How to specify timezones with DHCP

One thing that can be useful for locations where you have lots of travellers, is to have your DHCP server specify the timezone that people should be using. Some examples might be large conferences, or a busy office building of a multinational company.

While cellphones generally have GPS and cellular networks to tell them where they are, and therefore from there they can guess the correct timezone, laptops usually don't have this luxury.

Fortunately, there are two DHCP options that you can use to tell clients what timezone they should be expecting. These are specified in [RFC4833].

As usual, not many DHCP clients automatically know what to do with these options.

POSIX Timezone specifier

The first option (100) specifies a POSIX Timezone specifier (as documented in POSIX "Base Definitions", section 8 "Environment Variables", under TZ).

The format of this is <name><offset><dst name><dst offset>,<dstrule>

• name is the name of the timezone when not in daylight savings (eg GMT, PST, NZST)
• offset is the offset added to the local time to get UTC, specified as [+|-]hh[:mm[:ss]] (eg 0, 8, -12)
• dst name is the name of the timezone when in daylight savings (eg BST, PDT, NZDT)
• dst offset is the offset added to the local time to get UTC during daylight savings, similarly specified as [+|-]hh[:mm[:ss]]

The dstrule is interesting, it specifies the rule for when daylight savings starts and ends.

dstdate[/dsttime],stddate[/stdtime]

dstdate is the date (and optional time) that the change from standard timezone to dst timezone occurs. dsttime if not specified defaults to 02:00:00

stddate is the date (and optional time) that the change from dst timezone to standard timezone occurs. stdtime if not specified defaults to 02:00:00

• Jn specifies a 1 based Julian day of the year, skipping any leap Feb 29ths.
• n specifies a 1 based Julian day of the year, including any leap Feb 29ths.
• Mm.w.d The dth day of week w of the month m of the year. Week 5 means the last d of the month. d is specified as 0 == Sunday, 6 == Saturday. m and w are 1 indexed.

Some examples:

• London: GMT0BST1,M3.5.0/1:00:00,M10.5.0/2:00:00
• Los Angeles: PST8PDT,M3.2.0/2:00:00,M11.1.0/2:00:00
• New Zealand: NZST-12NZDT,M9.5.0/2:00:00,M4.1.0/3:00:00

Some hints:

• In most of Europe go forward 1 hour at 2am on the last Sunday in March, and back one hour at 3am on the last Sunday in October. This is specified as M3.5.0,M10.5.0/3
• In most of the US (and Canada) the clocks go forward 1 hour at 2am on the second Sunday in March, and back 1 hour at 2am on the first Sunday in November. This is specified as M3.2.0/2:00:00,M11.1.0/2:00:00
• Australia and most of South America enter daylight savings on the first Sunday in October, and change back to standard time, on the third Sunday in March. This is because they are in the Southern Hemisphere where mid Summer is December, instead of July.
• Most of Central America go forward 1 hour on the 1st Sunday of April at 2am, and back 1 hour on the last Sunday of October at 2am. This is specified as M4.1.0,M10.5.0

Timezone Name

The second option (101) specifies a Timezone Name. e.g. "Europe/London" or "Pacific/Auckland".

How to specify this in common DHCP configs:

Using dnsmasq

# Option 100 - POSIX TZ Code
dhcp-option=100,"NZST-12NZDT,M9.5.0/2,M4.1.0/3"
# Option 101 - TCode
dhcp-option=101,"Pacific/Auckland"

Using Junos Apply groups

set groups default-dhcp-attributes routing-instances <*> access address-assignment pool <*> family inet dhcp-attributes option 100 string NZST-12NZDT,M9.5.0/2,M4.1.0/3
set groups default-dhcp-attributes routing-instances <*> access address-assignment pool <*> family inet dhcp-attributes option 101 string Pacific/Auckland

Using ISC DHCP server

option PCode code 100 = text;
option TCode code 101 = text;
option PCode "NZST-12NZDT,M9.5.0/2,M4.1.0/3";
option TCode "Pacific/Auckland";