ctdb-doc: Document reasons for separating private and public networks

[metze/samba/wip.git] / ctdb / doc / ctdb.7.xml

<?xml version="1.0" encoding="iso-8859-1"?>
<!DOCTYPE refentry
        PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
        "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
<refentry id="ctdb.7">

<refmeta>
        <refentrytitle>ctdb</refentrytitle>
        <manvolnum>7</manvolnum>
        <refmiscinfo class="source">ctdb</refmiscinfo>
        <refmiscinfo class="manual">CTDB - clustered TDB database</refmiscinfo>
</refmeta>


<refnamediv>
        <refname>ctdb</refname>
        <refpurpose>Clustered TDB</refpurpose>
</refnamediv>

<refsect1>
  <title>DESCRIPTION</title>

  <para>
    CTDB is a clustered database component in clustered Samba that
    provides a high-availability load-sharing CIFS server cluster.
  </para>

  <para>
    The main functions of CTDB are:
  </para>

  <itemizedlist>
    <listitem>
      <para>
        Provide a clustered version of the TDB database with automatic
        rebuild/recovery of the databases upon node failures.
      </para>
    </listitem>

    <listitem>
      <para>
      Monitor nodes in the cluster and services running on each node.
      </para>
    </listitem>

    <listitem>
      <para>
        Manage a pool of public IP addresses that are used to provide
        services to clients.  Alternatively, CTDB can be used with
        LVS.
      </para>
    </listitem>
  </itemizedlist>

  <para>
    Combined with a cluster filesystem CTDB provides a full
    high-availablity (HA) environment for services such as clustered
    Samba, NFS and other services.
  </para>
</refsect1>

<refsect1>
  <title>ANATOMY OF A CTDB CLUSTER</title>

  <para>
    A CTDB cluster is a collection of nodes with 2 or more network
    interfaces.  All nodes provide network (usually file/NAS) services
    to clients.  Data served by file services is stored on shared
    storage (usually a cluster filesystem) that is accessible by all
    nodes.
  </para>
  <para>
    CTDB provides an "all active" cluster, where services are load
    balanced across all nodes.
  </para>
</refsect1>

  <refsect1>
    <title>Recovery Lock</title>

    <para>
      CTDB uses a <emphasis>recovery lock</emphasis> to avoid a
      <emphasis>split brain</emphasis>, where a cluster becomes
      partitioned and each partition attempts to operate
      independently.  Issues that can result from a split brain
      include file data corruption, because file locking metadata may
      not be tracked correctly.
    </para>

    <para>
      CTDB uses a <emphasis>cluster leader and follower</emphasis>
      model of cluster management.  All nodes in a cluster elect one
      node to be the leader.  The leader node coordinates privileged
      operations such as database recovery and IP address failover.
      CTDB refers to the leader node as the <emphasis>recovery
      master</emphasis>.  This node takes and holds the recovery lock
      to assert its privileged role in the cluster.
    </para>

    <para>
      The recovery lock is implemented using a file residing in shared
      storage (usually) on a cluster filesystem.  To support a
      recovery lock the cluster filesystem must support lock
      coherence.  See
      <citerefentry><refentrytitle>ping_pong</refentrytitle>
      <manvolnum>1</manvolnum></citerefentry> for more details.
    </para>

    <para>
      If a cluster becomes partitioned (for example, due to a
      communication failure) and a different recovery master is
      elected by the nodes in each partition, then only one of these
      recovery masters will be able to take the recovery lock.  The
      recovery master in the "losing" partition will not be able to
      take the recovery lock and will be excluded from the cluster.
      The nodes in the "losing" partition will elect each node in turn
      as their recovery master so eventually all the nodes in that
      partition will be excluded.
    </para>

    <para>
      CTDB does sanity checks to ensure that the recovery lock is held
      as expected.
    </para>

    <para>
      CTDB can run without a recovery lock but this is not recommended
      as there will be no protection from split brains.
    </para>
  </refsect1>

