IPSEC VPN using AH and ESP together

I was under the impression that when you use IPSEC, you choose either AH or ESP for encapsulation, with ESP being the better choice as it uses encryption on the data .

Yesterday I heard you can use them both at the same time, but I wonder why you would want to do this? ESP can do what AH can do plus encryption.

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jdancer

It's true that ESP offers AH but since IPSec doesn't proscribe a policy, you can combine ESP with AH. As for doing it, well, that's why it's called a policy decision.

FrankGuthrie

Yes, but why do it, that's my question.... Why would anyone choose to run IPSEC with AH AND ESP??

Is this bad design?

jdancer

I would consider ESP and AH a bad design and if I am not mistaken may slow things down. As to why it was configured that way, I can only think of one answer: the person who configured it probably doesn't know that ESP offers AH. So, chalk it up to lack of knowledge on how IPSec really works.

FrankGuthrie

jdancer wrote: »

I would consider ESP and AH a bad design and if I am not mistaken may slow things down. As to why it was configured that way, I can only think of one answer: the person who configured it probably doesn't know that ESP offers AH. So, chalk it up to lack of knowledge on how IPSec really works.

Ok so it's either ESP and AH and nowadays AH is not used anymore, because of no encryption, Correct?

jdancer

Correct, AH only provides integrity not confidentiality.

FrankGuthrie

So NO mixing of the 2 to get the best of both worlds?

fuz1on

Tunnel...

FrankGuthrie

fuz1on wrote: »

Tunnel...

Well that was a usefull comment...

JoJoCal19

I think fuz1on was meaning that in tunnel mode the entire packet is encrypted and authenticated, thus combining both functions of AH and ESP?

fuz1on

Yeah! I had a meeting so I couldn't post anymore.

This page really helped me with IPSec. I'm an extremely visual learner...

An Illustrated Guide to IPsec

jdancer

Here's a picture of ESP used in both transport and tunnel mode which includes encryption and authentication (integrity). Can't really get more basic than this illustration.

creamy_stew

I always thought that ESP/AH was choose 1. Where everyone would choose ESP. Is it at all possible to do ESP and Ah in the same tunnel?

fuz1on

creamy_stew wrote: »

I always thought that ESP/AH was choose 1. Where everyone would choose ESP. Is it at all possible to do ESP and Ah in the same tunnel?

It is possible but unpractical (NAT traversal); I just alluding to the fact that a tunnel would be like the best of both worlds.

creamy_stew

fuz1on wrote: »

It is possible but unpractical (NAT traversal); I just alluding to the fact that a tunnel would be like the best of both worlds.

I wasn't criticizing you. I really have no idea.

Can you elaborate on the NAT traversal part?

creamy_stew

"In the packet, the AH is located after the IP header but before the ESP (if present) or other higher level protocol, such as TCP. Like the ESP, the AH can implement tunneling mode. Also, like the ESP, IPsec requires specific algorithms to be available for the AH to be implemented."

Introduction to Cisco IPsec Technology - Cisco

So, it seems pretty clear that not only can you use AH+ESP together, but you can also use AH in tunnel mode. Weird. I've never seen this in the wild.

FrankGuthrie

creamy_stew wrote: »

So, it seems pretty clear that not only can you use AH+ESP together, but you can also use AH in tunnel mode. Weird. I've never seen this in the wild.

Yeha, but when is this used?

FrankGuthrie

I guess nobody knows and nobody will ever.

Maybe a desingnerror if it's both used.

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OctalDump

It seems that authentication is optional in ESP. Could there be a situation where one end point refuses authentication (or authentication method?) in ESP, but supports AH? This would allow AH + ESP to meet goals of authentication and encryption. It's a fringe case, and likely would only be caused by technical, rather than policy, limitations.