<info><title>Common Address Redundancy Protocol

    <info>

      (<acronym>CARP</acronym>)</title>

      <title>Common Address Redundancy Protocol

	(<acronym>CARP</acronym>)</title>

      same <acronym>IP</acronym> address.  In some configurations,

      same <acronym>IP</acronym> address(es) and can be used to

      this may be used for availability or load balancing.  Hosts

      achieve high availability.  Hosts should also have a unique

      may use separate <acronym>IP</acronym> addresses, as in the

      <acronym>IP</acronym> address for management and configuration,

      example provided here.</para>

      as in the example provided here.</para>

    <para>To enable support for <acronym>CARP</acronym>, the &os;

    <sect2 xml:id="carp-ha">

      kernel can be rebuilt as described in <xref linkend="kernelconfig"/> with the following option:</para>

      <title>Using <acronym>CARP</acronym> for High

	Availability</title>

    <programlisting>device	carp</programlisting>

      <para>One use of <acronym>CARP</acronym> is to provide server

	high availability for one or more services.  This example

	configures failover support for three hosts, all with

	unique <acronym>IP</acronym> addresses and providing the same

	web content.  These machines are load balanced with a Round

	Robin <acronym>DNS</acronym> configuration.  The master and

	backup machines should be configured identically other than

	their hostnames and management <acronym>IP</acronym>

	addresses.  These servers need to run the same services, such

	as the web server, with the same configuration so that when

	the failover happens, requests to that service will be

	answered correctly and with the same content.  The failover

	machine has two additional <acronym>CARP</acronym> interfaces,

	one for each of the master content server's

	<acronym>IP</acronym> addresses.  When a failure occurs, the

	failover server will pick up the failed machine's

	<acronym>IP</acronym> address.  This means that the failure

	should go completely unnoticed by the user.  This example has

	two different masters named

	<systemitem>hosta.example.org</systemitem> and

	<systemitem>hostb.example.org</systemitem> respectively, with

	a shared slave named

	<systemitem>hostc.example.org</systemitem>.</para>

    <para>Alternatively, the <filename>if_carp.ko</filename> module

      <para>Each virtual address has a unique

      can be loaded at boot time.  Add the following line to

	identification number known as a Virtual Host IDentification

      <filename>/boot/loader.conf</filename>:</para>

	(<acronym>VHID</acronym>) which is used to distinguish the

	virtual address across the various failover machines that

	share the address on the network.</para>

    <programlisting>if_carp_load="YES"</programlisting>

    </sect2>

    <para><acronym>CARP</acronym> functionality should now be

    <sect2 xml:id="carp-10x">

      available and may be tuned via several &man.sysctl.8;

      <title>Using <acronym>CARP</acronym> on &os; 10 and

      variables:</para>

	Later</title>

    <informaltable frame="none" pgwide="1">

      <para>To enable support for <acronym>CARP</acronym>, load the

      <tgroup cols="2">

	<filename>carp.ko</filename> kernel module by adding the

	<thead>

	following line to

	  <row>

	<filename>/boot/loader.conf</filename>:</para>

	    <entry>OID</entry>

	    <entry>Description</entry>

	  </row>

	</thead>

	<tbody>

      <programlisting>carp_load="YES"</programlisting>

	  <row>

	    <entry><varname>net.inet.carp.allow</varname></entry>

	    <entry>Accept incoming <acronym>CARP</acronym> packets.

