TCP and UDP Port 0 - Should an ISP or ITP Block it?

Tue Aug 25 12:17:16 UTC 2020

On Tue, Aug 25, 2020 at 07:27:33AM -0400, K. Scott Helms wrote:
> I think a fairly easy thing to do is see what other large retail ISPs
> have done.  Comcast, as an example, lists all of the ports they block
> and 0 is blocked.  I do recommend that port 0 be blocked by all of the
> ISPs I work with and frankly Comcast's list is a pretty good one to
> use in general, though you will get some pushback on things like SMTP.
> 
> https://www.xfinity.com/support/articles/list-of-blocked-ports

I may be reading the table incorrectly, but it seems to me Comcast is
*not* blocking UDP port 0 according to the above URL?

> Transit providers are a little bit different, but then again port 0 is
> also different since AFAIK it's never had a legitimate use case.  It's
> always been a reserved port.  I'd personally block it if I ran a
> transit, but I'd be more willing to open it up for one of my large
> customers (in a limited way) than I would on the retail side.
> 
> https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml

What about UDP IP fragmentation?

Kind regards,

Job