clarification

[John Wobus]

On Oct 22, 2010, at 8:31 AM, rams wrote:
> I have a record in BIND as follows:
>
> mxdomain.com. 86400 IN MX 65536 gmail.com.
>
> When I query "mxdomain.com." with type MX. What is the bind  
> response. Is there any RFC mentioned about this .

On the wire, the MX preference is carried in a 16-bit field,
which cannot store 65536: the field simply isn't big enough.  If you  
query
an MX record and get a preference of 65536, the software with which
you are doing the query has a bug in it and is displaying something
that did not come from the server.

If a zone file has a preference of 65536, dns server software (such
as bind) that attempts to load the zone file should reject it as
impossible to use.  If you have dns server software that doesn't reject
it, you will have to experiment to find out what it does with the input,
which should be easy to do.  It could conceivably use a legal number
instead, or it simply leave out that record.  RFCs merely say 65535
is the maximum allowed.  Specifying what to do when reading a
zone file that exceeds this maximum is one of an infinite
number of possible input errors that RFCs have nothing specific
about.

John Wobus