OpenVPN and "SSL VPNs"

[Florian Weimer]

* Stefan Mink:

> a) It would be good to hear from this community if there
> are any negative aspects of OpenVPN (vs. IPsec VPNs).

It's not standardized, and it only interoperates with itself (but this
is true for many IPsec implementations as well).  This is more than
compensated by its portability.  OpenVPN has a very interesting
feature set, including hybrid authentication and a HMAC-based
integrity check before TLS processing for the paranoid.  (Static key
mode is also possible and doesn't require TLS at all.)

Unfortunately, the protocol would have to be reverse-engineered from
the source code before it can be reviewed.  You've already mentioned
important aspects of the protocl (TSL on the control plane, ESP for
the payload).

What's still missing, though, is multicast support and PPPoE-style
multihop authentication.  PMUTD doesn't work for me at the moment, but
this could also be a local configuration problem.

> b) I still have a problem with the term "SSL/TLS VPN".

What is an "SSL VPN"?  A web application that runs over TLS? 8-)

Uh-oh, it looks as if this joke isn't too far off.  This reminds me of
the good old times when we tried to use TeraTerm and SSH port
forwarding to secure a Baan installation.

> What OpenVPN seems to do is use SSL for authentication and key
> exchange/rekeying, but does use "ESP similar" data protection
> schemes/formats. Does the usage of SSL on a "control plane" make
> OpenVPN an "SSL VPN"?

No, it certainly isn't.  OpenVPN doesn't work at the application
layer, as SSL VPNs usually do.  It's a real VPN, and you can choose
between layer 2 or layer 3 operation.

> This sounds to me like calling something a car just
> because it uses a steering wheel... So far I thought
> about SSL VPNs as doing everything over SSL (with
> the known disadvantages...).

At least OpenVPN uses a bit SSL and provides a VPN.  SSL VPNs use a
lot of SSL, but provide no VPN.

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