Ubuntu Manpage: carp — Common Address Redundancy Protocol

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NAME

     carp — Common Address Redundancy Protocol

SYNOPSIS

     device carp

DESCRIPTION

The CARP allows multiple hosts on the same local network to share a set of IPv4 and/or IPv6
addresses. Its primary purpose is to ensure that these addresses are always available.

To use carp, the administrator needs to configure at a minimum a common virtual host ID
(vhid), and attach at least one IP address to this vhid on each machine which is to take
part in the virtual group. Additional parameters can also be set on a per-vhid 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 values can be configured
using ifconfig(8), or through the SIOCSVH ioctl(2).

CARP virtual hosts can be configured on multicast-capable interfaces: Ethernet, layer 2
VLAN, FDDI and Token Ring. An arbitrary number of virtual host IDs can be configured on an
interface. An arbitrary number of IPv4 or IPv6 addresses can be attached to a particular
vhid. It is important that all hosts participating in a vhid have the same list of prefixes
configured on the vhid, since all the prefixes are included in the cryptographic checksum
supplied in each advertisement. Multiple vhids running on one interface participate in
master/backup elections independently.

Additionally, there are a number of global parameters which can be set using sysctl(8):

net.inet.carp.allow Allow carp operation. When disabled, virtual hosts
remain in initial state, neither sending nor receiving
announcements or traffic. Enabled by default.

net.inet.carp.preempt Allow virtual hosts to preempt each other. When
enabled, a vhid in a backup state would preempt a
master that is announcing itself with a lower advskew.
Disabled by default.

net.inet.carp.dscp DSCP value in carp packet. Valid Values are 0 to 63.
A value of 4 is equivalent to the old standard of TOS
LOW_DELAY. TOS values were deprecated and replaced by
DSCP in 1998. The default value is 56 (CS7/Network
Control).

net.inet.carp.log Determines what events relating to carp vhids are
logged. A value of 0 disables any logging. A value
of 1 enables logging state changes of carp vhids.
Values above 1 enable logging of bad carp packets.
The default value is 1.

net.inet.carp.demotion This value shows the current level of CARP demotion.
The value is added to the actual advskew sent in
announcements for all vhids. During normal system
operation the demotion factor is zero. However,
problematic conditions raise its level: when carp
experiences problem with sending announcements, when
an interface running a vhid goes down, or while the
pfsync(4) interface is not synchronized. The demotion
factor can be adjusted writing to the sysctl oid. The
signed value supplied to the sysctl(8) command is
added to current demotion factor. This allows to
control carp behaviour depending on some external
conditions, for example on the status of some daemon
utility.

net.inet.carp.ifdown_demotion_factor This value is added to net.inet.carp.demotion when an
interface running a vhid goes down. The default value
is 240 (the maximum advskew value).

net.inet.carp.senderr_demotion_factor
This value is added to net.inet.carp.demotion when
carp experiences errors sending its announcements.
The default value is 240 (the maximum advskew value).

STATE CHANGE NOTIFICATIONS

     Sometimes it is useful to get notified about carp status change events.  This can be
     accomplished by using devd(8) hooks.  Master/slave events are signalled under system CARP.
     The subsystem specifies the vhid and name of the interface where the master/slave event
     occurred.  The type of the message displays the new state of the vhid.  Please see
     devd.conf(5) and the EXAMPLES section for more information.

EXAMPLES

     For firewalls and routers with multiple interfaces, it is desirable to failover all of the
     addresses running carp together, when one of the physical interfaces goes down.  This is
     achieved by the use of the preempt option.  Enable it on both hosts A and B:

           sysctl net.inet.carp.preempt=1

     Assume that host A is the preferred master and we are running the 192.168.1.0/24 prefix on
     em0 and 192.168.2.0/24 on em1.  This is the setup for host A (advskew is above 0 so it could
     be overwritten in the emergency situation from the other host):

           ifconfig em0 vhid 1 advskew 100 pass mekmitasdigoat 192.168.1.1/24
           ifconfig em1 vhid 2 advskew 100 pass mekmitasdigoat 192.168.2.1/24

     The setup for host B is identical, but it has a higher advskew:

           ifconfig em0 vhid 1 advskew 200 pass mekmitasdigoat 192.168.1.1/24
           ifconfig em1 vhid 2 advskew 200 pass mekmitasdigoat 192.168.2.1/24

     When one of the physical interfaces of host A fails, advskew is demoted to a configured
     value on all its carp vhids.  Due to the preempt option, host B would start announcing
     itself, and thus preempt host A on both interfaces instead of just the failed one.

     Processing of carp status change events can be set up by using the following devd.conf rule:

           notify 0 {
                   match "system"          "CARP";
                   match "subsystem"       "[0-9]+@[0-9a-z]+";
                   match "type"            "(MASTER|BACKUP)";
                   action "/root/carpcontrol.sh $subsystem $type";
           };

     To see carp packets decoded in tcpdump(1) output, one needs to specify the -T carp option,
     otherwise tcpdump(1) will interpret them as VRRP packets:

           tcpdump -npi vlan0 -T carp

HISTORY