How to bind an http.Client in Go to an IP Address

Question

I have a client machine with multiple NICs, how do I bind an http.Client in Go to a certain NIC or to a certain SRC IP Address?

Say you have some very basic http cli

Answer 1

Here is a fully working example that incorporates the answer from Tim. I also broke out all of the nested pieces to make it easier to read and learn from.

package main

import (
    "fmt"
    "io/ioutil"
    "net"
    "net/http"
    "time"
)

func main() {
    localAddr, err := net.ResolveIPAddr("ip", "10.128.64.219")
    if err != nil {
        panic(err)
    }

    localTCPAddr := net.TCPAddr{
        IP: localAddr.IP,
    }

    d := net.Dialer{
        LocalAddr: &localTCPAddr,
        Timeout:   30 * time.Second,
        KeepAlive: 30 * time.Second,
    }

    tr := &http.Transport{
        Proxy:               http.ProxyFromEnvironment,
        Dial:                d.Dial,
        TLSHandshakeTimeout: 10 * time.Second,
    }

    webclient := &http.Client{Transport: tr}

    // Use NewRequest so we can change the UserAgent string in the header
    req, err := http.NewRequest("GET", "http://www.google.com:80", nil)
    if err != nil {
        panic(err)
    }

    res, err := webclient.Do(req)
    if err != nil {
        panic(err)
    }

    fmt.Println("DEBUG", res)
    defer res.Body.Close()

    content, err := ioutil.ReadAll(res.Body)
    if err != nil {
        panic(err)
    }
    fmt.Printf("%s", string(content))
}