  <refsect1>
    <title>Private vs Public addresses</title>

    <para>
      Each node in a CTDB cluster has multiple IP addresses assigned
      to it:

      <itemizedlist>
        <listitem>
          <para>
            A single private IP address that is used for communication
            between nodes.
          </para>
        </listitem>
        <listitem>
          <para>
            One or more public IP addresses that are used to provide
            NAS or other services.
          </para>
        </listitem>
      </itemizedlist>
    </para>

    <refsect2>
      <title>Private address</title>

      <para>
        Each node is configured with a unique, permanently assigned
        private address.  This address is configured by the operating
        system.  This address uniquely identifies a physical node in
        the cluster and is the address that CTDB daemons will use to
        communicate with the CTDB daemons on other nodes.
      </para>
      <para>
        Private addresses are listed in the file specified by the
        <varname>CTDB_NODES</varname> configuration variable (see
        <citerefentry><refentrytitle>ctdbd.conf</refentrytitle>
        <manvolnum>5</manvolnum></citerefentry>, default
        <filename>/usr/local/etc/ctdb/nodes</filename>).  This file contains the
        list of private addresses for all nodes in the cluster, one
        per line. This file must be the same on all nodes in the
        cluster.
      </para>
      <para>
        Private addresses should not be used by clients to connect to
        services provided by the cluster.
      </para>
      <para>
        It is strongly recommended that the private addresses are
        configured on a private network that is separate from client
        networks.  This is because the CTDB protocol is both
        unauthenticated and unencrypted.  If clients share the private
        network then steps need to be taken to stop injection of
        packets to relevant ports on the private addresses.  It is
        also likely that CTDB protocol traffic between nodes could
        leak sensitive information if it can be intercepted.
      </para>

      <para>
        Example <filename>/usr/local/etc/ctdb/nodes</filename> for a four node
        cluster:
      </para>
      <screen format="linespecific">
192.168.1.1
192.168.1.2
192.168.1.3
192.168.1.4
      </screen>
    </refsect2>

    <refsect2>
      <title>Public addresses</title>

      <para>
        Public addresses are used to provide services to clients.
        Public addresses are not configured at the operating system
        level and are not permanently associated with a particular
        node.  Instead, they are managed by CTDB and are assigned to
        interfaces on physical nodes at runtime.
      </para>
      <para>
        The CTDB cluster will assign/reassign these public addresses
        across the available healthy nodes in the cluster. When one
        node fails, its public addresses will be taken over by one or
        more other nodes in the cluster.  This ensures that services
        provided by all public addresses are always available to
        clients, as long as there are nodes available capable of
        hosting this address.
      </para>
      <para>
        The public address configuration is stored in a file on each
        node specified by the <varname>CTDB_PUBLIC_ADDRESSES</varname>
        configuration variable (see
        <citerefentry><refentrytitle>ctdbd.conf</refentrytitle>
        <manvolnum>5</manvolnum></citerefentry>, recommended
        <filename>/usr/local/etc/ctdb/public_addresses</filename>).  This file
        contains a list of the public addresses that the node is
        capable of hosting, one per line.  Each entry also contains
        the netmask and the interface to which the address should be
        assigned.
      </para>

      <para>
        Example <filename>/usr/local/etc/ctdb/public_addresses</filename> for a
        node that can host 4 public addresses, on 2 different
        interfaces:
      </para>
      <screen format="linespecific">
10.1.1.1/24 eth1
10.1.1.2/24 eth1
10.1.2.1/24 eth2
10.1.2.2/24 eth2
      </screen>

      <para>
        In many cases the public addresses file will be the same on
        all nodes.  However, it is possible to use different public
        address configurations on different nodes.
      </para>

      <para>
        Example: 4 nodes partitioned into two subgroups:
      </para>
      <screen format="linespecific">
Node 0:/usr/local/etc/ctdb/public_addresses
        10.1.1.1/24 eth1
        10.1.1.2/24 eth1

Node 1:/usr/local/etc/ctdb/public_addresses
        10.1.1.1/24 eth1
        10.1.1.2/24 eth1

