Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


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

	   # 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.

     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.
     However, the actual packet	contents may differ as the messages sent over
     the network are "compressed" where	possible, containing only the neces-
     sary information.

     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.

BSD			       November	29, 2002			   BSD


Want to link to this manual page? Use this URL:

home | help