Network: Avoid DHCP over-writing user specified DNS settings

* Adds parser to servers/net/nethost, to check current
   settings in resolv.conf before updating

   If user has specified DNS settings manually, DHCP derived
   settings are appended to resolv.conf, rather than overwriting.

   (If no user specified data is present, or in the event of
   a badly damaged resolv.conf, DHCP over-writes file in same
   manner as prior to this change)

Fixes #11565.

Change-Id: I43464909009d2416d57f66f7bd14d13c81f803e7
Reviewed-on: https://review.haiku-os.org/c/997
Reviewed-by: Axel Dörfler <[email protected]>
Reviewed-by: Stephan Aßmus <[email protected]>

src/add-ons/network_settings/dnsclient/DNSSettingsView.cpp

@@ -233,7 +233,7 @@ DNSSettingsView::_SaveDNSConfiguration()
 		return file.InitCheck();
 	}
 
-	BString content("# Generated by Network preferences\n");
+	BString content("# Static DNS entries\n# Generated by Network preferences\n");
 
 	for (int i = 0; i < fServerListView->CountItems(); i++) {
 		BString item = ((BStringItem*)fServerListView->ItemAt(i))->Text();
@@ -244,5 +244,7 @@ DNSSettingsView::_SaveDNSConfiguration()
 	if (strlen(fDomain->Text()) > 0)
 		content << "domain\t" << fDomain->Text() << "\n";
 
+	content << "# Dynamic DNS entries\n# may be altered by DHCP\n";
+
 	return file.Write(content.String(), content.Length());
 }

src/servers/net/NetServer.cpp

@@ -598,30 +598,73 @@ NetServer::_ConfigureInterface(BMessage& message)
 status_t
 NetServer::_ConfigureResolver(BMessage& resolverConfiguration)
 {
-	// TODO: resolv.conf should be parsed, all information should be
-	// maintained and it should be distinguished between user entered
-	// and auto-generated parts of the file, with this method only re-writing
-	// the auto-generated parts of course.
+	// Store resolver settings in resolv.conf file, while maintaining any
+	// user specified settings already present.
 
 	BPath path;
 	if (find_directory(B_SYSTEM_SETTINGS_DIRECTORY, &path) != B_OK
 		|| path.Append("network/resolv.conf") != B_OK)
 		return B_ERROR;
 
-	FILE* file = fopen(path.Path(), "w");
-	if (file != NULL) {
-		const char* nameserver;
-		for (int32 i = 0; resolverConfiguration.FindString("nameserver", i,
-				&nameserver) == B_OK; i++) {
-			fprintf(file, "nameserver %s\n", nameserver);
+	FILE* file = fopen(path.Path(), "r+");
+	// open existing resolv.conf if possible
+	if (file == NULL) {
+		// no existing resolv.conf, create a new one
+		file = fopen(path.Path(), "w");
+		if (file == NULL) {
+			fprintf(stderr, "Could not open resolv.conf: %s\n",
+				strerror(errno));
+			return errno;
+		}
+	} else {
+		// An existing resolv.conf was found, parse it for user settings
+		const char* staticDNS = "# Static DNS Only";
+		size_t sizeStaticDNS = strlen(staticDNS);
+		const char* dynamicDNS = "# Dynamic DNS entries";
+		size_t sizeDynamicDNS = strlen(dynamicDNS);
+		char resolveConfBuffer[80];
+		size_t sizeResolveConfBuffer = sizeof(resolveConfBuffer);
+
+		while (fgets(resolveConfBuffer, sizeResolveConfBuffer, file)) {
+			if (strncmp(resolveConfBuffer, staticDNS, sizeStaticDNS) == 0) {
+				// If DNS is set to static only, don't modify
+				fclose(file);
+				return B_OK;
+			} else if (strncmp(resolveConfBuffer, dynamicDNS, sizeDynamicDNS)
+					== 0) {
+				// Overwrite existing dynamic entries
+				break;
+			}
 		}
 
-		const char* domain;
-		if (resolverConfiguration.FindString("domain", &domain) == B_OK)
-			fprintf(file, "domain %s\n", domain);
-
-		fclose(file);
+		if (feof(file) != 0) {
+			// No static entries found, close and re-open as new file
+			fclose(file);
+			file = fopen(path.Path(), "w");
+			if (file == NULL) {
+				fprintf(stderr, "Could not open resolv.conf: %s\n",
+					strerror(errno));
+				return errno;
+			}
+		}
 	}
+
+	fprintf(file, "# Added automatically by DHCP\n");
+
+	const char* nameserver;
+	for (int32 i = 0; resolverConfiguration.FindString("nameserver", i,
+			&nameserver) == B_OK; i++) {
+		fprintf(file, "nameserver %s\n", nameserver);
+	}
+
+	const char* domain;
+	if (resolverConfiguration.FindString("domain", &domain) == B_OK)
+		fprintf(file, "domain %s\n", domain);
+
+	fprintf(file, "# End of automatic DHCP additions\n");
+
+	fclose(file);
+
 	return B_OK;
 }