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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
<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>2008-2012</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>
&gt;= 1.4.0. 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 have 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 have 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 if:</para>
<itemizedlist>
<listitem><para>You have a stateful rule-set that blocks traffic in INVALID state.</para></listitem>
<listitem><para>You have set <emphasis>/proc/sys/net/ipv4/netfilter/ip_conntrack_tcp_loose</emphasis> or <emphasis>/proc/sys/net/netfilter/nf_conntrack_tcp_loose</emphasis>, depending on your kernel version, to zero.</para></listitem>
</itemizedlist>
<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/ulogd/">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>
<note><title>Stateful firewall architectures</title>
<para>A good reading to extend the information about firewall architectures is <ulink url="http://1984.lsi.us.es/~pablo/docs/intcomp09.pdf">Demystifying cluster-based fault-tolerant firewalls</ulink> published in IEEE Internet Computing magazine.
</para>
</note>
<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-options"><title>Other configuration options</title>
<para>The daemon allows several configuration options that you may want to
enable. This section contains some information about them.</para>
<sect3 id="sync-disable-external"><title>Disabling external cache</title>
<para>It is possible to disable the external cache. Thus,
<emphasis>conntrackd</emphasis> directly injects the flow-states into the
in-kernel Connection Tracking System of the backup firewall. You can do it
by enabling the <emphasis>DisableExternalCache</emphasis> option in the
<emphasis>conntrackd.conf</emphasis> configuration file:
</para>
<programlisting>
Sync {
Mode FTFW {
[...]
DisableExternalCache Off
}
}
</programlisting>
<para>You can also use this option with the NOTRACK and ALARM modes. This
increases CPU consumption in the backup firewall but now you do not need
to commit the flow-states during the master failures since they are already
in the in-kernel Connection Tracking table. Moreover, you save memory in
the backup firewall since you do not need to store the foreign flow-states
anymore.
</para>
</sect3>
<sect3 id="sync-disable-internal"><title>Disabling internal cache</title>
<para>You can also disable the internal cache by means of the
<emphasis>DisableInternalCache</emphasis> option in the
<emphasis>conntrackd.conf</emphasis> configuration file:
</para>
<programlisting>
Sync {
Mode NOTRACK {
[...]
DisableInternalCache Off
}
}
</programlisting>
<para>However, this option is only available for the NOTRACK mode. This
mode provides unreliable flow-state synchronization between firewalls.
Thus, if flow-states are lost during the synchronization, the protocol
provides no way to recover them.</para>
</sect3>
<sect3 id="sync-transport-protocol">
<title>Using UDP, TCP or multicast for flow-state synchronization</title>
<para>You can use up to three different transport layer protocols to
synchronize flow-state changes between the firewalls: UDP, TCP and
Multicast. UDP and multicast are unreliable but together with the FT-FW
mode provide partial reliable flow-state synchronization.
</para>
<para>The preferred choice is FT-FW over UDP, or multicast alternatively.
TCP introduces latency in the flow-state synchronization due to the
congestion control. Under flow-state message are lost, the FIFO delivery
becomes also a problem since the backup firewall quickly gets out of
sync. For that reason, its use is discouraged. Note that using TCP only
makes sense with the NOTRACK mode.
</para>
</sect3>
<sect3 id="sync-redundant-link"><title>Redundant dedicated links</title>
<para>You can set redundant dedicated links without using bonding, you have
to configure as many redundant links as you want in the configuration file.
In case of failure of the master dedicated link, conntrackd failovers to one
of the backups. An example of this configuration is the following:
</para>
<programlisting>
Sync {
Mode FTFW {
[...]
}
# default master dedicated link
UDP Default {
IPv4_address 192.168.2.1
IPv4_Destination_Address 192.168.2.2
Port 3780
Interface eth3
SndSocketBuffer 24985600
RcvSocketBuffer 24985600
Checksum on
}
# backup dedicated link
UDP {
IPv4_address 192.168.1.3
IPv4_Destination_Address 192.168.1.4
Port 3780
Interface eth2
SndSocketBuffer 24985600
RcvSocketBuffer 24985600
Checksum on
}
[...]
