Protocol BOOTP
Bootstrap (RFC 951)
and
DHCP
Dynamic Host Configuration Protocol (RFC 2131)
Michael Sadowsky
CISC 856-010 University of Delaware October 12, 2004
Why BOOTP?
1. 2. 3. 4. What's my IP address? What's my subnet mask? Who's my router? Who's my DNS server?
DNS Server File Server
192.168.1.4 ee:ee:ee:ee:ee:01 192.168.1.3 ee:ee:ee:ee:ee:03 ee:ee:ee:ee:ee:02 ee:ee:ee:ee:ee:04
Diskless Workstations Router
192.168.1.1
Boot Server
192.168.1.2
Internet
BOOTP: Bootstrap Protocol
RFC 951 Designed for diskless workstations Supplies static configuration:
IP address Subnet mask Router IP address Name server IP address Boot image
BOOTP Operation
Port 68/UDP Port 67/UDP
BOOTP Client (A)
BOOTP Server (B)
BOOTP request
BOOTP request
67 | 68 255.255.255.255 | 0.0.0.0 ff:ff:ff:ff:ff:ff | ee:ee:ee:ee:ee:01
BOOTP reply
ee:ee:ee:ee:ee:05 | ee:ee:ee:ee:ee:01 192.168.1.2 | 255.255.255.255 67 | 68
BOOTP reply
BOOTP
UDP
IP
Ethernet
BOOTP PDU Format
Operation Code Hardware Type Hardware Length Hop Count Transaction ID Number of seconds Client IP address Your IP address Server IP address Gateway IP address Client hardware address (16 bytes) Server name (64 bytes) Boot file name (128 bytes) Options (up to 64 bytes) Unused
4 bytes
BOOTP Problem
ee:ee:ee:ee:ee:01 ee:ee:ee:ee:ee:02 ee:ee:ee:ee:ee:03 ee:ee:ee:ee:ee:04
DNS Server
192.168.1.3
File Server
192.168.1.4
Router New Roaming Client
What is my network configuration? 192.168.1.1
Boot Server
192.168.1.2 I don't know, I've never seen you here before.
Internet
BOOTP Limitations
Static configuration Does not dynamically allocate IP addresses Manual administrator intervention to add/remove clients
DHCP Motivations
Automatic network configuration for clients No administrator intervention
Effective allocation of limited addresses Support for transient/roaming systems
DHCP Evolution
DHCP is an extension of Bootstrap Protocol Uses same basic PDU format for backwards compatibility Introduces pool of IP addresses for dynamic assignment Concept of temporary leased addresses
DHCP PDU Format
Operation Code Hardware Type Hardware Length Hop Count Transaction ID Number of seconds Flag (1 bit) | (15 unused bits MBZ) Client IP address Your IP address Server IP address Gateway IP address Client hardware address (16 bytes) Server name (64 bytes) Boot file name (128 bytes) Options (up to 312 bytes)
4 bytes
DHCP PDU Format
Broadcast bit is to inform server if it can respond with unicast IP PDUs or if it must instead broadcast the reply to the entire network. DHCP PDU has 312 bytes for options versus 64 bytes in BOOTP PDU messages DHCP carried in options portion of the PDU
Typical Options
Tag(0) Padding Tag Length(N) Value N bytes
Tag (255)
End of options
Tag ID 1 37 69 54 3 4
Function Subnet Mask TCP Default TTL SMTP Server Server Identifier Time server DNS name server
Tag ID 13 72 61 66 53 55
Function Boot File size WWW Server Client Identifier TFTP Server DHCP Message Parameter Request List
Message Types
Type identified by value field of option with tag 53:
DHCPDISCOVER (1) DHCPOFFER (2) DHCPREQUEST (3) DHCPDECLINE (4) DHCPACK (5) DHCPNACK (6) DHCPRELEASE (7) DHCPINFORM (8)
DHCP Client State Diagram
Initializing
DHCPOFFER Lease Cancelled DHCPRELEASE Lease Expired DHCPNACK DHCPACK Lease 87.5% Expired DHCPREQUEST DHCPDISCOVER Lease Expired DHCPNACK
Selecting
DHCPREQUEST
Requesting
Lease 50% Expired DHCPREQUEST DHCPACK
BOUND
Renewing
DHCPACK
Rebinding
Allocating New Address
DHCP Server DHCP Client
DHCPDISCOVER DHCPOFFER DHCPOFFER
DHCP Server
Client attempts to discover available DHCP servers Servers reply with address offers Client selects which offer to accept
DHCPREQUEST
DHCPDISCOVER
DHCPREQUEST
DHCPACK
Client notifies servers of choice Server acknowledges client use of address
DHCPRELEASE
Client gives up use of address
Address Renewing Scenario
DHCP Server DHCP Client
Client makes request Server acknowledges request; lease begins Time passes; 50% of lease expires Client makes request to renew address No response from server, client times out and sends request again Server responds with negative acknowledgement, address can not ...