How to force IPv4 or IPv6 connections in Go\'s net/http Client?

Forcing IPv4 or IPv6 in Go's net/http Client

In Go, the net/http package provides a versatile HTTP client for making requests. It offers flexibility in customizing the behavior of the client through its Transport field. This article will explore how to force the client to use either IPv4 or IPv6 when establishing connections.

Detecting IPv6-Only Domains

To determine if a domain is IPv6-only, we can create a custom DialContext function that checks the network parameter passed to the Control method. If the network is "tcp4," this indicates an IPv4-only connection, and we can cancel the connection.

<code class="go">func ModifiedTransport() {
    var MyTransport = &amp;http.Transport{
        DialContext: (&amp;net.Dialer{
            DualStack: false,
            Control: func(network, address string, c syscall.RawConn) error {
                if network == "tcp4" {
                    return errors.New("you should not use ipv4")
                }
                return nil
            },
        }).DialContext,
        MaxIdleConns:          100,
        IdleConnTimeout:       90 * time.Second,
        TLSHandshakeTimeout:   10 * time.Second,
        ExpectContinueTimeout: 1 * time.Second,
    }

    var myClient = http.Client{Transport: MyTransport}
    resp, myerr := myClient.Get("http://www.github.com")
    if myerr != nil {
        fmt.Println("request error")
        return
    }
}</code>

Forcing IPv6 Connections

To force an IPv6 connection, we can specify "tcp6" as the network parameter in the Control function:

<code class="go">func ModifiedTransport() {
    var MyTransport = &amp;http.Transport{
        DialContext: (&amp;net.Dialer{
            DualStack: false,
            Control: func(network, address string, c syscall.RawConn) error {
                if network != "tcp6" {
                    return errors.New("you should use ipv6")
                }
                return nil
            },
        }).DialContext,
        MaxIdleConns:          100,
        IdleConnTimeout:       90 * time.Second,
        TLSHandshakeTimeout:   10 * time.Second,
        ExpectContinueTimeout: 1 * time.Second,
    }

    var myClient = http.Client{Transport: MyTransport}
}</code>

By controlling the network parameter in the Control function, we can enforce IPv4 or IPv6 connectivity as desired. This approach provides flexibility in customizing the behavior of Go's net/http client for specific networking requirements.

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