To access the certificate, you will have get access to the ConnectionState. The easiest way to do that is to provide your own version of DialTLS. In there you connect to the server using net.Dial, do the TLS handshake and then you are free to verify.
package main
import (
"crypto/tls"
"log"
"net"
"net/http"
"net/http/httputil"
"net/url"
)
func main() {
proxy := httputil.NewSingleHostReverseProxy(&url.URL{
Scheme: "https",
Host: "sha256.badssl.com",
})
// Set a custom DialTLS to access the TLS connection state
proxy.Transport = &http.Transport{DialTLS: dialTLS}
// Change req.Host so badssl.com host check is passed
director := proxy.Director
proxy.Director = func(req *http.Request) {
director(req)
req.Host = req.URL.Host
}
log.Fatal(http.ListenAndServe("127.0.0.1:3000", proxy))
}
func dialTLS(network, addr string) (net.Conn, error) {
conn, err := net.Dial(network, addr)
if err != nil {
return nil, err
}
host, _, err := net.SplitHostPort(addr)
if err != nil {
return nil, err
}
cfg := &tls.Config{ServerName: host}
tlsConn := tls.Client(conn, cfg)
if err := tlsConn.Handshake(); err != nil {
conn.Close()
return nil, err
}
cs := tlsConn.ConnectionState()
cert := cs.PeerCertificates[0]
// Verify here
cert.VerifyHostname(host)
log.Println(cert.Subject)
return tlsConn, nil
}