Added ability to enforce dm68-compatible stream along with new challenge format, can be enabled on both client and server-side by adding -compat suffix to protocol cvar in command line, for example:

+set protocol "71-compat"

code/client/cl_main.c

@@ -2700,27 +2700,27 @@ static qboolean CL_ConnectionlessPacket( const netadr_t *from, msg_t *msg ) {
 			return qfalse;
 		}
 
-		if ( !clc.compat )
-		{
-			c = Cmd_Argv(1);
-
-			if(*c)
-				challenge = atoi(c);
-			else
-			{
-				Com_Printf("Bad connectResponse received. Ignored.\n");
+		if ( !clc.compat ) {
+			c = Cmd_Argv( 1 );
+			if ( *c != '\0' ) {
+				challenge = atoi( c );
+			} else {
+				Com_Printf( "Bad connectResponse received. Ignored.\n" );
 				return qfalse;
 			}
 
-			if(challenge != clc.challenge)
-			{
-				Com_Printf("ConnectResponse with bad challenge received. Ignored.\n");
+			if ( challenge != clc.challenge ) {
+				Com_Printf( "ConnectResponse with bad challenge received. Ignored.\n" );
 				return qfalse;
 			}
+
+			if ( com_protocolCompat ) {
+				// enforce dm68-compatible stream
+				clc.compat = qtrue;
+			}
 		}
 
-		Netchan_Setup( NS_CLIENT, &clc.netchan, from, Cvar_VariableIntegerValue("net_qport"),
-			clc.challenge, clc.compat );
+		Netchan_Setup( NS_CLIENT, &clc.netchan, from, Cvar_VariableIntegerValue( "net_qport" ), clc.challenge, clc.compat );
 
 		cls.state = CA_CONNECTED;
 		clc.lastPacketSentTime = -9999;		// send first packet immediately

code/qcommon/common.c

@@ -68,6 +68,7 @@ cvar_t	*com_timescale;
 static cvar_t *com_fixedtime;
 cvar_t	*com_journal;
 cvar_t	*com_protocol;
+qboolean com_protocolCompat;
 #ifndef DEDICATED
 cvar_t	*com_maxfps;
 cvar_t	*com_maxfpsUnfocused;
@@ -3611,6 +3612,15 @@ void Com_Init( char *commandLine ) {
 	Cvar_Get( "sv_master3", "master.maverickservers.com", CVAR_INIT );
 
 	com_protocol = Cvar_Get( "protocol", XSTRING( PROTOCOL_VERSION ), 0 );
+	if ( Q_stristr( com_protocol->string, "-compat" ) > com_protocol->string ) {
+		// strip -compat suffix
+		Cvar_Set2( "protocol", va( "%i", com_protocol->integer ), qtrue );
+		// enforce legacy stream encoding but with new challenge format
+		com_protocolCompat = qtrue;
+	} else {
+		com_protocolCompat = qfalse;
+	}
+
 	Cvar_CheckRange( com_protocol, "0", NULL, CV_INTEGER );
 	com_protocol->flags &= ~CVAR_USER_CREATED;
 	com_protocol->flags |= CVAR_SERVERINFO | CVAR_ROM;

code/qcommon/qcommon.h

@@ -1004,6 +1004,7 @@ extern	cvar_t	*com_buildScript;		// for building release pak files
 extern	cvar_t	*com_journal;
 extern	cvar_t	*com_cameraMode;
 extern	cvar_t	*com_protocol;
+extern	qboolean com_protocolCompat;
 
 // both client and server must agree to pause
 extern	cvar_t	*sv_paused;

code/qcommon/vm_x86.c

@@ -4871,7 +4871,7 @@ int32_t VM_CallCompiled( vm_t *vm, int nargs, int32_t *args )
 		Com_Error( ERR_DROP, "%s(%s): opStack corrupted in compiled code", __func__, vm->name );
 	}
 
-	if ( vm->programStack != stackOnEntry - ( MAX_VMMAIN_CALL_ARGS + 2 ) * sizeof( int32_t ) ) {
+	if ( vm->programStack != (int32_t)( stackOnEntry - ( MAX_VMMAIN_CALL_ARGS + 2 ) * sizeof( int32_t ) ) ) {
 		Com_Error( ERR_DROP, "%s(%s): programStack corrupted in compiled code", __func__, vm->name );
 	}
 #endif

code/server/sv_client.c

@@ -536,6 +536,12 @@ void SV_DirectConnect( const netadr_t *from ) {
 		}
 	}
 
+	if ( com_protocolCompat )
+	{
+		// enforce dm68-compatible stream
+		compat = qtrue;
+	}
+
 	v = Info_ValueForKey( userinfo, "qport" );
 	if ( *v == '\0' )
 	{

code/server/sv_main.c

@@ -778,7 +778,7 @@ static void SVC_Info( const netadr_t *from ) {
 	// to prevent timed spoofed reply packets that add ghost servers
 	Info_SetValueForKey( infostring, "challenge", Cmd_Argv(1) );
 
-	Info_SetValueForKey( infostring, "protocol", com_protocol->string );
+	Info_SetValueForKey( infostring, "protocol", va( "%i", com_protocol->integer ) );
 	Info_SetValueForKey( infostring, "hostname", sv_hostname->string );
 	Info_SetValueForKey( infostring, "mapname", sv_mapname->string );
 	Info_SetValueForKey( infostring, "clients", va("%i", count) );