path: root/README.dbk

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<article>
  <articleinfo>
    <title>MAP66 (NAT from IPv6 to IPv6, NAT66) for Linux</title>

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

      <surname>Tuecke</surname>

      <affiliation>
        <orgname>Freifunk</orgname>
      </affiliation>
    </author>

    <pubdate>07-OCT-2010</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="http://tools.ietf.org/html/draft-mrw-behave-nat66-02">http://tools.ietf.org/html/draft-mrw-behave-nat66-02</ulink></para>

  <warning>
    <para>Using MAP66 rules together with connection tracking rules sich as
    <userinput>--ctstate</userinput> is currently untested and may not work or
    may cause oopses.</para>
  </warning>

  <section>
    <title>Installation</title>

    <para>MAP66 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 MAP66' target 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 ip6tables extension with the
    following command:</para>

    <programlisting>sudo make install</programlisting>

    <note>
      <para>The kernel module (<filename>ip6t_MAP66.ko</filename> for
      Linux-2.6 or <filename>ip6t_MAP66.o</filename> for Linux-2.4) 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>
    <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_MAP66 -v 0.4
sudo dkms build -m ip6t_MAP66 -v 0.4
sudo dkms install -m ip6t_MAP66 -v 0.4</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>
    <title>Configuration</title>

    <section>
      <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 -I POSTROUTING -o eth0 -s FD01:0203:0405::/48 -j MAP66 --to 2001:0DB8:0001::/48
ip6tables -t mangle -I PREROUTING  -i eth0 -d 2001:0DB8:0001::/48 -j MAP66 --to FD01:0203:0405::/48</programlisting>

      <para>This example is also printed to the screen if you issue
      <userinput>ip6tables -j MAP66 --help</userinput>. By design, you cannot
      use an arbitrary prefix length. Only /112, /96 .. /16 are
      supported.</para>

      <para>For each packet, the Linux kernel module also compares the
      packet's source address to all IPv6 addresses assigned to the outgoing
      interface. If a match is found, the packet's source address is not
      mapped. The same comparison happens on the incoming packet's destination
      address. The comparison requires some CPU resources, especially if the
      interface has a large number of assigned IPv6 addresses. If you are sure
      that the mapping cannot match the IPv6 address of the interface (e.g.
      the mapping rule defines a mapping prefix that cannot result in the
      interface address) you can switch off the comparison. Add the
      <userinput>--nocheck</userinput> parameter to the ip6tables command for
      this.</para>
    </section>

    <section>
      <title>Detailed Version</title>

      <para>The following explanation details a living example from the
      wireless mesh network that is mentioned under <xref
      endterm="motivation-title" linkend="motivation" /> (see below).
      Throughout the mesh network, a private IP address range is used. The ULA
      prefix is fdca:ffee:babe::/64. All mesh nodes derive their IPv6
      interface addresses by correlating the ULA prefix with the EUI48
      (<quote>MAC address</quote>) of the respective network adapter.</para>

      <para>There is a Debian based virtual machine that should act as one
      IPv6 Internet gateway for the mesh. You can reach the virtual machine's
      web service via IPv4 under <ulink
      url="http://bbb-vpn.freifunk.net">http://bbb-vpn.freifunk.net</ulink>.
      To experiment with IPv6, a <ulink
      url="http://www.sixxs.net/">SIXXS</ulink> static tunnel setup has been
      added and there is also an experimental 6-to-4 configuration. The
      following <filename>/etc/network/interfaces</filename> file provides the
      configuration for IPv6:</para>

      <programlisting>auto sixxs
iface sixxs inet6 v4tunnel
        address  2001:4dd0:ff00:2ee::2
        netmask 64
        local 77.87.48.7
        endpoint 78.35.24.124
        ttl 64
        up ip link set mtu 1280 dev $IFACE
        up ip route add default via  2001:4dd0:ff00:2ee::1 dev $IFACE
        up ip addr add 2001:4dd0:fe77::1/48 dev $IFACE

#auto tun6to4
iface tun6to4 inet6 v4tunnel
        # ipv6calc --quiet --action conv6to4 77.87.48.7
        address 2002:4d57:3007::1
        netmask 16
        local 77.87.48.7
        endpoint any
        ttl 64
        gateway ::192.88.99.1</programlisting>

      <para>As you can see, the virtual machine has an IPv6 prefix of
      2001:4dd0:fe77::/48 and is reachable via <ulink
      url="http://[2001:4dd0:fe77::1]/">http://[2001:4dd0:fe77::1]/</ulink>.
      For experimental purposes, the 6-to-4 tunnel can be activated by issuing
      <userinput>ifup tun6to4</userinput>. The netfilter setup of this machine
      includes the following command sequence to realize mapping from the
      private fdca:ffee:babe::/64 prefix to the globally valid IPv6
      addresses:</para>

      <programlisting>ip6tables -t mangle -F POSTROUTING
ip6tables -t mangle -F PREROUTING
ip6tables -t mangle -F FORWARD

grep -q ^ip6t_MAP66 /proc/modules &amp;&amp; rmmod ip6t_MAP66
insmod /usr/src/map66/ip6t_MAP66.ko

ip6tables -t mangle -A POSTROUTING -o sixxs -s fdca:ffee:babe::/64 -j MAP66 --to 2001:4dd0:fe77:1::/64 --nocheck
ip6tables -t mangle -A PREROUTING  -i sixxs -d 2001:4dd0:fe77:1::/64 -j MAP66 --to fdca:ffee:babe::/64 --nocheck
ip6tables -t mangle -A POSTROUTING -o tun6to4 -s fdca:ffee:babe::/64 -j MAP66 --to 2002:4d57:3007:1::/64 --nocheck
ip6tables -t mangle -A PREROUTING  -i tun6to4 -d 2002:4d57:3007:1::/64 -j MAP66 --to fdca:ffee:babe::/64 --nocheck
ip6tables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu</programlisting>

      <para>Because for both IPv6 networks the external prefix length is
      smaller than the internal prefix length, we can make sure that the
      mapped addresses cannot match the interface addresses. For example:
      2001:4dd0:fe77:1::/64 cannot be converted to 2001:4dd0:fe77:0::1/128 in
      this context. For this reason, we can use the
      <userinput>--nocheck</userinput> speedup here.</para>

      <para>You may stumble over the MSS-clamping rule. While IPv6 defines,
      that path MTU detection via ICMPv6 must be supported by any host,
      sometimes path MTU detection does not work. The SIXXS tunnel uses an MTU
      of 1280 byte. To get the following command working on my PC, I needed to
      add the above MSS-clamping rule on the gateway:</para>

      <programlisting>wget --prefer-family=IPv6 -O - http://6to4.nro.net/</programlisting>

      <note>
        <para>The tun6to4 tunnel interface is disabled normally, because of
        the implicit 2002::/16 network route configured for that interface.
        This network route ensures, that traffic between one 2002::/16 to
        another 2002::/16 travels directly between the IPv4 hosts. Without
        this network route, any IPv6 traffic will be routed via the 6-to-4
        gateways which may not work and place a higher load on those 6-to-4
        gateways.</para>

        <para>However, if you ping the SIXXS IP address from another host that
        has a 6-to-4 address, you will get the answer packet back via the
        6-to-4 interface. If the above address mapping is configured, you ping
        one IPv6 address and get the answer from another IPv6
        address...</para>
      </note>
    </section>
  </section>

  <section>
    <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>.</para>

    <note>
      <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>
    </note>

    <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="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>