Common Address Redundancy
carp interface is a pseudo-device
which 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
carp interface can be created at runtime
create command or by setting up a
hostname.if(5) configuration file for
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:
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. Finally
carpdev is used to specify which interface the
carp device attaches to. These configurations can be
ifconfig(8), or through the
carp can also be used in conjunction with
ifstated(8) to respond to changes in CARP state; however, for most
uses this will not be necessary. See the manual page for
ifstated(8) for more information.
Additionally, there are a number of global parameters which can be set using sysctl(8):
- Accept incoming
carppackets. Enabled by default.
- Allow virtual hosts to preempt each other. Disabled by default.
carplog state changes, bad packets, and other errors. May be a value between 0 and 7 corresponding with syslog(3) priorities. The default value is 2, which limits logging to changes in CARP state.
carp uses IP balancing to load balance
incoming traffic over a group of
carp hosts. IP
balancing is not dependent on ARP and therefore works for traffic that comes
over a router. However it requires the traffic that is destined towards the
load balanced IP addresses to be received by all
carp hosts. While this is always the case when
connected to a hub, it has to play some tricks in switched networks, which
will result in a higher network load.
To configure load balancing one has to specify multiple carp nodes
carpnodes option. Each node in a load
balancing cluster is represented by at least one
pair in a comma separated list.
carp tries to
distribute the incoming network load over all configured carpnodes. The
following example creates a load balancing group consisting of three nodes,
using vhids 3, 4 and 6:
# ifconfig carp0 carpnodes 3:0,4:0,6:100
The advskew value of the last node is set to 100, so that this
node is designated to the BACKUP state. It will only become MASTER if all
nodes with a lower advskew value have failed. By varying this value
throughout the machines in the cluster it is possible to decide which share
of the network load each node receives. Therefore, all carp interfaces in
the cluster are configured identically, except for a different
advskew value within the carpnodes
IP balancing works by utilizing the network itself to distribute
incoming traffic to all
carp nodes in the cluster.
Each packet is filtered on the incoming
interface so that only one node in the cluster accepts the packet. All the
other nodes will just silently drop it. The filtering function uses a hash
over the source and destination address of the IPv4 or IPv6 packet and
compares the result against the state of the carpnode.
IP balancing is activated by setting the balancing mode to
ip. This is the recommended default setting. In this
mode, carp uses a multicast MAC address, so that a switch sends incoming
traffic towards all nodes.
However, there are a few OS and routers that do not accept a
multicast MAC address being mapped to a unicast IP. This can be resolved by
using one of the following unicast options. For scenarios where a hub is
used it is not necessary to use a multicast MAC and it is safe to use the
ip-unicast mode. Manageable switches can usually be
tricked into forwarding unicast traffic to all cluster nodes ports by
configuring them into some sort of monitoring mode. If this is not possible,
ip-stealth mode is another option, which
should work on most switches. In this mode
never sends packets with its virtual MAC address as source. Stealth mode
prevents a switch from learning the virtual MAC address, so that it has to
flood the traffic to all its ports. Activating stealth mode on a
carp interface that has already been running might
not work instantly. As a workaround the VHID of the first carpnode can be
changed to a previously unused one, or just wait until the MAC table entry
in the switch times out. Some layer 3 switches do port learning based on ARP
packets. Therefore the stealth mode cannot hide the virtual MAC address from
these kind of devices.
If IP balancing is being used on a firewall, it is recommended to
carpnodes in a symmetrical manner.
This is achieved by simply using the same
list on all sides of the firewall. This ensures that packets of one
connection will pass in and out on the same host and are not routed
For most scenarios it is desirable to have a well-defined master,
achieved by enabling 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 carp should run on the physical interfaces em0 with the network 192.168.1.0/24 and em1 with network 192.168.2.0/24. This is the setup for host A:
# ifconfig carp0 192.168.1.1/24 carpdev em0 vhid 1 # ifconfig carp1 192.168.2.1/24 carpdev em1 vhid 2
The setup for host B is identical, but it has a higher
# ifconfig carp0 192.168.1.1/24 carpdev em0 vhid 1 advskew 100 # ifconfig carp1 192.168.2.1/24 carpdev em1 vhid 2 advskew 100
In order to set up a load balanced virtual host, it is necessary
to configure one
carpnodes entry for each physical
host. In the following example, two physical hosts are configured to provide
balancing and failover for the IP address 192.168.1.10.
carp interface on Host A is
advskew of 100 on the second
carpnode entry means that its advertisements will be sent out slightly less
frequently and will therefore become the designated backup.
# ifconfig carp0 192.168.1.10 carpdev em0 carpnodes 1:0,2:100 \ balancing ip
The configuration for host B is identical, except the skew is on the carpnode entry with virtual host 1 rather than virtual host 2.
# ifconfig carp0 192.168.1.10 carpdev em0 carpnodes 1:100,2:0 \ balancing ip
If a different mode of load balancing is desired, the
balancing mode can be adjusted accordingly.
sysctl(2), inet(4), pfsync(4), hostname.if(5), ifconfig(8), ifstated(8), netstart(8), sysctl(8)
carp device first appeared in
If load balancing is used in setups where the carpdev does not
share an IP in the same subnet as
carp, it is not
possible to use the IP of the
carp interface for
self originated traffic. This is because the return packets are also subject
to load balancing and might end up on any other node in the cluster.
If an IPv6 load balanced carp interface is taken down manually, it will accept all incoming packets for its address. This will lead to duplicated packets.