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