DHCP Messages
1) DHCPDISCOVER
A message that is transmitted by a DHCP client to find servers that are willing to offer an address and configuration parameters.
2) DHCPOFFER
A message that is transmitted by a DHCP server in response to a DHCPDISCOVER to offer a client an address and configuration parameters.
3) DHCPREQUEST
A message that is used by a DHCP client to request initial configuration parameters, to confirm the validity of an address already assigned to the client, and to obtain an extension on the lease for an address assigned to the client.
4) DHCPACK
A message that is transmitted by a DHCP server to a client to confirm the use of the parameters requested by the client in a DHCPREQUEST message.
The DHCP call flow is commonly known as the DORA process.
DORA stands for DHCPDISCOVER, DHCPOFFER, DHCPREQUEST, DHCPACK
Other DHCP Messages include:
• DHCPINFORM
̶ A message that is used by a DHCP client that doesn’t need an IP address to obtain other
configuration parameters
– NTP servers, DNS servers, proxy servers, etc.
• DHCPRELEASE
̶ A message that is transmitted by a DHCP client to a server to explicitly terminate the lease on the
client’s IP address prior to the expiration of the lease
• DHCPNAK
̶ A message that is transmitted by a DHCP server to a client to inform the client that it cannot use the parameters requested by the client in a DHCPREQUEST message
̶ Usually means “no you can’t have the IP address you asked for”
̶ Client has to stop what it’s doing and go back to DHCPDISCOVER
• DHCPDECLINE
̶ A message that is transmitted by a DHCP client to a server to decline an offered address because
the client has determined that it is already in use by another client
• DHCPLEASEQUERY
̶ Sent to the DHCP server by routers/relay agents or other devices to find out information about an IP address
̶ DHCP server looks up that IP address in the lease database and returns:
– DHCPLEASEACTIVE – Lease is present in database and active
– DHCPLEASEUNASSIGNED – Lease is present in database, but no longer in use
– DHCPLEASEUNKNOWN – No lease in database for that address
• DHCPFORCERENEW
̶ NEVER IMPLEMENTED!
̶ There is NO way for a DHCP server to force a client to renew it’s lease