From f29a7dd518f2119a5d26dfb96887b8303efcad8e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Hampus=20Sj=C3=B6berg?= <hampus.sjoberg@gmail.com>
Date: Mon, 5 Sep 2016 23:56:50 +0200
Subject: [PATCH] Qt: Ask user to use standard port on startup if specified
 port is in use

If the port (specified by -port or GUI setting) is already in use when
starting the GUI client, and it's not the standard port, ask the user if
they want to listen via the standard network port instead.

Github-Pull: #7107
Rebased-From: 1f37c87d8f2fbb7113c38674d1cc65933a222ccb
---
 src/net.cpp | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/src/net.cpp b/src/net.cpp
index e1206745a4..489cbead47 100644
--- a/src/net.cpp
+++ b/src/net.cpp
@@ -3224,7 +3224,29 @@ bool CConnman::InitBinds(const Options& options)
         inaddr_any.s_addr = htonl(INADDR_ANY);
         const CService ipv4_any{inaddr_any, GetListenPort()}; // 0.0.0.0
         if (!Bind(ipv4_any, BF_REPORT_ERROR, NetPermissionFlags::None)) {
-            return false;
+            int defaultPort = Params().GetDefaultPort();
+            // If listening failed and another port than the standard port was specified,
+            // ask if the user wants to connect via the standard port for the network instead
+            if (GetListenPort() != defaultPort) {
+                bool fRet = uiInterface.ThreadSafeQuestion(
+                    strprintf(_("Do you want to use the standard network port for %s (port %s) instead?"), PACKAGE_NAME, defaultPort),
+                    strprintf(_("Listen on port %s failed.").translated, GetListenPort()),
+                    "", CClientUIInterface::MSG_INFORMATION | CClientUIInterface::MODAL | CClientUIInterface::BTN_OK | CClientUIInterface::BTN_ABORT);
+
+                if (fRet) {
+                    // FIXME: Unbind IPv6 on the other port
+
+                    gArgs.ForceSetArg("-port", defaultPort);
+                    // Attempt to use standard port
+                    struct in6_addr inaddr6_any = IN6ADDR_ANY_INIT;
+                    Bind(CService(inaddr6_any, defaultPort), BF_NONE, NetPermissionFlags::None);
+                    struct in_addr inaddr_any;
+                    inaddr_any.s_addr = INADDR_ANY;
+                    if (!Bind(CService(inaddr_any, defaultPort), BF_REPORT_ERROR, NetPermissionFlags::None)) {
+                        return false;
+                    }
+                }
+            }
         }
     }
     return true;