210 lines
6.7 KiB
Groff
210 lines
6.7 KiB
Groff
.\" $NetBSD: carp.4,v 1.4 2006/12/23 06:58:20 wiz Exp $
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.\" $OpenBSD: carp.4,v 1.19 2005/08/09 09:52:12 jmc Exp $
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.\"
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.\" Copyright (c) 2003, Ryan McBride. All rights reserved.
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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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.Dd October 16, 2003
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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 "pseudo-device carp" Op Ar count
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.Sh DESCRIPTION
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The
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.Nm
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interface is a pseudo-device which implements and controls the
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CARP protocol.
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.Nm
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allows multiple hosts on the same local network to share a set of IP addresses.
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Its primary purpose is to ensure that these
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addresses are always available, but in some configurations
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.Nm
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can also provide load balancing functionality.
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.Pp
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A
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.Nm
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interface can be created at runtime using the
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.Ic ifconfig carp Ns Ar N Ic create
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command.
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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 minimum a common virtual host ID and
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virtual host IP address on each machine which is to take part in the virtual
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group.
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Additional parameters can also be set on a per-interface 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 carp advertisements.
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Finally
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.Cm carpdev
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is used to specify which interface the
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.Nm
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device attaches to.
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If unspecified, the kernel attempts to set carpdev by looking for
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another interface with the same subnet.
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These configurations can be done 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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ioctl.
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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 xxxxxxxxxxxxxxxxxxxxxxxxxx
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.It 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 net.inet.carp.preempt
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Allow virtual hosts to preempt each other.
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It is also used to failover
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.Nm
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interfaces as a group.
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When the option is enabled and one of the
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.Nm
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enabled physical interfaces
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goes down, advskew is changed to 240 on all
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.Nm
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interfaces.
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See also the first example.
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Disabled by default.
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.It net.inet.carp.log
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Log bad
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.Nm
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packets.
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Disabled by default.
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.It net.inet.carp.arpbalance
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Balance local traffic using ARP.
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Disabled by default.
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.El
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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
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.Nm
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interfaces together, when one of the physical interfaces goes down.
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This is achieved by the preempt option.
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Enable it on both host A and B:
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.Pp
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.Dl # sysctl -w net.inet.carp.preempt=1
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.Pp
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Assume that host A is the preferred master and 192.168.1.x/24 is
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configured on one physical interface and 192.168.2.y/24 on another.
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This is the setup for host A:
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.Bd -literal -offset indent
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# ifconfig carp0 create
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# ifconfig carp0 vhid 1 pass mekmitasdigoat 192.168.1.1 \e
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netmask 255.255.255.0
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# ifconfig carp1 create
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# ifconfig carp1 vhid 2 pass mekmitasdigoat 192.168.2.1/24 \e
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netmask 255.255.255.0
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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 advskew:
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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 \e
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192.168.1.1 netmask 255.255.255.0
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# ifconfig carp1 create
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# ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat \e
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192.168.2.1 netmask 255.255.255.0
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.Ed
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.Pp
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Because of the preempt option, when one of the physical interfaces of
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host A fails, advskew is adjusted to 240 on all its
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.Nm
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interfaces.
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This will cause host B to preempt on both interfaces instead of
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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 pass mekmitasdigoat 192.168.1.10 \e
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netmask 255.255.255.0
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# ifconfig carp1 create
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# ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat \e
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192.168.1.10 netmask 255.255.255.0
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.Ed
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.Pp
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The configuration for host B is identical, except the skew is on
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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 100 pass mekmitasdigoat \e
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192.168.1.10 netmask 255.255.255.0
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# ifconfig carp1 create
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# ifconfig carp1 vhid 2 pass mekmitasdigoat 192.168.1.10 \e
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netmask 255.255.255.0
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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 -w 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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Note: ARP balancing only works on the local network segment.
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It cannot balance traffic that crosses a router, because the router
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itself will always be balanced to the same virtual host.
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.Sh SEE ALSO
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.Xr netstat 1 ,
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.Xr sysctl 3 ,
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.Xr arp 4 ,
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.Xr arp 8 ,
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.Xr ifconfig 8 ,
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.Xr sysctl 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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