Fix client TLS termination bug

Problem had to do with certmagic. Details in code.

client.go

@@ -56,6 +56,19 @@ func NewBoringProxyClient() *BoringProxyClient {
 		}
 	}
 
+	// Use random unprivileged port for ACME challenges. This is necessary
+	// because of the way certmagic works, in that if it fails to bind
+	// HTTPSPort (443 by default) and doesn't detect anything else binding
+	// it, it fails. Obviously the boringproxy client is likely to be
+	// running on a machine where 443 isn't bound, so we need a different
+	// port to hack around this. See here for more details:
+	// https://github.com/caddyserver/certmagic/issues/111
+	var err error
+	certmagic.HTTPSPort, err = randomOpenPort()
+	if err != nil {
+		log.Fatal("Failed get random port for TLS challenges")
+	}
+
 	certmagic.DefaultACME.DisableHTTPChallenge = true
 
 	if *certDir != "" {

tunnel_manager.go

@@ -11,9 +11,7 @@ import (
 	"golang.org/x/crypto/ssh"
 	"io/ioutil"
 	"log"
-	"net"
 	"os/user"
-	"strconv"
 	"strings"
 	"sync"
 )
@@ -75,7 +73,7 @@ func (m *TunnelManager) RequestCreateTunnel(tunReq Tunnel) (Tunnel, error) {
 		return Tunnel{}, errors.New("Tunnel exists for domain " + tunReq.Domain)
 	}
 
-	port, err := randomPort()
+	port, err := randomOpenPort()
 	if err != nil {
 		return Tunnel{}, err
 	}
@@ -224,20 +222,3 @@ func MakeSSHKeyPair() (string, string, error) {
 
 	return pubKey, privKeyBuf.String(), nil
 }
-
-func randomPort() (int, error) {
-	listener, err := net.Listen("tcp", "127.0.0.1:0")
-	if err != nil {
-		return 0, err
-	}
-
-	addrParts := strings.Split(listener.Addr().String(), ":")
-	port, err := strconv.Atoi(addrParts[len(addrParts)-1])
-	if err != nil {
-		return 0, err
-	}
-
-	listener.Close()
-
-	return port, nil
-}

utils.go

@@ -6,7 +6,9 @@ import (
 	"errors"
 	"io/ioutil"
 	"math/big"
+	"net"
 	"net/http"
+	"strconv"
 	"strings"
 )
 
@@ -65,3 +67,20 @@ func genRandomCode(length int) (string, error) {
 	}
 	return id, nil
 }
+
+func randomOpenPort() (int, error) {
+	listener, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		return 0, err
+	}
+
+	addrParts := strings.Split(listener.Addr().String(), ":")
+	port, err := strconv.Atoi(addrParts[len(addrParts)-1])
+	if err != nil {
+		return 0, err
+	}
+
+	listener.Close()
+
+	return port, nil
+}