DHCP and timezone options

H. Peter Anvin hpa at zytor.com
Thu Dec 14 00:08:27 UTC 2017

Hello all,

I have been trying to figure out a sane way of dealing with timezone
options in DHCP, especially with the handling of the legacy DHCP
time-offset option.

In the end, I found that it is possible to find the following
information by looking at the zic-compiled output of the Olson/IANA TZ

1. The current TZ offset
2. The timestamp of the next TZ offset change (and the following offset)
3. A POSIX string corresponding to the future of the current timezone
   (at least to the extent that such a string can be composed.)

I was thinking it would be nice if one could configure DHCP to emit the
relevant options dynamically, and for the DHCPv4 case, it would also be
interesting to have the dhcp-renewal-time capped to the point of the
next TZ transition, to force it to renew the lease and thus get the
updated time-offset option.

I did write a timezone file parser to extract that above information.
It is not in ISC style; I will try at some point to implement this but
just in case someone feels motivated to look at it, this is my POC (not
anywhere close to ISC style I'm afraid):



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