Table of Contents
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:
https://tools.ietf.org/html/rfc6296
Using NPTv6 rules together with connection tracking rules such as
--ctstate is currently untested and may not work or
may cause dysfunctions.
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:
sudo apt-get install build-essential linux-headers iptables-dev
Unpack the source tgz archive below /usr/src,
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 /lib/xtables or
/usr/lib/iptables.
The kernel module (ip6t_MAP66.ko is not
automatically installed nor loaded into the kernel. You can copy the
kernel module file manually, e.g. with sudo cp ip6t_MAP66.ko
/lib/modules/$(uname -r)/.
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 dkms.conf
file is included with the MAP66 source file package. Install DKMS with the
following command:
sudo apt-get install dkms
If not already in place, move/unpack the MAP66 source file archive
below /usr/src/. To register the MAP66 source to DKMS
and compile/install, issue these commands:
sudo dkms add -m ip6t_NPTV6 -v 0.6 sudo dkms build -m ip6t_NPTV6 -v 0.6 sudo dkms install -m ip6t_NPTV6 -v 0.6
Read DKMS details here: https://wiki.kubuntu.org/Kernel/Dev/DKMSPackaging
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 “Address Mapping Example” given in the IETF discussion paper:
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
This example is also printed to the screen if you issue
ip6tables -j SNPTV6 --help. By design, you cannot
use prefix lengths longer than 64.
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:
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
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
/etc/gai.conf file will give you a hint on
this.
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 /etc/gai.conf (see Change gai.conf) or use another
prefix (see Use Changed Internal Address).
The getaddrinfo() library function manages lists of label,
precedence, and scope4 type entries. If the
/etc/gai.conf 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
“label” lines compare source addresses, the
“precedence” lines compare destination addresses.
Procedure 1. Change IPv6 Precedence
Open the /etc/gai.conf file as root user,
e.g. by executing sudo nano
/etc/gai.conf.
Remove the leading hash character from the 8 lines starting with “#label”.
Re-add the hash character to the line stating “#label fc00::/7 6”.
Save the file.
Restart your browser and re-try to browse to a multi-homed web site.
The above procedure removes the difference between standard IPv6 source addresses and ULA type private IPv6 source addresses. Anything else is unchanged.
As an alternative solution, you may use an arbitrary address
prefix in your LAN that is not mentioned in the
gai.conf 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.
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.
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:
Address amplification - something not necessary with IPv6 any more
Anonymization - nice to have as an option but not mission critical
ISP independence - no reverse routing, no "buy-a-number-range"
The last point is 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.
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).
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: “hey, some kind of address mapping may be nice to have”. 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".
// Sven-Ola