Created the Certainly-Client-Dn header...

main.go

@@ -9,6 +9,7 @@ import (
 	"net/url"
 	"io/ioutil"
 	"log"
+	"strings"
 )
 
 var serverAddress, serverCert, serverKey, clientCAFile, targetURL string
@@ -23,6 +24,43 @@ func init() {
 
 }
 
+// stolen from NewSingleHostReverseProxy
+func singleJoiningSlash(a, b string) string {
+	aslash := strings.HasSuffix(a, "/")
+	bslash := strings.HasPrefix(b, "/")
+	switch {
+	case aslash && bslash:
+		return a + b[1:]
+	case !aslash && !bslash:
+		return a + "/" + b
+	}
+	return a + b
+}
+
+// stolen from NewSingleHostReverseProxy
+func NewCertainlyReverseProxy(target *url.URL) *httputil.ReverseProxy {
+	targetQuery := target.RawQuery
+	director := func(req *http.Request) {
+		req.URL.Scheme = target.Scheme
+		req.URL.Host = target.Host
+		req.URL.Path = singleJoiningSlash(target.Path, req.URL.Path)
+		if targetQuery == "" || req.URL.RawQuery == "" {
+			req.URL.RawQuery = targetQuery + req.URL.RawQuery
+		} else {
+			req.URL.RawQuery = targetQuery + "&" + req.URL.RawQuery
+		}
+		if _, ok := req.Header["User-Agent"]; !ok {
+			// explicitly disable User-Agent so it's not set to default value
+			req.Header.Set("User-Agent", "")
+		}
+
+		// this is probably unsafe ;)
+		clientDN := req.TLS.PeerCertificates[len(req.TLS.PeerCertificates)-1].Subject.String()
+		req.Header.Set("Certainly-Client-DN", clientDN)
+	}
+	return &httputil.ReverseProxy{Director: director}
+}
+
 func main() {
 	clientCAData, err := ioutil.ReadFile(clientCAFile)
 	if err != nil {
@@ -49,11 +87,9 @@ func main() {
 		},
 	}
 
-	proxy := httputil.NewSingleHostReverseProxy(targetURL)
-
 	proxyServer := http.Server{
 		Addr: serverAddress,
-		Handler: proxy,
+		Handler: NewCertainlyReverseProxy(targetURL),
 		TLSConfig: serverTLSConfig,
 		TLSNextProto: make(map[string]func(*http.Server, *tls.Conn, http.Handler), 0),
 	}