Current head of GDAAsyncUdpSocket does not (also) set SO_REUSEPORT (it just does ADDRESS, which is quite enough for TCP).
I just popped in a quick fix:
https://github.com/dirkx/CocoaAsyncSocket/commit/5ec8c83d59ecb754824f706198eb62e29961c1be
https://github.com/dirkx/CocoaAsyncSocket
and it should get fixed upstream too:
https://github.com/robbiehanson/CocoaAsyncSocket/pull/159
Thanks,
Dw.