How to set which IP to use for a HTTP request?

I dont know if it's possible as the std lib does not state anything about the current address being used:

http://golang.org/pkg/net/http/

resp, err := http.Get("http://example.com/")
if err != nil {
	// handle error
}
defer resp.Body.Close()
body, err := ioutil.ReadAll(resp.Body)

What I'm trying to do is set the source address for that http request, why? because I don't want to use my primary ip address for that kind of stuff...

You can set a custom Dialer in the Client's Transport.

// Create a transport like http.DefaultTransport, but with a specified localAddr
transport := &http.Transport{
	Proxy: http.ProxyFromEnvironment,
	DialContext: (&net.Dialer{
		Timeout:   30 * time.Second,
		KeepAlive: 30 * time.Second,
		LocalAddr: localAddr,
		DualStack: true,
	}).DialContext,
	MaxIdleConns:          100,
	IdleConnTimeout:       90 * time.Second,
	TLSHandshakeTimeout:   10 * time.Second,
	ExpectContinueTimeout: 1 * time.Second,
}

client := &http.Client{
    Transport: transport,
}

Setting up a custom dialer with the specify IP works sometimes, but it is weird since sometimes it failed to work, while curl works normally.
After checking the source code of curl, I figured out why curl is able to specify what network interface to be used: SO_BINDTODEVICE. Unfortunately this is a Linux thing.

So compared to JimB's answer, my method has:

• Pro: Behaves as stable as curl, which is irrelevant from routing tables
• Con: Only support Linux. So you probably want to learn more about the build tags to write platform-specific code.

dialer := &net.Dialer{
	Control: func(network, address string, conn syscall.RawConn) error {
		var operr error
		if err := conn.Control(func(fd uintptr) {
			operr = unix.BindToDevice(int(fd), forceNetworkInterface)
		}); err != nil {
			return err
		}
		return operr
	},
}

client = http.Client{
	Transport = &http.Transport{
		DialContext: dialer.DialContext,
	},
}

In addition, curl performs SO_BINDTODEVICE which behaves like the above code. And for non-Linux platforms, or when SO_BINDTODEVICE fails, curl sets the local IP address just as JimB's answer does. So you can first try my code and then use JimB's answer as a fallback.

See the source code of curl for details.