Node 2:/usr/local/etc/ctdb/public_addresses
        10.1.2.1/24 eth2
        10.1.2.2/24 eth2

Node 3:/usr/local/etc/ctdb/public_addresses
        10.1.2.1/24 eth2
        10.1.2.2/24 eth2
      </screen>
      <para>
        In this example nodes 0 and 1 host two public addresses on the
        10.1.1.x network while nodes 2 and 3 host two public addresses
        for the 10.1.2.x network.
      </para>
      <para>
        Public address 10.1.1.1 can be hosted by either of nodes 0 or
        1 and will be available to clients as long as at least one of
        these two nodes are available.
      </para>
      <para>
        If both nodes 0 and 1 become unavailable then public address
        10.1.1.1 also becomes unavailable. 10.1.1.1 can not be failed
        over to nodes 2 or 3 since these nodes do not have this public
        address configured.
      </para>
      <para>
        The <command>ctdb ip</command> command can be used to view the
        current assignment of public addresses to physical nodes.
      </para>
    </refsect2>
  </refsect1>


  <refsect1>
    <title>Node status</title>

    <para>
      The current status of each node in the cluster can be viewed by the 
      <command>ctdb status</command> command.
    </para>

    <para>
      A node can be in one of the following states:
    </para>

    <variablelist>
      <varlistentry>
        <term>OK</term>
        <listitem>
          <para>
            This node is healthy and fully functional.  It hosts public
            addresses to provide services.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>DISCONNECTED</term>
        <listitem>
          <para>
            This node is not reachable by other nodes via the private
            network.  It is not currently participating in the cluster.
            It <emphasis>does not</emphasis> host public addresses to
            provide services.  It might be shut down.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>DISABLED</term>
        <listitem>
          <para>
            This node has been administratively disabled. This node is
            partially functional and participates in the cluster.
            However, it <emphasis>does not</emphasis> host public
            addresses to provide services.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>UNHEALTHY</term>
        <listitem>
          <para>
            A service provided by this node has failed a health check
            and should be investigated.  This node is partially
            functional and participates in the cluster.  However, it
            <emphasis>does not</emphasis> host public addresses to
            provide services.  Unhealthy nodes should be investigated
            and may require an administrative action to rectify.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>BANNED</term>
        <listitem>
          <para>
            CTDB is not behaving as designed on this node.  For example,
            it may have failed too many recovery attempts.  Such nodes
            are banned from participating in the cluster for a
            configurable time period before they attempt to rejoin the
            cluster.  A banned node <emphasis>does not</emphasis> host
            public addresses to provide services.  All banned nodes
            should be investigated and may require an administrative
            action to rectify.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>STOPPED</term>
        <listitem>
          <para>
            This node has been administratively exclude from the
            cluster.  A stopped node does no participate in the cluster
            and <emphasis>does not</emphasis> host public addresses to
            provide services.  This state can be used while performing
            maintenance on a node.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>PARTIALLYONLINE</term>
        <listitem>
          <para>
            A node that is partially online participates in a cluster
            like a healthy (OK) node.  Some interfaces to serve public
            addresses are down, but at least one interface is up.  See
            also <command>ctdb ifaces</command>.
          </para>
        </listitem>
      </varlistentry>

    </variablelist>
  </refsect1>

  <refsect1>
    <title>CAPABILITIES</title>

    <para>
      Cluster nodes can have several different capabilities enabled.
      These are listed below.
    </para>

    <variablelist>

      <varlistentry>
        <term>RECMASTER</term>
        <listitem>
          <para>
            Indicates that a node can become the CTDB cluster recovery
            master.  The current recovery master is decided via an
            election held by all active nodes with this capability.
          </para>
          <para>
            Default is YES.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>LMASTER</term>
        <listitem>
          <para>
            Indicates that a node can be the location master (LMASTER)
            for database records.  The LMASTER always knows which node
            has the latest copy of a record in a volatile database.
          </para>
          <para>
            Default is YES.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>LVS</term>
        <listitem>
          <para>
            Indicates that a node is configued in Linux Virtual Server
            (LVS) mode.  In this mode the entire CTDB cluster uses one
            single public address for the entire cluster instead of
            using multiple public addresses in failover mode.  This is
            an alternative to using a load-balancing layer-4 switch.
            See the <citetitle>LVS</citetitle> section for more
            details.
          </para>
        </listitem>
      </varlistentry>

