Ana Maria Chanaba Robert Huylo DHCP Ana Maria Chanaba Robert Huylo 11/17/2018

DHCP Client-Server Interaction The DHCP is composed of DHCP client asks the DHCP server for an allocation of resources. DHCP server allocates the network resource according to the request from the DHCP client. DHCP relay agent relays the request and the reply packets between the DHCP client and DHCP server. 11/17/2018

DHCP Model 11/17/2018

Fundamental Idea To look for dhcp servers, a client sends the DHCPDISCOVER message. The dhcp server that receives the DHCPDISCOVER message transmits a DHCPOFFER message to the client. The client selects a server and transmits a DHCPREQUEST message. The dhcp server that receives this DHCPREQUEST message sends back a DHCPPACK message with allocated resource information to the client. Finally the client can be connected to the network using the receive information. 11/17/2018

DHCP Message Flow 11/17/2018

Reusing a Previously Allocated Network Address If the client remembers and wishes to reuse a previously allocated address, the client may choose to omit some of the steps described in previous section. The DHCPREQUEST message includes the clients network address. Servers with knowledge of the clients configuration respond with a DHCPACK message. 11/17/2018

IP address allocation methods Dynamic allocation - DHCP assigns an IP address to a client for a limited period of time Manual allocation - Admin assigns IP address manually and DHCP is used only to convey the address to a client Automatic allocation - DHCP assigns a permanent IP address to a client 11/17/2018

Dynamic allocation The client's current address as recorded in the client's current binding, ELSE The client's previous address as recorded in the client's (now expired or released) binding, if that address is in the server's pool of available addresses and not already allocated, ELSE The address requested in the 'Requested IP Address' option, if that address is valid and not already allocated, ELSE A new address allocated from the server's pool of available addresses; the address is selected based on the subnet from which the message was received (if 'giaddr' is 0) or on the address of the relay agent that forwarded the message ('giaddr' when not 0). 11/17/2018

Manual allocation Binding between IP address and MAC address of a client Mapped manually EX: host lab13 { hardware ethernet 00:C0:F0:30:D7:11; fixed-address 134.198.161.50; } 11/17/2018

Automatic allocation Bindings between IP addresses and MAC addresses are mapped automatically. Similar to Dynamic allocation. Lease time is infinite. IP address can still be released (DHCPRELEASE) by client leaving network and later recycled 11/17/2018

References man pages – dhcpd.conf, dhcpcd, dhcpd, dhcpd.leases DHCP RFC 2131 -- R. Droms Text – Chp. 13.7 www.tcpipguide.com 11/17/2018