Common Address Redundancy Protocol
interface is a pseudo-device which
implements and controls the CARP protocol.
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
can also provide load balancing
interface can be created at runtime
command or by setting up a
configuration file for
, 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:
, which are used to control how
frequently the host sends advertisements when it is the master for a virtual
which is used to
authenticate carp advertisements. Finally
is used to specify which interface
device attaches 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
- Allow virtual hosts to preempt each other. Disabled by default.
carp log state changes, bad
packets, and other errors. May be a value between 0 and 7 corresponding
priorities. The default value is 2, which limits logging to changes in
provides two mechanisms to load balance
incoming traffic over a group of
hosts: ARP 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
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.
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
value 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
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
IP balancing is activated by setting the
. 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
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 configure the
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 asymmetrically.
For most scenarios it is desirable to have a well-defined master, achieved by
option. Enable it on
both host A and B:
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.
interface on Host A is
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
, it is not possible
to use the IP of the
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