1.\" $OpenBSD: carp.4,v 1.16 2004/12/07 23:41:35 jmc Exp $ 2.\" 3.\" Copyright (c) 2003, Ryan McBride. All rights reserved. 4.\" 5.\" Redistribution and use in source and binary forms, with or without 6.\" modification, are permitted provided that the following conditions 7.\" are met: 8.\" 1. Redistributions of source code must retain the above copyright 9.\" notice, this list of conditions and the following disclaimer. 10.\" 2. Redistributions in binary form must reproduce the above copyright 11.\" notice, this list of conditions and the following disclaimer in the 12.\" documentation and/or other materials provided with the distribution. 13.\" 14.\" THIS SOFTWARE IS PROVIDED BY THE PROJECT AND CONTRIBUTORS ``AS IS'' AND 15.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 16.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 17.\" ARE DISCLAIMED. IN NO EVENT SHALL THE PROJECT OR CONTRIBUTORS BE LIABLE 18.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 19.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 20.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 21.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 22.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 23.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 24.\" SUCH DAMAGE. 25.\" 26.\" $FreeBSD$ 27.\" 28.Dd May 15, 2005 29.Dt CARP 4 30.Os 31.Sh NAME 32.Nm carp 33.Nd Common Address Redundancy Protocol 34.Sh SYNOPSIS 35.Cd "device carp" 36.Sh DESCRIPTION 37The 38.Nm 39interface is a pseudo-device that implements and controls the 40CARP protocol. 41CARP allows multiple hosts on the same local network to share a set of IP addresses. 42Its primary purpose is to ensure that these 43addresses are always available, but in some configurations 44.Nm 45can also provide load balancing functionality. 46.Pp 47A 48.Nm 49interface can be created at runtime using the 50.Nm ifconfig Li carp Ns Ar N Cm create 51command or by configuring 52it via 53.Va cloned_interfaces 54in the 55.Pa /etc/rc.conf 56file. 57.Pp 58To use 59.Nm , 60the administrator needs to configure at minimum a common virtual host ID and 61virtual host IP address on each machine which is to take part in the virtual 62group. 63Additional parameters can also be set on a per-interface basis: 64.Cm advbase 65and 66.Cm advskew , 67which are used to control how frequently the host sends advertisements when it 68is the master for a virtual host, and 69.Cm pass 70which is used to authenticate 71.Nm 72advertisements. 73The 74.Cm advbase 75parameter stands for 76.Dq "advertisement base" . 77It is measured in seconds and specifies the base of the advertisement interval. 78The 79.Cm advskew 80parameter stands for 81.Dq "advertisement skew" . 82It is measured in 1/256 of seconds. 83It is added to the base advertisement interval to make one host advertise 84a bit slower that the other does. 85Both 86.Cm advbase 87and 88.Cm advskew 89are put inside CARP advertisements. 90These configurations can be done using 91.Xr ifconfig 8 , 92or through the 93.Dv SIOCSVH 94.Xr ioctl 2 . 95.Pp 96Additionally, there are a number of global parameters which can be set using 97.Xr sysctl 8 : 98.Bl -tag -width ".Va net.inet.carp.arpbalance" 99.It Va net.inet.carp.allow 100Accept incoming 101.Nm 102packets. 103Enabled by default. 104.It Va net.inet.carp.preempt 105Allow virtual hosts to preempt each other. 106It is also used to failover 107.Nm 108interfaces as a group. 109When the option is enabled and one of the 110.Nm 111enabled physical interfaces 112goes down, 113.Cm advskew 114is changed to 240 on all 115.Nm 116interfaces. 117See also the first example. 118Disabled by default. 119.It Va net.inet.carp.log 120Value of 0 disables any logging. 121Value of 1 enables logging of bad 122.Nm 123packets. 124Values above 1 enable logging state changes of 125.Nm 126interfaces. 127Default value is 1. 128.It Va net.inet.carp.arpbalance 129Balance local traffic using ARP. 130Disabled by default. 