypting the packet, IPsec translates the destination port back to the standard port number 1701, which the Windows 2000 client expects. IPsec encrypts the packet either with the SA to Windows 2000 Client #1 if the destination port is 1024 or with the SA to Windows 2000 Client #2 if the destination port is 1025. The traffic is now sent to the appropriate client, and multiple Windows clients can be connected to a Cisco IOS LNS router through a NAT server at the same time.

The connection is maintained until one of the following actions occurs:

• The IPsec connection is closed.
• The NAT or PAT router ends the session.
• The Cisco IOS LNS router closes the session.
• A Windows client closes the session.

The following example shows how to enable the L2TP IPsec Support for NAT and PAT Windows Clients feature for a dynamic crypto map:

!

version 12.3
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname 72_LNS
!
boot-start-marker
boot-end-marker
!
aaa new-model
!
!
aaa authentication ppp default local
aaa session-id common
ip subnet-zero
!
!
no ip cef
no ip domain lookup
ip domain name cisco.com
ip dhcp excluded-address 198.51.100.1
ip dhcp excluded-address 198.51.100.10
!
!
ip vrf VPN
 rd 1:1
!
! Enable virtual private networking. 
vpdn enable
vpdn ip udp ignore checksum
!
! Default L2TP VPDN group
vpdn-group L2TP
!
! Enables the LNS to accept dial in requests; specifies L2TP as the tunneling
! protocol; specifies the number of the virtual templates used to clone
! virtual-access interfaces.
 accept-dialin
  protocol l2tp
  virtual-template 1
 
! Disables L2TP tunnel authentication.
no l2tp tunnel authentication
!
!
crypto keyring L2TP
  pre-shared-key address 0.0.0.0 0.0.0.0 key *****
!
!Defines an Internet Key Exchange (IKE) policy and assigns priority 1.
crypto isakmp policy 1
 encr aes
 authentication pre-share
 group 14
 lifetime 3600
!
crypto isakmp key cisco hostname w2k01
crypto isakmp keepalive 3600
!
crypto ipsec security-association lifetime seconds 600
!
!Defines a transform set.
crypto ipsec transform-set TS1 esp-aes esp-sha-hmac
 mode transport
!
! Names the dynamic crypto map entry and enters crypto map configuration mode; Enables
! L2TP--IPSec support; Specifies which transform sets can be used with the crypto map
! entry.
crypto dynamic-map DYN_MAP 10
 set nat demux
 set transform-set TS1!
!
crypto map CRYP_MAP 6000 ipsec-isakmp dynamic DYN_MAP
!
interface Loopback0
 ip address 198.51.100.1 255.255.255.0
!
interface FastEthernet0/0
 ip address 198.51.100.110 255.255.255.0
 no ip route-cache
 duplex full
 speed 100
 crypto map CRYP_MAP
!
interface FastEthernet0/1
 ip address 198.51.100.1 255.255.255.0
 duplex full
 speed 100
!
interface FastEthernet2/0
 ip address 172.19.192.138 255.255.255.0
 duplex full
!
interface Virtual-Template1
 ip unnumbered Loopback0
 peer default ip address pool POOL
 ppp mtu adaptive
 ppp authentication chap ms-chap
!
router ospf 1
 log-adjacency-changes
 redistribute static subnets
 network 198.51.100.10 0.0.0.255 area 0
!
ip local pool POOL 198.51.100.100 198.51.100.110
ip classless
ip route 171.0.0.0 255.0.0.0 172.19.192.1
!
no ip http server
no ip http secure-server
!
!
control-plane
!
gatekeeper
 shutdown!
!
line con 0
 exec-timeout 0 0
 logging synchronous
 stopbits 1
line aux 0
 stopbits 1
line vty 0 4
!
end