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

     carp -- Common Address Redundancy Protocol

     device carp

     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 au-
     thenticate	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 defaults to using multicast messages,	but can	be configured to uni-
     cast announcements	to peers using the peer	and peer6 parameters. Default
     addresses can be restored using mcast and mcast6.	Note that TTL verifi-
     cation is disabled	if the peer address is not a multicast address.	 These
     values can	be configured using ifconfig(8), or through the	SIOCSPEER

     CARP virtual hosts	can be configured on multicast-capable interfaces:
     Ethernet, layer 2 VLAN, FDDI and Token Ring.  An arbitrary	number of vir-
     tual 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 im-
     portant that all hosts participating in a vhid have the same list of pre-
     fixes 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 indepen-

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

     net.inet.carp.allow		   Allow carp operation.  When dis-
					   abled, virtual hosts	remain in ini-
					   tial	state, neither sending nor re-
					   ceiving 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 de-

     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.	 Dur-
					   ing normal system operation the de-
					   motion factor is zero.  However,
					   problematic conditions raise	its
					   level: when carp experiences	prob-
					   lem 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 cur-
					   rent	demotion factor.  This allows
					   to control carp behaviour depending
					   on some external conditions,	for
					   example on the status of some dae-
					   mon utility.

     net.inet.carp.ifdown_demotion_factor  This	value is added to
					   net.inet.carp.demotion when an in-
					   terface running a vhid goes down.
					   The default value is	240 (the maxi-
					   mum advskew value).

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

     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.

     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	pre-
     empt 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 prefix on em0 and 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
	   ifconfig em1	vhid 2 advskew 100 pass	mekmitasdigoat

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

	   ifconfig em0	vhid 1 advskew 200 pass	mekmitasdigoat
	   ifconfig em1	vhid 2 advskew 200 pass	mekmitasdigoat

     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 in-
     terfaces instead of just the failed one.

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

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

	   tcpdump -npi	vlan0 -T carp

     tcpdump(1), inet(4), pfsync(4), devd.conf(5), rc.conf(5), ifconfig(8),

     The carp device first appeared in OpenBSD 3.5.  The carp device was im-
     ported into FreeBSD 5.4.  In FreeBSD 10.0,	carp was significantly rewrit-
     ten, and is no longer a pseudo-interface.

FreeBSD	13.0			 March 6, 2023			  FreeBSD 13.0


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