DHCP Option to Disable Stateless Auto-Configuration in IPv4 Clients
updated by
- rfc-8925 — IPv6-Only Preferred Option for DHCPv4
Extracted elements (22)
The option is scoped to the DHCPDISCOVER exchange only, because once a client has obtained a lease it already has an address and auto-configuration is irrelevant; subsequent message types (RENEW, INFORM, etc.) need no change.
The option was motivated by Windows 98 and NT5 assigning link-local addresses (in a specific subnet) when no DHCP server responds, breaking sites that rely on unrecognized hosts remaining address-less. The existing DHCP spec required servers to silently ignore unknown clients, giving those clients no way to distinguish 'no DHCP infrastructure' from 'DHCP infrastructure that chose not to assign an address.'
The server MAY include the Message option (RFC 2132 §9.9) in its DoNotAutoConfigure response to explain to the administrator why auto-configuration was disallowed, providing operational visibility without mandating a specific delivery mechanism.
DHCPOFFERs with 'yiaddr' of 0x00000000 are only sent by servers that support Auto-Configure and only in response to DISCOVERs that contain the option, so legacy clients never receive them and are unaffected.
DHCPRELEASE, DHCPDECLINE, and DHCPINFORM message flows are unaffected by the Auto-Configure option and are handled exactly as specified in RFC 2131.
When a client supports the Auto-Configure option but the server does not, the server either returns a valid DHCPOFFER or is silent; the client treats either case as permission to auto-configure, preserving backward compatibility.
When a server supports the Auto-Configure option but the client does not, the DHCPDISCOVER will not contain the option and the server ignores auto-configuration logic entirely, leaving the client to auto-configure if it receives no response.
Any IP stack that implements an auto-configuration mechanism MUST also implement the Auto-Configure DHCP option (option 116).
DHCP clients that have auto-configuration capabilities MUST add the Auto-Configure option (set to AutoConfigure) to the list of options in their initial DHCPDISCOVER message.
If a DHCP client receives any DHCPOFFER with 'yiaddr' of 0x00000000 and the Auto-Configure flag says DoNotAutoConfigure, and no valid-address DHCPOFFER arrives, the client MUST NOT generate a link-local IP address.
If the DHCP client receives a DHCPOFFER response that includes the Message option, the client MUST provide the message content to the administrator of the DHCP client.
If the DHCPDISCOVER contains the Auto-Configure option and the site administrator has disabled auto-configuration for that subnet or client, the server MUST send a DHCPOFFER with 'yiaddr' of 0x00000000 and Auto-Configure set to DoNotAutoConfigure.
If the site administrator allows auto-configuration on the originating subnet and the server has no address to offer, the DHCPDISCOVER is not answered (same as legacy behavior).
The Auto-Configure option only affects initial IP address selection and does not apply to subsequent DHCP messages such as RENEW, INFORM, or ACK.
When a DHCP server receives a DHCPDISCOVER and has no address to offer, if the DHCPDISCOVER does not contain the Auto-Configure option, the server MUST NOT answer it.
When a DHCPOFFER is received with 'yiaddr' of 0x00000000 and the Auto-Configure option is set to DoNotAutoConfigure, the DHCPOFFER MUST be ignored and the client MUST NOT generate a link-local IP address.
A DHCPOFFER with 'yiaddr' field set to 0x00000000 is the mechanism by which a supporting DHCP server signals that it has no address to offer while still communicating Auto-Configure policy to the client.
The Auto-Configure option defines two values: DoNotAutoConfigure (0) and AutoConfigure (1). These signal to the DHCP client whether it may generate a link-local IP address.
IANA assigned DHCP option number 116 to the Auto-Configure option defined by this document.
DHCP currently provides no authentication or security mechanisms; all exposures described in RFC 2131 §7 apply. This option adds an additional attack vector: malicious users on a subnet can flood DHCP clients with DoNotAutoConfigure responses, preventing all clients from obtaining link-local addresses on networks that require auto-configuration.
On lease expiry, the DHCP client returns to the INIT state and sends a fresh DHCPDISCOVER including the Auto-Configure option, re-triggering the full auto-configuration decision.
The Auto-Configure option has code 116, length 1, and carries an 8-bit value field ('a'). Its encoding is: Code=116, Len=1, Value=a.