	      Enabled by default.</entry>

	  </row>

	  <row>

      <para>Alternatively, the &os; kernel can be rebuilt as

	    <entry><varname>net.inet.carp.preempt</varname></entry>

	described in <xref linkend="kernelconfig"/> with the following

	    <entry>This option downs all of the

	option:</para>

	      <acronym>CARP</acronym> interfaces on the host when one

	      goes down.  Disabled by default.</entry>

	  </row>

	  <row>

      <programlisting>device	carp</programlisting>

	    <entry><varname>net.inet.carp.log</varname></entry>

	    <entry>A value of <literal>0</literal> disables any

	      logging.  A value of <literal>1</literal> enables

	      logging of bad <acronym>CARP</acronym> packets.  Values

	      greater than <literal>1</literal> enable logging of

	      state changes for the <acronym>CARP</acronym>

	      interfaces.  The default value is

	      <literal>1</literal>.</entry>

	  </row>

	  <row>

      <para>Set the hostname, configure the management

	    <entry><varname>net.inet.carp.arpbalance</varname></entry>

	<acronym>IP</acronym> address, then configure

	    <entry>Balance local network traffic using

	<acronym>CARP</acronym> and the <acronym>IP</acronym> address

	      <acronym>ARP</acronym>.  Disabled by default.</entry>

	to be shared by adding the required lines to

	  </row>

	<filename>/etc/rc.conf</filename>.  Here are example lines for

	<systemitem>hosta.example.org</systemitem>:</para>

	  <row>

      <programlisting>hostname="hosta.example.org"

	    <entry><varname>net.inet.carp.suppress_preempt</varname></entry>

ifconfig_em0="inet <systemitem class="ipaddress">192.168.1.3</systemitem> netmask 255.255.255.0"

	    <entry>A read-only variable showing the status of

ifconfig_em0_alias0="vhid 1 pass testpass alias <systemitem class="ipaddress">192.168.1.50</systemitem>/32"</programlisting>

	      preemption suppression.  Preemption can be suppressed

	      if the link on an interface is down.  A value of

	      <literal>0</literal> means that preemption is not

	      suppressed.  Every problem increments this

	      variable.</entry>

	  </row>

	</tbody>

      </tgroup>

    </informaltable>

    <para>The <acronym>CARP</acronym> devices themselves may be

      <para>On <systemitem>hostb.example.org</systemitem>, use the

      created using &man.ifconfig.8;:</para>

	following lines:</para>

    <screen>&prompt.root; <userinput>ifconfig carp0 create</userinput></screen>

      <programlisting>hostname="hostb.example.org"

ifconfig_em0="inet <systemitem class="ipaddress">192.168.1.4</systemitem> netmask 255.255.255.0"

ifconfig_em0_alias0="vhid 2 pass testpass alias <systemitem class="ipaddress">192.168.1.51</systemitem>/32"</programlisting>

    <para>In a real environment, each interface has a unique

      <note>

      identification number known as a Virtual Host IDentification

	<para>It is very important that the passwords, specified by

      (<acronym>VHID</acronym>) which is used to distinguish the

	  <option>pass</option> with &man.ifconfig.8;, are

      host on the network.</para>

	  identical.  The <filename>carp</filename> devices will

	  only listen to and accept advertisements from machines

	  with the correct password.  The <acronym>VHID</acronym>

	  must also be unique for each machine.</para>

      </note>

    <sect2>

      <para>The third machine,

      <title>Using <acronym>CARP</acronym> for Server

	<systemitem>hostc.example.org</systemitem>,

	Availability</title>

	should be prepared so that it may handle failover from either

	of the previous host.  This machine will be configured with

	two <acronym>CARP</acronym> <acronym>VHID</acronym>s, one to

	handle each host.  The additional <literal>advskew</literal>

	parameter controls the <acronym>CARP</acronym> advertising

	skew.  By making this host advertise later, it will become a

	backup, rather than the master.  The appropriate

	<filename>/etc/rc.conf</filename> configuration lines will be

	similar to the following:</para>

      <para>One use of <acronym>CARP</acronym> is to provide server

      <programlisting>hostname="hostc.example.org"

	availability.  This example configures failover support for

ifconfig_em0="inet <systemitem class="ipaddress">192.168.1.5</systemitem> netmask 255.255.255.0"

	three hosts, all with unique <acronym>IP</acronym>

ifconfig_em0_alias0="vhid 1 advskew 100 pass testpass alias <systemitem class="ipaddress">192.168.1.50</systemitem>/32"

	addresses and providing the same web content.  These machines

ifconfig_em0_alias1="vhid 2 advskew 100 pass testpass alias <systemitem class="ipaddress">192.168.1.51</systemitem>/32"</programlisting>

	act in conjunction with a Round Robin

	<acronym>DNS</acronym> configuration.  The failover machine

	has two additional <acronym>CARP</acronym> interfaces, one

	for each of the content server's

	<acronym>IP</acronym> addresses.  When a

	failure occurs, the failover server will pick up the failed

	machine's <acronym>IP</acronym> address.

