ares_expand_name() should escape more characters

RFC1035 5.1 specifies some reserved characters and escaping sequences
that are allowed to be specified.  Expand the list of reserved characters
and also escape non-printable characters using the \DDD format as
specified in the RFC.

Bug Reported By: philipp.jeitner@sit.fraunhofer.de
Fix By: Brad House (@bradh352)

Author: bradh352

src/lib/ares_expand_name.c

@@ -32,6 +32,26 @@
 static int name_length(const unsigned char *encoded, const unsigned char *abuf,
                        int alen);
 
+/* Reserved characters for names that need to be escaped */
+static int is_reservedch(int ch)
+{
+  switch (ch) {
+    case '"':
+    case '.':
+    case ';':
+    case '\\':
+    case '(':
+    case ')':
+    case '@':
+    case '$':
+      return 1;
+    default:
+      break;
+  }
+
+  return 0;
+}
+
 /* Expand an RFC1035-encoded domain name given by encoded.  The
  * containing message is given by abuf and alen.  The result given by
  * *s, which is set to a NUL-terminated allocated buffer.  *enclen is
@@ -111,9 +131,18 @@ int ares_expand_name(const unsigned char *encoded, const unsigned char *abuf,
           p++;
           while (len--)
             {
-              if (*p == '.' || *p == '\\')
+              if (!isprint(*p)) {
+                /* Output as \DDD for consistency with RFC1035 5.1 */
+                *q++ = '\\';
+                *q++ = '0' + *p / 100;
+                *q++ = '0' + (*p % 100) / 10;
+                *q++ = '0' + (*p % 10);
+              } else if (is_reservedch(*p)) {
                 *q++ = '\\';
-              *q++ = *p;
+                *q++ = *p;
+              } else {
+                *q++ = *p;
+              }
               p++;
             }
           *q++ = '.';
@@ -171,7 +200,13 @@ static int name_length(const unsigned char *encoded, const unsigned char *abuf,
           encoded++;
           while (offset--)
             {
-              n += (*encoded == '.' || *encoded == '\\') ? 2 : 1;
+              if (!isprint(*encoded)) {
+                n += 4;
+              } else if (is_reservedch(*encoded)) {
+                n += 2;
+              } else {
+                n += 1;
+              }
               encoded++;
             }
           n++;