DHCP

Reference notes.

DHCP (Dynamic Host Configuration Protocol) automatically assigns IP addresses and network configuration to devices on a network, eliminating the need for manual configuration.

DORA Process

DHCP uses a four-step process (all over UDP — client port 68, server port 67):

Client                          Server
  |--- DHCPDISCOVER (broadcast) --->|    1. "Any DHCP servers out there?"
  |<-- DHCPOFFER -------------------|    2. "Here's an IP you can use"
  |--- DHCPREQUEST (broadcast) ---->|    3. "I'll take that one, thanks"
  |<-- DHCPACK ---------------------|    4. "It's yours"

Discover — Client broadcasts (255.255.255.255) looking for DHCP servers. Client has no IP yet, uses 0.0.0.0 as source.
Offer — Server(s) respond with an available IP, subnet mask, gateway, and lease duration.
Request — Client broadcasts its chosen offer (broadcast so other servers know to withdraw their offers).
Acknowledge — Server confirms the lease. Client configures its interface.

The Discover and Request are broadcast so they work even when the client has no IP address.

Lease Lifecycle

Every DHCP assignment is a lease with a finite duration.

|--- Lease granted ---|--- T1 (50%) ---|--- T2 (87.5%) ---|--- Expiry ---|
                      Renew attempt     Rebind attempt      IP released
                      (unicast to       (broadcast to
                       original server)  any server)

T1 (renewal timer) — At 50% of lease time, client tries to renew with the original server via unicast
T2 (rebind timer) — At 87.5%, client broadcasts a renewal request to any server
Expiry — If no renewal succeeds, client releases the IP and starts DORA again

Typical lease durations: 1 hour (guest WiFi), 8-24 hours (office), 1-7 days (home).

DHCP Options

DHCP delivers more than just an IP address. Common options:

Option	Code	Description
Subnet mask	1	Network mask for the assigned IP
Router/gateway	3	Default gateway IP
DNS servers	6	Recursive DNS resolver addresses
Domain name	15	DNS search domain
Lease time	51	Duration in seconds
NTP servers	42	Time synchronisation servers
TFTP server	66	PXE boot server (network booting)

DHCP Relay

DHCP uses broadcast, which doesn’t cross router boundaries. In networks with centralised DHCP servers, a DHCP relay agent (often the router itself) forwards DHCP broadcasts from client subnets to the server as unicast.

Client (VLAN 10) --broadcast--> Router (relay) --unicast--> DHCP Server (VLAN 20)

The relay agent adds a giaddr (gateway IP address) field so the server knows which subnet the request came from and can assign an IP from the correct pool.

Static vs Dynamic Allocation

Type	Description	Use Case
Dynamic	Server assigns from a pool	Most devices
Reserved/Static	Server assigns a fixed IP based on MAC address	Servers, printers, IoT
Manual	No DHCP — configured directly on the device	Infrastructure (routers, switches)

DHCP reservations give the convenience of DHCP (centralised config) with the predictability of static IPs.

DHCP Snooping

A switch-level security feature that:

Builds a binding table mapping MAC → IP → port → VLAN for DHCP-assigned addresses
Trusted ports (uplinks to DHCP server) can send DHCP server messages (OFFER, ACK)
Untrusted ports (end devices) can only send client messages (DISCOVER, REQUEST)
Prevents rogue DHCP servers from assigning incorrect addresses
The binding table feeds Dynamic ARP Inspection

IPv6: DHCPv6 vs SLAAC

IPv6 offers two approaches to address configuration:

SLAAC (Stateless Address Autoconfiguration)

Devices generate their own IPv6 address from:

Network prefix (advertised by the router via Router Advertisement messages)
Interface identifier (derived from MAC address or randomly generated for privacy)

No DHCP server needed. The router only advertises the prefix, not individual addresses.

DHCPv6

Similar to DHCPv4 but with differences:

Stateful DHCPv6 — Server assigns addresses (like DHCPv4)
Stateless DHCPv6 — Device uses SLAAC for address, but gets DNS/NTP/domain from DHCPv6

In practice, many networks use SLAAC for addressing + stateless DHCPv6 for DNS configuration.

Self-Hosted DHCP

Software	Notes
ISC Kea	Modern, REST API, replaces ISC DHCP. Supports MySQL/PostgreSQL backends.
dnsmasq	Lightweight combined DNS + DHCP. Great for small networks and containers.
ISC DHCP	Legacy, end-of-life (2022). Migrate to Kea.

Most home/office routers run a built-in DHCP server.

Common Issues

Issue	Cause	Fix
169.254.x.x address (APIPA)	DHCP server unreachable	Check server, relay, network path
IP conflict	Two devices with same IP	Check for rogue static IPs, review DHCP pool
Rogue DHCP server	Unauthorised server on network	Enable DHCP snooping
Lease exhaustion	Pool too small for network	Expand pool, reduce lease time
Wrong gateway/DNS	DHCP misconfiguration	Audit DHCP options

Rai Notes

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