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I'm using openvpn (open source version). Each user authenticates using ssl certificates. The problem is that such a certificate can be used on any device. I'd want to restrict the usage of each certificate to only one device.

Is that feasible?

  • Are you trying to prevent users from logging in more than once at a given time (e.g: user has phone and laptop)? Or are you trying to "be secure" and ensure that a given key is used only by the authorized device? IMO, the former is a broken model (each device should have a separate and secret key), and there is no way to ensure the latter. – Attie Mar 11 '20 at 18:16
  • I'm trying the second one you mentioned – sebelk Mar 11 '20 at 18:20
  • I don't believe that is possible. – Attie Mar 11 '20 at 18:27
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You need to decide first how to uniquely identify a device in a satisfactory manner (for you, no silver bullet here).

That's the real question. Once you have that, then you can use a post-auth script in openvpn to make use of that information.

Here is the information on post_auth scripts and this is an example of MAC address checking.

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It's hardly foolproof, but if you are using TAP you can specify the MAC address in your configuration with:

lladdr=<MAC ADDRESS>

The reference manual has this to say about the option:

–lladdr address

Specify the link layer address, more commonly known as the MAC address. Only applied to TAP devices.

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