}
</programlisting>
</sect3>
<sect3 id="sync-iptables-filtering">
<title>Filtering Connection tracking events with iptables</title>
<para>Since Linux kernel &gt;= 2.6.34, iptables provides the
<emphasis>CT</emphasis> iptables target that allows to reduce the
amount of Connection Tracking events that are delivered to user-space.
However, you will have to use a Linux kernel &gt;= 2.6.38 to profit
from this feature, since several aspects of the event filtering were
broken.</para>
<para>The following example shows how to only generate the
<emphasis>assured</emphasis> and <emphasis>destroy</emphasis>
events:</para>
<programlisting>
# iptables -I PREROUTING -t raw -j CT --ctevents assured,destroy
</programlisting>
<note><title>Assured flows</title>
<para>One flow is assured if the firewall has seen traffic for it in
both directions.</para>
</note>
<para>Reducing the amount of events generated helps to reduce CPU
consumption in the active firewall.</para>
</sect3>
<sect3 id="sync-expect"><title>Synchronization of expectations</title>
<note><title>Check your Linux kernel version first</title>
<para>
The synchronization of expectations require a Linux kernel &gt;= 3.5
to work appropriately.
</para>
</note>
<para>The connection tracking system provides helpers that allows you to
filter multi-flow application protocols like FTP, H.323 and SIP among many
others. These protocols usually split the control and data traffic in
different flows. Moreover, the control flow usually announces layer 3 and
4 information to let the other peer know where the data flows will be
open. This sort of protocols require that the firewall inspects the
content of the packet, otherwise filtering by layer 3 and 4 selectors
like addresses and ports become a real nightmare. Netfilter already
provides the so-called <emphasis>helpers</emphasis> that track this
protocol aspects to allow deploying appropriate filtering. These
helpers create <emphasis>expectation</emphasis> entries that
represent expected traffic that will arrive to the firewall according
to the inspected packets.</para>
<para>In case that you have enabled tracking of these protocols, you
may want to enable the state-synchronization of expectation as well.
Thus, established flows for this specific protocols will not suffer
any disruption.</para>
<para>To enable the expectation support in the configuration file, you
have to use the following option:</para>
<programlisting>
Sync {
...
Options {
ExpectationSync {
ftp
sip
ras # for H.323
q.931 # for H.323
h.245 # for H.323
}
}
}</programlisting>
<para>The example above enables the synchronization of the expectations
for the FTP, SIP and H.323 helpers.</para>
<para>In my testbed, there are two firewalls in a primary-backup
configuration running keepalived. They use a couple of floating cluster
IP address (192.168.0.100 and 192.168.1.100) that are used by the client.
These firewalls protect one FTP server (192.168.1.2) that will be accessed
by one client.</para>
<para>In ASCII art, it looks like this:</para>
<programlisting>
192.168.0.100 192.168.1.100
eth1 eth2
fw-1
/ \ FTP
client ------ ------ server
192.168.0.2 \ / 192.168.1.2
fw-2
</programlisting>
<para>This is the rule-set for the firewalls:</para>
<programlisting>
-A FORWARD -m state --state RELATED -j ACCEPT
-A FORWARD -i eth2 -m state --state ESTABLISHED -j ACCEPT
-A FORWARD -i eth1 -p tcp -m tcp --dport 21 --tcp-flags FIN,SYN,RST,ACK SYN -m state --state NEW -j ACCEPT
-A FORWARD -i eth1 -p tcp -m state --state ESTABLISHED -j ACCEPT
-A FORWARD -m state --state INVALID -j LOG --log-prefix "invalid: "</programlisting>
