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CARP works by allowing a group of hosts on the same network segment to share an IP address. This group of hosts is referred to as a "redundancy group". The redundancy group is assigned an IP address that is shared amongst the group members. Within the group, one host is designated the "master" and the rest as "backups". The master host is the one that currently "holds" the shared IP; it responds to any traffic or ARP requests directed towards it. Each host may belong to more than one redundancy group at a time.
One common use for CARP is to create a group of redundant firewalls. The virtual IP that is assigned to the redundancy group is configured on client machines as the default gateway. In the event that the master firewall suffers a failure or is taken offline, the IP will move to one of the backup firewalls and service will continue unaffected.
It's possible for multiple CARP groups to exist on the same network segment. CARP advertisements contain the Virtual Host ID which allows group members to identify which redundancy group the advertisement belongs to.
In order to prevent a malicious user on the network segment from spoofing CARP advertisements, each group can be configured with a password. Each CARP packet sent to the group is then protected by an SHA1 HMAC.
Since CARP is its own protocol it should have an explicit pass rule in filter rulesets:
pass out on $carp_dev proto carp keep state
$carp_dev should be the physical interface that CARP is communicating over.
ifconfig carpN create
ifconfig carpN vhid vhid [pass password] [carpdev carpdev] \
[advbase advbase] [advskew advskew] [state state] ipaddress \
netmask mask
Further CARP behavior can be controlled via sysctl(8).
# sysctl -w net.inet.carp.allow=1
# ifconfig carp1 create
# ifconfig carp1 vhid 1 pass mekmitasdigoat carpdev em0 \
advskew 100 10.0.0.1 netmask 255.255.255.0
This sets up the following:
Running ifconfig on carp1 shows the status of the interface.
# ifconfig carp1
carp1: flags=8802<UP,BROADCAST,SIMPLEX,MULTICAST> mtu 1500
carp: BACKUP carpdev em0 vhid 1 advbase 1 advskew 100
groups: carp
inet 10.0.0.1 netmask 0xffffff00 broadcast 10.0.0.255
When pfsync(4) is set up to send and receive updates on the network, the default behavior is to multicast updates out on the local network. All updates are sent without authentication. Best common practice is either:
When updates are being sent and received on the network, pfsync packets should be passed in the filter ruleset:
pass on $sync_if proto pfsync
$sync_if should be the physical interface that pfsync(4) is communicating over.
ifconfig pfsyncN syncdev syncdev [syncpeer syncpeer]
# ifconfig pfsync0 syncdev em1This enables pfsync on the em1 interface. Outgoing updates will be multicast on the network allowing any other host running pfsync to receive them.
An example scenario.
Two firewalls, fw1 and fw2.
+----| WAN/Internet |----+
| |
em2| |em2
+-----+ +-----+
| fw1 |-em1----------em1-| fw2 |
+-----+ +-----+
em0| |em0
| |
---+-------Shared LAN-------+---
The firewalls are connected back-to-back using a crossover cable on em1. Both are connected to the LAN on em0 and to a WAN/Internet connection on em2. IP addresses are as follows:
The network policy is that fw1 will be the preferred master.
Configure fw1:
! enable preemption and group interface failover
# sysctl -w net.inet.carp.preempt=1
! configure pfsync
# ifconfig em1 10.10.10.1 netmask 255.255.255.0
# ifconfig pfsync0 syncdev em1
# ifconfig pfsync0 up
! configure CARP on the LAN side
# ifconfig carp1 create
# ifconfig carp1 vhid 1 carpdev em0 pass lanpasswd \
172.16.0.100 netmask 255.255.255.0
! configure CARP on the WAN/Internet side
# ifconfig carp2 create
# ifconfig carp2 vhid 2 carpdev em2 pass netpasswd \
192.0.2.100 netmask 255.255.255.0
|
Configure fw2:
! enable preemption and group interface failover
# sysctl -w net.inet.carp.preempt=1
! configure pfsync
# ifconfig em1 10.10.10.2 netmask 255.255.255.0
# ifconfig pfsync0 syncdev em1
# ifconfig pfsync0 up
! configure CARP on the LAN side
# ifconfig carp1 create
# ifconfig carp1 vhid 1 carpdev em0 pass lanpasswd \
advskew 128 172.16.0.100 netmask 255.255.255.0
! configure CARP on the WAN/Internet side
# ifconfig carp2 create
# ifconfig carp2 vhid 2 carpdev em2 pass netpasswd \
advskew 128 192.0.2.100 netmask 255.255.255.0
|
Examples:
To failover a particular CARP group, shut down the carp(4) interface on the master node. This will cause the master to advertise itself with an "infinite" advbase and advskew. The backup host(s) will see this and immediately take over the role of master.
# ifconfig carp1 down
An alternative is to increase the advskew to a value that's higher than the advskew on the backup host(s). This will cause a failover but still allow the master to participate in the CARP group.
Another method of failover is to tweak the CARP demotion counter. The demotion counter is a measure of how "ready" a host is to become master of a CARP group. For example, while a host is in the middle of booting up it's a bad idea for it to become the CARP master until all interfaces have been configured, all network daemons have been started, etc. Hosts advertising a high demotion value will be less preferred as the master.
A demotion counter is stored in each interface group that the CARP interface belongs to. By default, all CARP interfaces are members of the "carp" interface group. The current value of a demotion counter can be viewed using ifconfig(8):
# ifconfig -g carp
carp: carp demote count 0
In this example the counter associated with the "carp" interface group is shown. When a CARP host advertises itself on the network, it takes the sum of the demotion counters for each interface group the carp(4) interface belongs to and advertises that value as its demotion value.
Now assume the following example. Two firewalls running CARP with the following CARP interfaces:
The objective is to failover just the carp1 and carp2 groups to the secondary firewall.
First, assign each to a new interface group, in this case named "internal":
# ifconfig carp1 group internal
# ifconfig carp2 group internal
# ifconfig internal
carp1: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
carp: MASTER carpdev em0 vhid 1 advbase 1 advskew 100
groups: carp internal
inet 10.0.0.1 netmask 0xffffff00 broadcast 10.0.0.255
carp2: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
carp: MASTER carpdev em1 vhid 2 advbase 1 advskew 100
groups: carp internal
inet 10.0.1.1 netmask 0xffffff00 broadcast 10.0.1.255
Now increase the demotion counter for the "internal" group using ifconfig(8):
# ifconfig -g internal
internal: carp demote count 0
# ifconfig -g internal carpdemote 50
# ifconfig -g internal
internal: carp demote count 50
The firewall will now gracefully failover on the carp1 and carp2 groups to the other firewall in the cluster while still remaining the master on carp3 and carp4. If the other firewall started advertising itself with a demotion value higher than 50, or if the other firewall stopped advertising altogether, then this firewall would again take over mastership on carp1 and carp2.
To fail back to the primary firewall, reverse the changes:
# ifconfig -g internal -carpdemote 50
# ifconfig -g internal
internal: carp demote count 0
Network daemons such as OpenBGPD and sasyncd(8) make use of the demotion counter to ensure that the firewall does not become master until BGP sessions become established and IPsec SAs are synchronized.
pass in on fxp0 inet proto tcp from any to carp0 port 22but replacing the fxp0 with carp0 would not work as you desire.
DON'T forget to pass proto carp and proto pfsync!
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