I would like to know what are these numbers xD

151 is exact packet length for 8.6 and 9.7 (and probably all the clients in the middle)
clients that use checksums have packet lengths larger than 280 although I'm not sure how was that figured out
7.6 has packet lengths varying on user input for credentials fields, but it never goes above 100

This won't affect the gameplay because after login, it skips all these checks and just calls onRecvPacket

You can use a lambda here, I think it will be way cleaner, e.g.

		auto bufferLength = !receivedLastChar && receivedName && connectionState == CONNECTION_STATE_CONNECTING_STAGE2 ?  1 : NetworkMessage::HEADER_LENGTH
			// Read size of the first packet
		boost::asio::async_read(socket,
			boost::asio::buffer(msg.getBuffer(), bufferLength),
			[connection = shared_from_this()](auto packet) { connection->parseHeader(packet); });

Not 100% sure about the syntax but you'll get the idea

I don't understand this enough to make that change. I'm not that good at c++

I believe it is right now, just copy it over the whole if/else block

I see what you meant. There is same code with a small difference written twice. The code preview on github didn't show that so I didn't understood the context. Everything is clear now.

[connection = shared_from_this()](auto packet) { connection->parseHeader(packet); }); this line causes an error btw

C2780 "auto Connection::accept::<lambda_a28015d96ed371fa3d8a39af35f1cb78>::operator ()(_T1) const": expected 1 argument, got 2 theforgottenserver c:\vcpkg\installed\x64-windows\include\boost\asio\impl\read.hpp 480 so I'll use current tfs implementation instead