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<book id="conntrack-tools-how-to">
 <bookinfo>
  <title>The conntrack-tools user manual</title>
  
  <authorgroup>
   <author>
    <firstname>Pablo</firstname>
    <surname>Neira Ayuso</surname>
    <affiliation>
     <address>
      <email>pablo@netfilter.org</email>
     </address>
    </affiliation>
   </author>
  </authorgroup>

  <copyright>
   <year>2008</year>
   <holder>Pablo Neira Ayuso</holder>
  </copyright>

  <legalnotice>
   <para>
   Permission is granted to copy, distribute and/or modify this document
   under the terms of the GNU Free Documentation License, Version 1.2
   or any later version published by the Free Software Foundation;
   with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
   A copy of the license is included in the section entitled "GNU
   Free Documentation License".
   </para>
  </legalnotice>

  <releaseinfo>
  This document details how to install and configure the
  <ulink url="http://conntrack-tools.netfilter.org">conntrack-tools</ulink>
  0.9.8. This software is under development, for that reason, it is likely
  that this document will evolve in the future to cover new features and
  changes.</releaseinfo>

 </bookinfo>

 <toc></toc>

 <chapter id="introduction"><title>Introduction</title>

  <para>This document should be a kick-off point to install and configure the 
  <ulink url="http://conntrack-tools.netfilter.org">conntrack-tools</ulink>.
  If you find any error or imprecision in this document, please send an email
  to the author, it will be appreciated.</para>

  <para>In this document, the author assumes that the reader is familiar with firewalling concepts and iptables in general. If this is not your case, I suggest you to read the iptables documentation before going ahead. Moreover, the reader must also understand the difference between <emphasis>stateful</emphasis> and <emphasis>stateless</emphasis> firewalls. If this is not your case, I strongly suggest you to read the article <ulink url="http://people.netfilter.org/pablo/docs/login.pdf">Netfilter's Connection Tracking System</ulink> published in <emphasis>:login; the USENIX magazine</emphasis>. That document contains a general description that should help to clarify the concepts.</para>

<para>If you do not fulfill the previous requirements, this documentation is likely to be a source of frustration. Probably, you wonder why I'm insisting on these prerequisites too much, the fact is that if your iptables rule-set is <emphasis>stateless</emphasis>, it is very likely that the <emphasis>conntrack-tools</emphasis> will not be of any help for you. You have been warned!</para>

 </chapter>
 <chapter id="what"><title>What are the conntrack-tools?</title>

  <para>The conntrack-tools are a set of free software tools for GNU/Linux that allow system administrators interact, from user-space, with the in-kernel <ulink url="http://people.netfilter.org/pablo/docs/login.pdf">Connection Tracking System</ulink>, which is the module that enables stateful packet inspection for iptables. Probably, you did not hear about this module so far. However, if any of the rules of your rule-set use the <emphasis>state</emphasis> or <emphasis>ctstate</emphasis> iptables matches, you are indeed using it.
  
  </para>

<para>The <ulink url="http://conntrack-tools.netfilter.org">conntrack-tools</ulink> package contains two programs:</para>

  <itemizedlist>
   <listitem>
  	<para><emphasis>conntrack</emphasis> is command line interface conntrack provides a more flexible interface to the connnection tracking system than /proc/net/ip_conntrack. With conntrack, you can show, delete and update the existing state entries; and you can also listen to flow events.</para>
   </listitem>
   <listitem>
  	<para><emphasis>conntrackd</emphasis> is the user-space connection tracking daemon. This daemon can be used to deploy fault-tolerant GNU/Linux firewalls but you can also use it to collect flow-based statistics of the firewall use.</para>
   </listitem>
  </itemizedlist>

  <para>Although the name of both tools is very similar - and you can blame me for that, I'm not a marketing guy - they are used for very different tasks.</para>

