carp —
Common
Address Redundancy Protocol
device carp
The CARP allows multiple hosts on the same local network to share a set of IPv4
and/or IPv6 addresses. Its primary purpose is to ensure that these addresses
are always available.
To use
carp, the administrator needs to configure
at a minimum a common virtual host ID (vhid), and attach at least one IP
address to this vhid on each machine which is to take part in the virtual
group. Additional parameters can also be set on a per-vhid basis:
advbase and
advskew,
which are used to control how frequently the host sends advertisements when it
is the master for a virtual host, and
pass which
is used to authenticate
carp advertisements. The
advbase parameter stands for
“advertisement base”. It is measured in seconds and specifies
the base of the advertisement interval. The
advskew parameter stands for
“advertisement skew”. It is measured in 1/256 of seconds. It is
added to the base advertisement interval to make one host advertise a bit
slower that the other does. Both
advbase and
advskew are put inside CARP advertisements. These
values can be configured using
ifconfig(8), or
through the
SIOCSVH
ioctl(2).
CARP virtual hosts can be configured on multicast-capable interfaces: Ethernet,
layer 2 VLAN, FDDI and Token Ring. An arbitrary number of virtual host IDs can
be configured on an interface. An arbitrary number of IPv4 or IPv6 addresses
can be attached to a particular vhid. It is important that all hosts
participating in a vhid have the same list of prefixes configured on the vhid,
since all the prefixes are included in the cryptographic checksum supplied in
each advertisement. Multiple vhids running on one interface participate in
master/backup elections independently.
Additionally, there are a number of global parameters which can be set using
sysctl(8):
- net.inet.carp.allow
- Allow carp operation. When
disabled, virtual hosts remain in initial state, neither sending nor
receiving announcements or traffic. Enabled by default.
- net.inet.carp.preempt
- Allow virtual hosts to preempt each other. When enabled, a
vhid in a backup state would preempt a master that is announcing itself
with a lower advskew. Disabled by default.
- net.inet.carp.dscp
- DSCP value in carp packet. Valid Values are 0 to 63. A
value of 4 is equivalent to the old standard of TOS LOW_DELAY. TOS values
were deprecated and replaced by DSCP in 1998. The default value is 56
(CS7/Network Control).
- net.inet.carp.log
- Determines what events relating to
carp vhids are logged. A value of 0 disables
any logging. A value of 1 enables logging state changes of
carp vhids. Values above 1 enable logging of
bad carp packets. The default value is
1.
- net.inet.carp.demotion
- This value shows the current level of CARP demotion. The
value is added to the actual advskew sent in announcements for all vhids.
During normal system operation the demotion factor is zero. However,
problematic conditions raise its level: when
carp experiences problem with sending
announcements, when an interface running a vhid goes down, or while the
pfsync(4) interface is not synchronized. The
demotion factor can be adjusted writing to the sysctl oid. The signed
value supplied to the sysctl(8) command is
added to current demotion factor. This allows to control
carp behaviour depending on some external
conditions, for example on the status of some daemon utility.
- net.inet.carp.ifdown_demotion_factor
- This value is added to
net.inet.carp.demotion when an interface
running a vhid goes down. The default value is 240 (the maximum advskew
value).
- net.inet.carp.senderr_demotion_factor
- This value is added to
net.inet.carp.demotion when
carp experiences errors sending its
announcements. The default value is 240 (the maximum advskew value).
Sometimes it is useful to get notified about
carp
status change events. This can be accomplished by using
devd(8) hooks. Master/slave events are signalled
under system
CARP. The subsystem specifies
the vhid and name of the interface where the master/slave event occurred. The
type of the message displays the new state of the vhid. Please see
devd.conf(5) and the
EXAMPLES section for more
information.
For firewalls and routers with multiple interfaces, it is desirable to failover
all of the addresses running
carp together, when
one of the physical interfaces goes down. This is achieved by the use of the
preempt option. Enable it on both hosts A and B:
sysctl net.inet.carp.preempt=1
Assume that host A is the preferred master and we are running the 192.168.1.0/24
prefix on em0 and 192.168.2.0/24 on em1. This is the setup for host A (advskew
is above 0 so it could be overwritten in the emergency situation from the
other host):
ifconfig em0 vhid 1 advskew 100 pass mekmitasdigoat 192.168.1.1/24
ifconfig em1 vhid 2 advskew 100 pass mekmitasdigoat 192.168.2.1/24
The setup for host B is identical, but it has a higher
advskew:
ifconfig em0 vhid 1 advskew 200 pass mekmitasdigoat 192.168.1.1/24
ifconfig em1 vhid 2 advskew 200 pass mekmitasdigoat 192.168.2.1/24
When one of the physical interfaces of host A fails,
advskew is demoted to a configured value on all
its
carp vhids. Due to the preempt option, host B
would start announcing itself, and thus preempt host A on both interfaces
instead of just the failed one.
Processing of
carp status change events can be set
up by using the following devd.conf rule:
notify 0 {
match "system" "CARP";
match "subsystem" "[0-9]+@[0-9a-z]+";
match "type" "(MASTER|BACKUP)";
action "/root/carpcontrol.sh $subsystem $type";
};
To see
carp packets decoded in
tcpdump(1) output, one needs to specify the
-T carp option,
otherwise
tcpdump(1) will interpret them as VRRP
packets:
tcpdump -npi vlan0 -T carp
tcpdump(1),
inet(4),
pfsync(4),
devd.conf(5),
rc.conf(5),
ifconfig(8),
sysctl(8)
The
carp device first appeared in
OpenBSD 3.5. The
carp
device was imported into
FreeBSD 5.4. In
FreeBSD 10.0,
carp was
significantly rewritten, and is no longer a pseudo-interface.