Question

Cisco ASA VPN and NAT issue

Asked by: kevin_todd

Hi Experts,

I hope you can help.  We are in the process of switching our firewall vendor to cisco.  I am attempting to configure an ASA for our management environment.  We are also going to use it for vpn access.  I have an issue with address translation.

We have 4 networks on the ASA

Network A  a.a.a.a/24  security level 100
Network B  b.b.b.b/24  security level 50
Network C  c.c.c.c/24  security level 50
Netword D  d.d.d.d/28  security level 0

I have setup dynamic rules which allow Network A to communicate with Network B, C and D. (D being the outside world)  This all works perfectly.  My issue is with network B and C.  They both neet to talk to the outside world.  So I have create a dynamic nat policy to allow them out.  This worls fine when i use the network Dinterface and PAT, and also works when i configure a global pool on network D and use that for the translation.

My issue is with the VPN (IPSEC and SSL vpns are experiencing the same problem).  The vpn endpoint is on Network D.  (the external ip of the firewall.) The vpn client then gets an address on network A.  I can then connect to everything on network A, B, and C.  But when the translation rule on network C for outbound nat is put in place, the vpn client can no longer ping a host on network C.  The logs indicate that there is no translation rule.  When I remove the outbound nat rule for network C, the vpn client can then ping the hosts on network C again.

Can anyone shed some light, and tell me what Im doing wrong?  

Thanks in advance.

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Asked On
2009-09-29 at 03:15:08ID24769509
Tags

Cisco ASA

,

VPN

,

firewall

,

NAT

,

Address Translation

Topics

Virtual Private Networking (VPN)

,

Cisco PIX Firewall

Participating Experts
2
Points
500
Comments
5

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Answers

 

by: ikalmarPosted on 2009-09-29 at 03:21:31ID: 25447365

did you confiugured other address the VPN pools than 4 networks?

 

by: Darkstriker69Posted on 2009-09-29 at 05:17:28ID: 25448005

I find that Nating from one internal network to another is adds unnecissary complexity and extra processing for the ASA. What has worked well for me is if you remove all NAT statements. Add your dynamic NAT statement to get out to the internet through your outside interface. Then use Nat exempt statements so that your internal networks communicate directly. If your design is good and all your interal networks are in the same class (IE. 192.168.x.x) then you can do an easy blanket NAT exempt statement that would exempt 192.168.0.0 to 192.169.0.0, If not you may end up with 4 NAT exempt statements.

You then just need to make sure your VPN pool is NAT exempt to all you internal networks.

Hope this helps

Darkstriker

 

by: kevin_toddPosted on 2009-09-29 at 06:05:07ID: 25448498

Hi Darkstriker,

Interesting idea.  However, would I be right in saying that all the devices on each network would require their default gateway (or a static route) to point back to the asa?  (we have some multihomed systems)

Also, should I create a 5th network to home my vpn pool, or is it OK to have it in one of the existing networks? (Network A)

Thanks for the quick responses.

Kevin

 

by: kevin_toddPosted on 2009-09-29 at 07:23:51ID: 25449364

Hi,

I've also noticed that the nat exempt rule only works when connecting to a network who's internet has a lower security level.  If the securty level is the same the commection is blocked.  Is there any what round this?

Thanks,

 

by: Darkstriker69Posted on 2009-09-29 at 12:16:44ID: 25452330

"I've also noticed that the nat exempt rule only works when connecting to a network who's internet has a lower security level."

This is an "interface" security level, not an "internet" security level and what you have described is the expected behavior by design of interface security levels. You must create an access list to allow traffic to flow from higher to lower level.

20120131-EE-VQP-002

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