DHCP

1
DHCP

• Mike OConnor
• Eric Tallman
• Matt Yasiejko

2
Overview

• DHCP defined
• How it works
• Installation
• dhcpd.conf
• rc.conf

3
DHCP defined

• DHCP Dynamic Host Configuration Protocol
• Uses UDP (User Datagram Protocol)
• Broadcasts without error correction
• Allows systems to connect to a network and be
  assigned necessary network information
• IP address, subnet, lease period

4
DHCP defined

• Simplifies network administration
• Automatically assigns and keeps track of IP
  Addresses on the network
• Eliminates the necessity of assigning static IP
  Addresses
• Static IP can be assigned using DHCP in
  configuration
• Developed from BOOTP
• Introduced the idea of a lease and IP binding
• Purposes
• Provide persistent storage of network parameters
  for network clients
• Allocation of temporary or permanent network (IP)
  addresses to clients

5
DHCP How it works
http//www.tarunz.org/vassilii/TAU/protocols/dhcp
/timeline.htm
6
DHCPDISCOVER

• Client finds out which machines out there are
  providing a DHCP server

7
DHCPOFFER

• Server responds to DHCPDISCOVER with possible
  configuration parameters for the network
• The client collects various DHCPOFFER responses

8
DHCPREQUEST

• Chooses possible action
• Gets parameters from one server and declines all
  others
• Missing use defaults get all get only requested
• Verifies correctness of parameters of previous
  allocation
• Negotiates extended lease of a network address

9
DHCPACK / DHCPNAK

• Server responds with an ACK or NAK
• ACK initialization of client complete
• NAK client has invalid network information

10
Use a previous allocation

• Old configuration located
• Server sends DHCPACK to client
• Other DHCPACKs ignored
• IP-subnet-number, hardware-address ? key to find
  old configuration parameters

11
DHCP Message Format

• http//www.tarunz.org/vassilii/TAU/protocols/dhcp
  /frame.htm
• Format used in the handshake between client and
  server to pass network information
• DHCPINFORM

12
DHCP IP Allocation

• Automatic allocation DHCP assigns a permanent
  address, no human interference
• Dynamic allocation DHCP assigns an IP for a
  lease period
• Manual allocation Network administrator assigns
  an IP

13
DHCP Lease Database

• /var/lib/dhcp/dhcpd.leases stores the DHCP client
  lease database
• Lease length
• To whom the IP is assigned
• MAC address

14
DHCP Lease Database

• lease 134.198.161.99
• starts 2 2005/03/01 184539
• ends 2 2005/03/01 185539
• tstp 2 2005/03/01 185539
• binding state free
• hardware ethernet 00c0f030d711
• uid "\001\000\300\3600\327\021"
• lease 134.198.161.98
• starts 4 2005/03/03 030159
• ends 4 2005/03/03 031159
• binding state active
• next binding state free
• hardware ethernet 00e04c96f395
• client-hostname "lab112.research.cs.uofs.edu"
• courtesy of Robert Huylo and Ana Chanaba
  http//www.cs.uofs.edu/7Eplishka/cmps354/DHCP1.pp
  t

15
Defining the subnet
10000110.11000110.10100001.01101100
(134.198.161.106) BITWISE
AND 11111111.11111111.11111000.00000000
(255.255.248.0) ----------------------------------
---------------------------------------- 10000110.
11000110.10100000.00000000 (134.198.160.0)
Subnet
16
Installing DHCP Server

• Ensure that the bpf device is compiled in the
  kernel
• Berkeley Packet Filter
• Provides a raw interface to data link layers in a
  protocol independent fashion
• Install the net/isc-dhcp3-server port
• Make install clean

17
Installing DHCP Server

• cp /usr/local/etc/dhcpd.conf.sample
  /usr/local/etc/dhcpd.conf
• Modify dhcpd.conf

18
dhcpd.conf

• dhcpd.conf file
• Domain Name System (DNS)
• ddns-update-style none
• host mailhost
• hardware ethernet 020304050607
• fixed-address mailhost.example.com
• Assign IP Addresses based on MAC Addresses

19
Installing DHCP Server

• Start the server
• /usr/local/etc/rc.d/isc-dhcpd.sh start
• Modify rc.conf to make the daemon run on startup

20
rc.conf

• hostnamelab6.research.cs.uofs.edu
• ifconfig_dc0inet 134.198.161.106 netmask
  255.255.248.0
• defaultrouter134.198.161.254
• hostnamelab6.research.cs.uofs.edu
• dhcpd_enableYES
• dhcpd_conf/usr/local/etc/dhcpd.conf
• sshd_enableYES

21
Resources

• http//www.freebsd.org/doc/en_US.ISO8859-1/books/h
  andbook/network-dhcp.html
• http//www.tarunz.org/vassilii/TAU/protocols/dhcp
  /toc.htm
• RFC 2131