最近部署的web程序，服务器上出现不少time_wait的tcp连接状态，占用了tcp端口，花费几天时间排查。
之前我有结论：HTTP keep-alive 是在应用层对TCP连接的滑动续约复用，如果客户端、服务器稳定续约，就成了名副其实的长连接。
有关[Http持久连接]的一切，卷给你看
HTTP1.1 Keep-Alive到底算不算长连接?
目前所有的HTTP网络库(不论是客户端、服务端)都默认开启了HTTP Keep-Alive，通过Request/Response的Connection标头来协商复用连接。
01非常规的行为形成的短连接我手上有个项目，由于历史原因，客户端禁用了Keep-Alive，服务端默认开启了Keep-Alive，如此一来协商复用连接失败， 客户端每次请求会使用新的TCP连接， 也就是回退为短连接。
客户端强制禁用Keep-Alive
package mainimport ( "fmt" "io/ioutil" "log" "net/http" "time")func main() { tr := http.Transport{ DisableKeepAlives: true, } client := &http.Client{ Timeout: 10 * time.Second, Transport: &tr, } for { requestWithClose(client) time.Sleep(time.Second * 1) }}func requestWithClose(client *http.Client) { resp, err := client.Get("http://10.100.219.9:8081") if err != nil { fmt.Printf("error occurred while fetching page, error: %s", err.Error()) return } defer resp.Body.Close() c, err := ioutil.ReadAll(resp.Body) if err != nil { log.Fatalf("Couldn"t parse response body. %+v", err) } fmt.Println(string(c))}
web服务端默认开启Keep-Alive
package mainimport ( "fmt" "log" "net/http")// 根据RemoteAddr 知道客户端使用的持久连接func IndexHandler(w http.ResponseWriter, r *http.Request) { fmt.Println("receive a request from:", r.RemoteAddr, r.Header) w.Write([]byte("ok"))}func main() { fmt.Printf("Starting server at port 8081\n") // net/http 默认开启持久连接 if err := http.ListenAndServe(":8081", http.HandlerFunc(IndexHandler)); err != nil { log.Fatal(err) }}
从服务端的日志看，确实是短连接。
receive a request from: 10.22.34.48:54722 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]receive a request from: 10.22.34.48:54724 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]receive a request from: 10.22.34.48:54726 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]receive a request from: 10.22.34.48:54728 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]receive a request from: 10.22.34.48:54731 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]receive a request from: 10.22.34.48:54733 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]receive a request from: 10.22.34.48:54734 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]receive a request from: 10.22.34.48:54738 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]receive a request from: 10.22.34.48:54740 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]receive a request from: 10.22.34.48:54741 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]receive a request from: 10.22.34.48:54743 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]receive a request from: 10.22.34.48:54744 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]receive a request from: 10.22.34.48:54746 map[Accept-Encoding:[gzip] Connect02谁是主动断开方?
我想当然的以为 客户端是主动断开方，被现实啪啪打脸。
某一天服务器上超过300的time_wait报警，告诉我这tmd是服务器主动终断连接。
常规的TCP4次挥手， 主动断开方会进入time_wait状态，等待2MSL后释放占用的SOCKET
以下是从服务器上tcpdump抓取的tcp连接信息。
2,3红框显示：
Server端先发起TCP的FIN消息, 之后Client回应ACK确认收到Server的关闭通知; 之后Client再发FIN消息，告知现在可以关闭了， Server端最后发ACK确认收到，并进入time_wait状态，等待2MSL的时间关闭Socket。
特意指出，红框1表示TCP双端同时关闭[1]，此时会在Client,Server同时留下time_wait痕迹，发生概率较小。
03没有源码说个串串此种情况是服务端主动关闭，我们翻一翻golang httpServer的源码
http.ListenAndServe(":8081")server.ListenAndServe()srv.Serve(ln)go c.serve(connCtx) 使用go协程来处理每个请求服务器连接处理请求的简略源码如下：
func (c *conn) serve(ctx context.Context) { c.remoteAddr = c.rwc.RemoteAddr().String() ctx = context.WithValue(ctx, LocalAddrContextKey, c.rwc.LocalAddr()) defer func() { if !c.hijacked() { c.close() // go协程conn处理请求的协程退出时，主动关闭底层的TCP连接 c.setState(c.rwc, StateClosed, runHooks) } }() ...... // HTTP/1.x from here on. ctx, cancelCtx := context.WithCancel(ctx) c.cancelCtx = cancelCtx defer cancelCtx() c.r = &connReader{conn: c} c.bufr = newBufioReader(c.r) c.bufw = newBufioWriterSize(checkConnErrorWriter{c}, 4<<10) for { w, err := c.readRequest(ctx)..... serverHandler{c.server}.ServeHTTP(w, w.req) w.cancelCtx() if c.hijacked() { return } w.finishRequest() if !w.shouldReuseConnection() { if w.requestBodyLimitHit || w.closedRequestBodyEarly() { c.closeWriteAndWait() } return } c.setState(c.rwc, StateIdle, runHooks) c.curReq.Store((*response)(nil)) if !w.conn.server.doKeepAlives() { // We"re in shutdown mode. We might"ve replied // to the user without "Connection: close" and // they might think they can send another // request, but such is life with HTTP/1.1. return } if d := c.server.idleTimeout(); d != 0 { c.rwc.SetReadDeadline(time.Now().Add(d)) if _, err := c.bufr.Peek(4); err != nil { return } } c.rwc.SetReadDeadline(time.Time{}) }}
我们需要关注
①for循环，表示尝试复用该conn，用于处理迎面而来的请求
②w.shouldReuseConnection() = false， 表明读取到ClientConnection：Close标头，设置closeAfterReply=true，跳出for循环，协程即将结束，结束之前执行defer函数，defer函数内close该连接
c.close()......// Close the connection.func (c *conn) close() { c.finalFlush() c.rwc.Close()}
③如果 w.shouldReuseConnection() = true，则将该连接状态置为idle， 并继续走for循环，复用连接，处理后续请求。
04我的收获1. TCP 4次挥手的八股文
2. 短连接的效应：主动关闭方会在机器上产生 time_wait状态，需要等待2MSL时间才会关闭SOCKET
3.golang http keep-alive复用tcp连接的源码级分析
4.tcpdump抓包的姿势
引用链接[1] TCP双端同时关闭: https://blog.csdn.net/q1007729991/article/details/69950255
