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The Bootstrap Protocol (BOOTP) allows a client system to discover its own
IP address, the address of a bootpserver, and the name of a file to be loaded
into memory and executed. The bootstrap operation happens in two phases. In the first phase, address
determination and bootfile selection occur. This phase uses the BOOTP
server, bootpd. After the address and file name information is obtained,
control passes to the second phase of the bootstrap where a file transfer
occurs. This phase uses the TFTP server, tftpd. Address Determination and Bootfile Selection | |
The first phase involves a bootrequest packet that is broadcast by the
BOOTP client. A BOOTP server that receives the bootrequest can send a bootreply to the client if it finds the client's boot information in its database.
Or, it can relay the bootrequest to other BOOTP servers if it finds relay
information for the client in its database. The BOOTP client formulates a bootrequest that it will broadcast. Before sending
the bootrequest, the client does the following: It sets the hops field of the bootrequest packet to 0. Each time a BOOTP
server relays the client's bootrequest, the hops field is incremented by 1. If
the hops value exceeds the maximum hop value configured for this client on
a BOOTP server, the bootrequest is dropped. The hops value limits the
number of times a bootrequest can be relayed. It sets the secs field of the bootrequest packet to 0 for a first-time request.
If the client does not receive a reply to this request, it sets the value of this
field to the number of seconds since the first request was sent. If the value of
the secs field is less than the threshold value configured for this client on a
BOOTP server, the bootrequest is dropped. The threshold value ensures that
enough time is allowed for a bootreply to be received by the client before a
subsequent bootrequest for the same client is relayed. It sets the giaddr (gateway IP address) field to 0. If a BOOTP server finds
that this field is 0, it fills it with its own IP address.
The client broadcasts the bootrequest packet on its first LAN interface (lan0).
The bootrequest also contains the client's hardware address, and, if known, its IP
address. The BOOTP server checks to see if boot information for the client is in its
database. If boot information for the client is available in the server's database,
the server answers the bootrequest with a bootreply packet. If the BOOTP server does not find boot information for the client in its database,
it checks to see if there is relay information for the client. If there is no relay
information for the client in the database, the bootrequest is dropped. If there is
relay information available and the relay function is enabled for the client, the
server checks the following: Does the hops value in the bootrequest packet exceed the maximum
configured for the client? If it does, the request is dropped. If not, the hops
field in the bootrequest packet is incremented. Is the secs value in the bootrequest packet less than the threshold configured
on the server for the client? If it is, the request is dropped.
If the request has not been dropped during the above checks, the server then
relays the bootrequest to the BOOTP server(s) that have been configured for the
client. If the giaddr field of the bootrequest packet is 0, the server puts its IP
address in the field.
Steps 3 and 4 are repeated until either the bootrequest is received by a
BOOTP server that finds boot information about the client in its database, or
the request is dropped. When a server finds client information about a particular client in its
database, the server answers the bootrequest with a bootreply packet. The
client's IP address is placed into a field in the bootreply. The bootreply may
also contain a file name of a boot file, which the client should load with
TFTP. Other information that can be included in the bootreply are the
client's subnet mask, the addresses of nameservers, and the addresses of
gateways. If the bootrequest has been relayed to one or more BOOTP servers, the
bootreply is sent to the IP address in the giaddr field. This should be the IP
address of the BOOTP server that initially relayed the bootrequest. That
BOOTP server then sends the bootreply to the client. Figure 6-1 “Bootrequest Relay Example” shows an example of a bootrequest that is relayed from server A to server B to server C. Server C finds the client's boot information in its database, and sends the bootreply back to server A. Server A then sends the bootreply to the client. | | | |  | NOTE: BOOTP clients can be booted over a gateway; however, the BOOTP server with the
relay information for the client must be on the same side of the gateway as the client. | | | | |
File Transfer | |
The second phase, file transfer by the BOOTP client using TFTP, is optional.
Some BOOTP clients use BOOTP only for IP address resolution and do not
use TFTP. If the boot file is transferred, it must be publicly available. |
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