The most common cause of version incompatibility involves NAT rules. Historically, NAT and filtering were separate concepts. Modern PF has unified these syntaxes.
To grasp the error, you must understand two separate but interrelated parts of the PF system:
The error “configuration incompatible with program version” means the binary structure generated by your pfctl does not match what the kernel module expects. The kernel is effectively saying: “I don’t understand the format of the rules you just sent me.”
Many systems have multiple pfctl binaries. Use which and version checks:
which pfctl
pfctl -V
Compare this with the kernel module version: pf configuration incompatible with pf program version
sysctl net.pf.version
If the numbers do not match, you have a mismatch.
For developers and deep-divers: pf's internal interfaces change when structures like struct pf_rule, struct pf_state, or struct pf_status receive new fields. Between FreeBSD 12 and 13, for instance, the pf DIOCGETSTATUS ioctl changed its response layout. This is why pfctl compiled on 12 cannot correctly parse kernel responses on 13.
To view the exact ABI version expected by pfctl:
strings /sbin/pfctl | grep -i "pf version"
To view the kernel's exported version:
strings /boot/kernel/pf.ko | grep -i "pf version"
If these strings differ, you will see the incompatibility error.
If you must keep old config, compile matching pfctl from source (e.g., from OpenBSD ports history).
Situation: A FreeBSD 13.1 firewall was upgraded to 13.2 via freebsd-update. The administrator forgot to reboot. When they ran service pf start, they received:
/etc/pf.conf: pfctl: pf configuration incompatible with pf program version
Diagnosis:
Resolution: The kernel module was still the 13.1 version. A reboot loaded the 13.2 kernel module, and pf started correctly.
If you recently upgraded the kernel without updating userland, perform a complete upgrade.
For FreeBSD using freebsd-update:
freebsd-update fetch
freebsd-update install
# Reboot
shutdown -r now
# After reboot, update packages
pkg update && pkg upgrade
For OpenBSD (which is a unified system): The most common cause of version incompatibility involves
sysupgrade
After the upgrade, ensure both kernel and userland are synchronized.
