FreeBSD is even in year 2022 and 2023 a popular open-source operating system that is used as the basis for many different types of servers, including web servers, email servers, and database servers. One of the key features of FreeBSD is its support for the Common Address Redundancy Protocol (CARP), which provides high availability for network services.

CARP is a protocol that allows multiple servers to share a virtual IP address. When a server that is running CARP fails, another server takes over the virtual IP address and provides the network service. This allows administrators to create highly available services that can continue to operate even if one of the servers fails.

The problem with CARP is that it can only be used to provide high availability for services that are running on the same network segment as the servers. This means that if a server fails, it can only be taken over by another server that is on the same network segment.

This is where the Packet Filter Synchronization (pfsync) protocol comes in. pfsync is a protocol that is used to synchronize the state of packet filters between multiple servers. In the context of CARP, pfsync is used to synchronize the state of the virtual IP addresses between multiple servers.

When pfsync is used with CARP, each server maintains its own copy of the virtual IP addresses and their associated state. When one of the servers fails, the other servers detect the failure and take over the virtual IP addresses. This allows administrators to create highly available services that can continue to operate even if one of the servers fails, even if the servers are not on the same network segment.

To set up pfsync and CARP on FreeBSD, administrators first need to configure the network interfaces that will be used for the virtual IP addresses. They then need to configure pfsync to synchronize the state of the virtual IP addresses between the servers. Finally, they need to configure CARP to use the virtual IP addresses.

Once pfsync and CARP are set up, administrators can use them to provide high availability for network services. For example, they can use pfsync and CARP to provide high availability for a web server, an email server, or a database server.

In conclusion, pfsync is a powerful protocol that can be used to provide high availability for network services. When used in conjunction with CARP, pfsync provides administrators with a flexible and scalable solution for creating highly available services that can continue to operate even if one of the servers fails. If you’re looking to create a highly available network infrastructure, then pfsync and CARP are definitely worth considering.