 </chapter>

 <chapter id="requirements"><title>Requirements</title>

  <para>You have to install the following software in order to get the <emphasis>conntrack-tools</emphasis> working. Make sure that you have installed them correctly before going ahead:</para>

  <itemizedlist>
   <listitem>
  	<para><ulink url="http://www.kernel.org">Linux kernel</ulink> version &gt;= 2.6.18 that, at least, has support for:</para>
	<itemizedlist>
	 <listitem>
	 	<para>Connection Tracking System.</para>
		<itemizedlist>
		 <listitem>
		 <para>CONFIG_NF_CONNTRACK=m</para>
		 </listitem>
		 <listitem>
		 <para>CONFIG_NF_CONNTRACK_IPV4=m</para>
		 </listitem>
		 <listitem>
		 <para>CONFIG_NF_CONNTRACK_IPV6=m (if your setup supports IPv6)</para>
		 </listitem>
		</itemizedlist>
	 </listitem>
	 <listitem>
		<para>nfnetlink: the generic messaging interface for Netfilter.</para>
		<itemizedlist>
		 <listitem>
		 <para>CONFIG_NETFILTER_NETLINK=m</para>
		 </listitem>
		</itemizedlist>
	 </listitem>
	 <listitem>
		<para>nf_conntrack_netlink: the messaging interface for the Connection Tracking System.</para>
		<itemizedlist>
		 <listitem>
		 <para>CONFIG_NF_CT_NETLINK=m</para>
		 </listitem>
		</itemizedlist>
	 </listitem>
	 <listitem>
		<para>connection tracking event notification API: the flow-based event notification interface.</para>
		<itemizedlist>
		 <listitem>
		 <para>CONFIG_NF_CONNTRACK_EVENTS=y</para>
		 </listitem>
		</itemizedlist>
	 </listitem>
	</itemizedlist>
   <note><title>Verifying kernel support</title>
    <para>
     Make sure you have loaded <emphasis>nf_conntrack</emphasis>, <emphasis>nf_conntrack_ipv4</emphasis> (if your setup also supports IPv6, <emphasis>nf_conntrack_ipv6</emphasis>) and <emphasis>nf_conntrack_netlink</emphasis>.
    </para>
   </note>
   </listitem>
   <listitem>
   	<para>libnfnetlink: the netfilter netlink library use the official release available in <ulink url="http://www.netfilter.org">netfilter.org</ulink></para>
   </listitem>
   <listitem>
   	<para>libnetfilter_conntrack: the netfilter netlink library use the official release available in <ulink url="http://www.netfilter.org">netfilter.org</ulink></para>
   </listitem>
  </itemizedlist>
 </chapter>

 <chapter id="Installation"><title>Installation</title>

   <para>To compile and install the <emphasis>conntrack-tools</emphasis> run the following commands:</para>
   <programlisting>
	(non-root)$ tar xvjf conntrack-tools-x.x.x.tar.bz2
	(non-root)$ cd conntrack-tools-x.x.x
	(non-root)$ ./configure --prefix=/usr
	(non-root)$ make
	(root)    # make install</programlisting>

<note><title>Fedora Users</title>
  <para>If you are installing the libraries in /usr/local/, do not forget to do the following things:</para>
   <itemizedlist>
     <listitem><para>PKG_CONFIG_PATH=/usr/local/lib/pkgconfig; export PKG_CONFIG_PATH</para></listitem>
     <listitem><para>Add `/usr/local/lib' to your /etc/ld.so.conf file and run `ldconfig'</para></listitem>
   </itemizedlist>
   <para>Check `ldd' for trouble-shooting, read <ulink url="http://tldp.org/HOWTO/Program-Library-HOWTO/shared-libraries.html">this</ulink> for more information on how libraries work.</para>
</note>

<note><title>Verifying kernel support</title>
 <para>To check that the modules are enabled in the kernel, run <emphasis>`conntrack -E'</emphasis> and generate traffic, you should see flow events reporting new connections and updates.
 </para>
</note>

