NEFilterDataProvider - how to get socket's interface

I'm implementing NEFIlterDataProvider where I would like to drop sockets that will be bound to specific interface, but I can't find interface parameter anywhere. Is that even possible to do?

This is a good question as there is no readily apparent way to extract nw_interface_t from APIs like NEFilterSocketFlow or NEFilterFlow. One thing you could try here is to examine the localEndpoint of a NEFilterSocketFlow and then use a low level API like getifaddrs to examine if the local endpoint address matches anything interface on the machine with the same address. This seems like a lot more work than it should be though and probably best to open an Enhancement Request for this. Please follow up with the Feedback ID.

If you do end up going the getifaddrs route, keep in mind that the localEndpoint may not be available until methods like handleOutboundDataFromFlow are hit. There are several cases where handleNewFlow does not have the localEndpoint value set because the socket is not established yet.

Matt Eaton
DTS Engineering, CoreOS
meaton3@apple.com

Thanks for the suggestion. I managed to retrieve all interfaces on the machine with their addresses using getifaddrs, but localEndpoint of a NEFilterSocketFlow usually has 0.0.0.0 hostname so I can not match it with any interface. Is there a way to get actual IP address of a localEndpoint?

I wanted to elaborate on Matt’s comment:

Usually the localEndpoint will stay either the default address or nil until the actual connection is made.

To be clear, this isn’t a bug. Due to the way the our systems connect [1] we don’t know which local address will be used for a connection until it’s actually connected.

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[1] If you’re curious, read up on Happy Eyeballs.

Yes, in handleInboundDataFromFlow and handleOutboundDataFromFlow I can see actual address. Thank you