[ntp:questions] ISP bloked port 123

David Taylor david-taylor at blueyonder.co.uk.invalid
Wed Sep 18 08:53:44 UTC 2013

On 18/09/2013 08:55, Bert Gøtterup Petersen wrote:
> David,
> I understand that a Raspberry-Pi would do the trick, and I am sure that would work  for everyone reading this.
> However, to our customers and installers this would be rather invasive. They are buying/installing a TV not an IT system..
> Our accuracy requirement are not impressive, merely to allow the product to talk to our cloud service (UTC +-5min).
> Our assumption is that some ISP block the port to prevent their customers from running a public NTP server.
> At the moment our best bet seem to be using 'ntpdate' on a different port at regular intervals. From a SW perspective, this is not nice nor elegant, but it would do the trick...
> Cheers
> Bert


Thanks for that background.  Now you have expanded the problem, I can 
suggest a solution based on something I needed when writing a time 
monitoring program.  If you have guaranteed Internet access, but with 
123 blocked, then you could use:

- the HTTP protocol on port 80, and get the header information which 
includes the time from a known page on a known reliable server - one of 
your own, of course!  You could use the Last-Modified or Expires times, 
both of which I expect that you could program to return the current date 
and time.  Should be OK for 5-minute accuracy.

If the following ports are not blocked...

- use the time protocol on port 37

- use the daytime protocol on port 13

Also note Rob's comments about what time may already be present on a DVB 

I would guess the port blocking is rather due to an "enable the minimum" 
policy rather than any fear of customers running an NTP server.  Average 
customers running file sharing or watching a video is likely to use far 
more network capacity!
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