 </chapter>

 <chapter id="conntrack"><title>Using conntrack: the command line interface</title>

 <para>The <emphasis>/proc/net/ip_conntrack</emphasis> interface is very limited as it only allows you to display the existing flows, their state and other information:</para>

 <programlisting>
 # cat /proc/net/ip_conntrack
 tcp      6 431982 ESTABLISHED src=192.168.2.100 dst=123.59.27.117 sport=34846 dport=993 packets=169 bytes=14322 src=123.59.27.117 dst=192.168.2.100 sport=993 dport=34846 packets=113 bytes=34787 [ASSURED] mark=0 secmark=0 use=1
 tcp      6 431698 ESTABLISHED src=192.168.2.100 dst=123.59.27.117 sport=34849 dport=993 packets=244 bytes=18723 src=123.59.27.117 dst=192.168.2.100 sport=993 dport=34849 packets=203 bytes=144731 [ASSURED] mark=0 secmark=0 use=1
 </programlisting>

<para>The command line tool <emphasis>conntrack</emphasis> can be used to display the same information:</para>
 <programlisting>
 # conntrack -L
 tcp      6 431982 ESTABLISHED src=192.168.2.100 dst=123.59.27.117 sport=34846 dport=993 packets=169 bytes=14322 src=123.59.27.117 dst=192.168.2.100 sport=993 dport=34846 packets=113 bytes=34787 [ASSURED] mark=0 secmark=0 use=1
 tcp      6 431698 ESTABLISHED src=192.168.2.100 dst=123.59.27.117 sport=34849 dport=993 packets=244 bytes=18723 src=123.59.27.117 dst=192.168.2.100 sport=993 dport=34849 packets=203 bytes=144731 [ASSURED] mark=0 secmark=0 use=1
conntrack v0.9.7 (conntrack-tools): 2 flow entries has been shown.
 </programlisting>

<para>You can natively filter the output without using <emphasis>grep</emphasis>:</para>
<programlisting>
 # conntrack -L -p tcp --dport 34856
 tcp      6 431982 ESTABLISHED src=192.168.2.100 dst=123.59.27.117 sport=34846 dport=993 packets=169 bytes=14322 src=123.59.27.117 dst=192.168.2.100 sport=993 dport=34846 packets=113 bytes=34787 [ASSURED] mark=0 secmark=0 use=1
conntrack v0.9.7 (conntrack-tools): 1 flow entries has been shown.
 </programlisting>

<para>Update the mark based on a selection, this allows you to change the mark of an entry without using the CONNMARK target:</para>
<programlisting>
 # conntrack -U -p tcp --dport 3486 --mark 10
 tcp      6 431982 ESTABLISHED src=192.168.2.100 dst=123.59.27.117 sport=34846 dport=993 packets=169 bytes=14322 src=123.59.27.117 dst=192.168.2.100 sport=993 dport=34846 packets=113 bytes=34787 [ASSURED] mark=1 secmark=0 use=1
conntrack v0.9.7 (conntrack-tools): 1 flow entries has been updated.
 </programlisting>

<para>Delete one entry, this can be used to block traffic (you have to set <emphasis>/proc/sys/net/ipv4/netfilter/ip_conntrack_tcp_be_liberal</emphasis> to zero).</para>
<programlisting>
 # conntrack -D -p tcp --dport 3486
 tcp      6 431982 ESTABLISHED src=192.168.2.100 dst=123.59.27.117 sport=34846 dport=993 packets=169 bytes=14322 src=123.59.27.117 dst=192.168.2.100 sport=993 dport=34846 packets=113 bytes=34787 [ASSURED] mark=1 secmark=0 use=1
conntrack v0.9.7 (conntrack-tools): 1 flow entries has been deleted.
 </programlisting>

