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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
<!ENTITY VERSION "0.6">
]>
<article>
  <articleinfo>
    <title>NPTv6 (IPv6-to-IPv6 Network Prefix Translation) for Linux</title>

    <author>
      <firstname>Sven-Ola</firstname>

      <surname>Tuecke</surname>

      <affiliation>
        <orgname>Freifunk</orgname>
      </affiliation>
    </author>
    <author>
      <firstname>Matthias</firstname>

      <surname>Schiffer</surname>

      <affiliation>
        <orgname>Freifunk L├╝beck</orgname>
      </affiliation>
    </author>

    <pubdate>10-NOV-2011</pubdate>
  </articleinfo>

  <para>These files implement a Linux netfilter target that changes the IPv6
  address of packets. The address change is done checksum neutral, thus no
  checksum re-calculation for the packet is necessary. You can change the IPv6
  source address of outgoing packets as well as the IPv6 destination address
  of incoming packets. This allows you to map an internal IPv6 address range
  to a second, externally used IPv6 address range. IPv6 address mapping is not
  very similar to IPv4 network address translation, but one can describe it as
  some sort of stateless NAT. The implementation is based on the expired IETF
  discussion paper published here:</para>

  <para><ulink
  url="https://tools.ietf.org/html/rfc6296">https://tools.ietf.org/html/rfc6296</ulink></para>

  <warning>
    <para>Using NPTv6 rules together with connection tracking rules such as
    <userinput>--ctstate</userinput> is currently untested and may not work or
    may cause dysfunctions.</para>
  </warning>

  <section id="install">
    <title id="install-title">Installation</title>

    <para>NPTv6 implements two pieces of software: a shared library that
    extends the ip6tables command and a Linux kernel module. The shared
    library file adds the '-j SNPTV6' target (for source address translation)
    and the '-j DNPTV6' target (for destination address translation) to the
    ip6tables command. To build and install, you need ip6tables installed as
    well as the necessary headers. The Linux kernel module requires the Linux
    source file tree and kernel configuration files to compile. On a Debian/(EKU)buntu,
    the following command prepares the build environment:</para>

    <programlisting>sudo apt-get install build-essential linux-headers iptables-dev</programlisting>

    <para>Unpack the source tgz archive below <filename>/usr/src</filename>,
    change to the new sub-directory and issue "make" to build. If this
    compiles without errors, install the ip6tabless extension by copying
    libip6t_SNPTV6.so and libip6t_DNPTV6.so to the iptables module directory,
    which is probably located under <filename>/lib/xtables</filename> or
    <filename>/usr/lib/iptables</filename>.</para>

    <note>
      <para>The kernel module (<filename>ip6t_MAP66.ko</filename> is not
      automatically installed nor loaded into the kernel. You can copy the
      kernel module file manually, e.g. with <userinput>sudo cp ip6t_MAP66.ko
      /lib/modules/$(uname -r)/</userinput>.</para>
    </note>
  </section>

  <section id="dkms">
    <title id="dkms-title">DKMS Integration</title>

    <para>If the next system update needs to install a new kernel version, you
    also need to re-compile/re-install the MAP66 kernel module. With
    Debian/(EKU)buntu, this can be automated with the Dynamic Kernel Module
    Support Framework (DKMS). For this, the <filename>dkms.conf</filename>
    file is included with the MAP66 source file package. Install DKMS with the
    following command:</para>

    <programlisting>sudo apt-get install dkms</programlisting>

    <para>If not already in place, move/unpack the MAP66 source file archive
    below <filename>/usr/src/</filename>. To register the MAP66 source to DKMS
    and compile/install, issue these commands:</para>

    <programlisting>sudo dkms add -m ip6t_NPTV6 -v &VERSION;
sudo dkms build -m ip6t_NPTV6 -v &VERSION;
sudo dkms install -m ip6t_NPTV6 -v &VERSION;</programlisting>

    <para>Read DKMS details here: <ulink
    url="Read DKMS details here: https://wiki.kubuntu.org/Kernel/Dev/DKMSPackaging">https://wiki.kubuntu.org/Kernel/Dev/DKMSPackaging</ulink></para>
  </section>

