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!
The socket API is governed by IETF RFCs and should be the same on all platforms including windows WRT IPv6.
For IPv4/IPv6 applications it's ALL about getaddrinfo() and getnameinfo(). getaddrinfo is a genius - 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 you're going the dual-stack route and want it to return IPv4-mapped IPv6 addresses, it will do that too.
It provides a direct sockaddr * structure that can be plugged into bind(), recvfrom(), sendto() and the 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 it`s 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.
As Craig M. Brandenburg observes, getaddrinfo does all the heavy lifting to make dual IPv4/IPv6 possible. I have an experimental server and client on my localhost. I use this in the server:
hints.ai_family = AF_INET6;
hints.ai_socktype = SOCK_DGRAM;
hints.ai_flags = AI_PASSIVE;
...
The client can then connect to the server using any kind of address:
hints.ai_family = AF_UNSPEC;
hints.ai_socktype = SOCK_DGRAM;
host_port = "4950"; // whatever
// All of these work.
host_ip = "127.0.0.1"; // Pure IPv4 address
host_ip = "::ffff:127.0.0.1"; // IPv4 address expressed as IPv6
host_ip = "::1"; // Pure IPv6 address
host_ip = "localhost"; // Domain name
int rv = getaddrinfo(host_ip, host_port, &hints, &result);
...