path: root/share/man/man4/carp.4

.\"	$OpenBSD: carp.4,v 1.16 2004/12/07 23:41:35 jmc Exp $
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.\" Copyright (c) 2003, Ryan McBride.  All rights reserved.
.\" Copyright (c) 2011, Gleb Smirnoff <glebius@FreeBSD.org>
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.Dd December 20, 2011
.Dt CARP 4
.Os
.Sh NAME
.Nm carp
.Nd Common Address Redundancy Protocol
.Sh SYNOPSIS
.Cd "device carp"
.Sh 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.
.Pp
To use
.Nm ,
the administrator needs to configure at 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:
.Cm advbase
and
.Cm advskew ,
which are used to control how frequently the host sends advertisements when it
is the master for a virtual host, and
.Cm pass
which is used to authenticate
.Nm
advertisements.
The
.Cm advbase
parameter stands for
.Dq "advertisement base" .
It is measured in seconds and specifies the base of the advertisement interval.
The
.Cm advskew
parameter stands for
.Dq "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
.Cm advbase
and
.Cm advskew
are put inside CARP advertisements.
These configurations can be done using
.Xr ifconfig 8 ,
or through the
.Dv SIOCSVH
.Xr ioctl 2 .
.Pp
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 prefixes are included in the
cryptographic checksum supplied in each advertisement.
Multiple vhids running on one interface participate in master/backup
elections independently.
.Pp
Additionally, there are a number of global parameters which can be set using
.Xr sysctl 8 :
.Bl -tag -width ".Va net.inet.carp.preempt"
.It Va net.inet.carp.allow
Accept incoming
.Nm
packets.
Enabled by default.
.It Va net.inet.carp.preempt
Allow virtual hosts to preempt each other.
It is also used to failover
.Nm
interfaces as a group.
When the option is enabled and one of the
.Nm
enabled physical interfaces
goes down,
.Cm advskew
is changed to 240 on all
.Nm
interfaces.
See also the first example.
Disabled by default.
.It Va net.inet.carp.log
Value of 0 disables any logging.
Value of 1 enables logging state changes of
.Nm
interfaces.
Values above 1 enable logging of bad
.Nm
packets.
Default value is 1.
.It Va net.inet.carp.demotion
This value shows current level of CARP demotion.
The value is added to the actual advskew sent in announcements for
all vhids.
At normal system operation the demotion factor is zero.
However, problematic conditions raise its level: when
.Nm
experiences problem with sending announcements, when an interface
running a vhid goes down, or while the
.Xr pfsync 4
interface is not synchronized.
The demotion value is writable, so that user may alter it
depending on some external conditions, for example on status of some
daemon utility.
However, altering the value should be performed with care, do
not conflict with subsystems that adjust demotion factor
automatically:
.Nm
and
.Xr pfsync 4 .
.It Va net.inet.carp.ifdown_demotion_factor
Value added to
.Va net.inet.carp.demotion
when interface running a vhid goes down.
Default value is 240 (maximum advskew value).
.It Va net.inet.carp.senderr_demotion_factor
Value added to
.Va net.inet.carp.demotion
when
.Nm
experiences errors sending its announcements.
Default value is 240 (maximum advskew value).
.El
.\".Sh ARP level load balancing
.\"The
.\".Nm
.\"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
.\".Sx EXAMPLES
.\"section for a practical example of load balancing.
.\".Pp
.\"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
.\".Xr 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.
.Sh STATE CHANGE NOTIFICATIONS
Sometimes it is useful to get notified about
.Nm
status change events.
This can be accomplished by using
.Xr devd 8
hooks.
Master/slave events are signalled under system
.Dv CARP .
Subsystem specifies vhid and name of interface, where event occured.
Type of the message displays new state of vhid.
Please see
.Xr devd.conf 5
and 
.Sx EXAMPLES
section for more information.
.Sh EXAMPLES
For firewalls and routers with multiple interfaces, it is desirable to
failover all of the
.Nm
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:
.Pp
.Dl sysctl net.inet.carp.preempt=1
.Pp
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:
.Bd -literal -offset indent
ifconfig em0 vhid 1 pass mekmitasdigoat 192.168.1.1/24
ifconfig em1 vhid 2 pass mekmitasdigoat 192.168.2.1/24
.Ed
.Pp
The setup for host B is identical, but it has a higher
.Cm advskew :
.Bd -literal -offset indent
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
.Ed
.Pp
Because of the preempt option, when one of the physical interfaces of
host A fails,
.Cm advskew
is adjusted to 240 on all its
.Nm
interfaces.
This will cause host B to preempt on both interfaces instead of
just the failed one.
.\".Pp
.\"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.
.\".Pp
.\"First the
.\".Nm
.\"interfaces on host A are configured.
.\"The
.\".Cm advskew
.\"of 100 on the second virtual host means that its advertisements will be sent
.\"out slightly less frequently.
.\".Bd -literal -offset indent
.\"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
.\".Ed
.\".Pp
.\"The configuration for host B is identical, except the
.\".Cm advskew
.\"is on virtual host 1 rather than virtual host 2.
.\".Bd -literal -offset indent
.\"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
.\".Ed
.\".Pp
.\"Finally, the ARP balancing feature must be enabled on both hosts:
.\".Pp
.\".Dl sysctl net.inet.carp.arpbalance=1
.\".Pp
.\"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.
.\".Pp
.\"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.
.Pp
Processing of
.Nm
status change events can be set up by using the following devd.conf rule:
.Bd -literal -offset indent
notify 0 {
	match "system"          "CARP";
	match "subsystem"       "[0-9]+@";
	match "type"            "(MASTER|BACKUP)";
	action "/root/carpcontrol.sh $subsystem $type";
};
.Ed
.Sh SEE ALSO
.Xr inet 4 ,
.Xr pfsync 4 ,
.Xr rc.conf 5 ,
.Xr devd.conf 5 ,
.Xr ifconfig 8 ,
.Xr sysctl 8
.Sh HISTORY
The
.Nm
device first appeared in
.Ox 3.5 .
The
.Nm
device was imported into
.Fx 5.4 .
In
.Fx 10
the
.Nm
was significantly rewritten, and is no longer a pseudo-interface.