How does AlG work?

Question

VPN

There was a task to be connected to PPTP to the server being behind NAT. Until I turned on the application level gateway in my home router, the tunnel did not rise. I don't understand how this thing works?

Answer 1

PPTP uses not (only) TCP/UDP, but also the GRE protocol. NAT, in the general simple case, only works for TCP/UDP, parsing the IP address and port number, and establishing a mapping between them, which is not enough for PPTP. The ALG for PPTP is an additional code that monitors the transmission of data on the PPTP port, understands that the client has begun to establish a connection, and creates a NAT entry for the GRE protocol for this client.