<para>Before going ahead, make sure <emphasis>nf_conntrack_ftp</emphasis> is
loaded.</para>
<para>The following steps detail how to check that the expectation support
works fine with FTP traffic:</para>
<orderedlist>
<listitem>
<para>Switch to the client. Start one FTP control connection to one
server that is protected by the firewalls, enter passive mode:</para>
<programlisting>
(term-1) user@client$ nc 192.168.1.2 21
220 dummy FTP server
USER anonymous
331 Please specify the password.
PASS nothing
230 Login successful.
PASV
227 Entering Passive Mode (192,168,1,2,163,11).</programlisting>
<para>This means that port 163*256+11=41739 will be used for the data
traffic. I suggest you to read <ulink url="http://www.freefire.org/articles/ftpexample.php">djb's FTP protocol description</ulink> in case that you
don't understand how this calculation is done.</para>
</listitem>
<listitem>
<para> Switch to fw-1 (primary) to check that the expectation is in the
internal cache.</para>
<programlisting>
root@fw1# conntrackd -i exp
proto=6 src=192.168.0.2 dst=192.168.1.2 sport=0 dport=41739 mask-src=255.255.255.255 mask-dst=255.255.255.255 sport=0 dport=65535 master-src=192.168.0.2 master-dst=192.168.1.2 sport=36390 dport=21 helper=ftp [active since 5s]
</programlisting>
</listitem>
<listitem>
<para> Switch to fw-2 (backup) to check that the expectation has been
successfully replicated.</para>
<programlisting>
root@fw2# conntrackd -e exp
proto=6 src=192.168.0.2 dst=192.168.1.2 sport=0 dport=41739 mask-src=255.255.255.255 mask-dst=255.255.255.255 sport=0 dport=65535 master-src=192.168.0.2 master-dst=192.168.1.2 sport=36390 dport=21 [active since 8s]
</programlisting>
</listitem>
<listitem>
<para>Make the primary firewall fw-1 fail. Now fw-2 becomes primary.</para>
</listitem>
<listitem>
<para>Switch to fw-2 (primary) to commit the external cache into the
kernel. The logs should display that the commit was successful:</para>
<programlisting>
root@fw2# tail -100f /var/log/conntrackd.log
[Wed Dec 7 22:16:31 2011] (pid=19195) [notice] committing external cache: expectations
[Wed Dec 7 22:16:31 2011] (pid=19195) [notice] Committed 1 new entries
[Wed Dec 7 22:16:31 2011] (pid=19195) [notice] commit has taken 0.000366 seconds</programlisting>
</listitem>
<listitem>
<para> Switch to the client. Open a new terminal and connect to the port that
has been announced by the server:</para>
<programlisting>
(term-2) user@client$ nc -vvv 192.168.1.2 41739
(UNKNOWN) [192.168.1.2] 41739 (?) open</programlisting>
</listitem>
<listitem>
<para>Switch to term-1 and ask for the file listing:</para>
<programlisting>
[...]
227 Entering Passive Mode (192,168,1,2,163,11).
LIST</programlisting>
</listitem>
<listitem>
<para>Switch to term-2, it should display the listing. That means
everything has worked fine.</para>
</listitem>
</orderedlist>
<para>You may want to try disabling the expectation support and
repeating the steps to check that <emphasis>it does not work</emphasis>
without the state-synchronization.</para>
</sect3>
</sect2>
</sect1>
<sect1 id="helpers"><title>User-space helpers</title>
<note><title>Check your Linux kernel version first</title>
<para>
The user-space helper infrastructure requires a Linux kernel &gt;= 3.6
to work appropriately.
</para>
</note>
<para>Connection tracking helpers allows you to filter multi-flow protocols
that usually separate control and data traffic into different flows.
These protocols usually violate network layering by including layer 3/4
details, eg. IP address and TCP/UDP ports, in their application protocol
(which resides in layer 7). This is problematic for gateways since they
operate at packet-level, ie. layers 3/4, and therefore they miss this
important information to filter these protocols appropriately.</para>
<para>Helpers inspect packet content (at layer 7) and create the so-called
expectations. These expectations are added to one internal table
that resides in the gateway. For each new packet arriving to the
gateway, the gateway first looks up for matching expectations. If
there is any, then this flow is accepted since it's been expected.
