Question

From Beej’s Guide to Network programming

You can force it to use IPv4 or IPv6 in the ai_family field, or leave it as AF_UNSPEC to use whatever. This is cool because your code can be IP version-agnostic.

As the title says – what would be the disadvantages (or risks, if any) of always using AF_UNSPEC, instead of specifying IPv4 or IPv6?

Or it’s only for one reason – if the version is specified, this will guarantee that this and only this version is supported?

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A little background – I think about adding support for IPv6 in client-server (C++) applications and both versions should be supported. So I wondered if it’s fine to use AF_UNSPEC or it’s better to “recognize” the address from the string and use AF_INET6 or AF_INET, depending on the address.

Answer 1

You have to differentiate between client and server applications.

On the client, it is easy: just call getaddrinfo() and try each of the answers in sequence until you get a connection.

On the server, things are a little bit harder:

• There are systems whose IPv4 and v6 stacks are interconnected, there it is enough to just listen on IPv6. Maybe the socket has to be enabled to listen to both.
• Other systems, like Windows XP, have separated stacks where this connection is not possible. There you would have to work with several sockets at once. Let me concentrate on those in the following.

Even on servers, getaddrinfo() can be used. There you use the flag AI_PASSIVE in the hints. Then you get results. On these all you’ll have to listen, perhaps enabling the IPV6_V6ONLY flag.

accept() should either be done non-blocking or with select() or poll() (not sure if the latter is possible).