Hello Brian,

On 18.10.2016 21:05, Brian O'Connor wrote:
 
>   1.  Where in the diagram is NAT-T de-capsulation performed?
XFRM lookup.
> 
>   2.  Where in the diagram is NAT-T encapsulation performed?
XFRM lookup.
> 
>   3.  Does the NAT-T UDP header have to be removed so the iptables IPsec 
> policy module can operate?
Nope. This question sound suspiciously like you want to check if an ESP/NAT-T 
packet has a matching policy.
XFRM does that for you. IPsec policies also work the other way around. They 
don't just only allow protected traffic
to or from an IP address, they also implicitely drop unprotected matching 
datagrams. The kernel will and does also efficiently drop
spoofed ESP packets. The system design is sound. There's no reason to try to 
protect the kernel from invalid ESP packets.


> 
>   4.  Traffic from the topmost "local process" block flows to a "routing 
> decision" block.  Is this to prevent
>       a local IPsec connection (to loopback address, possibly ) from being 
> encrypted?
No, it's just there to fill in required routing information into the skb in the 
kernel.
XFRM is disabled on loopback via a sysctl value. You can enable it, if you 
want, but that makes no sense
and there's no need for that.

> 
>   [1]  http://inai.de/images/nf-packet-flow.png
> 
> TIA,
> Brian

-- 

Mit freundlichen Grüßen/Kind Regards,
Noel Kuntze

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