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-
tails).
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-
tails).
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-
face:
# 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 224.0.0.240.
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
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-
tomatically.
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 in-
terface, 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 pass foo
/etc/hostname.carp1:
inet 192.168.0.1 255.255.255.0 192.168.0.255 vhid 2 pass bar
/etc/hostname.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
/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
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