Add ability to autogenerate & serve LetsEncrypt SSL/TLS Certificate (#4)

This change uses the autocert package to add initial support for generating and serving a real live SSL certificate from LetsEncrypt via `ssl-proxy`. Now, serving your application with SSL is as easy as:

```sh
./ssl-proxy -from=0.0.0.0:443 -to=localhost:8080 -domain=mydomain.com
```

this will attempt to fetch a certificate for mydomain.com from LetsEncrypt (port 443 must be bindable by this process) and then begin proxying HTTPS traffic on :443 to any server running on :8080. Easy. This closes #1.

Gopkg.toml

@@ -0,0 +1,34 @@
+# Gopkg.toml example
+#
+# Refer to https://github.com/golang/dep/blob/master/docs/Gopkg.toml.md
+# for detailed Gopkg.toml documentation.
+#
+# required = ["github.com/user/thing/cmd/thing"]
+# ignored = ["github.com/user/project/pkgX", "bitbucket.org/user/project/pkgA/pkgY"]
+#
+# [[constraint]]
+#   name = "github.com/user/project"
+#   version = "1.0.0"
+#
+# [[constraint]]
+#   name = "github.com/user/project2"
+#   branch = "dev"
+#   source = "github.com/myfork/project2"
+#
+# [[override]]
+#   name = "github.com/x/y"
+#   version = "2.4.0"
+#
+# [prune]
+#   non-go = false
+#   go-tests = true
+#   unused-packages = true
+
+
+[[constraint]]
+  branch = "master"
+  name = "golang.org/x/crypto"
+
+[prune]
+  go-tests = true
+  unused-packages = true

main.go

@@ -12,13 +12,15 @@ import (
 	"strings"
 
 	"github.com/suyashkumar/ssl-proxy/gen"
+	"golang.org/x/crypto/acme/autocert"
 )
 
 var (
 	to       = flag.String("to", "http://127.0.0.1:80", "the address and port for which to proxy requests to")
 	fromURL  = flag.String("from", "127.0.0.1:4430", "the tcp address and port this proxy should listen for requests on")
 	certFile = flag.String("cert", "", "path to a tls certificate file. If not provided, ssl-proxy will generate one for you in ~/.ssl-proxy/")
 	keyFile  = flag.String("key", "", "path to a private key file. If not provided, ssl-proxy will generate one for you in ~/.ssl-proxy/")
+	domain   = flag.String("domain", "", "domain to mint letsencrypt certificates for. Usage of this parameter implies acceptance of the LetsEncrypt terms of service.")
 )
 
 const (
@@ -31,7 +33,12 @@ const (
 func main() {
 	flag.Parse()
 
-	if *certFile == "" || *keyFile == "" {
+	validCertFile := *certFile != ""
+	validKeyFile := *keyFile != ""
+	validDomain := *domain != ""
+
+	// Determine if we need to generate self-signed certs
+	if (!validCertFile || !validKeyFile) && !validDomain {
 		// Use default file paths
 		*certFile = DefaultCertFile
 		*keyFile = DefaultKeyFile
@@ -66,13 +73,40 @@ func main() {
 		log.Println("Assuming -to URL is using http://")
 	}
 
+	// Parse toURL as a URL
 	toURL, err := url.Parse(*to)
 	if err != nil {
 		log.Fatal("Unable to parse 'to' url: ", err)
 	}
 
+	// Setup ServeMux
 	localProxy := httputil.NewSingleHostReverseProxy(toURL)
-	http.Handle("/", localProxy)
+	mux := http.NewServeMux()
+	mux.Handle("/", localProxy)
+
 	log.Printf("Proxying calls from https://%s (SSL/TLS) to %s", *fromURL, toURL)
-	log.Fatal(http.ListenAndServeTLS(*fromURL, *certFile, *keyFile, nil))
+
+	// Determine if we should serve with autogenerated LetsEncrypt certificates or not
+	if validDomain {
+		// Domain is present, use autocert
+		// TODO: validate domain (though, autocert may do this)
+		// TODO: for some reason this seems to only work on :443
+		log.Printf("Domain specified, using LetsEncrypt to autogenerate and serve certs for %s\n", *domain)
+		if !strings.HasSuffix(*fromURL, ":443") {
+			log.Println("WARN: Right now, you must serve on port :443 to use autogenerated LetsEncrypt certs using the -domain flag, this may NOT WORK")
+		}
+		m := &autocert.Manager{
+			Cache:      autocert.DirCache("certs"),
+			Prompt:     autocert.AcceptTOS,
+			HostPolicy: autocert.HostWhitelist(*domain),
+		}
+		s := &http.Server{
+			Addr:      *fromURL,
+			TLSConfig: m.TLSConfig(),
+		}
+		s.Handler = mux
+		s.ListenAndServeTLS("", "")
+	} else {
+		log.Fatal(http.ListenAndServeTLS(*fromURL, *certFile, *keyFile, mux))
+	}
 }