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COMMON UPPER-LAYER PROTOCOLS - Chapter 9 from Practical Packet Analysis, 3rd Edition, Chris Sanders and Assignment Questions Notes

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DHCP (Dynamic Host Configuration Protocol)

BOOTP: Bootstrap Protocol (BOOTP) was created to automatically assign addresses to network-connected devices. BOOTP was later replaced with the more sophisticated Dynamic Host Configuration Protocol (DHCP).

DHCP Packet Structure:

Notes on DHCP Packet Structure Video

* OpCode: indicates whether the packet is a request or a reply
* Hardware Type: type of hardware address
* Hardware length: length of hardware address
* Hops: Used by relay agents to assist in finding a DHCP server
* Transaction ID: a random number to pair between DHCP requests and responses
* Seconds Elapsed: seconds since the client first requested an address from the DHCP server
* Flags: the types of traffic the DHCP client can accept (unicast, broadcast, etc.) 
* Client IP address: the client's IP address 
* Your IP address: The IP address offered by the DHCP server
* Server IP address: DHCP address
* Gateway IP address
* Client Hardware address (MAC) 
* Server host name (optional) 
* Boot file (optional) 
* Options 

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The DHCP Initialization Process

"The primary goal of DHCP is to assign addresses to clients during the initialization process." This is done through the DORA process

• DHCP relies on UDP as its transport layer protocol.
  • DHCP is very concerned with the speed at which a client receives the information it’s requesting.
  • DHCP has its own built-in reliability measures, which means UDP is a perfect fit.
• UDP 68 to 67 during Discover Packet

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DHCP In-Lease Renewal

Out-of-lease: when a device has not gone through the DORA Initialization process yet or the IP address that was assigned to it has an expired lease.

• The In-Lease Renewal occurs when a device is still in the lease window but reboots -- in this case, the discover and offer packets are not needed. All other cases of renewal require a complete DORA process again.

By default, DHCP renewal happens at 50% of the lease time

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DHCP Options and Message Types

Full List of DHCP Options: http://www.iana.org/assignments/bootp-dhcp-parameters/

• DHCP is very flexible since it has many available options
• The only option required in all DHCP packets is the Message type option (option 53)
  • Message Type Options GitHub Notes

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DHCP Version 6 (DHCPv6)

• DHCPv6 was made to accommodate IPv6 addressing
• Has simple packet structure since it is not based on BOOTP
• Options: http://www.iana.org/assignments/dhcpv6-parameters/dhcpv6-parameters.xhtml

• Solicit: An initial packet is sent from a client to a special multicast address (ff02::1:2) to attempt to locate available DHCPv6 servers on the network.
• Advertise: An available server responds directly to the client to indicate that it is available to provide addressing and configuration information.
• Request: The client sends a formal request for configuration information to the server via multicast.
• Reply: The server sends all available requested configuration information directly to the client, and the process is complete.