DHCP and BOOTP Interoperability
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BOOTP Clients Connecting to a DHCP Server
As indicated by the quote above from RFC 2131, DHCP was specifically intended to allow a DHCP server to handle requests from BOOTP clients. The protocol itself is set up to enable this, but it does require that the DHCP server be given certain intelligence to know how to deal with BOOTP clients. One of the most important issue is, of course, that BOOTP clients will follow the BOOTP configuration process and not the DHCP leasing processes. The DHCP server must use BOOTP messages with the BOOTP meanings for fields when dealing with BOOTP clients. A server determines that a client is using BOOTP instead of DHCP by looking for the presence of the DHCP Message Type option, which must be present in all DHCP messages but of course is not used for BOOTP.
If a DHCP server detects that it is dealing with a BOOTP client, it can respond with configuration information for the client. The server can use either manual or automatic allocation for the client. Automatic allocation of course means the server chooses an address from its pool of unused addresses, but assigns it permanently. BOOTP clients are not capable of dynamic allocation since BOOTP is static in nature.
A DHCP server may include BOOTP vendor information fields in its response to a BOOTP client, including ones defined since BOOTP was created. However, it obviously must not send any DHCP-specific options.
Now, for the other case. A DHCP client can obtain configuration information from a BOOTP server, because the server will respond to the client's initial DHCPDISCOVER message as if it were a BOOTP BOOTREQUEST message. The DHCP client can tell that a BOOTP reply has been received because there will be no DHCP Message Type option. A response from a BOOTP server should be treated as an infinite lease, since again, that's all that BOOTP supports. Note that if a DHCP client receives a response from both a BOOTP server and a DHCP server, it should use the DHCP response and not the BOOTP response (even if this means it gets a shorter lease).
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