The send buffer has data

tcp_rst.lua

local socket = require("socket")

local host, port = "180.101.50.242", 80
local sock = socket.connect(host, port)
sock:settimeout(1)
sock:send("GET / HTTP/1.1\r\n\r\n")
socket.sleep(1)
sock:close()

sock;close() will send TCP RST to passive closer, as the receiving buffer has data.

Fix:

local socket = require("socket")

local host, port = "www.baidu.com", 80
local sock = socket.connect(host, port)
sock:settimeout(1)
sock:send("GET / HTTP/1.1\r\n\r\n")

while true do
    local line, err = sock:receive()
    if err == "timeout" then
        break
    end
end
sock:close()

See #tcp-close