Use carp in FreeBSD to implement mutual hot standby and load balancing

29.13 common access redundancy protocol (CARP, shared address redundancy protocol)

Original Tom Rhodes.

Common Access redundancy protocol, or carp for shortAllows multiple hosts to share the same IP address.Address. In some configurations, this can improve availability or achieve load balancing. In the following example, these hosts can also use different IP addresses at the same time.Address.

To enable CarpYes. You must add the following options in the FreeBSD Kernel configuration and re-compile the kernel:

device carp

In this way, you can use carpFunction, some specific parameters can be passed through a seriesSysctlOid. Devices can useIfconfigCommand to load:

# ifconfig carp0 create

In a real environment, these interfaces needID. This vhidOr virtual host identification (virtual host ID) is used to differentiate hosts on the network.

29.13.1 use carp to improve service availability (CARP)

As mentioned above, CarpIs to improve service availability. In this example, three hosts will be provided with Failover services, each of which has an independent IP address.And provide the same web content. Three machines use DNSPoll to provide services. The machine used for failover has two CarpInterface to configure the IP addresses of the other two servers.Address. When a server fails, the machine automatically obtains the IP address of the faulty machine.Address. In this way, users do not feel any fault. The content and services provided by the Failover server should be consistent with those provided by the server for hot backup.

Configuration of the two machines, except the Host Name and vhidOtherwise, they should be completely consistent. In our example, the host names of these two machines areHosta.example.orgAndHostb.example.org. First, you needAdd configurationRC. conf. ForHosta.example.orgFor example,RC. confThe file should contain the following configurations:

hostname="hosta.example.org"
ifconfig_fxp0="inet 192.168.1.3 netmask 255.255.255.0"
cloned_interfaces="carp0"
ifconfig_carp0="vhid 1 pass testpast 192.168.1.50/24"

InHostb.example.org, CorrespondingRC. confThe configuration is:

hostname="hostb.example.org"
ifconfig_fxp0="inet 192.168.1.4 netmask 255.255.255.0"
cloned_interfaces="carp0"
ifconfig_carp0="vhid 2 pass testpass 192.168.1.51/24"

Note:On two machinesIfconfigOfpassThe password specified by the options must be consistent, which is very important.CarpThe device only listens to and accepts announcements from machines with the correct password. In addition, the vhid of different virtual hostsMust be different.

Third machine,Provider.example.orgConfiguration is required to take over when problems occur on the other two machines. This machine requires twoCarpDevice, which processes two machines respectively. CorrespondingRC. confThe configuration is similar to the following:

hostname="provider.example.org"
ifconfig_fxp0="inet 192.168.1.5 netmask 255.255.255.0"
cloned_interfaces="carp0 carp1"
ifconfig_carp0="vhid 1 advskew 100 pass testpass 192.168.1.50/24"
ifconfig_carp1="vhid 2 advskew 100 pass testpass 192.168.1.51/24"

Configure twoCarpDevice, enablingProvider.example.orgTake over the IP address immediately when either of the two machines stops respondingAddress.

Note:Default FreeBSD KernelPossibleInterhost preemption is enabled. If so,Provider.example.orgThe IP address may not be released when the formal content server is restored.Address. In this case, the administrator can "remind" the interface. The specific method isProvider.example.orgUse the following command:

# ifconfig carp0 down && ifconfig carp0 up

This operation must be performed onCarpInterface.

Now you have completed CarpAnd you can start testing. During the test, the network of the two machines can be restarted or cut off at any time.

For more details, see carp (4) online manual.

CARP(4)                FreeBSD Kernel Interfaces Manual                CARP(4)

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 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 inter-
val.  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 con-
figurations 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 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.

FreeBSD 6.2                      June 6, 2006                      FreeBSD 6.2