Root cause found via pcap analysis: MikroTik with connection-count=N
sends UDP from N different source ports (2257, 2258, 2259, ...) all
to our single server port 2001. A connect()'d UDP socket only accepts
packets from the one connected address, silently dropping ~75% of
traffic with conn_count=4.
Fix: when tcp_conn_count > 0, leave the UDP socket unconnected and
use send_to()/recv_from() instead of send()/recv(). This accepts
packets from all MikroTik source ports.
This bug also exists in the original C btest-opensource.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
