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DHCP DHCP • Dynamic Host Configuration Protocol (DHCP) • From 1993 • An extension of BOOTP, very similar to DHCP • Same port numbers as BOOTP • DHCP is the preferred mechanism for dynamic assignment of IP addresses • DHCP can interoperate with BOOTP clients. 2 DHCP • Dynamic Host Configuration Protocol • It is a method for assigning Internet Protocol (IP) addresses permanently or to individual computers in an organization’s network • DHCP lets a network administrator supervise and distribute IP addresses from a central point and automatically sends a new IP address when a computer is plugged into a different place in the network MOTIVATION FOR DHCP • Static Vs Dynamic IP • Configuration parameters for network hosts IP address Router Subnet Mask Others.. DYNAMIC HOST CONFIGURATION PROTOCOL (DHCP) • Four Key benefits to DHCP: • • • • Centralized administration of IP configuration. Dynamic host configuration. Seamless IP host configuration. Flexibility and scalability. DISADVANTAGES OF DHCP • When DHCP server is unavailable, client is unable to access enterprises network • Your machine name does not change when you get a new IP address • Uses UDP, an unreliable and insecure protocol. • DNS cannot be used for DHCP configured hosts. SECURITY PROBLEM • DHCP is an unauthenticated protocol When connecting to a network, the user is not required to provide credentials in order to obtain a lease Malicious users with physical access to the DHCPenabled network can instigate a denial-of-service attack on DHCP servers by requesting many leases from the server, thereby depleting the number of leases that are available to other DHCP clients LIMITATIONS • Some machines on your network need to be at fixed addresses, for example servers and routers • You need to be able to assign a machine to run the DHCP server continually as it must be available at all times when clients need IP access DHCP INTERACTION (SIMPLIFIED) Argon 128.143.137.144 00:a0:24:71:e4:44 DHCP Server DHCP Response: IP address: 128.143.137.144 Default gateway: 128.143.137.1 Netmask: 255.255.0.0 9 DHCP OPERATION DHCP Client 00:a0:24:71:e4:44 DHCP Server DHCPDISCOVER • DCHP DISCOVER Sent to 255.255.255.255 DHCP Server DHCP Client 00:a0:24:71:e4:44 DCHP OFFER DHCPOFFER DHCP Server DHCPOFFER DHCP Server 10 DHCP OPERATION DHCP Client 00:a0:24:71:e4:44 DHCP Server DHCPREQUEST DCHP DISCOVER DHCPACK At this time, the DHCP client can start to use the IP address DHCP Server DHCP Client 00:a0:24:71:e4:44 DHCP Server DHCPREQUEST Renewing a Lease (sent when 50% of lease has expired) If DHCP server sends DHCPNACK, then address is released. DHCPACK DHCP Server 11 DHCP OPERATION DHCP Client 00:a0:24:71:e4:44 DHCP Server DHCPRELEASE DCHP RELEASE At this time, the DHCP client has released the IP address DHCP Server 12 BOOTP/DHCP MESSAGE FORMAT OpCode (1 = Req, 2 = Reply) Hardware Type (1 = Ethernet) Number of Seconds Hardware Address Hop Count Length Unused (in BOOTP) Flags (in DHCP) Transaction ID Client IP address Your IP address Server IP address Gateway IP address Client hardware address (16 bytes) Server host name (64 bytes) Boot file name (128 bytes) Options (There are >100 different options) 13 DHCP MESSAGE TYPE • Message type is sent as an option. Value Message Type 1 DHCPDISCOVER 2 DHCPOFFER 3 DHCPREQUEST 4 DHCPDECLINE 5 DHCPACK 6 DHCPNAK 7 DHCPRELEASE 8 DHCPINFORM 14