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I'm currently working on a UDP socket application and I need to build in support so that IPV4 and IPV6 connections can send packets to a server.

I was hoping that someone could help me out and point me in the right direction; the majority of the documentation that I found was not complete. It'd also be helpful if you could point out any differences between Winsock and BSD sockets.

Thanks in advance!

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4 Answers

up vote 41 down vote accepted

The best approach is to create an IPv6 server socket that can also accept IPv4 connections. To do so, create a regular IPv6 socket, turn off the socket option IPV6_V6ONLY, bind it to the "any" address, and start receiving. IPv4 addresses will be presented as IPv6 addresses, in the IPv4-mapped format.

The major difference across systems is whether IPV6_V6ONLY is a) available, and b) turned on or off by default. It is turned off by default on Linux (i.e. allowing dual-stack sockets without setsockopt), and is turned on on most other systems.

In addition, the IPv6 stack on Windows XP doesn't support that option. In these cases, you will need to create two separate server sockets, and place them into select or into multiple threads.

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Thanks for this information, exactly what I was looking for. –  Charles Oct 24 '09 at 16:01
Saying that IPV6_V6ONLY is off by default on Linux is wrong: it depends on the operating system, not just on the kernel. For instance, on Debian GNU/Linux, it recently switched to on by default. –  bortzmeyer Sep 18 '10 at 17:21
OS X also has it off by default, but the best thing is to always set it explicitly. The local sysadmin might've changed it after all. –  Per Johansson Nov 13 '11 at 16:49
The default on Windows is enabled (just implemented this on Win7). –  tdistler Oct 2 '12 at 22:09
@Andrius Bentkus: as Windows XP demonstrates, it is well possible to have a system where you can simultaneously use IPv4 and IPv6, yet IPV6_V6ONLY is not available. Whether or not this is "dual stacking" depends on your definition of that term. –  Martin v. Löwis Dec 27 '12 at 23:29
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The socket API is governed by IETF RFCs and should be the same on all platforms including windows WRT IPv6.

For IPv4/IPv6 applications its ALL about getaddrinfo() and getnameinfo(). getaddrinfo is a genious - looks at DNS, port names and capabilities of the client to resolve the eternal question of can I use IPv4, IPv6 or both to reach a particular destination? Or if your going dualstack route and want it to return IPv4 mapped IPv6 addresses it will do that too.

It provides direct sockaddr * structure that can be plugged into bind(), recvfrom(), sendto() and address family for socket()... In many cases this means no messy sockaddr_in(6) structures to fill out and deal with.

For UDP implementations I would be careful about setting dual stack sockets or more generally binding to all interfaces (INADDR_ANY) The classic issue is that when addresses are not locked down (see bind()) to specific interfaces and the system has multiple interfaces requests responses may transit from different addresses for computers with multiple addresses based on the whims of the OS routing table confusing application protocols especially any systems with authentication requirements.

For UDP implementations where this is not a problem or TCP ... dual stack sockets can save a lot of time when IPv* enabling your system. One must be careful to not rely entirely on dual stack where its not absolutely necessary as there are no shortage of reasonable platforms (Old linux,BSD,Windows 2003) deployed with IPv6 stacks not capable of dual stack sockets.

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The RFCs don't really specify the existence of the IPV6_V6ONLY socket option, but, if it is absent, the RFCs are pretty clear that the implementation should be as though that option is FALSE.

Where the option is present, I would argue that it should default FALSE, but, for reasons passing understanding, BSD and Windows implementations default to TRUE. There is a bizarre claim that this is a security concern because an unknowing IPv6 programmer could bind thinking they were binding only to IN6ADDR_ANY for only IPv6 and accidentally accept an IPv4 connection causing a security problem. I think this is both far-fetched and absurd in addition to a surprise to anyone expecting an RFC-compliant implementation.

In the case of Windows, non-compiance won't usually be a surprise. In the case of BSD, this is unfortunate at best.

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The standard on IPv6 API, RFC 3493, describes IPV6_V6ONLY in its section 5.3 if you want to read all the details. –  bortzmeyer Sep 18 '10 at 17:24
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I've been playing with this under Windows and it actually does appear to be a security issue there, if you bind to the loopback address then the IPv6 socket is correctly bound to [::1] but the mapped IPv4 socket is bound to INADDR_ANY, so your (supposedly) safely local-only app is actually exposed to the world.

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