    </variablelist>

    <para>
      The RECMASTER and LMASTER capabilities can be disabled when CTDB
      is used to create a cluster spanning across WAN links. In this
      case CTDB acts as a WAN accelerator.
    </para>

  </refsect1>

  <refsect1>
    <title>LVS</title>

    <para>
      LVS is a mode where CTDB presents one single IP address for the
      entire cluster. This is an alternative to using public IP
      addresses and round-robin DNS to loadbalance clients across the
      cluster.
    </para>

    <para>
      This is similar to using a layer-4 loadbalancing switch but with
      some restrictions.
    </para>

    <para>
      In this mode the cluster selects a set of nodes in the cluster
      and loadbalance all client access to the LVS address across this
      set of nodes. This set of nodes are all LVS capable nodes that
      are HEALTHY, or if no HEALTHY nodes exists all LVS capable nodes
      regardless of health status.  LVS will however never loadbalance
      traffic to nodes that are BANNED, STOPPED, DISABLED or
      DISCONNECTED. The <command>ctdb lvs</command> command is used to
      show which nodes are currently load-balanced across.
    </para>

    <para>
      One of the these nodes are elected as the LVSMASTER. This node
      receives all traffic from clients coming in to the LVS address
      and multiplexes it across the internal network to one of the
      nodes that LVS is using.  When responding to the client, that
      node will send the data back directly to the client, bypassing
      the LVSMASTER node.  The command <command>ctdb
      lvsmaster</command> will show which node is the current
      LVSMASTER.
    </para>

    <para>
      The path used for a client I/O is:
      <orderedlist>
        <listitem>
          <para>
            Client sends request packet to LVSMASTER.
          </para>
        </listitem>
        <listitem>
          <para>
            LVSMASTER passes the request on to one node across the
            internal network.
          </para>
        </listitem>
        <listitem>
          <para>
            Selected node processes the request.
          </para>
        </listitem>
        <listitem>
          <para>
            Node responds back to client.
          </para>
        </listitem>
      </orderedlist>
    </para>

    <para> 
      This means that all incoming traffic to the cluster will pass
      through one physical node, which limits scalability. You can
      send more data to the LVS address that one physical node can
      multiplex. This means that you should not use LVS if your I/O
      pattern is write-intensive since you will be limited in the
      available network bandwidth that node can handle.  LVS does work
      wery well for read-intensive workloads where only smallish READ
      requests are going through the LVSMASTER bottleneck and the
      majority of the traffic volume (the data in the read replies)
      goes straight from the processing node back to the clients. For
      read-intensive i/o patterns you can acheive very high throughput
      rates in this mode.
    </para>

    <para>
      Note: you can use LVS and public addresses at the same time.
    </para>

    <para>
      If you use LVS, you must have a permanent address configured for
      the public interface on each node. This address must be routable
      and the cluster nodes must be configured so that all traffic
      back to client hosts are routed through this interface. This is
      also required in order to allow samba/winbind on the node to
      talk to the domain controller.  This LVS IP address can not be
      used to initiate outgoing traffic.
    </para>
    <para>
      Make sure that the domain controller and the clients are
      reachable from a node <emphasis>before</emphasis> you enable
      LVS.  Also ensure that outgoing traffic to these hosts is routed
      out through the configured public interface.
    </para>

    <refsect2>
      <title>Configuration</title>

      <para>
        To activate LVS on a CTDB node you must specify the
        <varname>CTDB_PUBLIC_INTERFACE</varname> and
        <varname>CTDB_LVS_PUBLIC_IP</varname> configuration variables.
        Setting the latter variable also enables the LVS capability on
        the node at startup.
      </para>
        
