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. If unspecified, the kernel
attempts to set it by looking for another interface with the same subnet.
These configurations can be done using
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 provides two mechanisms to load
balance incoming traffic over a group of
ARP balancing and IP balancing.
Which one to use mainly depends on the network environment
carp 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
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
See the EXAMPLES section for a practical example of load balancing.
For ARP balancing, one has to configure multiple
carpnodes and choose the
balancing mode arp.
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 pfsync(4) requires 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 different 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 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
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,
using the 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. Please note that activating stealth mode
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:
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 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 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
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
balancing mode can be adjusted accordingly.
sysctl(3), 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.