<para>Display the connection tracking events:</para>
<programlisting>
 # conntrack -E
     [NEW] udp      17 30 src=192.168.2.100 dst=192.168.2.1 sport=57767 dport=53 [UNREPLIED] src=192.168.2.1 dst=192.168.2.100 sport=53 dport=57767
  [UPDATE] udp      17 29 src=192.168.2.100 dst=192.168.2.1 sport=57767 dport=53 src=192.168.2.1 dst=192.168.2.100 sport=53 dport=57767
     [NEW] tcp      6 120 SYN_SENT src=192.168.2.100 dst=66.102.9.104 sport=33379 dport=80 [UNREPLIED] src=66.102.9.104 dst=192.168.2.100 sport=80 dport=33379
  [UPDATE] tcp      6 60 SYN_RECV src=192.168.2.100 dst=66.102.9.104 sport=33379 dport=80 src=66.102.9.104 dst=192.168.2.100 sport=80 dport=33379
  [UPDATE] tcp      6 432000 ESTABLISHED src=192.168.2.100 dst=66.102.9.104 sport=33379 dport=80 src=66.102.9.104 dst=192.168.2.100 sport=80 dport=33379 [ASSURED]
</programlisting>

<para>You can also display the existing flows in XML format, filter the output based on the NAT handling applied, etc.</para>

</chapter>

 <chapter id="settingup"><title>Setting up conntrackd: the daemon</title>

 <para>The daemon <emphasis>conntrackd</emphasis> supports two working modes:</para>

 <itemizedlist> 
  <listitem>
   <para><emphasis>State table synchronization</emphasis>: the daemon can be used to synchronize the connection tracking state table between several firewall replicas. This can be used to deploy fault-tolerant stateful firewalls. This is the main feature of the daemon.</para>
  </listitem>
  <listitem>
   <para><emphasis>Flow-based statistics collection</emphasis>: the daemon can be used to collect flow-based statistics. This feature is similar to what <ulink url="http://www.netfilter.org/projects/ulogd2/">ulogd-2.x</ulink> provides.</para>
  </listitem>
 </itemizedlist>

 <sect1 id="sync"><title>State table synchronization</title>

 <sect2 id="sync-requirements"><title>Requirements</title>

 <para>In order to get <emphasis>conntrackd</emphasis> working in synchronization mode, you have to fulfill the following requirements:</para>

 <orderedlist>
 <listitem>
 <para>A <emphasis>high availability manager</emphasis> like <ulink url="http://www.keepalived.org">keepalived</ulink> that manages the virtual IPs of the 
 firewall cluster, detects errors, and decide when to migrate the virtual IPs
 from one firewall replica to another. Without it, <emphasis>conntrackd</emphasis> will not work appropriately.</para>

 <para>The state synchronization setup requires a working installation of <ulink url="http://www.keepalived.org">keepalived</ulink>, preferibly a recent version. Check if your distribution comes with a recent packaged version. Otherwise, you may compile it from the sources.
 </para>

 <para>
 There is a very simple example file in the <emphasis>conntrackd</emphasis>
 sources to setup a simple HA cluster with keepalived (see the file 
 keepalived.conf under the doc/sync/ directory). This file can be used to 
 set up a simple VRRP cluster composed of two machines that hold the virtual
 IPs 192.168.0.100 on eth0 and 192.168.1.100 on eth1.</para>

 <para>If you are not familiar with <emphasis>keepalived</emphasis>, please
 read the official documentation available at the keepalived website 
 (<ulink url="http://www.keepalived.org">http://www.keepalived.org</ulink>).</para>

<para>If you use a different high availability manager, make sure it works correctly before going ahead.</para>

 </listitem>

 <listitem>
 <para>A dedicated link. The dedicated link between the firewalls is used
 to transmit and receive the state information. The use of a dedicated link
 is mandatory for security reasons as someone may pick the state information
 that is transfered between the firewalls.</para>
 </listitem>

