Ephemeral port exhaustion.

the two-minute refresher

• Each outbound connection needs a local ephemeral port: about 16,000 in the IANA range, about 28,000 by Linux default.
• A closed connection keeps its port in TIME_WAIT for 60-120 s.
• At 500 new connections/s × 60 s you need 30,000 ports: errors like 'Cannot assign requested address'.
• Fix: reuse connections (pooling), widen the port range, tune TIME_WAIT reuse.