DHCP, from zero: a map of this series

Every machine you have ever booted on a network got its address from something. On most networks, that something is a DHCP server.

It is one of those pieces of infrastructure that is invisible until it breaks, and then it takes everything with it. Nobody can reach anything, and the outage looks like DNS, or the switch, or the cloud provider — until someone checks the lease pool and finds it empty.

I am writing this series because I am working on network boot at OVHcloud, and I kept hitting the edges of what I actually understood about DHCP. Not the four-message handshake — that part is easy to look up. The parts underneath: why the protocol is shaped the way it is, what a relay really does to a packet, what happens to a lease when nobody renews it, and whether DHCPv6 is worth anyone's time.

This is the map.

What this series is

Fourteen posts, each one about a single idea, each one short enough to read in one sitting.

It starts from zero. If you know what an IP address is and roughly what a switch does, you have enough. It ends somewhere useful: by the last post you should be able to install a DHCP server, debug one that is misbehaving, decide where it belongs in a network, and have an opinion about IPv6 that is yours rather than borrowed.

What this series is not

It is not a replacement for the RFCs. RFC 2131 is 45 pages and surprisingly readable; where a detail matters I will point at it rather than paraphrase it badly.

It is also not about OVHcloud's network boot work. That is what got me reading, but nothing in here is specific to it. Everything I write about is public protocol behaviour and software you can install yourself.

And it is not a set of benchmarks. Where I run something, I will show the command and the output. Where I am repeating something I have read but not verified, I will say so.

The order

flowchart TD
  A["A host boots with no address"] --> B["1. How it gets one: DORA"]
  B --> C["2. The network underneath"]
  B --> D["3. Leases and their expiry"]
  C --> E["4. Relays: one server, many subnets"]
  D --> F["5. Options, and booting from the network"]
  F --> G["6. NTP and the cold clock"]
  E --> H["7-8. Practice: install it, then break it"]
  G --> H
  H --> I["9. Where the server lives"]
  I --> J["10. On-prem and hybrid"]
  I --> K["11. Inside a cloud provider"]
  J --> L["12. The market, and Go"]
  K --> L
  L --> M["13. DHCPv6: worth it?"]

The posts

Foundations — what the protocol does

#	Post	The one idea
1	How a host with no address gets one	The DORA exchange, message by message
2	The network underneath	Why DHCP needs broadcast, and what that constrains
3	Leases: the state everyone forgets	A lease is a timer, and timers expire

Beyond one flat network

#	Post	The one idea
4	Relays: one server, many subnets	How a broadcast protocol crosses a router
5	Options, and booting from the network	DHCP carries far more than an address
6	DHCP and NTP: the cold clock	A machine that just booted does not know what time it is

Practice

#	Post	The one idea
7	A DHCP server you can run in five minutes	Kea in a network namespace lab
8	Debugging DHCP	Read the wire, then work down a decision tree

Where it belongs

#	Post	The one idea
9	Where the server lives	Topology, hardware, and high availability
10	Outside the cloud: on-prem and hybrid	Campus networks, Active Directory, split scopes
11	Inside a cloud provider	Why providers do not just run dhcpd

Decisions

#	Post	The one idea
12	The market, and what Go offers	Kea, dnsmasq, CoreDHCP, and when to write your own
13	DHCPv6 is not DHCPv4 with bigger addresses	Two different protocols, one hard decision

The lab

Posts 7 and 8 build a lab you can run anywhere Docker runs: a client, a server, and a router, each in its own network namespace, with no virtual machines and no cloud account. Several of the earlier posts use it too, for captures I would rather show than describe.

It is a handful of ip netns commands wrapped in a container, so the only thing you need installed is Docker. The scripts are at tul1/dhcp-lab.

Where to start

Post 1. If you already know the DORA exchange cold, start at post 3 — leases are where the interesting failures live, and they are the part most people skip.

Next: How a host with no address gets one.