 <listitem>
 <para>A well-formed stateful rule-set. Otherwise you are likely to experience
 problems during the fail-over. An example of a well-formed stateful iptables
 rule-set is available in the <ulink url="http://conntrack-tools.netfilter.org/testcase.html">conntrack-tools website</ulink>.</para>
 </listitem>

 <listitem>
  <para>If your Linux kernel is &lt; 2.6.22, you have to disable TCP window
  tracking:
   <programlisting>
    # echo 1 > /proc/sys/net/ipv4/netfilter/ip_conntrack_tcp_be_liberal
   </programlisting>
  </para>
 </listitem>

 </orderedlist>

 </sect2>

 <sect2 id="sync-configure"><title>Configuring the daemon</title>

 <para>The daemon <emphasis>conntrackd</emphasis> in synchronization mode 
 supports up to three replication approaches:</para>

 <itemizedlist>
  <listitem>
   <para><emphasis>notrack</emphasis>: this approach is the most simple as 
   it is based on a best effort replication protocol, ie. unreliable
   protocol. This protocol sends and receives the state information
   without performing any specific checking.
   </para>
  </listitem>
  <listitem>
   <para><emphasis>ft-fw</emphasis>: this approach is based on a reliable 
   protocol that performs message tracking. Thus, the protocol can recover
   from message loss, re-ordering and corruption.</para>
  </listitem>
  <listitem>
   <para><emphasis>alarm</emphasis>: this approach is spamming. It is based 
   on a alarm-based protocol that periodically re-sends the flow state to
   the backup firewall replicas. This protocol consumes a lot of bandwidth
   but it resolves synchronization problems fast.</para>
  </listitem>
 </itemizedlist>

 <para>The three existing approaches are soft real-time asynchronous 
 replication protocols that are aimed to have negligible impact in terms
 of latency and bandwidth throughput in the stateful firewall filtering.</para>

 <para>To configure <emphasis>conntrackd</emphasis> in any of the existing
 synchronization modes, you have to copy the example configuration file to
 the directory /etc/conntrackd/ on every firewall replica. Note that 
 <emphasis>_type_</emphasis> is the synchronization type selected.</para>

<programlisting>
 (conntrack-tools-x.x.x)# cp doc/_type_/conntrackd.conf /etc/conntrackd/conntrackd.conf
</programlisting>

<para>
 Do not forget to edit the files before going ahead. There are several
 parameters that you have to tune to adapt the example configuration file
 to your setup.
</para>

<note><title>Configuration file location</title>
 <para>If you don't want to put the config file under /etc/conntrackd/, just tell conntrackd where to find it passing the option -C.</para>
</note>

</sect2>

<sect2 id="sync-pb"><title>Active-Backup setup</title>

 <para>In the Active-Backup setup, one of the stateful firewall replicas 
 filters traffic and the other acts as backup. If you use this approach, 
 you have to copy the script <emphasis>primary-backup.sh</emphasis> to:
 </para>

<programlisting>
 (conntrack-tools-x.x.x)# cp doc/sync/primary-backup.sh /etc/conntrackd/
</programlisting>

 <para>The HA manager invokes this script when a transition happens, ie. If
 a stateful firewall replica:</para>

 <itemizedlist>
  <listitem>
   <para>becomes active to recover the filtering.</para>
  </listitem>
  <listitem>
   <para>becomes backup.</para>
  </listitem>
  <listitem>
   <para>hits failure (this is available if the HA manager has a failure state, which is true for <ulink url="http://www.keepalived.org">keepalived</ulink>.</para>
  </listitem>
 </itemizedlist>

 <para>The script is simple, and it contains the different actions that 
 <emphasis>conntrackd</emphasis> performs to recover the filtering or
 purge obsolete entries from the state table, among others. The script is
 commented, you can have a look at it if you need further information.</para>

</sect2>

<sect2 id="sync-aa"><title>Active-Active setup</title>

 <para>The Active-Active setup consists of having more than one stateful
 firewall replicas actively filtering traffic. Thus, we reduce the resource
 waste that implies to have a backup firewall which does nothing.</para>

