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network and internet - Bootstrap Protocol) BootP)

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Bootstrap Protocol) BootP)

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BootP

The problem with using Reverse Address Resolution Protocol (RARP) (RFC 903) for assigning IP addresses to clients is that it operates at the data link layer and is therefore limited to the local LAN. Bootp however uses IP/UDP (port 67 for the server destination port, and port 68 for the client source port) and can cross routers. If a client does not know its own IP address when it boots up then it can utilise a Bootp server to obtain its IP address. It will use a IP broadcast address of 255.255.255.255. This IP broadcast is transmitted in a link-layer broadcast of FFFF.FFFF.FFFF. If the client is on the same LAN as the Bootp server, then the Bootp server will respond to the broadcast, by using the MAC address of the client, and the IP address will be given to the client. If the Bootp server is on another LAN i.e. on the other side of a router then we are left with the situation whereby broadcasts are being sent by the client and the router is designed not to pass broadcasts. Consider the following scenaro


The client requires an IP address and a default gateway address so it sends out an IP broadcast BOOTREQUEST.
No Bootp server exists locally so the router R1, which is configured to forward Bootp/DHCP requests, sends a unicast with the source IP address of it's LAN interface on which it received the initial broadcast. Note that this is different from TCP connections which use the WAN IP address on the router which owns the LAN interface from whence the packet came. The unicast contains the IP address of the Bootp server which R1 knows since this is part of the Bootp forwarding configuration.
R2 forwards the unicast to the Bootp server.
The server examines the packet and checks for the client's hardware address in its database. If the MAC address has been entered then the server can map the client MAC address to the IP address configured in the database.
The server then looks to see if the client is requesting a configuration file (boot file).
The server replies with a unicast BOOTREPLY containing the client's IP address and details of the configuration file.
R1 recognises the unicast reply and forwards the packet to the clients MAC address.
The client's TCP/IP stack then pings the IP address to check that it is not being used before taking it for itself.
The client then can use TFTP etc. to download the configuration file including information such as the default gateway to the LAN interface of R1.

Note that the TCP/IP stack needs to be in place on the client, since the client needs to be able to send an IP broadcast (not a layer 2 broadcast!).

Below is the frame format which is used for both Bootp and DHCP



Operation - indicates a request 1 or a reply 2.
Hardware Type - Ethernet is 1.
Hardware Length - length of the address
Hops - the client starts this off with 0 and then this increment by each Bootp server if the packet is passed on.
Transaction ID - diskless nodes use this number to match responses with the requests.
Seconds - this gives the elapsed time since the client started the boot process
Client IP Address - if a client knows its IP address it puts it in here, otherwise it is a 0.
Your IP Address - the server puts an IP address here if the Client IP address field is 0.
Server IP Address - if the client knows this then the client can place the server address in here, otherwise the server does.
Server Host Name - same as previously.
Gateway IP Address - the client initially sets this to 0. The router receiving the packet will put the IP address of the interface on which the packet was received (i.e. the LAN interface).
Client Hardware Address - so that the servers/routers know where to send responses back to.
Server Host Name - if the client puts a specific server name in here then it restricts itself to booting from that server alone. If no name is in here then the client can boot from any server that responds to a bootp request.
Boot File Name - the client can request a specific boot filename.
Vendor Specific Area - up to 64 octets long for Bootp. This is called the Options field in a DHCP packet and is up to 312 octets long. In DHCP this field contains optional information that the server may wish to give the client e.g. client identifier and server identifier.


نوشته شده در : یکشنبه 7 آذر 1389  توسط : حسین جمالی.    نظرات() .

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