A general VPN server has two connection modes, L2TP mode and PPTP mode. The difference between the two connection modes depends on the protocol and working mode.
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First, L2TP mode
(1) L2TP VPN connection. The VPN server maintains a UDP connection between port 1701 and client 1701. It will automatically create a certificate authentication IPsec policy for the L2TP connection. Therefore, the L2TP communication is wrapped in the IPsec policy. In the tunnel, ipsecmon can be used to see UDP traffic that is actually 1701<-->1701.
(2) When the VPN starts to connect, the two parties need to exchange keys. This is implemented by ISAKMP of UPD 500 port. From then on, all VPN communication, including establishment/disconnection request, user authentication, and data transmission are all through ESP. Transmitted on top.
Second, PPTP method
(1) VPN connection in PPTP mode. The VPN client establishes/disconnects the connection request through the PPTP protocol with the TCP 1723 port of the server. As for the specific user authentication and data transmission, the PPP protocol is used for communication. And the PPP protocol runs on top of the GRE.
(2) When using PPTP VPN connection, the VPN server maintains a TCP connection between port 1723 and an arbitrary port on the client. TCP port 1723 runs PPTP Control Message, including PPTP tunnel creation, maintenance, and termination. Daily management work, the client establishes a connection with the server 1723 port through TCP, enters GRE-based PPP negotiation, including user authentication, data transmission and other communication. When disconnecting the VPN connection, the PPTP Control Message based on port 1723 is used again. .
A VPN client connected by L2TP cannot use a private address to connect to a VPN server with a valid address, and a PPTP connection can be directly connected.
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