 <para>We can classify the type of Active-Active setups in several
 families:</para>

 <itemizedlist>
  <listitem>
   <para><emphasis>Symmetric path routing</emphasis>: The stateful firewall
   replicas share the workload in terms of flows, ie. the packets that are
   part of a flow are always filtered by the same firewall.</para>
   </listitem>
   <listitem>
   <para><emphasis>Asymmetric multi-path routing</emphasis>: The packets that 
   are part of a flow can be filtered by whatever stateful firewall in the
   cluster. Thus, every flow-states have to be propagated to all the firewalls
   in the cluster as we do not know which one would be the next to filter a
   packet. This setup goes against the design of stateful firewalls as we
   define the filtering policy based on flows, not in packets anymore.
   </para>
  </listitem>
 </itemizedlist>

 <para>As for 0.9.8, the design of <emphasis>conntrackd</emphasis> allows you
 to deploy an symmetric Active-Active setup based on a static approach. 
 For example, assume that you have two virtual IPs, vIP1 and vIP2, and two
 firewall replicas, FW1 and FW2. You can give the virtual vIP1 to the
 firewall FW1 and the vIP2 to the FW2.
 </para>

 <para>Unfortunately, you will have to wait for the support for the
 Active-Active setup based on dynamic approach, ie. a workload sharing setup
 without directors that allow the stateful firewall share the filtering.</para>

 <para>On the other hand, the asymmetric scenario may work if your setup 
 fulfills several strong assumptions. However, in the opinion of the author
 of this work, the asymmetric setup goes against the design of stateful
 firewalls and <emphasis>conntrackd</emphasis>. Therefore, you have two
 choices here: you can deploy an Active-Backup setup or go back to your
 old stateless rule-set (in that case, the conntrack-tools will not be
 of any help anymore, of course).</para>

</sect2>

<sect2 id="sync-launch"><title>Launching conntrackd</title>

 <para>
 Once you have configured <emphasis>conntrackd</emphasis>, you can run in 
 <emphasis>console mode</emphasis> which is an interactive mode, in that case 
 type 'conntrackd' as root.</para>
 
 <programlisting>(root)# conntrackd</programlisting>

 <para>If you want to run <emphasis>conntrackd</emphasis> in <emphasis>daemon
 mode</emphasis>, then type:</para>

 <programlisting>(root)# conntrackd -d</programlisting>

 <para>You can verify that conntrackd is running by checking the log messages 
 via <emphasis>ps</emphasis>. Moreover, if <emphasis>conntrackd</emphasis> is
 running fine, you can dump the current status of the daemon:</para>

 <programlisting>
 # conntrackd -s
 cache internal:
 current active connections:                4
 connections created:                       4    failed:            0
 connections updated:                       0    failed:            0
 connections destroyed:                     0    failed:            0

 cache external:
 current active connections:                0
 connections created:                       0    failed:            0
 connections updated:                       0    failed:            0
 connections destroyed:                     0    failed:            0

 traffic processed:
                    0 Bytes                         0 Pckts

 multicast traffic:
                  352 Bytes sent                    0 Bytes recv
                   22 Pckts sent                    0 Pckts recv
                    0 Error send                    0 Error recv

 multicast sequence tracking:
                    0 Pckts mfrm                    0 Pckts lost
 </programlisting>

 <para>This command displays the number of entries in the internal and
 external cache:</para>

 <itemizedlist>
  <listitem>
   <para>The internal cache contains the states that this firewall replica is filtering, ie. this is a cache of the kernel state table.
   </para>
  </listitem>
  <listitem>
   <para>The external cache contains the states that the other firewall replica is filtering.
   </para>
  </listitem>
 </itemizedlist>