      <para>
        Example:
        <screen format="linespecific">
CTDB_PUBLIC_INTERFACE=eth1
CTDB_LVS_PUBLIC_IP=10.1.1.237
        </screen>
      </para>

    </refsect2>
  </refsect1>

  <refsect1>
    <title>TRACKING AND RESETTING TCP CONNECTIONS</title>

    <para>
      CTDB tracks TCP connections from clients to public IP addresses,
      on known ports.  When an IP address moves from one node to
      another, all existing TCP connections to that IP address are
      reset.  The node taking over this IP address will also send
      gratuitous ARPs (for IPv4, or neighbour advertisement, for
      IPv6).  This allows clients to reconnect quickly, rather than
      waiting for TCP timeouts, which can be very long.
    </para>

    <para>
      It is important that established TCP connections do not survive
      a release and take of a public IP address on the same node.
      Such connections can get out of sync with sequence and ACK
      numbers, potentially causing a disruptive ACK storm.
    </para>

  </refsect1>

  <refsect1>
    <title>NAT GATEWAY</title>

    <para>
      NAT gateway (NATGW) is an optional feature that is used to
      configure fallback routing for nodes.  This allows cluster nodes
      to connect to external services (e.g. DNS, AD, NIS and LDAP)
      when they do not host any public addresses (e.g. when they are
      unhealthy).
    </para>
    <para>
      This also applies to node startup because CTDB marks nodes as
      UNHEALTHY until they have passed a "monitor" event.  In this
      context, NAT gateway helps to avoid a "chicken and egg"
      situation where a node needs to access an external service to
      become healthy.
    </para>
    <para>
      Another way of solving this type of problem is to assign an
      extra static IP address to a public interface on every node.
      This is simpler but it uses an extra IP address per node, while
      NAT gateway generally uses only one extra IP address.
    </para>

    <refsect2>
      <title>Operation</title>

      <para>
        One extra NATGW public address is assigned on the public
        network to each NATGW group.  Each NATGW group is a set of
        nodes in the cluster that shares the same NATGW address to
        talk to the outside world.  Normally there would only be one
        NATGW group spanning an entire cluster, but in situations
        where one CTDB cluster spans multiple physical sites it might
        be useful to have one NATGW group for each site.
      </para>
      <para>
        There can be multiple NATGW groups in a cluster but each node
        can only be member of one NATGW group.
      </para>
      <para>
        In each NATGW group, one of the nodes is selected by CTDB to
        be the NATGW master and the other nodes are consider to be
        NATGW slaves.  NATGW slaves establish a fallback default route
        to the NATGW master via the private network.  When a NATGW
        slave hosts no public IP addresses then it will use this route
        for outbound connections.  The NATGW master hosts the NATGW
        public IP address and routes outgoing connections from
        slave nodes via this IP address.  It also establishes a
        fallback default route.
      </para>
    </refsect2>

    <refsect2>
      <title>Configuration</title>

      <para>
        NATGW is usually configured similar to the following example configuration:
      </para>
      <screen format="linespecific">
CTDB_NATGW_NODES=/usr/local/etc/ctdb/natgw_nodes
CTDB_NATGW_PRIVATE_NETWORK=192.168.1.0/24
CTDB_NATGW_PUBLIC_IP=10.0.0.227/24
CTDB_NATGW_PUBLIC_IFACE=eth0
CTDB_NATGW_DEFAULT_GATEWAY=10.0.0.1
      </screen>

      <para>
        Normally any node in a NATGW group can act as the NATGW
        master.  Some configurations may have special nodes that lack
        connectivity to a public network.  In such cases, those nodes
        can be flagged with the "slave-only" option in the
        <varname>CTDB_NATGW_NODES</varname> file to limit the NATGW
        functionality of those nodes.
      </para>

      <para>
        See the <citetitle>NAT GATEWAY</citetitle> section in
        <citerefentry><refentrytitle>ctdbd.conf</refentrytitle>
        <manvolnum>5</manvolnum></citerefentry> for more details of
        NATGW configuration.
      </para>
    </refsect2>


