PFSYNC(4) | Device Drivers Manual | PFSYNC(4)

# NAME

`pfsync` — packet filter state table synchronisation interface

# SYNOPSIS

`pseudo-device pfsync`

# DESCRIPTION

The `pfsync` interface is a pseudo-device which exposes certain changes to the state table used by pf(4). State changes can be viewed by invoking tcpdump(8) on the `pfsync` interface. If configured with a physical synchronisation interface, `pfsync` will also send state changes out on that interface, and insert state changes received on that interface from other systems into the state table.

By default, all local changes to the state table are exposed via `pfsync`. State changes from packets received by `pfsync` over the network are not rebroadcast. Updates to states created by a rule marked with the no-sync keyword are ignored by the `pfsync` interface (see pf.conf(5) for details).

The `pfsync` interface will attempt to collapse multiple state updates into a single packet where possible. The maximum number of times a single state can be updated before a `pfsync` packet will be sent out is controlled by the ifconfig(8) maxupd parameter. The sending out of a `pfsync` packet will be delayed by a maximum of one second.

Where more than one firewall might actively handle packets, e.g. with certain ospfd(8), bgpd(8) or carp(4) configurations, it is beneficial to defer transmission of the initial packet of a connection. The `pfsync` state insert message is sent immediately; the packet is queued until either this message is acknowledged by another system, or a timeout has expired. This behaviour is enabled with the defer parameter to ifconfig(8).

# NETWORK SYNCHRONISATION

States can be synchronised between two or more firewalls using this interface, by specifying a synchronisation interface using ifconfig(8). For example, the following command configures an address on fxp0 and sets it as the synchronisation interface:

    # ifconfig fxp0 inet 172.19.13.1/28
    # ifconfig pfsync0 syncdev fxp0

By default, state change messages are sent out on the synchronisation interface using IP multicast packets to the 224.0.0.240 group address. An alternative destination address for `pfsync` packets can be specified using the `syncpeer` keyword.

It is important that the pfsync traffic be well secured as there is no authentication on the protocol and it would be trivial to spoof packets which create states, bypassing the pf ruleset. Only run the pfsync protocol on a trusted network - ideally a network dedicated to pfsync messages such as a crossover cable between two firewalls.

`pfsync` will increase the carp(4) demotion counter for any interface groups associated with the interface by 32 during initialisation, and by 1 if the `pfsync` link is down or if a bulk update fails.

# EXAMPLES

`pfsync` and carp(4) can be used together to provide automatic failover of a pair of firewalls configured in parallel. One firewall will handle all traffic until it dies, is shut down, or is manually demoted, at which point the second firewall will take over automatically.

Both firewalls in this example have three sis(4) interfaces. sis0 is the external interface, on the 10.0.0.0/24 subnet; sis1 is the internal interface, on the 192.168.0.0/24 subnet; and sis2 is the `pfsync` interface, using the 192.168.254.0/24 subnet. A crossover cable connects the two firewalls via their sis2 interfaces. On all three interfaces, firewall A uses the .254 address, while firewall B uses .253. The interfaces are configured as follows (firewall A unless otherwise indicated):

/etc/hostname.sis0:

`inet 10.0.0.254 255.255.255.0 NONE`

/etc/hostname.sis1:

`inet 192.168.0.254 255.255.255.0 NONE`

/etc/hostname.sis2:

`inet 192.168.254.254 255.255.255.0 NONE`

/etc/hostname.carp0:

    inet 10.0.0.1 255.255.255.0 10.0.0.255 \
    	vhid 1 carpdev sis0 pass foo

/etc/hostname.carp1:

    inet 192.168.0.1 255.255.255.0 192.168.0.255 \
    	vhid 2 carpdev sis1 pass bar

/etc/hostname.pfsync0:

    syncdev sis2
    up

pf(4) must also be configured to allow `pfsync` and carp(4) traffic through. The following should be added to the top of /etc/pf.conf:

    pass quick on { sis2 } proto pfsync keep state (no-sync)
    pass on { sis0 sis1 } proto carp keep state (no-sync)

It is preferable that one firewall handle the forwarding of all the traffic, therefore the advskew on the backup firewall's carp(4) interfaces should be set to something higher than the primary's. For example, if firewall B is the backup, its /etc/hostname.carp1 would look like this:

    inet 192.168.0.1 255.255.255.0 192.168.0.255 \
    	vhid 2 pass bar advskew 100

The following must also be added to /etc/sysctl.conf:

    net.inet.carp.preempt=1

# SEE ALSO

bpf(4), carp(4), inet(4), inet6(4), netintro(4), pf(4), hostname.if(5), pf.conf(5), protocols(5), ifconfig(8), ifstated(8), tcpdump(8)

# HISTORY

The `pfsync` device first appeared in OpenBSD 3.3.

The `pfsync` protocol and kernel implementation were significantly modified between OpenBSD 4.4 and OpenBSD 4.5. The two protocols are incompatible and will not interoperate.

# BUGS

`pfsync` does not currently work with ipsec(4).

January 31, 2024 | Debian