Add a remote service to test NAT compatability

Add a remote probetest service that will allow proxies to test their
compatability with symmetric NATs.

.gitignore

@@ -8,5 +8,6 @@ broker/broker
 client/client
 server/server
 proxy/proxy
+probetest/probetest
 snowflake.log
 ignore/

probetest/probetest.go

+/*
+Probe test server to check the reachability of Snowflake proxies from
+clients with symmetric NATs.
+
+The probe server receives an offer from a proxy, returns an answer, and then
+attempts to establish a datachannel connection to that proxy. The proxy will
+self-determine whether the connection opened successfully.
+*/
+package main
+
+import (
+	"crypto/tls"
+	"flag"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"log"
+	"net/http"
+	"os"
+	"strings"
+	"time"
+
+	"git.torproject.org/pluggable-transports/snowflake.git/common/messages"
+	"git.torproject.org/pluggable-transports/snowflake.git/common/safelog"
+	"git.torproject.org/pluggable-transports/snowflake.git/common/util"
+
+	"github.com/pion/webrtc/v2"
+	"golang.org/x/crypto/acme/autocert"
+)
+
+const (
+	readLimit          = 100000                         //Maximum number of bytes to be read from an HTTP request
+	dataChannelTimeout = 20 * time.Second               //time after which we assume proxy data channel will not open
+	stunUrl            = "stun:stun.l.google.com:19302" //default STUN URL
+)
+
+// Create a PeerConnection from an SDP offer. Blocks until the gathering of ICE
+// candidates is complete and the answer is available in LocalDescription.
+func makePeerConnectionFromOffer(sdp *webrtc.SessionDescription,
+	dataChan chan struct{}) (*webrtc.PeerConnection, error) {
+
+	config := webrtc.Configuration{
+		ICEServers: []webrtc.ICEServer{
+			{
+				URLs: []string{stunUrl},
+			},
+		},
+	}
+	pc, err := webrtc.NewPeerConnection(config)
+	if err != nil {
+		return nil, fmt.Errorf("accept: NewPeerConnection: %s", err)
+	}
+	pc.OnDataChannel(func(dc *webrtc.DataChannel) {
+		close(dataChan)
+	})
+
+	err = pc.SetRemoteDescription(*sdp)
+	if err != nil {
+		if inerr := pc.Close(); inerr != nil {
+			log.Printf("unable to call pc.Close after pc.SetRemoteDescription with error: %v", inerr)
+		}
+		return nil, fmt.Errorf("accept: SetRemoteDescription: %s", err)
+	}
+
+	answer, err := pc.CreateAnswer(nil)
+	if err != nil {
+		if inerr := pc.Close(); inerr != nil {
+			log.Printf("ICE gathering has generated an error when calling pc.Close: %v", inerr)
+		}
+		return nil, err
+	}
+
+	err = pc.SetLocalDescription(answer)
+	if err != nil {
+		if err = pc.Close(); err != nil {
+			log.Printf("pc.Close after setting local description returned : %v", err)
+		}
+		return nil, err
+	}
+
+	return pc, nil
+}
+
+func probeHandler(w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Access-Control-Allow-Origin", "*")
+	resp, err := ioutil.ReadAll(http.MaxBytesReader(w, r.Body, readLimit))
+	if nil != err {
+		log.Println("Invalid data.")
+		w.WriteHeader(http.StatusBadRequest)
+		return
+	}
+
+	offer, _, err := messages.DecodePollResponse(resp)
+	if err != nil {
+		log.Printf("Error reading offer: %s", err.Error())
+		w.WriteHeader(http.StatusBadRequest)
+		return
+	}
+	if offer == "" {
+		log.Printf("Error processing session description: %s", err.Error())
+		w.WriteHeader(http.StatusBadRequest)
+		return
+	}
+	sdp, err := util.DeserializeSessionDescription(offer)
+	if err != nil {
+		log.Printf("Error processing session description: %s", err.Error())
+		w.WriteHeader(http.StatusBadRequest)
+		return
+	}
+
+	dataChan := make(chan struct{})
+	pc, err := makePeerConnectionFromOffer(sdp, dataChan)
+	if err != nil {
