NAME¶
carp —
Common Address Redundancy
Protocol
SYNOPSIS¶
device carp
DESCRIPTION¶
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 carp
N
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 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. 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 that 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 interfaces. See also the first example. Disabled by
default.
- net.inet.carp.log
- Value of 0 disables any logging. Value of 1 enables logging
state changes of carp interfaces. Values above 1 enable
logging of bad carp packets. 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 preemption
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
connections 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 configured 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 configure
one virtual host for each physical host which would respond to ARP requests
and thus handle the traffic. In the following example, two virtual 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 virtual 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 ALSO¶
inet(4),
pfsync(4),
rc.conf(5),
ifconfig(8),
sysctl(8)
HISTORY¶
The
carp device first appeared in
OpenBSD
3.5. The
carp device was imported into
FreeBSD 5.4.