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

Tue Aug 25 12:39:13 UTC 2020

TCP vs. UDP. 

----- 
Mike Hammett 
Intelligent Computing Solutions 
http://www.ics-il.com 

Midwest-IX 
http://www.midwest-ix.com 

----- Original Message -----

From: "K. Scott Helms" <kscott.helms at gmail.com> 
To: "Job Snijders" <job at ntt.net> 
Cc: "NANOG list" <nanog at nanog.org> 
Sent: Tuesday, August 25, 2020 7:27:24 AM 
Subject: Re: TCP and UDP Port 0 - Should an ISP or ITP Block it? 

Job, 

Comcast is blocking it. From the table on that page. 

"Port 0 is a reserved port, which means it should not be used by 
applications. Network abuse has prompted the need to block this port." 

"What about UDP IP fragmentation?" 

I'm not sure I follow this. The IP packet will be fragmented with UDP 
inside it. When the IP packet gets put together the UDP PDU will have 
a port number. It's possible that some packet analyzers or network 
gear will improperly "see" a partial UDP flow as port 0 but that's a 
mischaracterization of the flow. 

Scott Helms 

Scott Helms 

On Tue, Aug 25, 2020 at 8:17 AM Job Snijders <job at ntt.net> wrote: 
> 
> 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 

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