Note this lookup only occurs for the first packet that is part of one
newly established flow, not for all packets.</para>
<para>Since 1.4.0, conntrackd provides the infrastructure to develop
helpers in user-space. The main features of the user-space infrastructure
for helpers are:</para>
<itemizedlist>
<listitem><para>Rapid connection tracking helper development, as developing code
in user-space is usually faster.</para></listitem>
<listitem><para>Reliability: A buggy helper does not crash the kernel. If the helper
fails, ie. the conntrackd crashes, Moreover, we can monitor the helper process
and restart it in case of problems.</para></listitem>
<listitem><para>Security: Avoid complex string matching and mangling in
kernel-space running in privileged mode. Going further, we can even think
about running user-space helper as a non-root process.</para></listitem>
<listitem><para>It allows the development of very specific helpers for
proprietary protocols that are not standard. This is the case of the SQL*net
helper. Implementing this in kernel-space may be problematic, since
this may not be accepted for ainline inclusion in the Linux kernel.
As an alternative, we can still distribute this support as separate
patches. However, my personal experience is that, given that the
kernel API/ABI is not stable, changes in the interface lead to the
breakage of the patch. This highly increase the overhead in the
maintainance.</para></listitem>
</itemizedlist>
<para>Currently, the infrastructure supports the following user-space helpers:
</para>
<itemizedlist>
<listitem><para>Oracle*TNS, to support its special <emphasis>Redirect</emphasis> message.</para></listitem>
<listitem><para>NFSv3, mind that version 4 does not require this helper.</para></listitem>
<listitem><para>FTP (this helper is also available in kernel-space).</para></listitem>
<listitem><para>SSDP.</para></listitem>
</itemizedlist>
<para>The following steps describe how to enable the RPC portmapper helper for NFSv3 (this is similar for other helpers):</para>
<orderedlist>
<listitem><para>Register user-space helper:
<programlisting>
nfct add helper rpc inet udp
nfct add helper rpc inet tcp
</programlisting>
This registers the portmapper helper for both UDP and TCP (NFSv3 traffic goes both over TCP and UDP).
</para></listitem>
<listitem><para>Add iptables rule using the CT target:
<programlisting>
# iptables -I OUTPUT -t raw -p udp --dport 111 -j CT --helper rpc
# iptables -I OUTPUT -t raw -p tcp --dport 111 -j CT --helper rpc
</programlisting>
With this, packets matching port TCP/UDP/111 are passed to user-space for
inspection. If there is no instance of conntrackd configured to support
user-space helpers, no inspection happens and packets are not sent to
user-space.</para></listitem>
<listitem><para>Add configuration to conntrackd.conf:
<programlisting>
Helper {
Type rpc inet udp {
QueueNum 1
QueueLen 10240
Policy rpc {
ExpectMax 1
ExpectTimeout 300
}
}
Type rpc inet tcp {
QueueNum 2
QueueLen 10240
Policy rpc {
ExpectMax 1
ExpectTimeout 300
}
}
}
</programlisting>
This configures conntrackd to use NFQUEUE queue numbers 1 and 2 to send traffic
for inspection to user-space</para>
<note><title>If you have some custom libnetfilter_queue application</title>
<para>
Make sure your queue numbers do not collide with those used in your
conntrackd.conf file.
</para>
</note>
</listitem>
</orderedlist>
<para>Now you can test this (assuming you have some working NFSv3 setup) with:
<programlisting>
mount -t nfs -onfsvers=3 mynfs.server.info:/srv/cvs /mnt/
</programlisting>
</para>
<para>You should see new expectations being added via:
<programlisting>
# conntrack -E expect
[NEW] 300 proto=17 src=1.2.3.4 dst=1.2.3.4 sport=0 dport=54834 mask-src=255.255.255.255 mask-dst=255.255.255.255 sport=0 dport=65535 master-src=1.2.3.4 master-dst=1.2.3.4 sport=58190 dport=111 PERMANENT class=0 helper=rpc
[NEW] 300 proto=6 src=1.2.3.4 dst=1.2.3.4 sport=0 dport=2049 mask-src=255.255.255.255 mask-dst=255.255.255.255 sport=0 dport=65535 master-src=1.2.3.4 master-dst=1.2.3.4 sport=55450 dport=111 PERMANENT class=0 helper=rpc
[NEW] 300 proto=17 src=1.2.3.4 dst=1.2.3.4 sport=0 dport=58031 mask-src=255.255.255.255 mask-dst=255.255.255.255 sport=0 dport=65535 master-src=1.2.3.4 master-dst=1.2.3.4 sport=56309 dport=111 PERMANENT class=0 helper=rpc
</programlisting>
</para>
</sect1>
<sect1 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>
<qandaentry>
<question>
<para>
Does conntrackd support TCP flow-recovery with window tracking enabled?
