Honor Null MX domains during delivery

include/xpldns.h

@@ -93,6 +93,7 @@ typedef struct{
 typedef struct {
 	XplDns_ResultCode status;
 	int preference;
+	int null_mx;
 	char current_mx[XPLDNS_NAMELEN + 1];
 	XplDns_Result *_mx_result;
 	XplDns_RecordList *_mx_current;
@@ -101,6 +102,7 @@ typedef struct {
 int		XplDnsInit(void);
 void		XplDnsResultFree(XplDns_Result *result);
 XplDns_Result *	XplDnsLookupIP(const char *domain);
+int             XplDnsMxRecordsAreNull(const XplDns_RecordList *records);
 XplDns_MxLookup *XplDnsNewMxLookup(const char *domain);
 XplDns_IpList * XplDnsNextMxLookupIpList(XplDns_MxLookup *mx);
 void		XplDnsFreeMxLookup(XplDns_MxLookup *mx);

src/agents/smtp/smtpc.c

@@ -494,6 +494,10 @@ Connection *LookupRemoteMX(RecipStruct *Recipient, char *hostname,
                 break;
         }
     }
+    if (mx->null_mx) {
+        Recipient->Result = DELIVER_HOST_UNKNOWN;
+        goto finish;
+    }
 
     while ((list = XplDnsNextMxLookupIpList(mx)) != NULL) {
         int x;

src/agents/smtp/smtpd.c

@@ -3601,6 +3601,9 @@ DeliverRemoteMessage (ConnectionStruct * Client, char *Sender,
         DELIVER_ERROR(RetVal);
         goto finish;
     }
+    if (mx->null_mx) {
+        DELIVER_ERROR(DELIVER_HOST_UNKNOWN);
+    }
 
     while ((list = XplDnsNextMxLookupIpList(mx)) != NULL) {
         int x;

src/libs/xpl/CMakeLists.txt

@@ -30,4 +30,8 @@ if(BUILD_TESTING)
 	add_executable(xpl-init-test tests/init-test.c)
 	target_link_libraries(xpl-init-test PRIVATE bongoxpl Threads::Threads)
 	add_test(NAME xpl-init COMMAND xpl-init-test)
+
+	add_executable(xpl-dns-mx-test tests/dns-mx-test.c)
+	target_link_libraries(xpl-dns-mx-test PRIVATE bongoxpl)
+	add_test(NAME xpl-dns-mx COMMAND xpl-dns-mx-test)
 endif()

src/libs/xpl/dns.c

@@ -192,6 +192,22 @@ XplDnsResultFree(XplDns_Result *result)
 	MemFree(result);
 }
 
+int
+XplDnsMxRecordsAreNull(const XplDns_RecordList *records)
+{
+	const XplDns_RecordList *record;
+	const XplDns_MxRecord *only_mx = NULL;
+	unsigned int mx_count = 0U;
+
+	for (record = records; record != NULL; record = record->next) {
+		if (record->type != XPLDNS_RR_MX) continue;
+		only_mx = &record->record.MX;
+		mx_count++;
+	}
+	return mx_count == 1U && only_mx->preference == 0 &&
+	       only_mx->mxname[0] == '\0';
+}
+
 XplDns_MxLookup *
 XplDnsNewMxLookup(const char *domain)
 {
@@ -248,7 +264,9 @@ restart_query:
 
 	result->status = mx_query->status;
 	result->_mx_result = mx_query;
-	result->_mx_current = mx_query->list;
+	result->null_mx = mx_query->status == XPLDNS_SUCCESS &&
+	                  XplDnsMxRecordsAreNull(mx_query->list);
+	result->_mx_current = result->null_mx ? NULL : mx_query->list;
 
 	return result;
 }

src/libs/xpl/tests/dns-mx-test.c

@@ -0,0 +1,53 @@
+/****************************************************************************
+ * <Novell-copyright>
+ * Copyright (c) 2001 Novell, Inc. All Rights Reserved.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of version 2 of the GNU General Public License
+ * as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, contact Novell, Inc.
+ *
+ * To contact Novell about this file by physical or electronic mail, you
+ * may find current contact information at www.novell.com.
+ * </Novell-copyright>
+ ****************************************************************************/
+
+#include <xpldns.h>
+
+#include <assert.h>
+#include <string.h>
+
+int
+main(void)
+{
+    XplDns_RecordList null_mx;
+    XplDns_RecordList regular_mx;
+
+    memset(&null_mx, 0, sizeof(null_mx));
+    null_mx.type = XPLDNS_RR_MX;
+    null_mx.record.MX.preference = 0;
+    assert(XplDnsMxRecordsAreNull(&null_mx));
+
+    null_mx.record.MX.preference = 10;
+    assert(!XplDnsMxRecordsAreNull(&null_mx));
+    null_mx.record.MX.preference = 0;
+    strcpy(null_mx.record.MX.mxname, "mail.example.test.");
+    assert(!XplDnsMxRecordsAreNull(&null_mx));
+
+    null_mx.record.MX.mxname[0] = '\0';
+    memset(&regular_mx, 0, sizeof(regular_mx));
+    regular_mx.type = XPLDNS_RR_MX;
+    regular_mx.record.MX.preference = 10;
+    strcpy(regular_mx.record.MX.mxname, "backup.example.test.");
+    null_mx.next = &regular_mx;
+    assert(!XplDnsMxRecordsAreNull(&null_mx));
+    assert(!XplDnsMxRecordsAreNull(NULL));
+    return 0;
+}