NAME¶
carp
—
Common Address Redundancy Protocol
SYNOPSIS¶
device carp
DESCRIPTION¶
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 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
- Accept incoming
carp
packets. 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.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 current level of CARP demotion. The value is added to the
actual advskew sent in announcements for all vhids. At 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).
STATE CHANGE NOTIFICATIONS¶
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.
EXAMPLES¶
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(8) output, one needs to specify
-T
carp
option, otherwise
tcpdump(8) tries to interpret
them as VRRP packets:
tcpdump -npi vlan0 -T carp
SEE ALSO¶
inet(4),
pfsync(4),
rc.conf(5),
devd.conf(5),
ifconfig(8),
sysctl(8) tcpdump(8)
HISTORY¶
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.