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FreeBSD IPv6-only Support

IPv4, Dual-Stack and what is "IPv6-only"?

For multiple decades the Internet has been running on the Internet Protocol version 4 (IPv4). You may know IPv4 addresses like IPv6, the Internet Protocol version 6, is the successor to that and has a larger address space and longer addresses like 2001:db8:4672:6565:2026:5043:2d42:5344. A dual-stack (DS) system supports both address families, IPv4 and IPv6. Dual-stack is the default for FreeBSD releases shipping at this time and has been since 2003. An IPv6-only system is one that has been configured to use only IPv6 support; IPv4 support is completely removed from the system. Providing IPv6-only snapshots of FreeBSD shows that it has no strict internal dependence on Internet Protocol version 4, and is ready to run in a pure IPv6 environment.

IPv6-only history

The IPv6 only kernel has started as a research project in 2008 when FreeBSD Jails gained IPv6 support and it became possible to have jails without IPv4.

The FreeBSD kernel has long been able to be compiled for dual-stack or without INET6 support. It was time for feature parity in that area as well. As an intermediate step there were efforts to get the kernel compiling without any IP support at all, which helped to narrow down the problematic cases.

As of SVN r221353 it is now possible to compile the kernel without INET but with INET6 support.

How can I get a system without INET support?

There are two ways to get this:

  • The easy way is to download an IPv6-only snapshot we provide and test that put. See the IPv6-only wiki page for the latest snapshot builds and download links.
  • Alternatively you can install any FreeBSD HEAD (9.x or later) as of May 2nd 2011 or later and compile your own kernel. See the Handbook for how to do this.

Doing IPv6-only when the world tries to get to dual-stack?

In a time where you hear numbers that about 4% of end users could actually successfully access IPv6-only services it may indeed sound strange. The BSD network stack however was used as a reference implementation for the first time of TCP/IP and again was for IPv6 and there are some parts of the world already that are limited to IPv6-only. Over time, as IPv6 deployment proceeds, we expect to see a lot more of this.

On the other hand having the IPv4 fall back option of dual-stack hides a lot of IPv6 errors and omissions. Supposedly IPv6-ready software breaks when running without IPv4. We want to help early in the process to catch and fix these problems and want to encourage other software developers to do the same. FreeBSD is used in embedded device and targeting servers while at the same time people build desktop systems with it. This entirety allows us to provide a turnkey solution, an ideal platform for thorough testing.


Do you have a sample kernel configuration for me?

Yes we do. It is as simple as:

include GENERIC
nooptions INET
nodevice gre

I am not doing kernels but desktop applications. What about me?

FreeBSD ships with a lot of applications running fine on command line. If you prefer to test on a preconfigured graphical desktop, PC-BSD, a FreeBSD derived desktop distribution is providing IPv6-only snapshots as well.

Is this limited to FreeBSD?

No. While we provide the kernel and parts of user space for you to start with, a lot of open sources and commercial software running on a UNIX® or UNIX®-like operating system (e.g. ®Linux) should be able to compile and run on FreeBSD with minimal efforts. Also see the next question.

Does everything just work on IPv6-only in FreeBSD

While are doing our best, some things are not yet working without IPv4. Very few parts of the kernel still depend on IPv4 and we are working on these. In user space you can find three different categories:

  1. Software that is maintained as FreeBSD base. We are actively working on these as we find problems. Patches from the community are always welcome.
  2. Software that is shipped with FreeBSD base but imported from a 3rd party project. We are trying our best to get any problems solved and are working with upstream vendors.
    If you are part of such a project you may want to test on IPv6-only yourself to be really IPv6-ready with your next release! Try our snapshots.
  3. Software that is part of FreeBSD ports or any other open source or commercial software running on FreeBSD. We will lend a hand if needed and possible but are hoping for the community to make best use of our snapshots and improve the overall IPv6 readiness of software.