I've been exploring custom IP protocols with Network.framework on macOS and have run into a couple of problems: Loopback & bridge interfaces let parameters = NWParameters(customIPProtocolNumber: 123) let destination = NWEndpoint.hostPort(host: "127.0.0.1", port: 0) let connection = NWConnection(to: destination, using: parameters) connection.start(queue: .main) connection.send(content: Data([1, 2, 3, 4]), completion: .contentProcessed({ error in print("error", error) })) Yields this error when host is directly connected to a virtual interface like bridge100 (the other end of my connection is a Linux VM) or lo0 (as in the example above): [connection] nw_socket_connect [C1:1] connectx(12 (guarded), [srcif=0, srcaddr=<NULL>, dstaddr=127.0.0.1], SAE_ASSOCID_ANY, 0, NULL, 0, NULL, SAE_CONNID_ANY) failed: [49: Can't assign requested address] If I change host to an address directly connected to en0 (my MacBook's Wi-Fi), I get no errors, and I can see the packet in Wireshark. Multicast listeners guard let allSPFRouters = try? NWMulticastGroup(for: [NWEndpoint.hostPort(host: "224.0.0.5", port: 0)]) else { print("Couldn't create NWMulticastGroup") return } let parameters = NWParameters(customIPProtocolNumber: 123) let group = NWConnectionGroup(with: allSPFRouters, using: parameters) group.setReceiveHandler(maximumMessageSize: 16384, rejectOversizedMessages: true) { (message, content, isComplete) in print("received", message, content, isComplete) } group.stateUpdateHandler = { print("state", $0) } group.start(queue: .main) Yields a number of errors, of which this one seems like the most relevant: -[nw_listener_inbox_socket initWithParameters:delegate:] Cannot create listener with IP Protocol 123, dumping backtrace: I did some spelunking in Ghidra, and it seems like nw_listener_inbox_socket_initWithParameters:delegate: might only allows you to join a multicast group if your protocol number is 6 (TCP) or 17 (UDP).  So, the questions are: Is there something obvious I’m doing wrong? Are there any clever workarounds? Here’s a repo with a repro. All the relevant code is in AppDelegate.swift. Tested on macOS 13.0.1 on a 14-inch MacBook Pro, with App Sandbox and Hardened Runtime disabled and an Xcoded-managed provisioning profile with the com.apple.developer.networking.custom-protocol entitlement. Thanks in advance!