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33 .Nd Common Address Redundancy Protocol
39 interface is a pseudo-device that implements and controls the
41 CARP allows multiple hosts on the same local network to share a set of IP addresses.
42 Its primary purpose is to ensure that these
43 addresses are always available, but in some configurations
45 can also provide load balancing functionality.
49 interface can be created at runtime using the
50 .Nm ifconfig Li carp Ns Ar N Cm create
51 command or by configuring
60 the administrator needs to configure at minimum a common virtual host ID and
61 virtual host IP address on each machine which is to take part in the virtual
63 Additional parameters can also be set on a per-interface basis:
67 which are used to control how frequently the host sends advertisements when it
68 is the master for a virtual host, and
70 which is used to authenticate
76 .Qq advertisement base .
77 It is measured in seconds and specifies the base of the adverisement interval.
81 .Qq advertisement skew .
82 It is measured in 1/256 of seconds.
83 It is added to the base advertisement interval to make one host advertise
84 a bit slower that the other does.
89 are put inside CARP advertisments.
90 These configurations can be done using
96 Additionally, there are a number of global parameters which can be set using
98 .Bl -tag -width ".Va net.inet.carp.arpbalance"
99 .It Va net.inet.carp.allow
104 .It Va net.inet.carp.preempt
105 Allow virtual hosts to preempt each other.
106 It is also used to failover
108 interfaces as a group.
109 When the option is enabled and one of the
111 enabled physical interfaces
114 is changed to 240 on all
117 See also the first example.
119 .It Va net.inet.carp.log
120 Value of 0 disables any logging.
121 Value of 1 enables logging of bad
124 Values above 1 enable logging state changes of
128 .It Va net.inet.carp.arpbalance
129 Balance local traffic using ARP.
131 .It Va net.inet.carp.suppress_preempt
132 A read only value showing the status of preemption suppression.
133 Preemption can be suppressed if link on an interface is down
136 interface is not synchronized.
137 Value of 0 means that preemption is not suppressed, since no
138 problems are detected.
139 Every problem increments suppression counter.
142 For firewalls and routers with multiple interfaces, it is desirable to
145 interfaces together, when one of the physical interfaces goes down.
146 This is achieved by the preempt option.
147 Enable it on both host A and B:
149 .Dl sysctl net.inet.carp.preempt=1
151 Assume that host A is the preferred master and 192.168.1.x/24 is
152 configured on one physical interface and 192.168.2.y/24 on another.
153 This is the setup for host A:
154 .Bd -literal -offset indent
155 ifconfig carp0 create
156 ifconfig carp0 vhid 1 pass mekmitasdigoat 192.168.1.1/24
157 ifconfig carp1 create
158 ifconfig carp1 vhid 2 pass mekmitasdigoat 192.168.2.1/24
161 The setup for host B is identical, but it has a higher
163 .Bd -literal -offset indent
164 ifconfig carp0 create
165 ifconfig carp0 vhid 1 advskew 100 pass mekmitasdigoat 192.168.1.1/24
166 ifconfig carp1 create
167 ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat 192.168.2.1/24
170 Because of the preempt option, when one of the physical interfaces of
173 is adjusted to 240 on all its
176 This will cause host B to preempt on both interfaces instead of
179 In order to set up an ARP balanced virtual host, it is necessary to configure
180 one virtual host for each physical host which would respond to ARP requests
181 and thus handle the traffic.
182 In the following example, two virtual hosts are configured on two hosts to
183 provide balancing and failover for the IP address 192.168.1.10.
187 interfaces on host A are configured.
190 of 100 on the second virtual host means that its advertisements will be sent
191 out slightly less frequently.
192 .Bd -literal -offset indent
193 ifconfig carp0 create
194 ifconfig carp0 vhid 1 pass mekmitasdigoat 192.168.1.10/24
195 ifconfig carp1 create
196 ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat 192.168.1.10/24
199 The configuration for host B is identical, except the
201 is on virtual host 1 rather than virtual host 2.
202 .Bd -literal -offset indent
203 ifconfig carp0 create
204 ifconfig carp0 vhid 1 advskew 100 pass mekmitasdigoat 192.168.1.10/24
205 ifconfig carp1 create
206 ifconfig carp1 vhid 2 pass mekmitasdigoat 192.168.1.10/24
209 Finally, the ARP balancing feature must be enabled on both hosts:
211 .Dl sysctl net.inet.carp.arpbalance=1
213 When the hosts receive an ARP request for 192.168.1.10, the source IP address
214 of the request is used to compute which virtual host should answer the request.
215 The host which is master of the selected virtual host will reply to the
216 request, the other(s) will ignore it.
218 This way, locally connected systems will receive different ARP replies and
219 subsequent IP traffic will be balanced among the hosts.
220 If one of the hosts fails, the other will take over the virtual MAC address,
221 and begin answering ARP requests on its behalf.
223 Note: ARP balancing only works on the local network segment.
224 It cannot balance traffic that crosses a router, because the router
225 itself will always be balanced to the same virtual host.
235 device first appeared in
239 device was imported into