+		log.Printf("Error making WebRTC connection: %s", err)
+		w.WriteHeader(http.StatusInternalServerError)
+		return
+	}
+
+	sdp = &webrtc.SessionDescription{
+		Type: pc.LocalDescription().Type,
+		SDP:  util.StripLocalAddresses(pc.LocalDescription().SDP),
+	}
+	answer, err := util.SerializeSessionDescription(sdp)
+	if err != nil {
+		log.Printf("Error making WebRTC connection: %s", err)
+		w.WriteHeader(http.StatusInternalServerError)
+		return
+	}
+	body, err := messages.EncodeAnswerRequest(answer, "")
+	if err != nil {
+		log.Printf("Error making WebRTC connection: %s", err)
+		w.WriteHeader(http.StatusInternalServerError)
+		return
+	}
+
+	w.Write(body)
+	// Set a timeout on peerconnection. If the connection state has not
+	// advanced to PeerConnectionStateConnected in this time,
+	// destroy the peer connection and return the token.
+	select {
+	case <-dataChan:
+		if err := pc.Close(); err != nil {
+			log.Printf("Error calling pc.Close: %v", err)
+		}
+	case <-time.After(dataChannelTimeout):
+		if err := pc.Close(); err != nil {
+			log.Printf("Error calling pc.Close: %v", err)
+		}
+	}
+	return
+
+}
+
+func main() {
+	var acmeEmail string
+	var acmeHostnamesCommas string
+	var acmeCertCacheDir string
+	var addr string
+	var disableTLS bool
+	var certFilename, keyFilename string
+	var unsafeLogging bool
+
+	flag.StringVar(&acmeEmail, "acme-email", "", "optional contact email for Let's Encrypt notifications")
+	flag.StringVar(&acmeHostnamesCommas, "acme-hostnames", "", "comma-separated hostnames for TLS certificate")
+	flag.StringVar(&acmeCertCacheDir, "acme-cert-cache", "acme-cert-cache", "directory in which certificates should be cached")
+	flag.StringVar(&certFilename, "cert", "", "TLS certificate file")
+	flag.StringVar(&keyFilename, "key", "", "TLS private key file")
+	flag.StringVar(&addr, "addr", ":8443", "address to listen on")
+	flag.BoolVar(&disableTLS, "disable-tls", false, "don't use HTTPS")
+	flag.BoolVar(&unsafeLogging, "unsafe-logging", false, "prevent logs from being scrubbed")
+	flag.Parse()
+
+	var logOutput io.Writer = os.Stderr
+	if unsafeLogging {
+		log.SetOutput(logOutput)
+	} else {
+		// Scrub log output just in case an address ends up there
+		log.SetOutput(&safelog.LogScrubber{Output: logOutput})
+	}
+
+	log.SetFlags(log.LstdFlags | log.LUTC)
+
+	http.HandleFunc("/probe", probeHandler)
+
+	server := http.Server{
+		Addr: addr,
+	}
+
+	var err error
+	if acmeHostnamesCommas != "" {
+		acmeHostnames := strings.Split(acmeHostnamesCommas, ",")
+		log.Printf("ACME hostnames: %q", acmeHostnames)
+
+		var cache autocert.Cache
+		if err = os.MkdirAll(acmeCertCacheDir, 0700); err != nil {
+			log.Printf("Warning: Couldn't create cache directory %q (reason: %s) so we're *not* using our certificate cache.", acmeCertCacheDir, err)
+		} else {
+			cache = autocert.DirCache(acmeCertCacheDir)
+		}
+
+		certManager := autocert.Manager{
+			Cache:      cache,
+			Prompt:     autocert.AcceptTOS,
+			HostPolicy: autocert.HostWhitelist(acmeHostnames...),
+			Email:      acmeEmail,
+		}
+		// start certificate manager handler
+		go func() {
+			log.Printf("Starting HTTP-01 listener")
+			log.Fatal(http.ListenAndServe(":80", certManager.HTTPHandler(nil)))
+		}()
+
+		server.TLSConfig = &tls.Config{GetCertificate: certManager.GetCertificate}
+		err = server.ListenAndServeTLS("", "")
+	} else if certFilename != "" && keyFilename != "" {
+		err = server.ListenAndServeTLS(certFilename, keyFilename)
+	} else if disableTLS {
+		err = server.ListenAndServe()
+	} else {
+		log.Fatal("the --cert and --key, --acme-hostnames, or --disable-tls option is required")
+	}
+
+	if err != nil {
+		log.Println(err)
+	}
+}