||AerieBSD 1.0 Refernce Manual
Common Address Redundancy Protocol
.Cd "pseudo-device carp"
interface is a pseudo-device which implements and controls the
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 functionality.
interface can be created at runtime using the
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
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 host, and
which is used to authenticate carp advertisements.
is used to specify which interface the
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
or through the
can also be used in conjunction with
to respond to changes in CARP state;
however, for most uses this will not be necessary.
See the manual page for
for more information.
Additionally, there are a number of global parameters which can be set using
Enabled by default.
Allow virtual hosts to preempt each other.
It is also used to failover
interfaces as a group.
When the option is enabled and one of the
enabled physical interfaces
is changed to 240 on all
See also the first example.
Disabled by default.
log state changes, bad packets, and other errors.
May be a value between 0 and 7 corresponding with
The default value is 2, which limits logging to changes in CARP state.
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
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
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
Each node in a load balancing cluster is represented
by at least one
pair in a comma separated list.
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
value within the carpnodes specification.
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
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
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
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
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
Managable 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
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 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
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 firewalls and routers with multiple interfaces, it is desirable to
failover all of the
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 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
Because of the preempt option, when one of the physical interfaces of
host A fails,
is adjusted to 240 on all its
This will cause host B to preempt on both interfaces instead of
just the failed one.
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 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
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
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.
| AerieBSD 1.0 Reference Manual
|| August 26 2008