Connect two LAN networks with FreeBSD IPSEC tunnel mode

In actual work, we often encounter the requirement to connect two local area networks in different locations. The use of FreeBSD IPSec tunnel method can easily connect two local area networks, and has good security. Here's a case to tell how to connect two local area networks in this way.

Assume the following network structure:

The two FreeBSD machines are configured with firewalls of two subnets, and IPSec support is added to the kernel configuration, with the associated kernel configuration parameters as follows:

# IP Security (Crypto define W/IPSEC)

Options IPSEC

Options Ipsec_esp

Options Ipsec_debug

# Generic Tunnel Interface

Pseudo-device gif 4

# Berkeley packet filter used by DHCP server.

Pseudo-device BPF 4

# Firewall Flags

Options Ipfirewall

Options Ipdivert

Options IPFilter

Options Ipfilter_log