what is difference between simple TUN and VPNs like PPTP and L2TP

Question

In respect of VPNs and tunneling, as far as i know, TUN is a virtual interface works on network Layer 3 (IP packets), and is nothing more than a simple more likely UDP socket link in practice (I think).

Here are my questions:

1- What difference is between a simple TUN and VPN protocols like PPTP and L2TP?
2- With high possibility a simple TUN is penetrable. Do you acknowledge it?
3- Are the routers able to detect a TUN link as a VPN tunnel? or is detected as a simple socket link?

Thank you in advance

Answer 1

The tun interface in the linux is just pseudo-interface to pass network packets between kernel network stack and userspace program, not more. It's not a VPN protocol.

The answers follow from this fact:

1. You cannot compare the TUN interface entity with VPN protocols. These things are totally different by nature.

2. You cannot talk about penetrable of the TUN.

3. You cannot talk about any detection on intermediate devices.