MirBSD manpage: pfsync(4)

PFSYNC(4)                  BSD Programmer's Manual                   PFSYNC(4)


     pfsync - packet filter state table logging interface


     pseudo-device pfsync


     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 syn-
     chronisation interface, pfsync will also 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 de-

     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 de-

     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 is defined
     in <net/if_pfsync.h> as:

           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 in-
     terface, by specifying a synchronisation interface using ifconfig(8). For
     example, the following command sets fxp0 as the synchronisation inter-

           # 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 trivi-
     al to spoof packets which create states, bypassing the pf ruleset. Ideal-
     ly, this is a network dedicated to pfsync messages, i.e. a crossover ca-
     ble between two firewalls.

     There is a one-to-one correspondence between packets seen by bpf(4) on
     the pfsync interface, and packets sent out on the synchronisation inter-
     face, i.e. a packet with 4 state deletion messages on pfsync means that
     the same 4 deletions were sent out on the synchronisation interface. How-
     ever, the actual packet contents may differ as the messages sent over the
     network are "compressed" where possible, containing only the necessary


     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 au-

     Both firewalls in this example have three sis(4) interfaces. sis0 is the
     external interface, on the subnet; sis1 is the internal in-
     terface, on the subnet; and sis2 is the pfsync interface,
     using the 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):


           inet NONE


           inet NONE


           inet NONE


           inet vhid 1 pass foo


           inet vhid 2 pass bar


           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
     /etc/hostname.carp1 would look like this:

           inet vhid 2 pass bar \
                   advskew 100

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



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


     The pfsync device first appeared in OpenBSD 3.3.

MirBSD #10-current            November 29, 2002                              1

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