Re: #2391 Network.listenOn (PortNumber n) Sometimes Picks IPv6

On 2008-07-21 10:09 +0000 (Mon), Aaron Denney wrote:

On 2008-07-21, Curt Sampson wrote:

...

1. Listening on both IPV4 and IPV6, if I recall correctly, introduces all sorts of interesting problems, and basically, you don't want to go there. Or am I wrong about that?

I don't see why it would introduce any more problems than listening on multiple IPv4 addresses, which is routine.

If I recall correctly: If you're listening on interface '*', you just call accept() and block. If you want to listen on two separate sockets, you can't do this. You can listen on IPv4 and IPv6 addresses using one socket by using IPv4 to IPv6 translation, but that was what got interesting. I think that this was more or less what you were saying. Anyway, I'm quite happy with having listenOn listen on all interfaces that implement v4 and all interfaces that implement v6; it's just whether it's implementable in some reasonable way that I worry about. And you'll note, for example, we already distinguish between a PortID for listening to "one of v4 or v6, depending on OS" (PortNumber Int) and for listening on Unix domain sockets (UnixSocket String).

Basically, my position is that listenOn is the wrong interface -- it should take the host addresses as well as the port.

Well, perhaps the idea there was that if you wanted to start doing this, you should be using Network.Socket instead of Network, anyway.

The ports are the same namespace -- it shouldn't matter (much) whether TCP is running over IPv4 or IPv6.

If you bind by hostname (e.g., "localhost") they have separate namespaces. I suppose you could take only addresses, but then you need some way to construct them, and before you know it you're starting to get as complex as Network.Socket.

We could get around this by returning a composite pseudo-socket, representing both, and modifying Network.accept to take this pseudo-socket and wait for connections on any of the real sockets. But it's a bit ugly. And these pseudo-sockets won't work with Network.Socket.accept. If we give it a new type, I'm not entirely sure it's that bad a thing though.

That's not an unreasonable option. But it's a lot more work to implement. I think at this point I'm preferring the simple thing: just go for PortNumber Int and PortNumber6 Int data constructors, and if you want to do better than that, use Network.Socket. cjs -- Curt Sampson +81 90 7737 2974 Mobile sites and software consulting: http://www.starling-software.com