--- name: "pfsync" section: "4" platform: "freebsd" package: "base" version: "16.0-CURRENT-20261027-61c70dc16a79" release: "FreeBSD 16.0-CURRENT (2026-10-27)" platforms: ["linux", "freebsd", "openbsd"] url: "https://new.carta.tech/pfsync" --- # pfsync(4) packet filter state table synchronisation interface ## NAME **pfsync** – packet filter state table synchronisation interface ## SYNOPSIS **device pfsync** In [loader.conf(5)](/loader.conf/5.md): **net.pfsync.pfsync\_buckets** In [sysctl.conf(5)](/sysctl.conf/5.md): **net.pfsync.carp\_demotion\_factor** ## DESCRIPTION The **pfsync** interface is a pseudo-device which exposes certain changes to the state table used by [pf(4)](/pf/4.md). State changes can be viewed by invoking [tcpdump(1)](/tcpdump/1.md) 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)](/pf.conf/5.md) 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 *maxupd* parameter to ifconfig (see [ifconfig(8)](/ifconfig/8.md) and the example below for more details). The sending out of a **pfsync** packet will be delayed by a maximum of one second. ## NETWORK SYNCHRONISATION States can be synchronised between two or more firewalls using this interface, by specifying a synchronisation interface using [ifconfig(8)](/ifconfig/8.md). For example, the following command sets fxp0 as the synchronisation interface: ``` # 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. This can be used in combination with [ipsec(4)](/ipsec/4.md) to protect the synchronisation traffic. In such a configuration, the syncdev should be set to the [enc(4)](/enc/4.md) interface, as this is where the traffic arrives when it is decapsulated, e.g.: ``` # ifconfig pfsync0 syncpeer 10.0.0.2 syncdev enc0 ``` 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. Either run the pfsync protocol on a trusted network - ideally a network dedicated to pfsync messages such as a crossover cable between two firewalls, or specify a peer address and protect the traffic with [ipsec(4)](/ipsec/4.md). Support for **pfsync** transport over IPv6 was introduced in FreeBSD 14.0. To set up **pfsync** using multicast with IPv6 link-local addresses, the **syncpeer** must be set to the **pfsync** multicast address and the **syncdev** to the interface where **pfsync** traffic is expected. ``` # ifconfig pfsync0 syncpeer ff12::f0 syncdev vtnet0 ``` When new features are introduced to [pf(4)](/pf/4.md) the format of messages used by **pfsync** might change. **pfsync** will by default use the latest format. If synchronization with a peer running an older version of FreeBSD is needed the *version* parameter can be used. E.g.: ``` # ifconfig pfsync0 version 1301 ``` Currently the following versions are supported: - **1301** FreeBSD releases 13.2 and older. Compatibility with FreeBSD 13.1 has been verified. - **1400** FreeBSD release 14.0. - **1500** FreeBSD release 15.0. ## SYSCTL VARIABLES The following variables can be entered at the [loader(8)](/loader/8.md) prompt, set in [loader.conf(5)](/loader.conf/5.md), or changed at runtime with [sysctl(8)](/sysctl/8.md): - *net.pfsync.carp\_demotion\_factor* Value added to *net.inet.carp.demotion* while **pfsync** tries to perform its bulk update. See [carp(4)](/carp/4.md) for more information. Default value is 240. ## LOADER TUNABLES The following tunable may be set in [loader.conf(5)](/loader.conf/5.md) or at the [loader(8)](/loader/8.md) prompt: - *net.pfsync.pfsync\_buckets* The number of **pfsync** buckets. This affects the performance and memory tradeoff. Defaults to twice the number of CPUs. Change only if benchmarks show this helps on your workload. ## EXAMPLES **pfsync** and [carp(4)](/carp/4.md) 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)](/sis/4.md) 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): Interfaces configuration in */etc/rc.conf*: ``` network_interfaces="lo0 sis0 sis1 sis2" ifconfig_sis0="10.0.0.254/24" ifconfig_sis0_alias0="inet 10.0.0.1/24 vhid 1 pass foo" ifconfig_sis1="192.168.0.254/24" ifconfig_sis1_alias0="inet 192.168.0.1/24 vhid 2 pass bar" ifconfig_sis2="192.168.254.254/24" pfsync_enable="YES" pfsync_syncdev="sis2" ``` [pf(4)](/pf/4.md) must also be configured to allow **pfsync** and [carp(4)](/carp/4.md) 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)](/carp/4.md) vhids should be set to something higher than the primary's. For example, if firewall B is the backup, its carp1 configuration would look like this: ``` ifconfig_sis1_alias0="inet 192.168.0.1/24 vhid 2 pass bar advskew 100" ``` The following must also be added to */etc/sysctl.conf*: ``` net.inet.carp.preempt=1 ``` ## SEE ALSO [tcpdump(1)](/tcpdump/1.md), [bpf(4)](/bpf/4.md), [carp(4)](/carp/4.md), [enc(4)](/enc/4.md), [inet(4)](/inet/4.md), [inet6(4)](/inet6/4.md), [ipsec(4)](/ipsec/4.md), [netintro(4)](/netintro/4.md), [pf(4)](/pf/4.md), [pf.conf(5)](/pf.conf/5.md), [protocols(5)](/protocols/5.md), [rc.conf(5)](/rc.conf/5.md), [ifconfig(8)](/ifconfig/8.md) ## HISTORY The **pfsync** device first appeared in OpenBSD 3.3. It was first imported to FreeBSD 5.3. The **pfsync** protocol and kernel implementation were significantly modified in FreeBSD 9.0. The newer protocol is not compatible with older one and will not interoperate with it.