Detailed understanding of the operation mechanism of vrovpn VPN

Although VPN is rarely used in a general home network, VPN is a very important part for an enterprise network. With VPN, the security and stability of enterprise data transmission can be ensured, this article uses Huawei router as an example to introduce the knowledge about VPN.

Currently, two VPN connection methods are generally used: L2TP and PPTP. The following is a detailed description.

1. For an L2TP VPN connection, the VPN Server maintains a UDP connection between port 1701 and port 1701 of the client. A certificate-based authentication IPsec Policy is automatically created for the L2TP connection, therefore, L2TP communication is wrapped in the IPsec tunnel created by the Ipsec Policy. You can use ipsecmon to see whether the UDP communication is actually 1701 <--> 1701.

When the VPN starts to connect, both parties need to exchange the key, which is achieved through the isakmp of the upd 500 port. From then on, all VPN communication will be performed, this includes establishing/disconnecting requests, user authentication, and data transmission over ESP.

2. For pptp vpn connection, the establishment/disconnection requests of the VPN Client are all sent through the PPTP protocol with the TCP port 1723 of the server, for specific user authentication and data transmission, the PPP protocol is based on GRE.

3. When using the pptp vpn connection, the VPN Server maintains a TCP connection between port 1723 and any port of the client. The TCP port 1723 runs the PPTP Control Message, this includes the daily management of PPTP tunnel creation, maintenance, and termination. After the client establishes a connection with port 1723 of the server through TCP, it enters the GRE-based PPP negotiation.

4. User Authentication and data transmission are included. when the VPN connection is disconnected, PPTP Control Message based on port 1723 is used. The VPN Client Connected in L2TP mode cannot use a private address to connect the VPN Server with a valid address, the PPTP connection method can be directly connected.