131.It Va net.inet.carp.suppress_preempt 132A read only value showing the status of preemption suppression. 133Preemption can be suppressed if link on an interface is down 134or when 135.Xr pfsync 4 136interface is not synchronized. 137Value of 0 means that preemption is not suppressed, since no 138problems are detected. 139Every problem increments suppression counter. 140.El 141.Sh EXAMPLES 142For firewalls and routers with multiple interfaces, it is desirable to 143failover all of the 144.Nm 145interfaces together, when one of the physical interfaces goes down. 146This is achieved by the preempt option. 147Enable it on both host A and B: 148.Pp 149.Dl sysctl net.inet.carp.preempt=1 150.Pp 151Assume that host A is the preferred master and 192.168.1.x/24 is 152configured on one physical interface and 192.168.2.y/24 on another. 153This is the setup for host A: 154.Bd -literal -offset indent 155ifconfig carp0 create 156ifconfig carp0 vhid 1 pass mekmitasdigoat 192.168.1.1/24 157ifconfig carp1 create 158ifconfig carp1 vhid 2 pass mekmitasdigoat 192.168.2.1/24 159.Ed 160.Pp 161The setup for host B is identical, but it has a higher 162.Cm advskew : 163.Bd -literal -offset indent 164ifconfig carp0 create 165ifconfig carp0 vhid 1 advskew 100 pass mekmitasdigoat 192.168.1.1/24 166ifconfig carp1 create 167ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat 192.168.2.1/24 168.Ed 169.Pp 170Because of the preempt option, when one of the physical interfaces of 171host A fails, 172.Cm advskew 173is adjusted to 240 on all its 174.Nm 175interfaces. 176This will cause host B to preempt on both interfaces instead of 177just the failed one. 178.Pp 179In order to set up an ARP balanced virtual host, it is necessary to configure 180one virtual host for each physical host which would respond to ARP requests 181and thus handle the traffic. 182In the following example, two virtual hosts are configured on two hosts to 183provide balancing and failover for the IP address 192.168.1.10. 184.Pp 185First the 186.Nm 187interfaces on host A are configured. 188The 189.Cm advskew 190of 100 on the second virtual host means that its advertisements will be sent 191out slightly less frequently. 192.Bd -literal -offset indent 193ifconfig carp0 create 194ifconfig carp0 vhid 1 pass mekmitasdigoat 192.168.1.10/24 195ifconfig carp1 create 196ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat 192.168.1.10/24 197.Ed 198.Pp 199The configuration for host B is identical, except the 200.Cm advskew 201is on virtual host 1 rather than virtual host 2. 202.Bd -literal -offset indent 203ifconfig carp0 create 204ifconfig carp0 vhid 1 advskew 100 pass mekmitasdigoat 192.168.1.10/24 205ifconfig carp1 create 206ifconfig carp1 vhid 2 pass mekmitasdigoat 192.168.1.10/24 207.Ed 208.Pp 209Finally, the ARP balancing feature must be enabled on both hosts: 210.Pp 211.Dl sysctl net.inet.carp.arpbalance=1 212.Pp 213When the hosts receive an ARP request for 192.168.1.10, the source IP address 214of the request is used to compute which virtual host should answer the request. 215The host which is master of the selected virtual host will reply to the 216request, the other(s) will ignore it. 217.Pp 218This way, locally connected systems will receive different ARP replies and 219subsequent IP traffic will be balanced among the hosts. 220If one of the hosts fails, the other will take over the virtual MAC address, 221and begin answering ARP requests on its behalf. 222.Pp 223Note: ARP balancing only works on the local network segment. 224It cannot balance traffic that crosses a router, because the router 225itself will always be balanced to the same virtual host. 226.Sh SEE ALSO 227.Xr inet 4 , 228.Xr pfsync 4 , 229.Xr rc.conf 5 , 230.Xr ifconfig 8 , 231.Xr sysctl 8 232.Sh HISTORY 233The 234.Nm 235device first appeared in 236.Ox 3.5 . 237The 238.Nm 239device was imported into 240.Fx 5.4 . 241