PFSYNC(4) | Device Drivers Manual | PFSYNC(4) |
pfsync
— packet
filter state table synchronisation 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
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) 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) and the example below for
more details). The sending out of a pfsync
packet
will be delayed by a maximum of one second.
Where more than one firewall might actively handle packets, e.g.
with certain ospfd(8),
bgpd(8) or
carp(4) configurations, it is beneficial to
defer transmission of the initial packet of a connection. The
pfsync
state insert message is sent immediately; the
packet is queued until either this message is acknowledged by another
system, or a timeout has expired. This behaviour is enabled with the
defer parameter to
ifconfig(8).
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 configures an address on fxp0 and sets it as the synchronisation interface:
# ifconfig fxp0 inet 172.19.13.1/28 # 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.
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. Only run the pfsync protocol on a trusted network - ideally a network dedicated to pfsync messages such as a crossover cable between two firewalls.
pfsync
will increase the
carp(4) demotion counter for any interface
groups associated with the interface by 32 during initialisation, and by 1
if the pfsync
link is down or if a bulk update
fails.
pfsync
and
carp(4) 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) 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):
/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 carpdev sis0 pass foo
/etc/hostname.carp1:
inet 192.168.0.1 255.255.255.0 192.168.0.255 \ vhid 2 carpdev sis1 pass bar
/etc/hostname.pfsync0:
up syncdev 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 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) 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), carp(4), inet(4), inet6(4), netintro(4), pf(4), hostname.if(5), pf.conf(5), protocols(5), ifconfig(8), ifstated(8), tcpdump(8)
The pfsync
device first appeared in
OpenBSD 3.3.
The pfsync
protocol and kernel
implementation were significantly modified between OpenBSD
4.4 and OpenBSD 4.5. The two protocols are
incompatible and will not interoperate.
pfsync
does not currently work with
ipsec(4).
February 1, 2021 | OpenBSD-current |