Re: [Qemu-devel] [PATCH] tun: orphan an skb on tx

[Steffen Klassert]

On Mon, Feb 02, 2015 at 07:27:10AM +0000, David Woodhouse wrote:
> On Sun, 2015-02-01 at 21:07 -0800, David Miller wrote:
> 
> > We might as well have not have implemented the IPSEC stack at all,
> > because as a result of the userland VPN stuff our IPSEC stack is
> > largely unused except by a very narrow group of users.
> 
> Well, I'd love to make better use of it if I can. I do suspect it makes
> most sense for userspace to continue to manage the probing of UDP
> connectivity, and the fallback to TCP mode — and I suspect it also makes
> sense to continue to use tun for passing packets up to the VPN client
> when it's using the TCP transport.
> 
> So the question would be how we handle redirecting the packet flow to
> the optional UDP transport, when the VPN client determines that it's
> available. For the sake of the user setting up firewall and routing
> rules, I do think it's important that it continues to appear to
> userspace as the *same* device for the entire lifetime of the session,
> regardless of which transport the packets happen to be using at a given
> moment in time. It doesn't *have* to be tun, though. 
> 
> You don't seem to like my suggestion of somehow pushing down an XFRM
> state to the tun device to direct the packets out there instead of up to
> userspace. Do you have an alternative suggestion... or a specific
> concern that would help me come up with something you like better?

Maybe you want to use a virtual tunnel interface (vti) what we have
already. Everything that is routed through such an interface is
guaranteed to be either encrypted if a matching xfrm state is present
or dropped. Same on the rceive side, everything that is received by
this interface is guaranteed to be IPsec processed. So you can do
a routing based decision about the IPsec processing.

While I'm sure it could handle the ESP in UDP encapsulation, I'm not that
sure about your TCP fallback because this requires a valid xfrm state
to allow packets to pass. Using the same interface for both is probably
not possible.