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How the components work together
CARP, pfsync, and ifstated address different failure problems. CARP (the Common Address Redundancy Protocol) provides shared IP addresses and master/backup selection. pfsync distributes PF state-table changes to a peer. ifstated monitors interface state or runs external tests, then executes commands or changes state in response. The OpenBSD PF FAQ describes combining CARP and pfsync as a way to build a highly available, redundant firewall cluster.
- CARP: Clients and neighboring routers use a shared address, not a node-specific address. The CARP master owns that address and advertises it; a backup can take over if advertisements cease.
- pfsync: Peer firewalls exchange PF connection-state updates, allowing a newly active node to have state for existing connections. It does not copy the operating system or configuration files.
- ifstated: Adds health-aware actions. For example, a node can be demoted when an important path or upstream test fails, even if its CARP interface is still up.
CARP supports IPv4 and IPv6. A dual-stack deployment needs the appropriate shared addresses and network configuration for each protocol; a CARP address on one network does not automatically provide redundancy on another.
Plan the network and failover roles
Use two nodes connected to the same LAN and WAN segments, plus a dedicated inter-firewall path for pfsync where possible. Configure a CARP address on each protected network. Set clients’ default gateway to the LAN CARP address rather than either firewall’s physical address. The WAN-side shared address must also fit the upstream network’s addressing and routing arrangement.
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The OpenBSD documentation’s example uses separate LAN, WAN, and sync interfaces. Its addresses are illustrative only, not a production addressing plan:
| Interface or address | fw1 (preferred master) | fw2 (backup) | Shared CARP address |
|---|---|---|---|
| LAN, em0 | 172.16.0.1 | 172.16.0.2 | 172.16.0.100 |
| WAN, em1 | 10.10.10.1 | 10.10.10.2 | 10.10.10.100 |
| Sync, em2 | 192.0.2.1 | 192.0.2.2 | None |
The example’s shared WAN address is 192.0.2.100, on the sync-network row’s address range rather than the table’s illustrative WAN physical addresses. In a real design, choose interface addresses and prefixes that match the actual networks; do not copy the example’s addresses or infer that the shared WAN address belongs on the sync network.
For a straightforward first deployment, prefer active/standby: one node is preferred and the other takes over. Active/active routing adds topology and routing requirements; pfsync’s defer option can delay a new connection’s first packet until a peer acknowledges the state or a timeout occurs, but that delay is a trade-off, not a general requirement for a standby pair.
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Configure CARP virtual addresses
Create a CARP interface for each shared network address on each node. Set a matching VHID for the corresponding CARP interface on both firewalls, select the physical interface with carpdev, configure the shared IP address and netmask, and use a higher advskew on the less-preferred node. Lower skew is preferred. The documented example uses fw1 as preferred master and sets fw2 to advskew 128.
CARP parameters include vhid, pass, carpdev, advbase, advskew, and state. The documented default advertisement base is 1 second, with an allowed range of 1–255 seconds; advskew ranges from 0 to 254. A CARP password protects advertisements using SHA1 HMAC, but it is not a substitute for isolating the sync network.
Persist the interfaces across boots with /etc/hostname.carpN files; OpenBSD’s netstart creates and configures them at startup. Check the installed system’s ifconfig and interface manual pages when translating the design into interface-file syntax for a specific release.
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Configure and protect pfsync
Configure a pfsync interface on each node and bind it to the dedicated sync interface with syncdev. For a direct, back-to-back connection, the OpenBSD PF FAQ recommends using that interface as the sync device. A unicast arrangement can use syncpeer; the FAQ recommends protecting unicast pfsync traffic with IPsec.
pfsync updates are unauthenticated by default. Prefer an isolated direct link, or protect a unicast sync path with IPsec, and restrict access to the required traffic. Persist the interface in /etc/hostname.pfsyncN. The optional defer mode delays a new connection’s first packet while awaiting peer acknowledgment or timeout; assess its latency impact before using it.
