BOOTP Bootstrap Protocol RFC 951 and DHCP Dynamic

BOOTP Bootstrap Protocol (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. 3 192. 168. 1. 4 ee: ee: ee: 01 ee: ee: ee: 03 ee: ee: ee: 02 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 PDU Format Operation Code Hardware Type Hardware Length Transaction ID Number of seconds Unused 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) 4 bytes Hop Count

BOOTP Problem ee: ee: ee: 01 ee: ee: ee: 03 ee: ee: ee: 02 DNS Server 192. 168. 1. 3 ee: ee: ee: 04 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 Internet I don’t know, I’ve never seen you here before.

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 • DHCP messages carried in options portion of the PDU

Typical Options Tag(0) Tag Padding Length(N) Value N bytes Tag (255) End of options Tag ID Function 1 Subnet Mask 13 Boot File size 37 TCP Default TTL 72 WWW Server 69 SMTP Server 61 Client Identifier 54 Server Identifier 66 TFTP Server 3 Time server 53 DHCP Message 4 DNS name server 55 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 Lease Expired DHCPNACK DHCPOFFER DHCPDISCOVER Selecting DHCPREQUEST Requesting Lease 50% Expired DHCPREQUEST Renewing DHCPACK Lease Cancelled DHCPRELEASE BOUND DHCPACK Lease 87. 5% Expired DHCPREQUEST Lease Expired DHCPNACK Rebinding

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

Address Renewing Scenario DHCP Server DHCP Client makes request Server acknowledges request; lease begins DHCPREQUEST DHCPACK 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 DHCPDISCOVER DHCPREQUEST DHCPNACK DHCPDISCOVER Client begins discovery phase to find a new address to lease

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

DHCP Problem DNS Server 192. 168. 1. 3 ee: ee: ee: 01 ee: ee: ee: 02 ee: ee: ee: 04 Router 192. 168. 1. 1 File Server 192. 168. 1. 4 DHCP Server 192. 168. 1. 2 • What is the problem here? • Routers do not forward IP broadcast PDUs

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

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