add post

Author: Liudon

content/posts/cannot assign requested address in golang http client/index.md

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+---
+title: "Golang默认Http Client导致的cannot assign requested address错误"
+date: 2025-04-29T20:20:29+08:00
+draft: false
+tags:
+- golang
+---
+
+### 问题表现
+
+重现代码：
+
+```
+package main
+
+import (
+	"fmt"
+	"io"
+	"net/http"
+	"time"
+)
+
+func main() {
+
+	client := &http.Client{
+		Timeout: time.Duration(3) * time.Second,
+	}
+
+	for i := 0; i < 100; i++ {
+		go func() {
+			for {
+				req, _ := http.NewRequest(http.MethodGet, "https://baidu.com", nil)
+
+				rsp, err := client.Do(req)
+				if err != nil {
+					fmt.Println("request failed", err)
+					continue
+				}
+
+				rsp.Body.Close()
+
+				body, err := io.ReadAll(rsp.Body)
+				if err != nil {
+					fmt.Println("read body failed", err)
+					continue
+				}
+
+				fmt.Println(string(body))
+			}
+		}()
+	}
+
+	select {}
+}
+```
+
+启动后，随着请求越来越多，很快就出现了"cannot assign requested address"错误，服务器出现大量TIME_WAIT连接。
+
+### 问题原因
+
+
+[net/http代码](https://cs.opensource.google/go/go/+/refs/tags/go1.24.2:src/net/http/client.go;l=61)
+
+```
+type Client struct {
+	// Transport specifies the mechanism by which individual
+	// HTTP requests are made.
+	// If nil, DefaultTransport is used.
+	Transport RoundTripper
+```
+
+未配置Transport时，使用默认的DefaultTransport。
+
+```
+var DefaultTransport RoundTripper = &Transport{
+	Proxy: ProxyFromEnvironment,
+	DialContext: defaultTransportDialContext(&net.Dialer{
+		Timeout:   30 * time.Second,
+		KeepAlive: 30 * time.Second,
+	}),
+	ForceAttemptHTTP2:     true,
+	MaxIdleConns:          100,
+	IdleConnTimeout:       90 * time.Second,
+	TLSHandshakeTimeout:   10 * time.Second,
+	ExpectContinueTimeout: 1 * time.Second,
+}
+```
+
+这里指定了最大空闲连接未100，未指定单个host的最大空闲连接。
+
+```
+func (t *Transport) maxIdleConnsPerHost() int {
+	if v := t.MaxIdleConnsPerHost; v != 0 {
+		return v
+	}
+	return DefaultMaxIdleConnsPerHost
+}
+```
+
+如果未配置MaxIdleConnsPerHost，则使用默认的DefaultMaxIdleConnsPerHost配置。
+
+```
+// DefaultMaxIdleConnsPerHost is the default value of [Transport]'s
+// MaxIdleConnsPerHost.
+const DefaultMaxIdleConnsPerHost = 2
+```
+
+这下清楚了：
+
+100个协程，请求同一个地址，只能保留2个空闲连接，超出的请求完就会退出，产生一个TIME_WAIT；
+
+然后再创建一个连接，请求完关闭，又产生一个TIME_WAIT，直至耗尽端口。
+
+### 解决方案
+
+创建Http.Client时，配置MaxIdleConnsPerHost即可。