    <refsect2>
      <title>Implementation details</title>

      <para>
        When the NATGW functionality is used, one of the nodes is
        selected to act as a NAT gateway for all the other nodes in
        the group when they need to communicate with the external
        services.  The NATGW master is selected to be a node that is
        most likely to have usable networks.
      </para>

      <para>
        The NATGW master hosts the NATGW public IP address
        <varname>CTDB_NATGW_PUBLIC_IP</varname> on the configured public
        interfaces <varname>CTDB_NATGW_PUBLIC_IFACE</varname> and acts as
        a router, masquerading outgoing connections from slave nodes
        via this IP address.  If
        <varname>CTDB_NATGW_DEFAULT_GATEWAY</varname> is set then it
        also establishes a fallback default route to the configured
        this gateway with a metric of 10.  A metric 10 route is used
        so it can co-exist with other default routes that may be
        available.
      </para>

      <para>
        A NATGW slave establishes its fallback default route to the
        NATGW master via the private network
        <varname>CTDB_NATGW_PRIVATE_NETWORK</varname>with a metric of 10.
        This route is used for outbound connections when no other
        default route is available because the node hosts no public
        addresses.  A metric 10 routes is used so that it can co-exist
        with other default routes that may be available when the node
        is hosting public addresses.
      </para>

      <para>
        <varname>CTDB_NATGW_STATIC_ROUTES</varname> can be used to
        have NATGW create more specific routes instead of just default
        routes.
      </para>

      <para>
        This is implemented in the <filename>11.natgw</filename>
        eventscript.  Please see the eventscript file and the
        <citetitle>NAT GATEWAY</citetitle> section in
        <citerefentry><refentrytitle>ctdbd.conf</refentrytitle>
        <manvolnum>5</manvolnum></citerefentry> for more details.
      </para>

    </refsect2>
  </refsect1>

  <refsect1>
    <title>POLICY ROUTING</title>

    <para>
      Policy routing is an optional CTDB feature to support complex
      network topologies.  Public addresses may be spread across
      several different networks (or VLANs) and it may not be possible
      to route packets from these public addresses via the system's
      default route.  Therefore, CTDB has support for policy routing
      via the <filename>13.per_ip_routing</filename> eventscript.
      This allows routing to be specified for packets sourced from
      each public address.  The routes are added and removed as CTDB
      moves public addresses between nodes.
    </para>

    <refsect2>
      <title>Configuration variables</title>

      <para>
        There are 4 configuration variables related to policy routing:
        <varname>CTDB_PER_IP_ROUTING_CONF</varname>,
        <varname>CTDB_PER_IP_ROUTING_RULE_PREF</varname>,
        <varname>CTDB_PER_IP_ROUTING_TABLE_ID_LOW</varname>,
        <varname>CTDB_PER_IP_ROUTING_TABLE_ID_HIGH</varname>.  See the
        <citetitle>POLICY ROUTING</citetitle> section in
        <citerefentry><refentrytitle>ctdbd.conf</refentrytitle>
        <manvolnum>5</manvolnum></citerefentry> for more details.
      </para>
    </refsect2>

    <refsect2>
      <title>Configuration</title>

      <para>
        The format of each line of
        <varname>CTDB_PER_IP_ROUTING_CONF</varname> is:
      </para>
      
      <screen>
&lt;public_address&gt; &lt;network&gt; [ &lt;gateway&gt; ]
      </screen>

      <para>
        Leading whitespace is ignored and arbitrary whitespace may be
        used as a separator.  Lines that have a "public address" item
        that doesn't match an actual public address are ignored.  This
        means that comment lines can be added using a leading
        character such as '#', since this will never match an IP
        address.
      </para>

      <para>
        A line without a gateway indicates a link local route.
      </para>

