setting ntp with dhcp

Fri Oct 1 15:24:27 UTC 2021

Hi Giovane, a time server is a required DHCP option for DOCSIS devices. 
This uses the older TIME protocol (UDP port 37, RFC 868). However, it's 
common for DOCSIS devices like MTAs, STBs, etc to also request and use 
NTP server addresses received via DHCP (they may apply this using SNTP 
rather than a fully fledged NTP client).

As one data point, Google Fiber (a large GPON based ISP in USA) provides 
NTP servers to customers via DHCP. We actually had to contact them as 
they were providing a set of server addresses where one or more of the 
servers was no longer reachable. Some of our VoIP telephones used by 
employees did not like receiving bad info and would intermittently fail 
to register. Google Fiber was able to remove the bad info from their 
DHCP replies. We've seen some similar behavior in other networks so now 
I believe we prefer to ignore/override the DHCP provided NTP info with 
known good server addresses (it seems too common for NTP to simply be 
ignored/overlooked by network operators and users).

--Blake

On 10/1/2021 9:12 AM, Giovane C. M. Moura via NANOG wrote:
> hello folks,
>
> So DHCP can also be used to set NTP servers on clients, for both
> IPv4[rfc2132] and IPv6[rfc5908].
>
> I'm looking for statistics on setting NTP servers on clients using DHCP,
> in the wild. Does anyone know if there is any available somewhere?
>
> I'm also looking for reports from operators and their experiences on
> this, and why they use (or not) this DHCP feature, and what types of
> networks is this deployed, and their motivations, etc.
>
> Thanks in advance,
>
> /giovane
>