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DHCP Dynamic Host Configuration Protocol (RFC 2131)

BOOTP Bootstrap Protocol (RFC 951). DHCP Dynamic Host Configuration Protocol (RFC 2131). and. Michael Sadowsky CISC 856-010 University of Delaware October 12, 2004. Why BOOTP?. What’s my IP address? What’s my subnet mask? Who’s my router? Who’s my DNS server?. DNS Server 192.168.1.3.

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DHCP Dynamic Host Configuration Protocol (RFC 2131)

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  1. BOOTPBootstrap Protocol(RFC 951) DHCPDynamic Host Configuration Protocol(RFC 2131) and Michael Sadowsky CISC 856-010 University of Delaware October 12, 2004

  2. Why BOOTP? • What’s my IP address? • What’s my subnet mask? • Who’s my router? • Who’s my DNS server? DNS Server 192.168.1.3 FileServer 192.168.1.4 ee:ee:ee:ee:ee:01 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

  3. 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

  4. BOOTP request ff:ff:ff:ff:ff:ff | ee:ee:ee:ee:ee:01 255.255.255.255 | 0.0.0.0 67 | 68 BOOTP request 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 UDP IP Ethernet BOOTP BOOTP Operation Port 68/UDP Port 67/UDP BOOTP Client (A) BOOTP Server (B)

  5. BOOTP PDU Format 4 bytes

  6. New Roaming Client What is my network configuration? BOOTP Problem DNS Server 192.168.1.3 FileServer 192.168.1.4 ee:ee:ee:ee:ee:03 ee:ee:ee:ee:ee:01 ee:ee:ee:ee:ee:02 ee:ee:ee:ee:ee:04 Router 192.168.1.1 Boot Server 192.168.1.2 I don’t know, I’ve never seen you here before. Internet

  7. BOOTP Limitations • Static configuration • Does not dynamically allocate IP addresses • Manual administrator intervention to add/remove clients

  8. DHCP Motivations • Automatic network configuration for clients • No administrator intervention • Effective allocation of limited addresses • Support for transient/roaming systems

  9. 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

  10. DHCP PDU Format 4 bytes

  11. 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 • DHCP messages carried in options portion of the PDU

  12. Typical Options Tag(0) Padding Tag Length(N) Value N bytes Tag (255) End of options

  13. 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)

  14. Lease Cancelled DHCPRELEASE DHCP Client State Diagram Initializing DHCPDISCOVER DHCPOFFER Lease Expired Selecting DHCPNACK DHCPREQUEST Requesting DHCPACK Lease 50% ExpiredDHCPREQUEST BOUND Lease Expired DHCPNACK DHCPACK DHCPACK Renewing Rebinding Lease 87.5% ExpiredDHCPREQUEST

  15. DHCPDISCOVER DHCPDISCOVER DHCPOFFER DHCPOFFER DHCPREQUEST DHCPREQUEST DHCPACK DHCPRELEASE Allocating New Address DHCP Server DHCP Server DHCP Client Client attempts to discover available DHCP servers Servers reply with address offers Client selects which offer to accept Client notifies servers of choice Server acknowledges client use of address Client gives up use of address

  16. DHCPDISCOVER DHCPDISCOVER DHCPREQUEST DHCPREQUEST DHCPREQUEST DHCPNACK DHCPACK Address Renewing Scenario DHCP Server 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 be renewed Client begins discovery phase to find a new address to lease

  17. DHCPREQUEST DHCPREQUEST DHCPACK DHCPACK Renewing a Previous Address DHCP Server DHCP Server DHCP Client Client requests its previously assigned address. Servers respond granting the request. Ignore DHCPACKs when a connection has been established.

  18. DHCP Problem DNS Server 192.168.1.3 FileServer 192.168.1.4 • What is the problem here? • Routers do not forward IP broadcast PDUs ee:ee:ee:ee:ee:03 ee:ee:ee:ee:ee:01 ee:ee:ee:ee:ee:02 ee:ee:ee:ee:ee:04 Router 192.168.1.1 DHCP Server 192.168.1.2

  19. DHCP Infrastructure • Use relay agents to transmit DHCP messages between physical networks • Prohibitive/costly to have DHCP server on each physical LAN segment

  20. DHCP Security Considerations • Hostile environments with open physical access to network • Rouge DHCP server on network • Denial of service by exhausting address pool • Authentication introduced in RFC 3118 but not implemented

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