Support for IPSec NAT-T in transoprt mode

Bjoern A. Zeeb bzeeb-lists at
Wed May 2 20:00:30 UTC 2012

On 2. May 2012, at 18:50 , Zmiter wrote:

> 24.04.2012 23:10, Andreas Longwitz ?????:
>> There is one limitation I would like to get over. From man 8 setkey:
>> System that do not perform the port check cannot support multiple
>> endpoints behind the same NAT. I think this is a FreeBSD kernel restriction:
>> For the first incoming L2TP packet the IPSEC part of the kernel does not
>> save the source port in the corresponding SA (maybe a field like
>> natt_l2tp_port). So the kernel does for outgoing L2TP packets not know
>> the correct SA, if two ore more SA's with the same IP exists.
>> I would like to know if the patch mentioned in this thread adresses this
>> problem.
> Thank you very much for your attention.
> I've been testing those patches (actually, without your part) and YES it's a big problem with clients (Android, Windows Mobile) behind the same NAT. I cannot find the solution yet, but I'm very interested in it.
> So, my Androids is some sort of stupid bricks, they do not send NAT-OA payloads at phase 2, and ipsec-tools fills the SPD with IPs taken from IDs. But this is not the correct way. IDs contain LAN (which is behind the NAT) addresses, and FreeBSD cannot route packets to the IPSec crypto part.
> I've made some quick patching of IPSec tools to get my devices working, but I don't know if they accomodate to the RFCs and ISAKMP. The main idea is to take NAT-OAi and NAT-OAr addresses not from IDs when we are using NAT-T, but from real source and destination addresses of the server and client NATs.
> Here is my ipsec-tools patch (i've call it patch-zz-local-2.diff and place at /usr/ports/security/ipsec-tools/files with two other patches from kern /146190)


> It differs from that in kern/146190 in one simple thing. I have problems with the original patch from kern/146190. When there was no NAT-OAi or NAT-OAr values in the kernel space, checksums was calculated at 0, but they were not ignored despite of the sysctl net.inet.esp.esp_ignore_natt_cksum value. The improvement allows to ignore every checksum in esp packets when net.inet.esp.esp_ignore_natt_cksum=1.

Just replying to the last one -- you all need to make sure that this will work with a double-NAT (both i and r sitting behind a NAT) and not just i behind a NAT and r sitting there with a globally routable IP.

The changes suddenly become a lot more complex.  Just my 5cts.


Bjoern A. Zeeb                                 You have to have visions!
   It does not matter how good you are. It matters what good you do!

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