	This means that the failure should go completely unnoticed

	by the user.  The failover server requires identical content

	and services as the other content servers it is expected to

	pick up load for.</para>

	than their hostnames and <acronym>VHID</acronym>s.  This

	than their hostnames, unique <acronym>IP</acronym> addresses

	example calls these machines

	and <acronym>VHID</acronym>s.  This example calls these

	<systemitem>hosta.example.org</systemitem> and

	machines <systemitem>hosta.example.org</systemitem> and

	<systemitem>hostb.example.org</systemitem> respectively.  First, the

	<systemitem>hostb.example.org</systemitem> respectively.

	required lines for a <acronym>CARP</acronym> configuration

	First, the required lines for a <acronym>CARP</acronym>

	have to be added to <filename>/etc/rc.conf</filename>.  Here

	configuration have to be added to

	are the lines for

	<filename>/etc/rc.conf</filename>.  Here are example lines for

ifconfig_fxp0="inet 192.168.1.3 netmask 255.255.255.0"

ifconfig_fxp0="inet <systemitem class="ipaddress">192.168.1.3</systemitem> netmask 255.255.255.0"

ifconfig_carp0="vhid 1 pass testpass 192.168.1.50/24"</programlisting>

ifconfig_carp0="vhid 1 pass testpass <systemitem class="ipaddress">192.168.1.50</systemitem>/24"</programlisting>

ifconfig_fxp0="inet 192.168.1.4 netmask 255.255.255.0"

ifconfig_fxp0="inet <systemitem class="ipaddress">192.168.1.4</systemitem> netmask 255.255.255.0"

ifconfig_carp0="vhid 2 pass testpass 192.168.1.51/24"</programlisting>

ifconfig_carp0="vhid 2 pass testpass <systemitem class="ipaddress">192.168.1.51</systemitem>/24"</programlisting>

	  the <option>pass</option> option to &man.ifconfig.8;, are

	  <option>pass</option> with &man.ifconfig.8;, are

      <para>The third machine, <systemitem>provider.example.org</systemitem>,

      <para>The third machine, <systemitem>hostc.example.org</systemitem>,

      <programlisting>hostname="provider.example.org"

      <programlisting>hostname="hostc.example.org"

ifconfig_fxp0="inet 192.168.1.5 netmask 255.255.255.0"

ifconfig_fxp0="inet <systemitem class="ipaddress">192.168.1.5</systemitem> netmask 255.255.255.0"

ifconfig_carp0="vhid 1 advskew 100 pass testpass 192.168.1.50/24"

ifconfig_carp0="vhid 1 advskew 100 pass testpass <systemitem class="ipaddress">192.168.1.50</systemitem>/24"

ifconfig_carp1="vhid 2 advskew 100 pass testpass 192.168.1.51/24"</programlisting>

ifconfig_carp1="vhid 2 advskew 100 pass testpass <systemitem class="ipaddress">192.168.1.51</systemitem>/24"</programlisting>

	allow <systemitem>provider.example.org</systemitem> to notice and pick

	allow <systemitem>hostc.example.org</systemitem> to notice and pick

	it stop responding.</para>

	it become unavailable.</para>

	  <systemitem>provider.example.org</systemitem> may not relinquish the

	  <systemitem>hostc.example.org</systemitem> may not relinquish the

	  <systemitem>provider.example.org</systemitem>:</para>

	  <systemitem>hostc.example.org</systemitem>:</para>