Address Redundancy Protocol
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
also provide load balancing functionality.
interface can be created at runtime using
command or by setting up a
configuration file for
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
, which are used to control how frequently
the host sends advertisements when it is the master for a virtual host, and
which is used to authenticate carp
advertisements. Finally carpdev
is used to
specify which interface the carp
to. If unspecified, the kernel attempts to set it by looking for another
interface with the same subnet. These configurations can be done using
can also be used in conjunction with
respond to changes in CARP state; however, for most uses this will not be
necessary. See the manual page for
Additionally, there are a number of global parameters which can be set using
- Accept incoming carp packets.
Enabled by default.
- Allow virtual hosts to preempt each other. Disabled by
- Make carp log state changes,
bad packets, and other errors. May be a value between 0 and 7
priorities. The default value is 2, which limits logging to changes in
provides two mechanisms to load balance
incoming traffic over a group of carp
balancing and IP balancing.
Which one to use mainly depends on the network environment
is being used in. ARP balancing has limited
abilities for load balancing the incoming connections between hosts in an
Ethernet network. It only works for clients in the local network, because ARP
balancing spreads the load by varying ARP replies based on the source MAC
address of the host sending the query. Therefore it cannot balance traffic
that crosses a router, because the router itself will always be balanced to
the same virtual host.
IP balancing is not dependent on ARP and therefore also works for traffic that
comes over a router. This method should work in all environments and can also
provide more fine grained load balancing than ARP balancing. The downside of
IP balancing is that 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.
A rule of thumb might be to use ARP balancing if there are many hosts on the
same network segment and to use IP balancing for all other cases.
To configure load balancing one has to specify multiple carp nodes using the
option. Each node in a load balancing
cluster is represented by at least one
pair in a comma separated list. carp
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
within the carpnodes specification.
See the EXAMPLES
section for a
practical example of load balancing.
For ARP balancing, one has to configure multiple
and choose the
Once an ARP request is received, the CARP protocol will use a hashing function
against the source MAC address in the ARP request to determine which carpnode
the request belongs to. If the corresponding carpnode is in master state, the
ARP request will be answered, otherwise it will be ignored.
The ARP load balancing has some limitations. Firstly, 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 carpnode.
Secondly, ARP load balancing can lead to asymmetric routing of incoming and
outgoing traffic, thus combining it with
special care, because this can create a race condition between balanced
routers and the host they are serving. ARP balancing can be safely used with
pfsync if the pf(4)
ruleset translates the source address to an unshared address on the outgoing
interface using a NAT rule. This requires multiple CARP groups with
IP addresses on the outgoing interface,
configured so that each host is the master of one group.
ARP balancing also works for IPv6, but instead of ARP the Neighbor Discovery
Protocol (NDP) is used.
IP load 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 carp
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
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, using the ip-stealth
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. Please note
that activating stealth mode on a carp
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 configure the
in a symmetrical manner. This is
achieved by simply using the same carpnodes
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 asymmetrically.
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 192.168.1.x/24 is configured on
one physical interface and 192.168.2.y/24 on another. This is the setup for
# ifconfig carp0 192.168.1.1 vhid 1
# ifconfig carp1 192.168.2.1 vhid 2
The setup for host B is identical, but it has a higher
# ifconfig carp0 192.168.1.1 vhid 1 advskew 100
# ifconfig carp1 192.168.2.1 vhid 2 advskew 100
In order to set up a load balanced virtual host, it is necessary to configure
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.
First the carp
interface on Host A is configured.
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 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 carpnodes
1:100,2:0 balancing ip
If ARP balancing or a different mode of IP balancing is desired the
mode can be adjusted accordingly.
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