Server sends FIN immediately after TCP handshake

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Hi, I am writing my own small TCP stack and making an intiial TCP handshake and it looks correct, but after the third packet the server sends a FIN.

I have enabled checksum checking on TCP and they are correct. I cannot see any additional hints why this may be happening.

Could somebody please help me understand what is happening?

I'm new member so unfortunately cannot attach an image. I have pasted the summaries showing seq numbers below.

192.168.1.84    163.70.151.35   TCP 74  0 → 80 [SYN] Seq=0 Win=65535 Len=0 MSS=46085 SACK_PERM=1 TSval=2601039236 TSecr=0 WS=16384
163.70.151.35   192.168.1.84    TCP 74  80 → 0 [SYN, ACK] Seq=0 Ack=1 Win=65535 Len=0 MSS=1392 SACK_PERM=1 TSval=916456970 TSecr=2601039236 WS=256
192.168.1.84    163.70.151.35   TCP 54  0 → 80 [ACK] Seq=1 Ack=1 Win=1073725440 Len=0

The server sends FIN milliseconds later:

163.70.151.35   192.168.1.84    TCP 66  80 → 0 [FIN, ACK] Seq=1 Ack=1 Win=65536 Len=0 TSval=916456984 TSecr=2601039236