适用于公司内网请求外网网站，所以需要通过代理转发出去，这里配置一下环境变量COMPANY_HTTP_PROXY、COMPANY_HTTPS_PROXY，通过指定ip请求外网。

// envOnce looks up an environment variable (optionally by multiple
// names) once. It mitigates expensive lookups on some platforms
// (e.g. Windows).
type envOnce struct {
    names []string
    once  sync.Once
    val   string
}

func (e *envOnce) Get() string {
    e.once.Do(e.init)
    return e.val
}

func (e *envOnce) init() {
    for _, n := range e.names {
        e.val = os.Getenv(n)
        if e.val != "" {
            return
        }
    }
}

// reset is used by tests
func (e *envOnce) reset() {
    e.once = sync.Once{}
    e.val = ""
}

var (
    httpProxyEnv = &envOnce{
        names: []string{"COMPANY_HTTP_PROXY", "company_http_proxy"},
    }
    httpsProxyEnv = &envOnce{
        names: []string{"COMPANY_HTTPS_PROXY", "company_https_proxy"},
    }
)

func CompanyProxyFromEnvironment(req *http.Request) (*url.URL, error) {
    var proxy string
    if req.URL.Scheme == "https" {
        proxy = httpsProxyEnv.Get()
    }
    if proxy == "" {
        proxy = httpProxyEnv.Get()
        if proxy != "" && os.Getenv("REQUEST_METHOD") != "" {
            return nil, errors.New("net/http: refusing to use HTTP_PROXY value in CGI environment; see golang.org/s/cgihttpproxy")
        }
    }
    if proxy == "" {
        return nil, nil
    }
    /*默认使用代理
    if !useProxy(canonicalAddr(req.URL)) {
        return nil, nil
    }*/
    proxyURL, err := url.Parse(proxy)
    if err != nil || !strings.HasPrefix(proxyURL.Scheme, "http") {
        // proxy was bogus. Try prepending "http://" to it and
        // see if that parses correctly. If not, we fall
        // through and complain about the original one.
        if proxyURL, err := url.Parse("http://" + proxy); err == nil {
            return proxyURL, nil
        }
    }
    if err != nil {
        return nil, fmt.Errorf("invalid proxy address %q: %v", proxy, err)
    }
    return proxyURL, nil
}