Definition
The Dynamic Host Configuration Protocol automatically supplies hosts with IP addressing and other configuration. DHCPv4 and DHCPv6 are separate protocols with different message formats, transports, identifiers, and state models.
Follow a DHCPv4 client from discovery to lease
The familiar DORA exchange is only the beginning; relays, options, timers, and server state determine whether the resulting configuration remains usable.
A new client broadcasts DHCPDISCOVER because it does not yet know its address or DHCP server.
A server proposes an address and configuration in DHCPOFFER, using relay information when the client is on another subnet.
The client identifies its selected offer in DHCPREQUEST, and the server commits the binding with DHCPACK.
The client later renews with the granting server, attempts rebinding if necessary, and stops using the address if the lease expires.
What this does not mean
- DHCP only provides an IP address.
- A DHCP binding can include the subnet mask, gateway, DNS servers, lease duration, and many other options required for normal operation.
- Client broadcasts naturally cross routed boundaries.
- Enterprise networks normally use relay agents to pass DHCP messages between client subnets and centralized servers.
- A successful DHCP lease proves DNS registration succeeded.
- DHCP and DNS updates are separate operations. RFC 2131 does not make DNS registration part of the base DHCP transaction.
A practical review order
- Capture the Discover, Offer, Request, and Acknowledge sequence and match transaction identifiers.
- Verify the relay address and relay information identify the intended client subnet.
- Check pool availability, exclusions, reservations, client identifiers, policy, and offered options.
- Confirm lease timers, the granting server, renewal attempts, and whether the client is selecting an unexpected offer.
- Validate DNS updates separately from address allocation. DHCPv6 uses a different message and state model from DHCPv4.