NWMulticastGroup crash when socket address already in use

Question

Created Jun ’20

Replies 3

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Participants 3

iOS 14 introduces multicast support in the Network framework in the form of NWMulticastGroup.

Following Apple's guide ( https://developer.apple.com/news/?id=0oi77447 ) I am trying to join the multicast group for SSDP using the following code:

Code Block swiftlet host = NWEndpoint.Host(ipFamily.ssdpGroupName)
let multicastGroup = try NWMulticastGroup(for: [.hostPort(host: host, port: .ssdp)])
let ssdpGroup = NWConnectionGroup(with: multicastGroup, using: .udp)
ssdpGroup.stateUpdateHandler = { newState in
	Logger.info("Reached new state: \(newState)")
}
ssdpGroup.setReceiveHandler(maximumMessageSize: 16384, rejectOversizedMessages: false, handler: {[weak self] (message, content, isComplete) in
	Logger.info("SSDP message received, do something here...")
})
ssdpGroup.start(queue: delegateQueue)

However, it seems that the port is already in use by another application.

Now I have the following issues:

It reaches the .ready state and then crashes, instead of reaching the error state...
Is there a way to configure the socket to allow socket reuse? This is important to prevent clashes with e.g. the Spotify app.
Is it possible to send a message to the multicast group from a different port? (destination port and source port are different)

Boost

Answer 1

phirk OP

Aug ’20

This is what we see in Xcode (where 192.168.1.165 is the IP address of the device that we are trying to discover via SSDP):

Code Block  lang-none2020-08-18 16:36:44.804834+0200 DemoApp[632:112993] [] nw_path_evaluator_create_flow_inner NECP_CLIENT_ACTION_ADD_FLOW <some-UUID> [48: Address already in use]
2020-08-18 16:36:44.806020+0200 DemoApp[632:112993] [connection] nw_endpoint_flow_setup_channel [C8 192.168.1.165:1900 initial channel-flow (satisfied (Path is satisfied), interface: en0, scoped, ipv4, dns)] failed to request add nexus flow
2020-08-18 16:36:44.808119+0200 DemoApp[632:112993] [connection] nw_endpoint_handler_create_from_protocol_listener [C8 192.168.1.165:1900 failed channel-flow (satisfied (Path is satisfied), interface: en0, scoped, ipv4, dns)] nw_endpoint_flow_pre_attach_protocols
2020-08-18 16:36:44.808930+0200 DemoApp[632:112993] [connection] nw_connection_create_from_protocol_listener_on_nw_queue [C8] Failed to create connection from listener
2020-08-18 16:36:44.809410+0200 DemoApp[632:112993] [] nw_ip_channel_inbox_handle_new_flow nw_connection_create_from_protocol_listener_on_nw_queue failed

And this is the crash (Thread 10: EXC_BAD_ACCESS (code=1, address=0x10)) stack trace that we see after a while:

Code Block  lang-nonelibnetwork.dylib`__57-[NWConcrete_nw_listener handleInbound:addProtocolInbox:]_block_invoke:
    0x19e1530c8 <+0>:  ldp    x8, x1, [x0, #0x20]
    0x19e1530cc <+4>:  mov    x2, x8
->  0x19e1530d0 <+8>:  ldr    x3, [x2, #0x10]!
    0x19e1530d4 <+12>: mov    x0, x8
    0x19e1530d8 <+16>: braa   x3, x2

Is this an iOS 14 Beta 4 issue? Or are we doing something wrong?

0

Answer 2

phirk OP

Aug ’20

We had the crash with iOS 14 Beta 4 and Xcode 12 Beta 4, but can't reproduce it with iOS 14 Beta 5 and Xcode 12 Beta 5 (released a few hours ago) so far.

However, SSDP is still failing for us due to the address already being in use ([48: Address already in use]) with Beta 5.

0

Answer 3

DTS Engineer OP

Apple

Sep ’20

However, SSDP is still failing for us due to the address already being
in use ([48: Address already in use]) with Beta 5.

What port number are you using for SSDP?

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0