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

[Pim van Stam <pim () vanstam-ict nl>]

> On 25 Aug 2020, at 14:27, K. Scott Helms <kscott.helms () gmail com> wrote:
>
> 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.

a. some systems show UDP fragments as UDP port 0. So if the filter also handles fragments as UDP port 0, then you have 
a problem
b. if you don’t reassemble UDP fragments and filter on port number, like 11212 (memcache) or 389 (ldap), then fragments 
will be forwarded and still be a problem

I think in general you can say that problems with UDP port 0 are in fact fragments. Ohter opinions on this?

Best regards,

Pim van Stam



> Scott Helms
>
> Scott Helms
>
>
>
> On Tue, Aug 25, 2020 at 8:17 AM Job Snijders <job () 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