Allow '/' as a valid character for a returned name

As of c-ares 1.17.2, a CNAME an in-addr.arpa delegation broke due
to not allowing '/'.  This needs to be allowed to not break valid
functionality.

Fixes Bug: #427
Reported By: Adrian (@leftshift)
Fix By: Brad House (@bradh352)

src/lib/ares_expand_name.c

@@ -60,15 +60,18 @@ static int ares__isprint(int ch)
 }
 
 /* Character set allowed by hostnames.  This is to include the normal
- * domain name character set plus underscores which are used in SRV
- * records.  While RFC 2181 section 11 does state not to do validation,
+ * domain name character set plus:
+ *  - underscores which are used in SRV records.
+ *  - Forward slashes such as are used for classless in-addr.arpa 
+ *    delegation (CNAMEs)
+ * While RFC 2181 section 11 does state not to do validation,
  * that applies to servers, not clients.  Vulnerabilities have been
  * reported when this validation is not performed.  Security is more
  * important than edge-case compatibility (which is probably invalid
  * anyhow). */
 static int is_hostnamech(int ch)
 {
-  /* [A-Za-z0-9-._]
+  /* [A-Za-z0-9-._/]
    * Don't use isalnum() as it is locale-specific
    */
   if (ch >= 'A' && ch <= 'Z')
@@ -77,7 +80,7 @@ static int is_hostnamech(int ch)
     return 1;
   if (ch >= '0' && ch <= '9')
     return 1;
-  if (ch == '-' || ch == '.' || ch == '_')
+  if (ch == '-' || ch == '.' || ch == '_' || ch == '/')
     return 1;
 
   return 0;