324 lines
11 KiB
Groff
324 lines
11 KiB
Groff
.\" $OpenBSD: carp.4,v 1.16 2004/12/07 23:41:35 jmc Exp $
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.\"
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.\" Copyright (c) 2003, Ryan McBride. All rights reserved.
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.\" Copyright (c) 2011, Gleb Smirnoff <glebius@FreeBSD.org>
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE PROJECT AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE PROJECT OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd February 21, 2013
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.Dt CARP 4
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.Os
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.Sh NAME
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.Nm carp
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.Nd Common Address Redundancy Protocol
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.Sh SYNOPSIS
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.Cd "device carp"
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.Sh DESCRIPTION
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The CARP allows multiple hosts on the same local network to share a set of
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IPv4 and/or IPv6 addresses.
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Its primary purpose is to ensure that these
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addresses are always available.
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.Pp
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To use
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.Nm ,
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the administrator needs to configure at a minimum a common virtual host ID
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(vhid), and attach at least one IP address to this vhid on each machine which
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is to take part in the virtual group.
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Additional parameters can also be set on a per-vhid basis:
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.Cm advbase
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and
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.Cm advskew ,
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which are used to control how frequently the host sends advertisements when it
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is the master for a virtual host, and
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.Cm pass
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which is used to authenticate
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.Nm
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advertisements.
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The
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.Cm advbase
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parameter stands for
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.Dq "advertisement base" .
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It is measured in seconds and specifies the base of the advertisement interval.
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The
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.Cm advskew
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parameter stands for
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.Dq "advertisement skew" .
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It is measured in 1/256 of seconds.
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It is added to the base advertisement interval to make one host advertise
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a bit slower that the other does.
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Both
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.Cm advbase
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and
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.Cm advskew
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are put inside CARP advertisements.
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These values can be configured using
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.Xr ifconfig 8 ,
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or through the
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.Dv SIOCSVH
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.Xr ioctl 2 .
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.Pp
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CARP virtual hosts can be configured on multicast-capable interfaces: Ethernet,
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layer 2 VLAN, FDDI and Token Ring.
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An arbitrary number of virtual host IDs can be configured on an interface.
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An arbitrary number of IPv4 or IPv6 addresses can be attached to a particular
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vhid.
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It is important that all hosts participating in a vhid have the same list
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of prefixes configured on the vhid, since all prefixes are included in the
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cryptographic checksum supplied in each advertisement.
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Multiple vhids running on one interface participate in master/backup
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elections independently.
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.Pp
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Additionally, there are a number of global parameters which can be set using
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.Xr sysctl 8 :
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.Bl -tag -width ".Va net.inet.carp.ifdown_demotion_factor"
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.It Va net.inet.carp.allow
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Accept incoming
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.Nm
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packets.
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Enabled by default.
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.It Va net.inet.carp.preempt
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Allow virtual hosts to preempt each other.
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When enabled, a vhid in a backup state would preempt a master that
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is announcing itself with a lower advskew.
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Disabled by default.
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.It Va net.inet.carp.log
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Determines what events relating to
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.Nm
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vhids are logged.
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A value of 0 disables any logging.
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A value of 1 enables logging state changes of
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.Nm
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vhids.
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Values above 1 enable logging of bad
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.Nm
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packets.
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The default value is 1.
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.It Va net.inet.carp.demotion
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This value shows current level of CARP demotion.
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The value is added to the actual advskew sent in announcements for
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all vhids.
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At normal system operation the demotion factor is zero.
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However, problematic conditions raise its level: when
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.Nm
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experiences problem with sending announcements, when an interface
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running a vhid goes down, or while the
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.Xr pfsync 4
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interface is not synchronized.
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The demotion factor can be adjusted writing to the sysctl oid.
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The signed value supplied to the
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.Xr sysctl 8
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command is added to current demotion factor.
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This allows to control
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.Nm
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behaviour depending on some external conditions, for example on the status
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of some daemon utility.
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.It Va net.inet.carp.ifdown_demotion_factor
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This value is added to
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.Va net.inet.carp.demotion
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when an interface running a vhid goes down.
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The default value is 240 (the maximum advskew value).
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.It Va net.inet.carp.senderr_demotion_factor
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This value is added to
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.Va net.inet.carp.demotion
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when
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.Nm
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experiences errors sending its announcements.
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The default value is 240 (the maximum advskew value).
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.El
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.\".Sh ARP level load balancing
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.\"A
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.\".Nm
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.\"interface has limited abilities for load balancing incoming connections
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.\"between hosts in an Ethernet network.
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.\"For load-balancing operation, one needs several CARP interfaces that
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.\"are configured to the same IP address, but to a different vhids.
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.\"Once an ARP request is received, the CARP protocol will use a hashing
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.\"function against the source IP address in the ARP request to determine
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.\"which vhid the request will be assigned to.
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.\"If the corresponding CARP interface is the current
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.\"master interface, a reply will
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.\"be sent to the ARP request;
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.\"otherwise it will be ignored.
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.\"See the
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.\".Sx EXAMPLES
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.\"section for a practical example of load balancing.
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.\".Pp
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.\"The ARP load balancing implemented in
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.\".Nm
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.\"has some limitations.
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.\"First, ARP balancing only works on the local network segment.
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.\"It cannot balance traffic that crosses a router, because the
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.\"router itself will always be balanced to the same virtual host.
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.\"Second, ARP load balancing can lead to asymmetric routing
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.\"of incoming and outgoing traffic, and thus combining it with
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.\".Xr pfsync 4
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.\"is dangerous, because this creates a race condition between
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.\"balanced routers and a host they are serving.
