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 **Contents**  This page was moved to:
   [The NetBSD Guide - Introduction to the Common Address Redundancy Protocol (CARP)](//www.NetBSD.org/docs/guide/en/chap-carp.html)
 [[!toc levels=3]]  
 # Introduction to the Common Address Redundancy Protocol (CARP)  
 See [[below|guide/caro#license]] for the license of this chapter.  
 CARP is the *Common Address Redundancy Protocol*. Its primary purpose is to  
 allow multiple hosts on the same network segment to share an IP address. CARP is  
 a secure, free alternative to the  
 [Virtual Router Redundancy Protocol](http://www.ietf.org/rfc/rfc3768.txt)  
 and the  
 [Hot Standby Router Protocol](http://www.ietf.org/rfc/rfc2281.txt).  
 CARP works by allowing a group of hosts on the same network segment to share an  
 IP address. This group of hosts is referred to as a *redundancy group*. The  
 redundancy group is assigned an IP address that is shared amongst the group  
 members. Within the group, one host is designated the *master* and the rest as  
 *backups*. The master host is the one that currently *holds* the shared IP; it  
 responds to any traffic or ARP requests directed towards it. Each host may  
 belong to more than one redundancy group at a time.  
 One common use for CARP is to create a group of redundant firewalls. The virtual  
 IP that is assigned to the redundancy group is configured on client machines as  
 the default gateway. In the event that the master firewall suffers a failure or  
 is taken offline, the IP will move to one of the backup firewalls and service  
 will continue unaffected.  
 While highly redundant and fault-tolerant hardware minimizes the need for CARP,  
 it doesn't erase it. There is no hardware fault tolerance that is capable of  
 helping if someone knocks out a power cord, or if your system administrator  
 types reboot in the wrong window. CARP also makes it easier to make the patch  
 and reboot cycle transparent to users, and easier to test a software or hardware  
 upgrade -- if it doesn't work, you can fall back to your spare until fixed.  
 There are, however, situations in which CARP won't help. CARP's design does  
 require that the members of a group be on the same physical subnet with a static  
 IP address, although with the introduction of the carpdev directive, there is no  
 more need for IP addresses on the physical interfaces. Similarly, services that  
 require a constant connection to the server (such as SSH or IRC) will not be  
 transparently transferred to the other system -- though in this case, CARP can  
 help with minimizing downtime. CARP by itself does not synchronize data between  
 applications, for example, manually duplicating data between boxes with rsync,  
 or whatever is appropriate for your application.  
 CARP supports both IPv4 and IPv6.  
 ## CARP Operation  
 The master host in the group sends regular advertisements to the local network  
 so that the backup hosts know it's still alive. If the backup hosts don't hear  
 an advertisement from the master for a set period of time, then one of them will  
 take over the duties of master (whichever backup host has the lowest configured  
 advbase and advskew values). It is possible for multiple CARP groups to exist on  
 the same network segment. CARP advertisements contain the Virtual Host ID which  
 allows group members to identify which redundancy group the advertisement  
 belongs to.  
 In order to prevent a malicious user on the network segment from spoofing CARP  
 advertisements, each group can be configured with a password. Each CARP packet  
 sent to the group is then protected by an SHA1 HMAC.  
 ## Configuring CARP  
 Each redundancy group is represented by a  
 [[!template id=man name="carp" section="4"]] virtual  
 network interface. As such, CARP is configured using  
 [[!template id=man name="ifconfig" section="8"]]  
 The follow options are available:  
  * `carpN` -- The name of the carp virtual interface where N is an integer that  
    represents the interface's number (e.g., carp 0).  
  * `vhid` -- The Virtual Host ID. This is a unique number that is used to  
    identify the redundancy group to other nodes on the network. Acceptable  
    values are from 1 to 255. This allows for multiple redundancy groups to exist  
    on the same network.  
  * `password` -- The authentication password to use when talking to other  
    CARP-enabled hosts in this redundancy group. This must be the same on all  
    members of the redundancy group.  
  * `carpdev` -- This optional parameter specifies the physical network interface  
    that belongs to this redundancy group. By default, CARP will try to determine  
    which interface to use by looking for a physical interface that is in the  
    same subnet as the `ipaddress` and `mask` combination given to the carp  
  * `advbase` -- This optional parameter specifies how often, in seconds, to  
    advertise that we're a member of the redundancy group. The default is 1  
    second. Acceptable values are from 1 to 255.  
  * `advskew` -- This optional parameter specifies how much to skew the advbase  
    when sending CARP advertisements. By manipulating advbase, the master CARP  
    host can be chosen. The higher the number, the less preferred the host will  
    be when choosing a master. The default is 0. Acceptable values are from 1  
    to 254.  
  * `state` -- Force a carp interface into a certain state. Valid bits are  
    `init`, `backup` and `master`.  
  * `ipaddress` -- This is the shared IP address assigned to the redundancy  
    group. This address does not have to be in the same subnet as the IP address  
    on the physical interface (if present). This address needs to be the same on  
    all hosts in the group, however.  
  * `mask` -- The subnet mask of the shared IP.  
