How to Enforce IPv4 or IPv6 Usage in Go HTTP Client?

Enforcing IPv4/IPv6 Usage in Go HTTP Client

In Go, the http.Client provides a transport for HTTP requests that handles network connections. To force the client to use IPv4 or IPv6 exclusively, leverage the DialContext function within a custom transport implementation.

ModifiedTransport Implementation

The modified transport snippet aims to detect IPv6-only domains by returning an error when an attempt is made to establish an IPv4 connection.

<code class="go">type MyTransport struct {
    http.Transport
    Dialer net.Dialer
}

func (t *MyTransport) DialContext(ctx context.Context, network, address string) (net.Conn, error) {
    if network == "ipv4" {
        return nil, errors.New("IPv4 not permitted")
    }
    return t.Dialer.DialContext(ctx, network, address)
}</code>

HTTP Client Configuration

To use the custom transport, instantiate an http.Client object:

<code class="go">myClient := http.Client{
    Transport: &MyTransport{
        Dialer: net.Dialer{
            DualStack: false, // Disable IPv4/IPv6 dual-stack
        },
    },
}</code>

Enforcing IPv6-Only Connections

By setting DualStack to false, the Dialer disallows IPv4 connections. This modification effectively forces the client to use IPv6-only connections.

Closing the Connection

The connections established using the modified transport behave as regular connections. Closing them is managed by the underlying HTTP client, so there's no need to handle it manually.

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