How to do proxy and TLS in golang - go

Question

I am trying to route my requests through a proxy and also sending cert.pem in TLS config. Below code is throwing this error - proxyconnect tcp: tls: first record does not look like a TLS handshake. When I change the proxy URL from https to HTTP, the same code works. However proxy URL with https works in python. Below is my code so far
certs := x509.NewCertPool()
pemFile, err := ioutil.ReadFile("cert.pem")
if err != nil {
return
}
certs.AppendCertsFromPEM(pemFile)
tlsConfig := &tls.Config{
RootCAs: certs,
}
proxyUrl, err := url.Parse("https://someproxyurlhere.com:8080")
if err != nil {
return
}
t := &http.Transport{
TLSClientConfig: tlsConfig,
Proxy: http.ProxyURL(proxyUrl),
}
client := http.Client{
Transport: t,
}
reqBody := "some JSON body here"
buff, err := json.Marshal(reqBody)
if err != nil {
return
}
req, err := http.NewRequest(http.MethodPost, "https://someurlhere.com", bytes.NewBuffer(buff))
if err != nil {
return
}
res, err := client.Do(req)
if err != nil {
// Error here - proxyconnect tcp: tls: first record does not look like a TLS handshake
return
}
defer res.Body.Close()
Python code
import requests
os.environ['HTTPS_PROXY'] = 'https://someproxyurlhere.com:8080'
response = requests.post("https://someurlhere.com",
json={'key': 'value'},
verify='cert.pem')
print(str(response.content))

Answer 1

When I change the proxy URL from https to HTTP, the same code works.
This is because you are using a HTTP proxy which need to be accessed by HTTP and not HTTPS even for https://.. URLs. This is how HTTPS proxying usually works. Note that while in theory a proxy could sniff if it gets a plain or a TLS connection in practice proxies (and servers) use different ports for HTTP and HTTPS - thus if it works on one port with HTTP it will very likely not work on the same port with HTTPS.
proxyconnect tcp: tls: first record does not look like a TLS handshake.
This is because the proxy answers with an plain HTTP error to the strange HTTP request (which is actually the start of the TLS handshake).
However proxy URL with https works in python.
While it looks that it works it actually doesn't. Python requests will still use plain HTTP to the proxy even if https:// was given as URL.
You can try to do a simple TLS connection to the proxy itself, for example with Python or with openssl s_client. It will very likely fail with some handshake error because the proxy isn't actually expecting TLS.