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pfSense® is one of very few open source solutions offering enterprise-class high availability capabilities with stateful failover, allowing the elimination of the firewall as a single point of failure. High Availability is achieved through a combination of features:
CARP for IP address redundancy
XMLRPC for configuration synchronization
pfsync for state table synchronization
With this configuration, units act as an “active/passive” cluster with the primary node working as the master unit and the secondary node in a backup role, taking over as needed if the primary node fails.
Though often erroneously called a “CARP Cluster”, two or more redundant pfSense firewalls are more aptly titled a “High Availability Cluster” or “HA Cluster”, since CARP is only one of several technologies used to achieve High Availability with pfSense, and in the future CARP could be swapped for a different redundancy protocol.
One interface on each cluster node will be dedicated for synchronization tasks. This is typically referred to as the “Sync” interface, and it is used for configuration synchronization and pfsync state synchronization. Any available interface may be used.
Some call this the “CARP” interface but that is incorrect and very misleading. CARP heartbeats happen on each interface with a CARP VIP; CARP traffic and failover actions do not utilize the Sync interface.
The most common High Availability cluster configuration includes only two nodes. It is possible to have more nodes in a cluster, but they do not provide a significant advantage.
It is important to distinguish between the three functions (IP address redundancy, configuration synchronization, and state table synchronization), because they happen in different places. Configuration synchronization and state synchronization happen on the sync interface, directly communicating between firewall units. CARP heartbeats are sent on each interface with a CARP VIP. Failover signaling does not happen on the sync interface, but rather it happens on every CARP-enabled interface.
pfSense Hangouts on Youtube which contains the June 2015 Hangout also covering High Availability.
Common Address Redundancy Protocol (CARP) was created by OpenBSD developers as a free, open redundancy solution for sharing IP addresses among a group of network devices. Similar solutions already existed, primarily the IETF standard for Virtual Router Redundancy Protocol (VRRP). However Cisco claims VRRP is covered by its patent on their Hot Standby Router Protocol (HSRP), and told the OpenBSD developers that it would enforce its patent. Hence, the OpenBSD developers created a new free, open protocol to accomplish essentially the same result without infringing on Cisco’s patent. CARP became available in October 2003 in OpenBSD, and was later added to FreeBSD as well.
A CARP type Virtual IP address (VIP) is shared between nodes of a cluster. One node is master and receives traffic for the IP address, and the other nodes maintain backup status and monitor for heartbeats to see if they need to assume the master role if the previous master fails. Since only one member of the cluster at a time is using the IP address, there is no IP address conflict for CARP VIPs.
In order for failover to work properly it is important that inbound traffic coming to the cluster, such as routed upstream traffic, VPNs, NAT, local client gateway, DNS requests, etc., be sent to a CARP VIP and for outgoing traffic such as Outbound NAT to be sent from a CARP VIP. If traffic is addressed to a node directly and not a CARP VIP, then that traffic will not be picked up by other nodes.
CARP works similar to VRRP and HSRP, and may even conflict in some cases. Heartbeats are sent out on each interface containing a CARP VIP, one heartbeat per VIP per interface. At the default values for skew and base, a VIP sends out heartbeats about once per second. The skew determines which node is master at a given point in time. Whichever node transmits heartbeats the fastest assumes the master role. A higher skew value causes heartbeats to be transmitted with more delay, so a node with a lower skew will be the master unless a network or other issue causes the heartbeats to be delayed or lost.
Never access the firewall GUI, SSH, or other management mechanism using a CARP VIP. For management purposes, only use the actual IP address on the interface of each separate node and not the VIP. Otherwise it cannot be determined beforehand which unit is being accessed.
IP Address Requirements for CARP¶
A High Availability cluster using CARP needs three IP addresses in each subnet along with a separate unused subnet for the Sync interface. For WANs, this means that a /29 subnet or larger is required for an optimal configuration. One IP address is used by each node, plus a shared CARP VIP address for failover. The synchronization interface only requires one IP address per node.
It is technically possible to configure an interface with a CARP VIP as the only IP address in a given subnet, but it is not generally recommended. When used on a WAN, this type of configuration will only allow communication from the primary node to the WAN, which greatly complicates tasks such as updates, package installations, gateway monitoring, or anything that requires external connectivity from the secondary node. It can be a better fit for an internal interface, however internal interfaces do not typically suffer from the same IP address limitations as a WAN, so it is still preferable to configure IP addresses on all nodes.
Switch/Layer 2 Concerns¶
CARP heartbeats utilize multicast and may require special handling on the switches involved with the cluster. Some switches filter, rate limit, or otherwise interfere with multicast in ways that can cause CARP to fail. Also, some switches employ port security methods which may not work properly with CARP.
At a minimum, the switch must:
Allow Multicast traffic to be sent and received without interference on ports using CARP VIPs.
Allow traffic to be sent and received using multiple MAC addresses.
Allow the CARP VIP MAC address to move between ports.
Nearly all problems with CARP failing to properly reflect the expected status are failures of the switch or other layer 2 issues, so be sure the switches are properly configured before continuing.