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.\"Imagine an incoming packet creating state on the first router, being
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.\"forwarded to its destination, and the destination replying faster
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.\"than the state information is packed and synced with the second router.
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.\"If the reply would be load balanced to second router, it will be
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.\"dropped since the second router has not yet received information about
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.\"the connection state.
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.Sh STATE CHANGE NOTIFICATIONS
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Sometimes it is useful to get notified about
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.Nm
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status change events.
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This can be accomplished by using
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.Xr devd 8
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hooks.
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Master/slave events are signalled under system
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.Dv CARP .
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The subsystem specifies the vhid and name of the interface where
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the master/slave event occurred.
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The type of the message displays the new state of the vhid.
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Please see
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.Xr devd.conf 5
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and the
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.Sx EXAMPLES
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section for more information.
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.Sh EXAMPLES
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For firewalls and routers with multiple interfaces, it is desirable to
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failover all of the addresses running
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.Nm
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together, when one of the physical interfaces goes down.
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This is achieved by the use of the preempt option.
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Enable it on both hosts A and B:
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.Pp
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.Dl sysctl net.inet.carp.preempt=1
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.Pp
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Assume that host A is the preferred master and we are running the
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192.168.1.0/24 prefix on em0 and 192.168.2.0/24 on em1.
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This is the setup for host A (advskew is above 0 so it could be overwritten
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in the emergency situation from the other host):
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.Bd -literal -offset indent
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ifconfig em0 vhid 1 advskew 100 pass mekmitasdigoat 192.168.1.1/24
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ifconfig em1 vhid 2 advskew 100 pass mekmitasdigoat 192.168.2.1/24
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.Ed
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.Pp
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The setup for host B is identical, but it has a higher
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.Cm advskew :
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.Bd -literal -offset indent
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ifconfig em0 vhid 1 advskew 200 pass mekmitasdigoat 192.168.1.1/24
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ifconfig em1 vhid 2 advskew 200 pass mekmitasdigoat 192.168.2.1/24
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.Ed
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.Pp
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When one of the physical interfaces of host A fails,
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.Cm advskew
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is demoted to a configured value on all its
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.Nm
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vhids.
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Due to the preempt option, host B would start announcing itself, and thus
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preempt host A on both interfaces instead of just the failed one.
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.\".Pp
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.\"In order to set up an ARP balanced virtual host, it is necessary to configure
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.\"one virtual host for each physical host which would respond to ARP requests
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.\"and thus handle the traffic.
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.\"In the following example, two virtual hosts are configured on two hosts to
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.\"provide balancing and failover for the IP address 192.168.1.10.
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.\".Pp
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.\"First the
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.\".Nm
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.\"interfaces on host A are configured.
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.\"The
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.\".Cm advskew
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.\"of 100 on the second virtual host means that its advertisements will be sent
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.\"out slightly less frequently.
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.\".Bd -literal -offset indent
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.\"ifconfig carp0 create
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.\"ifconfig carp0 vhid 1 advskew 100 pass mekmitasdigoat 192.168.1.10/24
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.\"ifconfig carp1 create
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.\"ifconfig carp1 vhid 2 advskew 200 pass mekmitasdigoat 192.168.1.10/24
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.\".Ed
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.\".Pp
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.\"The configuration for host B is identical, except the
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.\".Cm advskew
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.\"is on virtual host 1 rather than virtual host 2.
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.\".Bd -literal -offset indent
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.\"ifconfig carp0 create
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.\"ifconfig carp0 vhid 1 advskew 200 pass mekmitasdigoat 192.168.1.10/24
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.\"ifconfig carp1 create
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.\"ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat 192.168.1.10/24
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.\".Ed
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.\".Pp
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.\"Finally, the ARP balancing feature must be enabled on both hosts:
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.\".Pp
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.\".Dl sysctl net.inet.carp.arpbalance=1
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.\".Pp
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.\"When the hosts receive an ARP request for 192.168.1.10, the source IP address
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.\"of the request is used to compute which virtual host should answer the request.
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.\"The host which is master of the selected virtual host will reply to the
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.\"request, the other(s) will ignore it.
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.\".Pp
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.\"This way, locally connected systems will receive different ARP replies and
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.\"subsequent IP traffic will be balanced among the hosts.
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.\"If one of the hosts fails, the other will take over the virtual MAC address,
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.\"and begin answering ARP requests on its behalf.
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.Pp
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Processing of
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.Nm
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status change events can be set up by using the following devd.conf rule:
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.Bd -literal -offset indent
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notify 0 {
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match "system" "CARP";
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match "subsystem" "[0-9]+@[0-9a-z]+";
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match "type" "(MASTER|BACKUP)";
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action "/root/carpcontrol.sh $subsystem $type";
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};
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.Ed
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.Pp
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To see
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.Nm
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packets decoded in
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.Xr tcpdump 8
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output, one needs to specify
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.Fl T Ar carp
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option, otherwise
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.Xr tcpdump 8
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tries to interpret them as VRRP packets:
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.Bd -literal -offset indent
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tcpdump -npi vlan0 -T carp
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.Ed
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.Sh SEE ALSO
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.Xr inet 4 ,
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.Xr pfsync 4 ,
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.Xr devd.conf 5 ,
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.Xr rc.conf 5 ,
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.Xr ifconfig 8 ,
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.Xr sysctl 8 ,
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.Xr tcpdump 8
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.Sh HISTORY
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The
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.Nm
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device first appeared in
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.Ox 3.5 .
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The
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.Nm
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device was imported into
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.Fx 5.4 .
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In
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.Fx 10.0 ,
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.Nm
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was significantly rewritten, and is no longer a pseudo-interface.
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