19-DHCP - BootP and DHCP Flexible and Scalable Host...

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1 2005/03/11 (C) Herbert Haas BootP and DHCP Flexible and Scalable Host Configuration
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2 Who's General Failure and why's he reading my disk? Anonymous
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3 3 (C) Herbert Haas 2005/03/11 Shortcomings of RARP ° Reverse Address Resolution Protocol ° Only IP Address distribution ° No subnet mask ° Using hardware address for identification ° New methods needed: BOOTP, DHCP RARP was one of the first protocols which offers automatically an IP Address to a new connected client. But RARP is an old protocol with many disadvantages. It can only distribute an IP Address without a subnet mask. RARP uses the hardware address for identification. This make it impossible to connect new clients to the network without some administrative work.
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4 Bootstrap Protocol (BOOTP) A static solution with many parameters
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5 5 (C) Herbert Haas 2005/03/11 Goal ° Clients request IP address and other parameters from server ± Subnet mask, configuration filename, ... ° IP addresses are predefined in a list ± Fixed mapping MAC address ² IP address ° Defined in RFC 951 and RFC 1048 The Bootstrap Protocol can offer many important parameters to the client. The most important parameters are the subnet mask and the configuration filename. With the configuration filename it is possible to connect a no-disk client. Also BOOTP uses a fixed mapping via hardware address (Ethernet Mac Address).
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6 6 (C) Herbert Haas 2005/03/11 Bootstrap Here is MAC A, I need an IP address, and something to boot! Request-ID = 77 Client IP = 0.0.0.0 MAC = A Your IP = ? Server IP = ? Image File = ? Eth2 DA = FFFF.FFFF.FFFF IP DA = 255.255.255.255 SA = 0.0.0.0 DPort = 67 SPort = 68 UDP B O O T P BOOTP Client BOOTP Server TFTP Server In the picture above you see the classic bootstrap principle. The are 2 important servers. The TFTP server with the configuration file and the BOOTP server. After a new client connect to a network he needs an IP address and something to boot. Via an IP broadcast (BOOTP works with UPD, Ports 67 and 68) he sends out a request.
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7 7 (C) Herbert Haas 2005/03/11 Bootstrap Request-ID = 77 Client IP = 0.0.0.0 MAC = A Your IP = 192.60.30.10 Server IP = 192.60.30.20 Image File = /tftpboot/dl.img Eth2 DA = FFFF.FFFF.FFFF IP DA = 255.255.255.255 SA = 192.60.30.100 DPort = 68 SPort = 67 UDP B O O T P TFTP Server 192.60.30.20 BOOTP Server 192.60.30.100 Thank You ! 192.60.30.10 BOOTP Client After the BOOTP server receipt the request from the BOOTP client, he uses his fixed mapping method (MAC A address = .. IP address) to offer the client an IP address. The BOOTP server also sends the client information about the TFTP server and the name of the configuration file.
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8 8 (C) Herbert Haas 2005/03/11 Principles ° Separation of the boot task into a BOOTP-part and a TFTP-part ° BOOTP server only needs to maintain a small database ! ° Image- and configuration-files can be stored on another machine ° BOOTP client is responsible for error detection After an error detection (timeout) there will be a retransmission. The timeout is selected randomly from a special interval, which is increased as error last on -> avoiding network overload. For the error detection the UDP and a checksum is used. Also the IP datagram has the Do Not Fragment Bit set to 1.
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