Allow CARP on the physical interfaces carrying CARP advertisements and pfsync on the sync interface in PF. PF processes forwarded traffic on the physical interface, so write forwarding rules against that interface rather than assuming a rule on carpN will match it. Both nodes need compatible PF policy: state replication cannot compensate for divergent rules or other configuration.
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Use ifstated to react to path failures
The OpenBSD ifstated.conf(5) manual describes ifstated as a daemon that responds to network-state changes determined by monitoring interface link state or running external tests. In ifstated.conf, define tests and states, then give states an initialization block and event-driven actions. Link tests can report up, down, or unknown; external tests can run periodically using an every interval.
- Identify what failure matters. Monitor a link or test a meaningful upstream destination. A link being up proves only that the local interface reports link, not that the entire route to the internet works.
- Define the healthy and degraded states. Use a state transition when a test fails, and return to the healthy state only when recovery is sufficiently reliable. Treat an unknown result deliberately instead of silently interpreting it as success.
- Demote the affected node. On entering the degraded state, run an idempotent action that increases CARP demotion; when health returns, restore the intended demotion. OpenBSD’s manual example adjusts the carp group demotion with
ifconfig -g carp -carpdemotewhen entering a state. - Log and observe transitions. Record why a node changed state and verify that recovery does not leave an unintended demotion in place.
Make external tests conservative enough to avoid flapping: a transient failed probe should not repeatedly transfer the gateway role. The test destination, interval, and recovery policy depend on the network and should be chosen to represent the path whose failure should trigger a handoff.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Keep configuration consistent on both nodes
CARP and pfsync do not distribute PF rules, interface configuration, DHCP or DNS data, certificates, or application and service state. Maintain those separately with a controlled configuration deployment or synchronization process. Include a way to check that both nodes have compatible interface mappings, addresses, PF policy, and service configuration before relying on failover.
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Choose the distribution method so that a change is applied and verified on the standby as well as the active node. Avoid treating a successful pfsync link as evidence that the rest of the cluster is configured consistently.
Test failure and recovery before relying on the cluster
Plan controlled tests that cover node, path, synchronization, and configuration behavior. OpenBSD’s documented interfaces support inspecting the relevant state; the following checks are operational guidance, not a claim of a measured failover time.
- Node loss: Confirm that the backup takes over when the preferred firewall stops advertising or is shut down.
- LAN and WAN link loss: Check whether the healthy node remains preferred, and whether ifstated demotes a node whose important path has failed.
- Sync-link loss: Verify that you detect the loss of replication and understand the risk to connections established while peers cannot exchange state.
- Preferred-node return: Reboot or restore the preferred node and verify the intended role and demotion behavior, rather than assuming it will return to master without disruption.
- PF reload and asymmetric routing: Check connection behavior when rules reload and when traffic can take different paths through the pair.
Use ifconfig to inspect CARP state and interface configuration, tcpdump on the pfsync interface to confirm state-update traffic, and pfctl to inspect PF rules and states. Test planned maintenance by taking the master CARP interface down; OpenBSD documents that backups then take over immediately. Raising advskew or using the carpdemote interface-group mechanism can also shift preference. Restore the original preference after maintenance.
What failover does—and does not—guarantee
The design can move shared gateway addresses and replicate PF connection state, but it cannot promise that every session survives every failure. Whether an existing connection continues depends on the failure, whether the state reached the peer, the traffic path, and the surrounding network’s behavior. The OpenBSD sources cited here publish no universal failover-latency, throughput, or connection-survival figure. Any such figure would need to be measured on the actual hardware, OpenBSD release, topology, PF rules, and traffic mix.
In practical terms, CARP handles gateway-address ownership, pfsync handles PF state updates, and ifstated can trigger a preference change based on health checks. A complete redundant service still requires separately managed configuration and a tested network design.
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