 <para>You can dump the internal cache with the following command:</para>

 <programlisting>
 # conntrackd -i
 tcp      6 ESTABLISHED src=192.168.2.100 dst=139.174.175.20 sport=58491 dport=993 src=139.174.175.20 dst=192.168.2.100 sport=993 dport=58491 [ASSURED] mark=0 secmark=0 [active since 536s]
 tcp      6 ESTABLISHED src=192.168.2.100 dst=123.59.27.117 sport=38211 dport=993 src=123.59.27.117 dst=192.168.2.100 sport=993 dport=38211 [ASSURED] mark=0 secmark=0 [active since 536s]
 tcp      6 ESTABLISHED src=192.168.2.100 dst=123.59.27.117 sport=38209 dport=993 src=123.59.27.117 dst=192.168.2.100 sport=993 dport=38209 [ASSURED] mark=0 secmark=0 [active since 536s]
 tcp      6 TIME_WAIT src=192.168.2.100 dst=74.125.45.166 sport=42593 dport=80 src=74.125.45.166 dst=192.168.2.100 sport=80 dport=42593 [ASSURED] [active since 165s]
 tcp      6 ESTABLISHED src=192.168.2.100 dst=139.174.175.20 sport=37962 dport=993 src=139.174.175.20 dst=192.168.2.100 sport=993 dport=37962 [ASSURED] mark=0 secmark=0 [active since 536s]
 </programlisting>

 <para>You can dump the external cache with the following command:</para>

 <programlisting># conntrackd -e</programlisting>

 <para>If the replication works fine, <emphasis>conntrackd -s</emphasis>
 displays the active's internal cache should display the same number of
 entries than the backup's external cache and vice-versa.</para>

 <para>To verify that the recovery works fine, if you trigger a fail-over,
 the log files should display the following information:</para>

 <programlisting>
 [Thu Sep 18 18:03:02 2008] (pid=9759) [notice] committing external cache
 [Thu Sep 18 18:03:02 2008] (pid=9759) [notice] Committed 1545 new entries</programlisting>

 <para>This means that the state entries have been injected into the kernel correctly.</para>

</sect2>

<sect2 id="sync-trouble"><title>Troubleshooting</title>

 <para>Problems with <emphasis>conntrackd</emphasis>? The following list 
 of questions should help for troubleshooting:</para>

 <qandaset>

  <qandaentry>
   <question>
    <para>
    I see <emphasis>packets lost</emphasis> in <emphasis>conntrackd -s</emphasis>
    </para>
   </question>
   <answer>
    <para>
    You can rise the value of <emphasis>McastRcvSocketBuffer</emphasis> and <emphasis>McastRcvSocketBuffer</emphasis>, if the problem is due to buffer overruns in the multicast sender or the receiver, the problem should disapear.
    </para>
   </answer>
  </qandaentry>
 
  <qandaentry>
   <question>
    <para>
    The log messages report that the <emphasis>maximum netlink socket buffer has been reached</emphasis>.
    </para>
   </question>
   <answer>
    <para>
    You can increase the values of <emphasis>SocketBufferSize</emphasis> and <emphasis>SocketBufferSizeMaxGrown</emphasis>.
    </para>
   </answer>
  </qandaentry>

 <qandaentry>
   <question>
    <para>
    I see <emphasis>can't open multicast server</emphasis> in the log messages
    </para>
   </question>
   <answer>
    <para>
    Make sure that the <emphasis>IPv4_interface</emphasis> clause has the IP of the dedicated link.
    </para>
   </answer>
  </qandaentry>

 <qandaentry>
   <question>
    <para>
    Can I use <ulink url="http://www.backhand.org/wackamole/">wackamole</ulink>, heartattack or any other HA manager?
    </para>
   </question>
   <answer>
    <para>
    Absolutely, you can. But before reporting issues, make sure that your HA manager is not the source of the problems.
    </para>
   </answer>
  </qandaentry>

 </qandaset>

</sect2>

</sect1>

</chapter>

</book>