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An error message was shown when I was connecting with my openvpn server through tcp or udp connection (on an openvpn client windows), the message was "Authenticate/Decrypt packet error".

Sat Jun 10 03:08:08 2017 Authenticate/Decrypt packet error: bad packet ID (may be a replay): [ #235853 ] -- see the man page entry for --no-replay and --replay-window for more info or silence this warning with --mute-replay-warnings
Sat Jun 10 03:08:36 2017 Authenticate/Decrypt packet error: bad packet ID (may be a replay): [ #491688 ] -- see the man page entry for --no-replay and --replay-window for more info or silence this warning with --mute-replay-warnings
Sat Jun 10 03:08:36 2017 Authenticate/Decrypt packet error: bad packet ID (may be a replay): [ #491701 ] -- see the man page entry for --no-replay and --replay-window for more info or silence this warning with --mute-replay-warnings

During this repeated message, about 700MB uploaded from my host!. Is this issue classified as Man In The Middle Attack (MITM)?

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It depends. It could be a replay attack, or a badly configured router sending packets again even after they've been received.

Essentially, OpenVPN signs every packet with a unique ID. When the client receives the packet, it records the packet ID. When the client receives a packet with a same unique ID, it drops the packet and raises a red flag.

Badly configured routers (and other network middle-ware) could resend packets over and over even though they have arrived at the client (which could be a reason why you saw the 700MB traffic), which would obviously cause a false alert with OpenVPN.

A malicious actor usually uses replay attacks to gain authentication. Usually, this is not much of an issue, because OpenVPN just drops duplicate packets and raises a warning. It is not so much of a MITM attack, because the actor still doesn't know how to decrypt those packets.

One more possibility is your system clock. When you turn back time, OpenVPN may confuse the unique IDs received and raise a false alarm. Make sure your NTP is working fine.

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