Tech Notes

1. 20.1 - DHCPv4 DORA Exchange

DHCP lets a host with no address discover local configuration. The client starts from0.0.0.0:68, broadcasts to server port 67, and uses the transaction ID plus client hardware address to match the final ACK to its boot attempt.

A lease is not permanent ownership. T1 is normal renewal with the original server; T2 is rebinding when the original server may be down; after expiry the client must stop using the address.

Minimal C Demo - DORA and Renewal

DHCP DORA Exchange — C Demo

stdin (optional)

2. 20.2 - DHCP Options Deep Dive

DHCP options are TLV records: one byte code, one byte length, and length bytes of data. That simple format carries routers, DNS, vendor data, PXE boot paths, relay metadata, and classless routes.

Option	Name	Operational meaning
1 / 3 / 6	Mask / router / DNS	Basic IPv4 host configuration.
43 / 60	Vendor-specific / class	Client identifies a vendor class; server returns matching vendor payload.
50 / 51 / 54 / 55	Requested IP, lease, server ID, PRL	DORA control fields and requested option list.
66 / 67	TFTP server / bootfile	Classic PXE boot server and first bootloader filename.
82	Relay agent info	Circuit ID, remote ID, and link selection for relay or snooping policy.
121	Classless static routes	Prefix/gateway tuples; when present, it supersedes option 3 default routing logic.

3. 20.3 - DHCP Relay Agent

Client broadcasts do not cross routers, so an SVI or router interface relays the packet to a central server. The giaddr field tells the server which client subnet should be used for scope selection.

4. 20.4 - DHCP Snooping

DHCP snooping turns the access switch into a DHCP firewall. Server messages are allowed only from trusted uplinks; successful ACKs populate a binding table that DAI and IP Source Guard reuse.

5. 20.5 - DHCP Server Implementations

Server	Best fit	Important traits
ISC dhcpd	Legacy enterprise networks	Classic config file and lease log; mature but aging and single-threaded.
Kea DHCP	Modern enterprise and ISP	Multi-threaded, API controlled, database backends, hooks, built-in HA modes.
dnsmasq	SOHO, labs, containers	Small DNS plus DHCP daemon; simple, not a large enterprise lease platform.
Windows DHCP	Active Directory shops	GUI/admin tooling and AD authorization to reduce rogue server risk.
Cloud DHCP	AWS/GCP/Azure VPCs	Provider-managed link service; tenants configure options indirectly.

6. 20.6 - DHCP HA and Failover

DHCP HA is mainly a lease-state consistency problem. MCLT limits how long one server may extend a lease without partner confirmation, preventing two servers from assigning the same address after a split.

Mode	How clients are served	Failure behavior
Hot standby	Primary answers; standby synchronizes.	Standby takes over after partner failure detection.
Load balancing	Client hash splits requests across peers.	Surviving peer can cover the full pool after failover rules allow it.
Shared database	Servers use a replicated lease backend.	Depends on database consistency and write conflict handling.
Anycast DHCP	Nearest server receives broadcast via relay/unicast design.	Simple reachability, harder global lease synchronization.

7. 20.7 - DHCPv6

DHCPv6 uses link-local multicast and identity associations instead of DHCPv4 broadcast fields. Stateful mode assigns addresses with IA_NA; stateless mode provides only non-address options.

Prefix delegation is the ISP and enterprise router case: the CPE receives a larger prefix and advertises or delegates smaller LAN prefixes downstream.

8. 20.8 - PXE / iPXE Boot

PXE boot begins before the machine has a local OS. DHCP supplies both the normal lease and boot metadata: option 60 identifies a PXE client, option 66 names the TFTP server, and option 67 names the bootfile.

iPXE is often chainloaded by classic PXE, then switches to HTTP or HTTPS for faster, scriptable provisioning.

Minimal C Demo - PXE Options

PXE and iPXE Boot Sequence — C Demo