From 8d4ac771383f254b7a68cd89aee37e2ac19de18e Mon Sep 17 00:00:00 2001
From: lolandhold <77760965+lolandhold@users.noreply.github.com>
Date: Sat, 13 Mar 2021 22:57:05 -0500
Subject: [PATCH] upnp: More accurate getOurIP.

This makes getOurIP return a local IP that is on the same subnet as
the serviceIP used for uPnP.  Verified getOurIP doesn't return a local
IPv6 IP when the serviceIP is IPv4.  Didn't test with IPv6 serviceIP.

Co-authored-by: lol_and_hold <lolandhold@protonmail.com>
---
 upnp.go | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/upnp.go b/upnp.go
index a1291e48..b5cbf806 100644
--- a/upnp.go
+++ b/upnp.go
@@ -40,6 +40,7 @@ import (
 	"fmt"
 	"net"
 	"net/http"
+	"net/url"
 	"strconv"
 	"strings"
 	"time"
@@ -113,8 +114,9 @@ func discover(ctx context.Context) (*upnpNAT, error) {
 		if err != nil {
 			return nil, err
 		}
+		var serviceIP string = getServiceIP(serviceURL)
 		var ourIP string
-		ourIP, err = getOurIP()
+		ourIP, err = getOurIP(serviceIP)
 		if err != nil {
 			return nil, err
 		}
@@ -197,12 +199,18 @@ func getChildService(d *device, serviceType string) *service {
 	return nil
 }
 
-// getOurIP returns a best guess at what the local IP is.
-func getOurIP() (ip string, err error) {
+func getServiceIP(serviceURL string) (routerIP string) {
+	url, _ := url.Parse(serviceURL)
+	return url.Hostname()
+}
+
+// getOurIP returns the local IP that is on the same subnet as the serviceIP.
+func getOurIP(serviceIP string) (ip string, err error) {
+	_, serviceNet, _ := net.ParseCIDR(serviceIP + "/24")
 	addrs, err := net.InterfaceAddrs()
 	for _, addr := range addrs {
 		ip, _, _ := net.ParseCIDR(addr.String())
-		if !ip.IsLoopback() {
+		if serviceNet.Contains(ip) {
 			return ip.String(), nil
 		}
 	}