DHCP

1
DHCP

• Ana Maria Chanaba
• Robert Huylo

2
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.

3
DHCP Model
4
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.

5
DHCP Message Flow
6
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.

7
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

8
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).

9
Manual allocation

• Binding between IP address and MAC address of a
  client
• Mapped manually
• EX
• host lab13
• hardware ethernet 00C0F030D711
• fixed-address 134.198.161.50

10
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
References

• man pages dhcpd.conf, dhcpcd,
• dhcpd, dhcpd.leases
• DHCP RFC 2131 -- R. Droms
• Text Chp. 13.7
• www.tcpipguide.com