 Further CARP behaviour can be controlled via  
 [[!template id=man name="sysctl" section="8"]]:  
  * `net.inet.carp.allow` -- Accept incoming CARP packets or not. Default is 1  
  * `net.inet.carp.preempt` -- Allow hosts within a redundancy group that have a  
    better advbase and advskew to preempt the master. In addition, this option  
    also enables failing over all interfaces in the event that one interface goes  
    down. If one physical CARP-enabled interface goes down, CARP will change  
    advskew to 240 on all other CARP-enabled interfaces, in essence, failing  
    itself over. This option is 0 (disabled) by default.  
  * `net.inet.carp.log` -- Log bad CARP packets. Default is 0 (disabled).  
  * `net.inet.carp.arpbalance` -- Load balance traffic across multiple redundancy  
    group hosts. Default is 0 (disabled). See  
    [[!template id=man name="carp" section="4"]] for  
    more information.  
 ## Enabling CARP Support  
 CARP support is not enabled by default.  
 To use  
 [[!template id=man name="carp" section="4"]],  
 you need a kernel with support for the `carp` pseudo-device. Make sure the  
 following line is in your kernel configuration file:  
     pseudo-device   carp        # CARP  
 After configuring the `carp` pseudo-device in your kernel configuration, you  
 must recompile your kernel and reboot to enable carp support.  
 ## CARP Example  
 An example CARP configuration:  
     # sysctl -w net.inet.carp.allow=1  
     # ifconfig carp0 create  
     # ifconfig carp0 vhid 1 pass lanpasswd \  
          carpdev em0 advskew 100  
 This sets up the following:  
  * Enables receipt of CARP packets (this is the default setting)  
  * Creates an carp interface.  
  * Configures carp0 for virtual host `#1`, enables a password (`lanpasswd`), sets  
    `em0` as the interface belonging to the group, and makes this host a backup due  
    to the advskew of 100 (assuming of course that the master is set up with an  
    advskew less than 100). The shared IP assigned to this group is  
 Running ifconfig on carp0 shows the status of the interface:  
     # ifconfig carp0  
     carp0: flags=8802<UP,BROADCAST,SIMPLEX,MULTICAST> mtu 1500  
          carp: BACKUP carpdev em0 vhid 1 advbase 1 advskew 100  
          inet netmask 0xffffff00 broadcast  
 ## Advanced CARP configuration  
 The following example creates a cluster of two highly-available, redundant  
 firewalls. The following diagram presents what we're trying to achieve:  
              +----| WAN/Internet |----+  
              |                        |  
           em1|                        |em1  
           +-----+                  +-----+  
           | fw1 |                  | fw2 |  
           +-----+                  +-----+  
           em0|                        |em0  
              |                        |  
           ---+-------Shared LAN-------+---  
 Both firewalls are connected to the LAN on em0 and to a WAN/Internet connection  
 on em1. IP addresses are as follows:  
  * Firewall 1 (fw1) em0:  
  * Firewall 1 (fw1) em1:  
  * Firewall 2 (fw2) em0:  
  * Firewall 2 (fw2) em1:  
 The IP addresses we wish to share between the redundancy groups:  
  * WAN/Internet Shared IP:  
  * LAN Shared IP:  
 The network policy is that Firewall 1 (fw1) will be the preferred master.  
 The following configuration is for Firewall 1 (fw1):  
     #Enable preemption and group interface failover  
     # sysctl -w net.inet.carp.preempt=1  
     #Configure CARP on the LAN side  
     # ifconfig carp0 create  
     # ifconfig carp0 vhid 1 pass lanpasswd carpdev em0 \  
     #Configure CARP on the WAN side  
     # ifconfig carp1 create  
     # ifconfig carp1 vhid 2 pass wanpasswd carpdev em1 \  
 As mentioned before, our policy is for Firewall 1 to be the preferred master.  
 When configuring Firewall 2 we make the `advskew` a higher value since it's less  
 preferred to be the master.  
 The following configuration is for Firewall 2 (fw2):  
     #Enable preemption and group interface failover  
     # sysctl -w net.inet.carp.preempt=1  
     #Configure CARP on the LAN side  
     # ifconfig carp0 create  
     # ifconfig carp0 vhid 1 pass lanpasswd carpdev em0 \  
          advskew 128  
     #Configure CARP on the WAN side  
     # ifconfig carp1 create  
     # ifconfig carp1 vhid 2 pass wanpasswd carpdev em1 \  
          advskew 128  
 ## Forcing Failover of the Master  
 There can be times when it is necessary to failover or demote the master node on  
 purpose. Examples include taking the master node down for maintenance or when  
 troubleshooting a problem. The objective here is to gracefully fail over traffic  
 to one of the backup hosts so that users do not notice any impact.  
 To failover, shut down the carp interface on the master node. This will cause  
 the master to advertise itself with an *infinite* advbase and advskew. The  
 backup host(s) will see this and immediately take over the role of master.  
     # ifconfig carp0 down  
 ## License  
 Copyright (c) 2005 Joel Knight <enabled@myrealbox.com>    
 Permission to use, copy, modify, and distribute this documentation for    
 any purpose with or without fee is hereby granted, provided that the    
 above copyright notice and this permission notice appear in all copies.    

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