  <section id="config">
    <title id="config-title">Configuration</title>

    <section id="config-brief">
      <title id="config-brief-title">Brief Version</title>

      <para>You always need to add two ip6tables-rules to your netfilter
      configuration. One rule matches outgoing packets and changes their IPv6
      source address. The second rule matches incoming packets and reverts the
      address change by altering their IPv6 destination address. To following
      commands correspond to the <quote>Address Mapping Example</quote> given
      in the IETF discussion paper:</para>

      <programlisting>ip6tables -t mangle -A PREROUTING  -i eth0 -d 2001:0DB8:0001::/48 -j DNPTV6 --to-destination FD01:0203:0405::/48
ip6tables -t mangle -A POSTROUTING -o eth0 -s FD01:0203:0405::/48 -j SNPTV6 --to-source 2001:0DB8:0001::/48</programlisting>

      <para>This example is also printed to the screen if you issue
      <userinput>ip6tables -j SNPTV6 --help</userinput>. By design, you cannot
      use prefix lengths longer than 64.</para>
    </section>

    <section id="config-nat-behaviour">
      <title id="config-nat-behaviour-title">NAT Behavioral Requirements</title>

      <para>RFC 6296 states that NPTv6 translators must support hairpinning behaviour.
      This means that when an NPTv6 Translator receives a datagram on the
      internal interface that has a destination address that matches the
      site's external prefix, it will translate the datagram and forward it
      internally. While it is possible that the translator works correctly
      without this depending on the network configuration, it is desiarable
      to have hairpinning behaviour. The following iptables rules will enable
      this:</para>

      <programlisting>ip6tables -t mangle -A PREROUTING -d 2001:0DB8:0001::/48 -j MARK --set-mark 42
ip6tables -t mangle -A PREROUTING -d 2001:0DB8:0001::/48 -j DNPTV6 --to-destination FD01:0203:0405::/48
ip6tables -t mangle -A POSTROUTING -m mark --mark 42 -s FD01:0203:0405::/48 -j SNPTV6 --to-source 2001:0DB8:0001::/48
ip6tables -t mangle -A POSTROUTING -o eth0 -s FD01:0203:0405::/48 -j SNPTV6 --to-source 2001:0DB8:0001::/48</programlisting>
    </section>
  </section>

  <section id="precedence">
    <title id="precedence-title">IPv6/IPv4 Precedence</title>

    <para>With (EKU)buntu and eventually with RedHat, you will notice that
    your browser does not show the IPv6 version of a web site that is
    multi-homed when using ULA addresses for your IPv6 Internet connection.
    The reason for this is an add on to the RFC 3484 rules that is compiled
    into the (EKU)buntu libc. The pre-installed
    <filename>/etc/gai.conf</filename> file will give you a hint on
    this.</para>

    <para>In short: the getaddrinfo() library function rates a private IPv4
    address higher than the ULA IPv6 address when choosing the transport
    protocol for a new Internet connection if this add on to the RFC 3484
    rules is compiled in. For this reason, you may want to change the
    precedence rules within <filename>/etc/gai.conf</filename> (see <xref
    endterm="precedence-gai-title" linkend="precedence-gai" />) or use another
    prefix (see <xref endterm="precedence-addrs-title"
    linkend="precedence-addrs" />).</para>

    <section id="precedence-gai">
      <title id="precedence-gai-title">Change gai.conf</title>

      <para>The getaddrinfo() library function manages lists of label,
      precedence, and scope4 type entries. If the
      <filename>/etc/gai.conf</filename> file does not provide a single entry
      for a particular type, the compiled-in list is used. For this reason,
      you cannot uncomment a single entry to overwrite the default. You need
      to uncomment all entries of a particular type for this. The
      <quote>label</quote> lines compare source addresses, the
      <quote>precedence</quote> lines compare destination addresses.</para>

