How to Force net/http Client to Use IPv4/IPv6 in Golang?

How to Force net/http Client to Use IPv4/IPv6 in Golang

In Golang, the net/http library provides a flexible transport layer that governs how HTTP requests are made. One common requirement is to enforce the use of either IPv4 or IPv6 for outgoing connections. This is particularly useful for scenarios where a domain's reachability needs to be tested specifically over one protocol or the other.

To achieve this, you can modify the underlying HTTP transport by specifying a custom DialContext function. This function allows you to intercept and potentially alter the connection establishment process. Here's how you can do it:

<code class="go">import (
    "errors"
    "fmt"
    "net"
    "net/http"
    "syscall"
    "time"
)

func modifiedTransport() {
    var myTransport = &http.Transport{
        DialContext: (&net.Dialer{
            Timeout:   30 * time.Second,
            KeepAlive: 30 * time.Second,
            DualStack: false,
            Control: func(network, address string, c syscall.RawConn) error {
                if network == "ipv4" {
                    // Here you can return an error to prevent connection establishment over IPv4.
                    return errors.New("ipv4 connections are not allowed")
                }
                return nil
            },
        }).DialContext,
        MaxIdleConns:          100,
        IdleConnTimeout:       90 * time.Second,
        TLSHandshakeTimeout:   10 * time.Second,
        ExpectContinueTimeout: 1 * time.Second,
    }

    var myClient = http.Client{Transport: myTransport}
    resp, err := myClient.Get("http://www.github.com")
    if err != nil {
        fmt.Println("Request error:", err)
        return
    }

    var buffer = make([]byte, 1000)
    resp.Body.Read(buffer)
    fmt.Println(string(buffer))
}</code>

In this example, we create a custom DialContext function within the modified transport. When a connection attempt is initiated, this function will be invoked, and it intercepts the network type that is being used. If the network is IPv4 (indicated by "ipv4" as the value of the network parameter), you can specify appropriate actions, such as returning an error to prevent the connection from being established over IPv4. This allows you to enforce the use of IPv6 for the request.

By utilizing the custom transport and its modified DialContext function, you can effectively control whether your HTTP requests are made over IPv4 or IPv6, providing greater flexibility in managing network connectivity.

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