Why the server answer RST right after receiving SYN from the client?

Question

Why the server(i.e modbus tcp slave) answer RST right after receiving SYN from the client?

Here are the packets captured by Wireshark:

  1 09:58:55.773863730 192.168.0.168         192.168.0.21          TCP      66     39110 → 502 [SYN] Seq=0 Win=29200 Len=0 MSS=1460 SACK_PERM=1 WS=128
  2 09:58:55.774090423 192.168.0.21          192.168.0.168         TCP      60     502 → 39110 [RST, ACK] Seq=1 Ack=1 Win=0 Len=0
  3 09:58:55.774194303 192.168.0.168         192.168.0.21          TCP      66     39112 → 502 [SYN] Seq=0 Win=29200 Len=0 MSS=1460 SACK_PERM=1 WS=128
  4 09:58:55.774298582 192.168.0.21          192.168.0.168         TCP      60     502 → 39112 [RST, ACK] Seq=1 Ack=1 Win=0 Len=0
  5 10:01:35.548023265 192.168.0.168         192.168.0.21          TCP      66     39118 → 502 [SYN] Seq=0 Win=29200 Len=0 MSS=1460 SACK_PERM=1 WS=128
  6 10:01:35.548248696 192.168.0.21          192.168.0.168         TCP      60     502 → 39118 [RST, ACK] Seq=1 Ack=1 Win=0 Len=0
  7 10:01:35.548340993 192.168.0.168         192.168.0.21          TCP      66     39120 → 502 [SYN] Seq=0 Win=29200 Len=0 MSS=1460 SACK_PERM=1 WS=128
  8 10:01:35.548446706 192.168.0.21          192.168.0.168         TCP      60     502 → 39120 [RST, ACK] Seq=1 Ack=1 Win=0 Len=0
  9 10:02:09.279784987 192.168.0.168         192.168.0.21          TCP      66     39126 → 502 [SYN] Seq=0 Win=29200 Len=0 MSS=1460 SACK_PERM=1 WS=128
 10 10:02:09.279974571 192.168.0.21          192.168.0.168         TCP      60     502 → 39126 [RST, ACK] Seq=1 Ack=1 Win=0 Len=0
 11 10:02:09.280092319 192.168.0.168         192.168.0.21          TCP      66     39128 → 502 [SYN] Seq=0 Win=29200 Len=0 MSS=1460 SACK_PERM=1 WS=128
 12 10:02:09.280196486 192.168.0.21          192.168.0.168         TCP      60     502 → 39128 [RST, ACK] Seq=1 Ack=1 Win=0 Len=0

Update: The server really listens to the port. I confirm that by netstat -atn | grip 502( and returns tcp 0 0 0.0.0.0:502 0.0.0.0:* LISTEN ) on the server side.

And telnet 192.168.0.21 502 returns Connected to 192.168.02.21.