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PFSYNC(4)                BSD Kernel Interfaces Manual                PFSYNC(4)

     pfsync -- packet filter states table logging interface

     device pfsync

     The pfsync interface is a pseudo-device which exposes certain changes to
     the state table used by pf(4).  If configured with a physical synchroni-
     sation interface, pfsync will send state changes out on that interface
     using IP multicast, 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.
     However, state changes from packets received by pfsync over the network
     are not rebroadcast.  States created by a rule marked with the no-sync
     keyword are omitted from the pfsync interface (see pf.conf(5) for

     The pfsync interface will attempt to collapse multiple updates of the
     same state into one message where possible.  The maximum number of times
     this can be done before the update is sent out is controlled by the
     maxupd to ifconfig.  (see ifconfig(8) and the example below for more

     Each packet retrieved on this interface has a header associated with it
     of length PFSYNC_HDRLEN.  The header indicates the version of the proto-
     col, address family, action taken on the following states and the number
     of state table entries attached in this packet.  This structure, defined
     in <net/if_pfsync.h> looks like:

           struct pfsync_header {
                   u_int8_t version;
                   u_int8_t af;
                   u_int8_t action;
                   u_int8_t count;

     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 sets fxp0 as the synchronisation

           # ifconfig pfsync0 syncif fxp0

     State change messages are sent out on the synchronisation interface using
     IP multicast packets.  The protocol is IP protocol 240, PFSYNC, and the
     multicast group used is

     It is important that the synchronisation interface be on a trusted net-
     work as there is no authentication on the protocol and it would be triv-
     ial to spoof packets which create states, bypassing the pf ruleset.  Ide-
     ally, this is a network dedicated to pfsync messages, i.e. a crossover
     cable between two firewalls.

     pfsync and carp(4) can be used together to provide automatic failover of
     a pair of firewalls configured in parallel.  One firewall handles all
     traffic - if it dies or is shut down, the second firewall takes over

     Both firewalls in this example have three sis(4) interfaces.  sis0 is the
     external interface, on the subnet, sis1 is the internal
     interface, on the subnet, and sis2 is the pfsync inter-
     face, using the subnet.  A crossover cable connects the
     two firewalls via their sis2 interfaces.  On all three interfaces, fire-
     wall 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:

           cloned_interfaces="carp0 carp1"
           network_interfaces="lo0 sis0 sis1 sis2 carp0 carp1 pfsync0"
           ifconfig_carp0="vhid 1 pass foo"
           ifconfig_carp1="vhid 2 pass bar"
           ifconfig_pfsync0="up syncif sis2"

     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
           pass on { sis0 sis1 } proto carp keep state

     If it is preferable that one firewall handle the traffic, 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
     carp1 configuration would look like this:

           ifconfig_carp1="vhid 2 pass bar advskew 100"

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


     Possibility to view state changes using tcpdump(8) has not been ported
     from OpenBSD yet.

     carp(4), ifconfig(8), inet(4), inet6(4), netintro(4), pf(4), pf.conf(5),
     protocols(5), rc.conf(5)

     The pfsync device first appeared in OpenBSD 3.3.  The pfsync device was
     imported to FreeBSD 5.3.

BSD                            February 23, 2005                           BSD