      <para>
        For example, consider the configuration line:
      </para>

      <screen>
  192.168.1.99  192.168.1.1/24
      </screen>

      <para>
        If the corresponding public_addresses line is:
      </para>

      <screen>
  192.168.1.99/24     eth2,eth3
      </screen>

      <para>
        <varname>CTDB_PER_IP_ROUTING_RULE_PREF</varname> is 100, and
        CTDB adds the address to eth2 then the following routing
        information is added:
      </para>

      <screen>
  ip rule add from 192.168.1.99 pref 100 table ctdb.192.168.1.99
  ip route add 192.168.1.0/24 dev eth2 table ctdb.192.168.1.99
      </screen>

      <para>  
        This causes traffic from 192.168.1.1 to 192.168.1.0/24 go via
        eth2.
      </para>

      <para>
        The <command>ip rule</command> command will show (something
        like - depending on other public addresses and other routes on
        the system):
      </para>

      <screen>
  0:            from all lookup local 
  100:          from 192.168.1.99 lookup ctdb.192.168.1.99
  32766:        from all lookup main 
  32767:        from all lookup default 
      </screen>

      <para>
        <command>ip route show table ctdb.192.168.1.99</command> will show:
      </para>

      <screen>
  192.168.1.0/24 dev eth2 scope link
      </screen>

      <para>
        The usual use for a line containing a gateway is to add a
        default route corresponding to a particular source address.
        Consider this line of configuration:
      </para>

      <screen>
  192.168.1.99  0.0.0.0/0       192.168.1.1
      </screen>

      <para>
        In the situation described above this will cause an extra
        routing command to be executed:
      </para>

      <screen>
  ip route add 0.0.0.0/0 via 192.168.1.1 dev eth2 table ctdb.192.168.1.99
      </screen>

      <para>
        With both configuration lines, <command>ip route show table
        ctdb.192.168.1.99</command> will show:
      </para>

      <screen>
  192.168.1.0/24 dev eth2 scope link 
  default via 192.168.1.1 dev eth2 
      </screen>
    </refsect2>

    <refsect2>
      <title>Sample configuration</title>

      <para>
        Here is a more complete example configuration.
      </para>

      <screen>
/usr/local/etc/ctdb/public_addresses:

  192.168.1.98  eth2,eth3
  192.168.1.99  eth2,eth3

/usr/local/etc/ctdb/policy_routing:

  192.168.1.98 192.168.1.0/24
  192.168.1.98 192.168.200.0/24 192.168.1.254
  192.168.1.98 0.0.0.0/0        192.168.1.1
  192.168.1.99 192.168.1.0/24
  192.168.1.99 192.168.200.0/24 192.168.1.254
  192.168.1.99 0.0.0.0/0        192.168.1.1
      </screen>

      <para>
        The routes local packets as expected, the default route is as
        previously discussed, but packets to 192.168.200.0/24 are
        routed via the alternate gateway 192.168.1.254.
      </para>

    </refsect2>
  </refsect1>

  <refsect1>
    <title>NOTIFICATION SCRIPT</title>

    <para>
      When certain state changes occur in CTDB, it can be configured
      to perform arbitrary actions via a notification script.  For
      example, sending SNMP traps or emails when a node becomes
      unhealthy or similar.
    </para>
    <para>
      This is activated by setting the
      <varname>CTDB_NOTIFY_SCRIPT</varname> configuration variable.
      The specified script must be executable.  
    </para>
    <para>
      Use of the provided <filename>/usr/local/etc/ctdb/notify.sh</filename>
      script is recommended.  It executes files in
      <filename>/usr/local/etc/ctdb/notify.d/</filename>.
    </para>
    <para>
      CTDB currently generates notifications after CTDB changes to
      these states:
    </para>

    <simplelist>
      <member>init</member>
      <member>setup</member>
      <member>startup</member>
      <member>healthy</member>
      <member>unhealthy</member>
    </simplelist>

  </refsect1>

  <refsect1>
    <title>DEBUG LEVELS</title>

    <para>
      Valid values for DEBUGLEVEL are:
    </para>

    <simplelist>
      <member>ERR (0)</member>
      <member>WARNING (1)</member>
      <member>NOTICE (2)</member>
      <member>INFO (3)</member>
      <member>DEBUG (4)</member>
    </simplelist>
  </refsect1>


