GCDAsyncUDPSocket：无法使IPv6上的UDP多播正常工作

Question

I cannot get UDP multicast over IPv6 to work. The platform on which I'm trying to do this is iOS (using Swift). I have a GCDAsyncUdpSocket and configure it like this:

self.socket!.setIPv4Enabled(false)
self.socket!.setIPv6Enabled(true)
do {
    try self.socket!.bindToPort(Announcement.ipv6BroadcastPort, interface: "en0")
}
catch let error {
    print("bind failed: \(error)")
    throw error
}
do {
    try self.socket!.joinMulticastGroup("ff32::5222", onInterface: "en0")
}
catch let e {
    print("join multicast failed: \(e)")
}

After that, I try to send some data to a multicast address:

socket.sendData(data, toHost:"ff32::5222", port: 21026, withTimeout: 5, tag: 0)

At that point, when stepping through the GCDAsyncUDPSocket code, I end up in a call to sendto in the method doSend (line 3919). This call returns -1 and the error is "no route to host". I'm running this in the iOS simulator. When I enumerate all available network interfaces, I get this:

Name: lo0
Address: Optional("::1ebc:680b:100:0")
Family: NetUtils.Interface.Family.ipv6
Supports multicast: true

Name: lo0
Address: Optional("::2bc:680b:100:0")
Family: NetUtils.Interface.Family.ipv4
Supports multicast: true

Name: lo0
Address: Optional("fe80::1ebc:680b:100:0")
Family: NetUtils.Interface.Family.ipv6
Supports multicast: true

Name: en0
Address: Optional("fe80::1ebc:680b:100:0")
Family: NetUtils.Interface.Family.ipv6
Supports multicast: true

Name: en0
Address: Optional("2001:984:3427:1:1ebc:680b:100::")
Family: NetUtils.Interface.Family.ipv6
Supports multicast: true

Name: en0
Address: Optional("2001:984:3427:1:1ebc:680b:100::")
Family: NetUtils.Interface.Family.ipv6
Supports multicast: true

Name: en0
Address: Optional("::2bc:680b:100:0")
Family: NetUtils.Interface.Family.ipv4
Supports multicast: true

Name: awdl0
Address: Optional("fe80::1ebc:680b:100:0")
Family: NetUtils.Interface.Family.ipv6
Supports multicast: true

Name: utun0
Address: Optional("fe80::1ebc:680b:100:0")
Family: NetUtils.Interface.Family.ipv6
Supports multicast: true

Name: utun0
Address: Optional("fde6:4b33:67e4:9d5e:1ebc:680b:100::")
Family: NetUtils.Interface.Family.ipv6
Supports multicast: true

Any help is much appreciated!

Answer 1

I have solved my issue, I succeeded in doing a send/receive over IPv6 UDP Multicast. The problem turned out to be a combination of issues.

• For sending these messages, it is required to set the socket option IPV6_MULTICAST_IF , which GCDAsyncUdpSocket does not do. (Receiving doesn't seem to require it.)
• When sending a message, you do have to bind before you can send (but don't bind to a specific port, because what you're binding is the source port, not the destination port).
• When you try to set a Swift object as a delegate of an Objective-C object, make sure your Swift object inherits from NSObject , or be screwed! No warnings, no errors, the delegate simply doesn't get called.
• Wireshark was my friend again :). I could see that the socket started listening because of the ICMPv6 "Multicast Listener Report v2" coming by, with contents: "Multicast Address Record Changed to exclude: ff12::5222". Note that "exclude" means that it does not get filtered away, in other words: that it is passed on. (Thanks Paul!)

Working proof of concept here: https://source.ind.ie/project/pulse-swift/blob/28bd0f48e3d4fa99f6095321a89036863c3f7a83/pulse-swift/discovery/Greeter.swift