[ntp:questions] why does asterisk still show after the ntp server is shutdown?

Rob nomail at example.com
Fri Nov 16 22:51:30 UTC 2012

David Woolley <david at ex.djwhome.demon.invalid> wrote:
> Rob wrote:
>> The next router upstream will send an ICMP "destination unreachable"
>> when it cannot resolve the IP address into a hardware address, which
>> normally happens when it gets no reply to its ARP requests.
> Only once the ARP cache times out, and only if the link level protocol 
> uses ARP.

When the link does not use ARP, it probably is a point-to-point link
that has immediate information about the link state.
What matters is that the sending side of the link knows the receiving
side is no longer there, and then can generate an ICMP informing the
source of the packet about that.

>> However, those ICMP "destination unreachable" are often ignored by
>> applications, including ntpd.  Some people even block them at their
>> firewalls.
> They are difficult to handle for UDP based protocols, as the application 
> has to specifically receive them and to correlate them with the specific 
> request.

I already wrote, it does not listen for this information.
It is not like the information is not there, it is not used.
Apparently because it is deemed too difficult to do that.

(which is a fault of the socket interface in Unix/Linux)

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