The End-To-End Internet (was Re: Blocking MX query)

David Miller dmiller at
Tue Sep 4 20:07:06 UTC 2012

On 9/4/2012 2:22 PM, Jay Ashworth wrote:
> ----- Original Message -----
>> From: "Owen DeLong" <owen at>
>> I am confused... I don't understand your comment.
> It is regularly alleged, on this mailing list, that NAT is bad *because it 
> violates the end-to-end principle of the Internet*, where each host is a
> full-fledged host, able to connect to any other host to perform transactions.
> We see it now alleged that the opposite is true: that a laptop, say, like
> mine, which runs Linux and postfix, and does not require a smarthost to
> deliver mail to a remote server *is a bad actor* *precisely because it does
> that* (in attempting to send mail directly to a domain's MX server) *from 
> behind a NAT router*, and possibly different ones at different times.
> I find these conflicting reports very conflicting.  Either the end-to-end
> principle *is* the Prime Directive... or it is *not*.

The end-to-end design principle pushes application functions to
endpoints instead of placing these functions in the network itself.
This principle requires that endpoints be *capable* of creating
connections to each other.  Network system design must support these
connections being initiated by either side - which is where NAT
implementations usually fail.

There is no requirement that all endpoints be *permitted* to connect to
and use any service of any other endpoint.  The end-to-end design
principle does not require a complete lack of authentication or

I can refuse connections to port 25 on my endpoint (mail server) from
hosts that do not conform to my requirements (e.g. those that do not
have forward-confirmed reverse DNS) without violating the end-to-end
design principle in any way.

Thus it is a false chain of conclusions to say that:
- end-to-end is violated by restricting connections to/from certain hosts
- the end-to-end design principle is not important
- NAT is good

...which I believe is the argument that was being made? ...

Ref -

> Cheers,
> -- jra

More information about the NANOG mailing list