How Can I Bind an HTTP Client to a Specific IP Address in Go?

Binding an http.Client to an IP Address in Go

In Go, by default, HTTP requests do not bind to a specific IP address or network interface card (NIC). This can be problematic if you have multiple NICs and need to specify which one to use for outgoing HTTP requests.

To resolve this, you can modify the Transport field of the http.Client struct. By setting it to an instance of net.Transport, you gain access to the DialContext method of net.Dialer, which allows you to override the default local address.

Here's an example that demonstrates how to bind an http.Client to a specific IP address:

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

func main() {
    // Parse the local IP address you want to use
    localAddr, err := net.ResolveIPAddr("ip", "<my local address>")
    if err != nil {
        panic(err)
    }

    // Create a TCPAddr from the IP address
    localTCPAddr := net.TCPAddr{
        IP: localAddr.IP,
    }

    // Initialize an http.Client with a custom DialContext
    webclient := &http.Client{
        Transport: &http.Transport{
            Proxy: http.ProxyFromEnvironment,
            DialContext: (&net.Dialer{
                LocalAddr:    &localTCPAddr,
                Timeout:     30 * time.Second,
                KeepAlive:   30 * time.Second,
                DualStack:   true,
            }).DialContext,
            MaxIdleConns:          100,
            IdleConnTimeout:       90 * time.Second,
            TLSHandshakeTimeout:   10 * time.Second,
            ExpectContinueTimeout: 1 * time.Second,
        },
    }

    // Make an HTTP request
    req, _ := http.NewRequest("GET", "http://www.google.com", nil)
    httpResponse, _ := webclient.Do(req)
    defer httpResponse.Body.Close()
}</code>

By using this approach, you can ensure that HTTP requests made using the modified http.Client will originate from a specific IP address.

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