  <refsect1>
    <title>REMOTE CLUSTER NODES</title>
    <para>
It is possible to have a CTDB cluster that spans across a WAN link. 
For example where you have a CTDB cluster in your datacentre but you also
want to have one additional CTDB node located at a remote branch site.
This is similar to how a WAN accelerator works but with the difference 
that while a WAN-accelerator often acts as a Proxy or a MitM, in 
the ctdb remote cluster node configuration the Samba instance at the remote site
IS the genuine server, not a proxy and not a MitM, and thus provides 100%
correct CIFS semantics to clients.
    </para>

    <para>
        See the cluster as one single multihomed samba server where one of
        the NICs (the remote node) is very far away.
    </para>

    <para>
        NOTE: This does require that the cluster filesystem you use can cope
        with WAN-link latencies. Not all cluster filesystems can handle
        WAN-link latencies! Whether this will provide very good WAN-accelerator
        performance or it will perform very poorly depends entirely
        on how optimized your cluster filesystem is in handling high latency
        for data and metadata operations.
    </para>

    <para>
        To activate a node as being a remote cluster node you need to set
        the following two parameters in /etc/sysconfig/ctdb  for the remote node:
        <screen format="linespecific">
CTDB_CAPABILITY_LMASTER=no
CTDB_CAPABILITY_RECMASTER=no
        </screen>
    </para>

    <para>
        Verify with the command "ctdb getcapabilities" that that node no longer
        has the recmaster or the lmaster capabilities.
    </para>

  </refsect1>


  <refsect1>
    <title>SEE ALSO</title>

    <para>
      <citerefentry><refentrytitle>ctdb</refentrytitle>
      <manvolnum>1</manvolnum></citerefentry>,

      <citerefentry><refentrytitle>ctdbd</refentrytitle>
      <manvolnum>1</manvolnum></citerefentry>,

      <citerefentry><refentrytitle>ctdbd_wrapper</refentrytitle>
      <manvolnum>1</manvolnum></citerefentry>,

      <citerefentry><refentrytitle>ltdbtool</refentrytitle>
      <manvolnum>1</manvolnum></citerefentry>,

      <citerefentry><refentrytitle>onnode</refentrytitle>
      <manvolnum>1</manvolnum></citerefentry>,

      <citerefentry><refentrytitle>ping_pong</refentrytitle>
      <manvolnum>1</manvolnum></citerefentry>,

      <citerefentry><refentrytitle>ctdbd.conf</refentrytitle>
      <manvolnum>5</manvolnum></citerefentry>,

      <citerefentry><refentrytitle>ctdb-statistics</refentrytitle>
      <manvolnum>7</manvolnum></citerefentry>,

      <citerefentry><refentrytitle>ctdb-tunables</refentrytitle>
      <manvolnum>7</manvolnum></citerefentry>,

      <ulink url="http://ctdb.samba.org/"/>
    </para>
  </refsect1>

  <refentryinfo>
    <author>
      <contrib>
        This documentation was written by
        Ronnie Sahlberg,
        Amitay Isaacs,
        Martin Schwenke
      </contrib>
    </author>

    <copyright>
      <year>2007</year>
      <holder>Andrew Tridgell</holder>
      <holder>Ronnie Sahlberg</holder>
    </copyright>
    <legalnotice>
      <para>
        This program is free software; you can redistribute it and/or
        modify it under the terms of the GNU General Public License as
        published by the Free Software Foundation; either version 3 of
        the License, or (at your option) any later version.
      </para>
      <para>
        This program is distributed in the hope that it will be
        useful, but WITHOUT ANY WARRANTY; without even the implied
        warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
        PURPOSE.  See the GNU General Public License for more details.
      </para>
      <para>
        You should have received a copy of the GNU General Public
        License along with this program; if not, see
        <ulink url="http://www.gnu.org/licenses"/>.
      </para>
    </legalnotice>
  </refentryinfo>

</refentry>