CARP(4) BSD Kernel Interfaces Manual CARP(4)NAMEcarp — Common Address Redundancy Protocol
SYNOPSIS
options CARP
pseudo-device carpDESCRIPTION
The carp interface is a pseudo-device that 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 functionality.
A carp interface can be created at runtime using the ifconfig carpN
create command or by configuring it via cloned_interfaces in the
/etc/rc.conf file.
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 and advskew, which are used to con‐
trol how frequently the host sends advertisements when it is the master
for a virtual host, and pass which is used to authenticate carp adver‐
tisements. The advbase parameter stands for “advertisement base”. It is
measured in seconds and specifies the base of the advertisement interval.
The advskew parameter stands for “advertisement skew”. It is measured in
1/256 of seconds. It is added to the base advertisement interval to make
one host advertise a bit slower than the other does. Both advbase and
advskew are put inside CARP advertisements. These configurations can be
done using ifconfig(8), or through the SIOCSVH ioctl(2).
Additionally, there are a number of global parameters which can be set
using sysctl(8):
net.inet.carp.allow Accept incoming carp packets. Enabled by
default.
net.inet.carp.preempt Allow virtual hosts to preempt each other. It
is also used to failover carp interfaces as a
group. When the option is enabled and one of
the carp enabled physical interfaces goes down,
advskew is changed to 240 on all carp inter‐
faces. See also the first example. Disabled
by default.
net.inet.carp.log Value of 0 disables any logging. Value of 1
enables logging of bad carp packets. Values
above 1 enable logging state changes of carp
interfaces. Default value is 1.
net.inet.carp.arpbalance Balance local traffic using ARP (see below).
Disabled by default.
net.inet.carp.suppress_preempt
A read only value showing the status of preemp‐
tion suppression. Preemption can be suppressed
if link on an interface is down or when
pfsync(4) interface is not synchronized. Value
of 0 means that preemption is not suppressed,
since no problems are detected. Every problem
increments suppression counter.
ARP LEVEL LOAD BALANCING
The carp has limited abilities for load balancing the incoming connec‐
tions between hosts in Ethernet network. For load balancing operation,
one needs several CARP interfaces that are configured to the same IP
address, but to a different VHIDs. Once an ARP request is received, the
CARP protocol will use a hashing function against the source IP address
in the ARP request to determine which VHID should this request belong to.
If the corresponding CARP interface is in master state, the ARP request
will be replied, otherwise it will be ignored. See the EXAMPLES section
for a practical example of load balancing.
The ARP load balancing has some limitations. First, 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 virtual host. Second, ARP load balancing can lead to asymmetric
routing of incoming and outgoing traffic, and thus combining it with
pfsync(4) is dangerous, because this creates a race condition between
balanced routers and a host they are serving. Imagine an incoming packet
creating state on the first router, being forwarded to its destination,
and destination replying faster than the state information is packed and
synced with the second router. If the reply would be load balanced to
second router, it will be dropped due to no state.
EXAMPLES
For firewalls and routers with multiple interfaces, it is desirable to
failover all of the carp 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 config‐
ured on one physical interface and 192.168.2.y/24 on another. This is
the setup for host A:
ifconfig carp0 create
ifconfig carp0 vhid 1 pass mekmitasdigoat 192.168.1.1/24
ifconfig carp1 create
ifconfig carp1 vhid 2 pass mekmitasdigoat 192.168.2.1/24
The setup for host B is identical, but it has a higher advskew:
ifconfig carp0 create
ifconfig carp0 vhid 1 advskew 100 pass mekmitasdigoat 192.168.1.1/24
ifconfig carp1 create
ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat 192.168.2.1/24
Because of the preempt option, when one of the physical interfaces of
host A fails, advskew is adjusted to 240 on all its carp interfaces.
This will cause host B to preempt on both interfaces instead of just the
failed one.
In order to set up an ARP balanced virtual host, it is necessary to con‐
figure one virtual host for each physical host which would respond to ARP
requests and thus handle the traffic. In the following example, two vir‐
tual hosts are configured on two hosts to provide balancing and failover
for the IP address 192.168.1.10.
First the carp interfaces on host A are configured. The advskew of 100
on the second virtual host means that its advertisements will be sent out
slightly less frequently.
ifconfig carp0 create
ifconfig carp0 vhid 1 pass mekmitasdigoat 192.168.1.10/24
ifconfig carp1 create
ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat 192.168.1.10/24
The configuration for host B is identical, except the advskew is on vir‐
tual host 1 rather than virtual host 2.
ifconfig carp0 create
ifconfig carp0 vhid 1 advskew 100 pass mekmitasdigoat 192.168.1.10/24
ifconfig carp1 create
ifconfig carp1 vhid 2 pass mekmitasdigoat 192.168.1.10/24
Finally, the ARP balancing feature must be enabled on both hosts:
sysctl net.inet.carp.arpbalance=1
When the hosts receive an ARP request for 192.168.1.10, the source IP
address of the request is used to compute which virtual host should
answer the request. The host which is master of the selected virtual
host will reply to the request, the other(s) will ignore it.
This way, locally connected systems will receive different ARP replies
and subsequent IP traffic will be balanced among the hosts. If one of
the hosts fails, the other will take over the virtual MAC address, and
begin answering ARP requests on its behalf.
SEE ALSOinet(4), pfsync(4), rc.conf(5), ifconfig(8), sysctl(8)HISTORY
The carp device first appeared in OpenBSD 3.5. It was imported into
DragonFly 1.11.
BSD April 9, 2007 BSD
