MX Record Theories

David Conrad drc at
Thu May 28 16:09:03 CDT 2009

On May 28, 2009, at 5:04 AM, Bobby Mac wrote:
> If you add enough recipients to an email, each domain within the  
> send line
> needs to have an associated MX record.

Well, it needs to resolve to an A RR somehow, but for each domain  
name, you get a different query.

> DNS by default starts with UDP which
> has a limit to the datagram size (64bit).

The UDP minimum datagram size that must be supported by DNS  
implementations is 512 bytes.  The maximum is 64K bytes.  Obviously if  
you try to send a 64K byte packet, it's going to fragment and as we  
all know, fragments are bad.

> A flag is placed in the
> header which then requires the request to be sent via TCP (160bit V4).

If the response to a query won't fit in the UDP buffer (512 by  
default, although modern client implementations can advertise a larger  
buffer with EDNS0), the server will signal truncation in the response  
(with the TC bit), typically resulting in the client retransmitting  
the request via TCP.

> Now
> that single query can be split up into many different packets  
> providing that
> the request is more than the 160 bit and obviously IPV6 offers even  
> more
> information contained in a single packet.

IPv6 packets are a bit larger, but not that much.  DNSSEC is where the  
fun starts.


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