      <procedure>
        <title>Change IPv6 Precedence</title>

        <step>
          <para>Open the <filename>/etc/gai.conf</filename> file as root user,
          e.g. by executing <userinput>sudo nano
          /etc/gai.conf</userinput>.</para>
        </step>

        <step>
          <para>Remove the leading hash character from the 8 lines starting
          with <quote>#label</quote>.</para>
        </step>

        <step>
          <para>Re-add the hash character to the line stating <quote>#label
          fc00::/7 6</quote>.</para>
        </step>

        <step>
          <para>Save the file.</para>
        </step>

        <step>
          <para>Restart your browser and re-try to browse to a multi-homed web
          site.</para>
        </step>
      </procedure>

      <para>The above procedure removes the difference between standard IPv6
      source addresses and ULA type private IPv6 source addresses. Anything
      else is unchanged.</para>
    </section>

    <section id="precedence-addrs">
      <title id="precedence-addrs-title">Use Changed Internal Address</title>

      <para>As an alternative solution, you may use an arbitrary address
      prefix in your LAN that is not mentioned in the
      <filename>gai.conf</filename> file nor compiled in. This will work but
      introduces a double mapping: one map (Inet-ULA) on the Internet gateway
      router and a second map (ULA-Intern) on the internal router. </para>

      <note>
        <para>While the well known IPv4 addresses 10.0.0.0/8, 172.16.0.0/12,
        and 192.168.0.0/16 still exist, it is unlikely that their 6to4
        counterparts 2002:0a00::/24, 2002:ac10::/28, and 2002:c0a8::/32 will
        be routed on the Internet. Sadly, the (EKU)buntu defaults penalize
        6to4 addresses also.</para>
      </note>
    </section>
  </section>

  <section id="motivation">
    <title id="motivation-title">Motivation</title>

    <para>My Internet access at home is realized by a wireless community mesh
    network not owned by me. The mesh is operated with small embedded devices
    (nodes aka. WLAN routers) that are interconnected via radio links (WLAN
    IBSS / AdHoc). Routing is done with a specialized protocol such as Batman
    or OLSR. The routing protocol selects the nearest out of a dozen Internet
    gateways and configures a default route or an IPIP tunnel accordingly.
    Each Internet gateway is connected to a different ISP and provides the
    service with the help of IPv4 network address translation (NAT). Using NAT
    has the following effects:</para>

    <itemizedlist>
      <listitem>
        <para>Address amplification - something not necessary with IPv6 any
        more</para>
      </listitem>

      <listitem>
        <para>Anonymization - nice to have as an option but not mission
        critical</para>
      </listitem>

      <listitem>
        <para>ISP independence - no reverse routing, no
        "buy-a-number-range"</para>
      </listitem>
    </itemizedlist>

    <para>The last point <emphasis role="bold">is</emphasis> mission critical.
    One can obtain a provider independent IPv6 address range, but you need the
    cooperation of an ISP to use that address range for Internet connectivity.
    If you e.g. move to another ISP you need your address range to be
    re-routed to your new location.</para>

    <para>ISP independence is also possible with some tunneling technique,
    such as VPN or mobile IP. Tunneling can be implemented on client PCs and
    Internet gateways/servers one day. But there is no need to implement the
    same tunneling technique on every mesh node. Why? Because the mesh nodes
    can use private IP addresses (or "ULA") to transport the tunnel data
    between the client PC and the gateway/server. Each tunneling technique
    typically needs a single instance (the "server") which forms a single
    point of failure. Rule-of-thumb1: avoid a SPOF for the infrastructure.
    Rule-of-thumb2: KISS (keep it simple stupid).</para>

    <para>Using private IP addresses on the mesh nodes has a drawback: mesh
    node software updates e.g. a download via HTTP from an Internet server is
    not possible. This is where I start to think: <quote>hey, some kind of
    address mapping may be nice to have</quote>. While opening Pandora's NAT66
    box, I discovered that IPv6 nerds do not like the acronym. It is always a
    good tactic in info wars to rename, hence the name "MAP66".</para>

    <para>// Sven-Ola</para>
  </section>
</article>