Tcp reverse proxy by openresty+lua can not transfter data after link established?

My TCP-based protocol needs to use openresty+lua as a gateway. After tcp client establishes a link with the gateway, the first 1536 bytes packet will tell the destination server address. The lua script parses and obtains the server address in preread_by_lua_block, and sets it to the ngx.ctx.hsr_server_ip variable. Then I set the server address as upstream in balancer_by_lua_block through balancer.set_current_peer. In actual running, the step of set_current_peer is executed, the final tcp server behind the gateway also received the connect request, the tcp link was successfully established. But the subsequent tcp data sent by the client was never transferred to the server.

The openresty config file is below:

events {
    worker_connections  1024;

stream {
    lua_code_cache on;

    init_by_lua_block {

    upstream lua_dispatcher {
        # just an invalid address as a placeholder

        balancer_by_lua_block {
            local balancer = require('ngx.balancer')
            local host = ngx.ctx.hsr_server_ip
            local port = ngx.ctx.hsr_server_port

            -- set peer
            local ok, err = balancer.set_current_peer(host, port)
            if ok then
              ngx.log(ngx.NOTICE, "Forward a client to server: ", host)
              ngx.log(ngx.ERR, err)

    # proxy
    server {
        listen 5701;
        proxy_pass lua_dispatcher;

        # Parse HSR Server IP from first packet
        preread_by_lua_block {
            local sock = ngx.req.socket()
            local data, err = sock:receive(1536)
            if not data then
            ngx.log(ngx.ERR, err)

            local datastr = tostring(data)

            -- get x.x.x.x from first tcp packet with length of 1536
            local ipStrTable = {}
            local tempStr = '\0'
            for i=1,#datastr do
              tempStr = string.sub(datastr,i,i)
              if tonumber(tempStr) or tempStr == "." then
                  table.insert(ipStrTable, tempStr)

            -- save upstream server ip to ngx.ctx
            local ipstr = table.concat(ipStrTable)
            ngx.ctx.hsr_server_ip = ipstr
            ngx.ctx.hsr_server_port = 5701
            ngx.log(ngx.NOTICE, "Connect to hsr server: " ,ngx.ctx.hsr_server_ip)

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