</para>
</question>
<answer>
<para>
Yes, but you require a Linux kernel &gt;= 2.6.36 and the conntrack-tools &gt;= 0.9.15. To enable it, check the TCPWindowTracking clause in the example configuration files.
</para>
</answer>
</qandaentry>
<qandaentry>
<question>
<para>
Does conntrackd support the H.323 and SIP connection tracking helpers?
</para>
</question>
<answer>
<para>
Yes, conntrackd includes expectation support since version 1.2.0.
</para>
</answer>
</qandaentry>
<qandaentry>
<question>
<para>
Is there any way to set up a more verbose mode in the log message for debugging?
</para>
</question>
<answer>
<para>
No, but conntrackd provides lots of information that you can look up in
runtime via -s option.</para>
<para>You can check network statistics to find anomalies:</para>
<programlisting>
# conntrackd -s network
network statistics:
recv:
Malformed messages: 0
Wrong protocol version: 0
Malformed header: 0
Malformed payload: 0
Bad message type: 0
Truncated message: 0
Bad message size: 0
send:
Malformed messages: 0
sequence tracking statistics:
recv:
Packets lost: 42726
Packets before: 0
UDP traffic (active device=eth3):
564232 Bytes sent 1979844 Bytes recv
2844 Pckts sent 8029 Pckts recv
0 Error send 0 Error recv
</programlisting>
<para>You can check cache statistics:</para>
<programlisting>
# conntrackd -s cache
cache:internal active objects: 0
active/total entries: 0/ 0
creation OK/failed: 11068/ 0
no memory available: 0
no space left in cache: 0
update OK/failed: 4128/ 0
entry not found: 0
deletion created/failed: 11068/ 0
entry not found: 0
cache:external active objects: 0
active/total entries: 0/ 0
creation OK/failed: 10521/ 0
no memory available: 0
no space left in cache: 0
update OK/failed: 8832/ 0
entry not found: 0
deletion created/failed: 10521/ 0
entry not found: 0
</programlisting>
<para>You can check runtime miscelaneous statistics:</para>
<programlisting>
# conntrackd -s runtime
daemon uptime: 14 min
netlink stats:
events received: 24736
events filtered: 0
events unknown type: 0
catch event failed: 0
dump unknown type: 0
netlink overrun: 0
flush kernel table: 1
resync with kernel table: 0
current buffer size (in bytes): 8000000
runtime stats:
child process failed: 0
child process segfault: 0
child process termsig: 0
select failed: 0
wait failed: 0
local read failed: 0
local unknown request: 0
</programlisting>
<para>You can check dedicated link statistics:</para>
<programlisting>
# conntrackd -s link
UDP traffic device=eth3 status=RUNNING role=ACTIVE:
566848 Bytes sent 1982612 Bytes recv
3018 Pckts sent 8203 Pckts recv
0 Error send 0 Error recv
</programlisting>
<para>You can check network queue statistics:</para>
<programlisting>
# conntrackd -s queue
allocated queue nodes: 1
queue txqueue:
current elements: 0
maximum elements: 2147483647
not enough space errors: 0
queue errorq:
current elements: 0
maximum elements: 128
not enough space errors: 0
queue rsqueue:
current elements: 1
maximum elements: 131072
not enough space errors: 0
</programlisting>
</answer>
</qandaentry>
</qandaset>
</sect1>
</chapter>
</book>