1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
|
.\" $OpenBSD: carp.4,v 1.16 2004/12/07 23:41:35 jmc Exp $
.\"
.\" Copyright (c) 2003, Ryan McBride. All rights reserved.
.\" Copyright (c) 2011, Gleb Smirnoff <glebius@FreeBSD.org>
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE PROJECT AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE PROJECT OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\" $FreeBSD$
.\"
.Dd January 26, 2012
.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.
When enabled, a vhid in a backup state would preempt a master that
is announcing itself with a lower advskew.
Disabled by default.
.It Va net.inet.carp.log
Value of 0 disables any logging.
Value of 1 enables logging state changes of
.Nm
vhids.
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 occurred.
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 addresses running
.Nm
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
When one of the physical interfaces of host A fails,
.Cm advskew
is demoted to a configured value on all its
.Nm
vhids.
Due to the preempt option, host B would start announcing itself, and thus
preempt host A 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.
|