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last update 19/02/2003 |
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DHCP requests and messages
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We could think a simple return message is enough for the protocol to work fine. Actually, several DHCP messages are used to complete a configuration, renew it... These messages can be sent both by a client or a DHCP server:
| name | description |
|---|---|
| DHCPDISCOVER (1) | to localise available DHCP servers and ask for a first configuration |
| DHCPOFFER (2) | reply of a server to a DHCPDISCOVER. It contains the first configuration elements. |
| DHCPREQUEST (3) | reply of a client to confirm it configuration (or lease) |
| DHCPDECLINE (4) | the client says the address is already in use |
| DHCPACK (5) | reply of a server that contains parameters and an IP address |
| DHCPNAK (6) | message used by a server to tell a client its lease is over or that the configuration it has chosen is wrong |
| DHCPRELEASE (7) | the client release its current configuration |
| DHCPINFORM (8) | the client is looking for local parameters, and already has its configuration set |
The value between brackets is used to identify the message in DHCP packets. See DHCP options.
The first request sent by the client is a DHCPDISCOVER. The server then replies with a DHCPOFFER, by sending a first IP address. The client then sets its configuration, may ask for other parameters and finally sends a DHCPREQUEST to validate its configuration. The server replies by a DHCPACK with the final IP address to confirm and things are over. Usually, this is enough for a client to get an efficient network configuration, but this may be more or less long, depending on the fact that the client or the server may refuse its configuration, ask for additional information...
To ask for a brand new address, things are like this:
To renew an address, things a like this (we assume that both servers know the client):
printable format working |
messages  |
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