Client: disable pre v25 init sending by default

Disable the ability to connect to old servers by default to
improve password security.

If people still want to connect to old (0.4.12 and earlier)
servers, they can flip the send_pre_v25_init setting.

Add the ability to detect if we've tried to connect
to a server which only supports the pre v25 init protocol,
and show an apropriate error message. Most times the error
will already be catched at the serverlist level, the
detection mechanism only acts as last resort, because the
"Connection timed out" error message that would be shown
otherwise would be very confusing.

Automatic "fixing" of this condition is not desired,
as it would allow for downgrade attacks.

As already 161 of the 167 servers on the serverlist
support the new srp based auth protocol (> 96%),
the breakage should be minimal.

Follow up of commit

af30183124d40a969040d7de4b3a487feec466e4 "Add option to not send pre v25 init packet"

Also change the pessimistic assumption of masterlist
server versions to optimistic, in order to avoid buggy
behaviour (favourites not in the serverlist would be
denied to connect to, etc).

3 files changed, 24 insertions, 2 deletions

diff --git a/src/client.h b/src/client.h
index a7eb22ad9..b479062a0 100644
--- a/src/client.h
+++ b/src/client.h
@@ -499,6 +499,9 @@ public:
 	u8 getProtoVersion()
 	{ return m_proto_ver; }
 
+	bool connectedToServer()
+	{ return m_con.Connected(); }
+
 	float mediaReceiveProgress();
 
 	void afterContentReceived(IrrlichtDevice *device);
diff --git a/src/defaultsettings.cpp b/src/defaultsettings.cpp
index 42b232afc..7c6f7ef3d 100644
--- a/src/defaultsettings.cpp
+++ b/src/defaultsettings.cpp
@@ -190,7 +190,7 @@ void set_default_settings(Settings *settings)
 	settings->setDefault("minimap_shape_round", "true");
 	settings->setDefault("minimap_double_scan_height", "true");
 
-	settings->setDefault("send_pre_v25_init", "true");
+	settings->setDefault("send_pre_v25_init", "false");
 
 	settings->setDefault("curl_timeout", "5000");
 	settings->setDefault("curl_parallel_limit", "8");
diff --git a/src/game.cpp b/src/game.cpp
index 1a036d03a..5a3b10879 100644
--- a/src/game.cpp
+++ b/src/game.cpp
@@ -2403,7 +2403,26 @@ bool Game::connectToServer(const std::string &playername,
 			wait_time += dtime;
 			// Only time out if we aren't waiting for the server we started
 			if ((*address != "") && (wait_time > 10)) {
-				*error_message = "Connection timed out.";
+				bool sent_old_init = g_settings->getFlag("send_pre_v25_init");
+				// If no pre v25 init was sent, and no answer was received,
+				// but the low level connection could be established
+				// (meaning that we have a peer id), then we probably wanted
+				// to connect to a legacy server. In this case, tell the user
+				// to enable the option to be able to connect.
+				if (!sent_old_init &&
+						(client->getProtoVersion() == 0) &&
+						client->connectedToServer()) {
+					*error_message = "Connection failure: init packet not "
+					"recognized by server.\n"
+					"Most likely the server uses an old protocol version (<v25).\n"
+					"Please ask the server owner to update to 0.4.13 or later.\n"
+					"To still connect to the server in the meantime,\n"
+					"you can enable the 'send_pre_v25_init' setting by editing minetest.conf,\n"
+					"or by enabling the 'Client -> Network -> Support older Servers'\n"
+					"entry in the advanced settings menu.";
+				} else {
+					*error_message = "Connection timed out.";
+				}
 				errorstream << *error_message << std::endl;
 				break;
 			}