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NAME
carp — Common Address Redundancy Protocol
SYNOPSIS
device carp
DESCRIPTION
The carp interface is a pseudo-device that implements and controls the CARP protocol. CARP
allows multiple hosts on the same local network to share a set of IP addresses. Its primary
purpose is to ensure that these addresses are always available, but in some configurations
carp can also provide load balancing functionality.
A carp interface can be created at runtime using the ifconfig carpN create command or by
configuring it via cloned_interfaces in the /etc/rc.conf file.
To use carp, the administrator needs to configure at minimum a common virtual host ID (VHID)
and virtual host IP address on each machine which is to take part in the virtual group.
Additional parameters can also be set on a per-interface 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 configurations can be done using ifconfig(8), or through the
SIOCSVH ioctl(2).
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. It is also used to
failover carp interfaces as a group. When the option is enabled
and one of the carp enabled physical interfaces goes down, advskew
is changed to 240 on all carp interfaces. See also the first
example. Disabled by default.
net.inet.carp.log Value of 0 disables any logging. Value of 1 enables logging state
changes of carp interfaces. Values above 1 enable logging of bad
carp packets. Default value is 1.
net.inet.carp.arpbalance Balance local traffic using ARP (see below). Disabled by default.
net.inet.carp.suppress_preempt
A read only value showing the status of preemption suppression.
Preemption can be suppressed if link on an interface is down or
when pfsync(4) interface is not synchronized. Value of 0 means
that preemption is not suppressed, since no problems are detected.
Every problem increments suppression counter.
ARP level load balancing
The carp has limited abilities for load balancing the incoming connections between hosts in
Ethernet network. For load balancing operation, one needs several CARP interfaces that are
configured to the same IP address, but to a different VHIDs. Once an ARP request is
received, the CARP protocol will use a hashing function against the source IP address in the
ARP request to determine which VHID should this request belong to. If the corresponding
CARP interface is in master state, the ARP request will be replied, otherwise it will be
ignored. See the EXAMPLES section for a practical example of load balancing.
The ARP load balancing has some limitations. First, ARP balancing only works on the local
network segment. It cannot balance traffic that crosses a router, because the router itself
will always be balanced to the same virtual host. Second, ARP load balancing can lead to
asymmetric routing of incoming and outgoing traffic, and thus combining it with pfsync(4) is
dangerous, because this creates a race condition between balanced routers and a host they
are serving. Imagine an incoming packet creating state on the first router, being forwarded
to its destination, and destination replying faster than the state information is packed and
synced with the second router. If the reply would be load balanced to second router, it
will be dropped due to no state.
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 as carp interface
LINK_UP or LINK_DOWN event. Please see devd.conf(5) and EXAMPLES section for more
information.
EXAMPLES
For firewalls and routers with multiple interfaces, it is desirable to failover all of the
carp interfaces together, when one of the physical interfaces goes down. This is achieved
by the preempt option. Enable it on both host A and B:
sysctl net.inet.carp.preempt=1
Assume that host A is the preferred master and 192.168.1.x/24 is configured on one physical
interface and 192.168.2.y/24 on another. This is the setup for host A:
ifconfig carp0 create
ifconfig carp0 vhid 1 pass mekmitasdigoat 192.168.1.1/24
ifconfig carp1 create
ifconfig carp1 vhid 2 pass mekmitasdigoat 192.168.2.1/24
The setup for host B is identical, but it has a higher advskew:
ifconfig carp0 create
ifconfig carp0 vhid 1 advskew 100 pass mekmitasdigoat 192.168.1.1/24
ifconfig carp1 create
ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat 192.168.2.1/24
Because of the preempt option, when one of the physical interfaces of host A fails, advskew
is adjusted to 240 on all its carp interfaces. This will cause host B to preempt on both
interfaces instead of just the failed one.
In order to set up an ARP balanced virtual host, it is necessary to configure one virtual
host for each physical host which would respond to ARP requests and thus handle the traffic.
In the following example, two virtual hosts are configured on two hosts to provide balancing
and failover for the IP address 192.168.1.10.
First the carp interfaces on host A are configured. The advskew of 100 on the second
virtual host means that its advertisements will be sent out slightly less frequently.
ifconfig carp0 create
ifconfig carp0 vhid 1 pass mekmitasdigoat 192.168.1.10/24
ifconfig carp1 create
ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat 192.168.1.10/24
The configuration for host B is identical, except the advskew is on virtual host 1 rather
than virtual host 2.
ifconfig carp0 create
ifconfig carp0 vhid 1 advskew 100 pass mekmitasdigoat 192.168.1.10/24
ifconfig carp1 create
ifconfig carp1 vhid 2 pass mekmitasdigoat 192.168.1.10/24
Finally, the ARP balancing feature must be enabled on both hosts:
sysctl net.inet.carp.arpbalance=1
When the hosts receive an ARP request for 192.168.1.10, the source IP address of the request
is used to compute which virtual host should answer the request. The host which is master
of the selected virtual host will reply to the request, the other(s) will ignore it.
This way, locally connected systems will receive different ARP replies and subsequent IP
traffic will be balanced among the hosts. If one of the hosts fails, the other will take
over the virtual MAC address, and begin answering ARP requests on its behalf.
Processing of carp status change events can be set up by using the following devd.conf
rules:
notify 0 {
match "system" "IFNET";
match "type" "LINK_UP";
match "subsystem" "carp*";
action "/root/carpcontrol.sh $type $subsystem";
};
notify 0 {
match "system" "IFNET";
match "type" "LINK_DOWN";
match "subsystem" "carp*";
action "/root/carpcontrol.sh $type $subsystem";
};
SEE ALSO
inet(4), pfsync(4), rc.conf(5), devd.conf(5), ifconfig(8), sysctl(8)
HISTORY
The carp device first appeared in OpenBSD 3.5. The carp device was imported into
FreeBSD 5.4.