DHCP Message Types Reference

Quick reference for DHCPv4 and DHCPv6 message types — what each message means, when it appears, and how the two protocols line up.

DHCPv4 Messages

Message Table

Type	Code	Direction	Description
DISCOVER	1	Client → Server	Client broadcasts to find available DHCP servers
OFFER	2	Server → Client	Server offers IP address configuration
REQUEST	3	Client → Server	Client requests offered configuration
DECLINE	4	Client → Server	Client indicates the offered address is already in use
ACK	5	Server → Client	Server confirms configuration
NAK	6	Server → Client	Server denies the configuration request
RELEASE	7	Client → Server	Client releases the assigned address
INFORM	8	Client → Server	Client requests configuration without an IP

Normal Address Assignment (DORA)

ASCII fallback

Client                          Server
   |                               |
   |-------- DISCOVER ------------>|
   |<-------- OFFER ---------------|
   |-------- REQUEST ------------->|
   |<-------- ACK -----------------|

Lease Renewal

ASCII fallback

Client                          Server
   |-------- REQUEST ------------->|
   |<-------- ACK -----------------|

Renewal Failure

ASCII fallback

Client                          Server
   |-------- REQUEST ------------->|
   |<-------- NAK -----------------|
   |-------- DISCOVER ------------>|
   ... (new DORA cycle)

Per-Message Notes

DISCOVER — source IP 0.0.0.0, destination 255.255.255.255. Usually carries the Parameter Request List (Option 55), Client Identifier (Option 61), and Hostname (Option 12).

OFFER — contains the offered IP address, lease time, server identifier, and the options the client requested.

REQUEST — appears in four flavours: selecting (accepting an offer), renewing (unicast to the assigned server), rebinding (broadcast when renewal fails), and init-reboot (after a host reboot with a remembered lease).

ACK — confirms the configuration with the full set of parameters.

NAK — rejects the request. Common causes: invalid requested address, client on the wrong subnet, administrative policy.

RELEASE — sent when a client shuts down or moves networks.

INFORM — used by hosts with statically assigned addresses that still need configuration parameters (DNS, NTP, etc.).

DHCPv6 Messages

Message Table

Type	Code	Direction	Description
SOLICIT	1	Client → Server	Client seeks DHCP servers
ADVERTISE	2	Server → Client	Server responds to a solicit
REQUEST	3	Client → Server	Client requests configuration
CONFIRM	4	Client → Server	Client verifies its address is still valid
RENEW	5	Client → Server	Client renews addresses
REBIND	6	Client → Server	Client rebinds addresses when its server is unreachable
REPLY	7	Server → Client	Server provides configuration
RELEASE	8	Client → Server	Client releases addresses
DECLINE	9	Client → Server	Client declines an offered address
RECONFIGURE	10	Server → Client	Server triggers reconfiguration
INFORMATION-REQUEST	11	Client → Server	Client requests configuration only
RELAY-FORW	12	Relay → Server	Relay forwards a client message
RELAY-REPL	13	Server → Relay	Server replies via the relay

Normal Address Assignment (SARR)

ASCII fallback

Client                          Server
   |-------- SOLICIT ------------->|
   |<-------- ADVERTISE -----------|
   |-------- REQUEST ------------->|
   |<-------- REPLY ---------------|

Rapid Commit (2-Message Exchange)

ASCII fallback

Client                          Server
   |-------- SOLICIT ------------->|
   |       (rapid commit)          |
   |<-------- REPLY ---------------|

Lease Renewal

ASCII fallback

Client                          Server
   |-------- RENEW --------------->|
   |<-------- REPLY ---------------|

Per-Message Notes

SOLICIT — sent to ff02::1:2 (All_DHCP_Relay_Agents_and_Servers). Carries Client DUID (Option 1), IA_NA / IA_TA / IA_PD identity associations, and the Option Request list.

ADVERTISE — the server’s reply with its DUID, available addresses or prefixes, and configuration options.

REQUEST — the client picks a server and includes the chosen server’s DUID.

REPLY — provides the requested configuration.

RENEW vs REBIND:

Message	When	Destination
RENEW	Before the T1 timer expires	Unicast to the assigned server
REBIND	After T1, before T2 — when RENEW gets no answer	Multicast to all servers

CONFIRM — sent when a client reconnects to a network and wants to know whether its remembered address is still appropriate.

Relay Messages

DHCPv6 relays don’t translate client messages — they wrap them:

+------------------+
| RELAY-FORW       |
|   - hop-count    |
|   - link-address |
|   - peer-address |
|   +------------+ |
|   | Client Msg | |
|   +------------+ |
+------------------+

The server unwraps the inner message, processes it, and sends a RELAY-REPL back the same way.

DHCPv4 vs DHCPv6 — Side by Side

DHCPv4	DHCPv6	Purpose
DISCOVER	SOLICIT	Find servers
OFFER	ADVERTISE	Server’s first response
REQUEST	REQUEST	Request configuration
ACK	REPLY	Confirm configuration
NAK	(Status Code option in REPLY)	Deny request
RELEASE	RELEASE	Release addresses
DECLINE	DECLINE	Decline an offered address
INFORM	INFORMATION-REQUEST	Configuration only, no address

Filtering by Message Type

In automation rules, DHCP Stream filters, and processor rules, message types are referenced by their textual name.

DHCPv4:

msg_type equals DISCOVER
msg_type in DISCOVER,REQUEST,RENEW

DHCPv6:

msg_type equals SOLICIT
msg_type in SOLICIT,REQUEST,RENEW,REBIND

Related Topics

• DHCP Stream — Real-time monitoring with message-type filters.
• Automation — Filtering by message type in automation rules.
• Glossary — Term definitions.