1. DHC P Dynamic Host Configuration Protocol

2. DHCP Overview
Used to automatically deliver IP addresses and Network settings
The purpose of DHCP is to assign network settings centrally from a server rather than configuring them locally on each and every workstation.
DHCP Server service must be installed and configured on machine that will function as DHCP server
Reduces the amount of time you spend configuring computers on your network
Used by client by default unless you specify otherwise
When configuring a newly detected card it auto sets itself as a dhcp client

3. Leasing an IP Address An IP address is leased during the boot process
The overall process is composed of four broadcast packets:
DHCPDISCOVER
DHCPOFFER
DHCPREQUEST
DHCPACK

4. Leasing an IP Address (continued)

5. Leasing an IP Address (continued)
A DHCP transaction begins when a client sends out a DHCP DISCOVER packet which is a broadcast packet. The packet contains only the client’s hardware address.
The DHCP server receives the DHCP DISCOVER packet from the client. If an IP address on the client subnet is available it makes an offer by sending a DHCP OFFER packet to the client. The offer packet contains the following information:
Proposed IP address for the client
Server information: i.e, name / IP
Other configuration information

6. Leasing an IP Address (continued)
The client receives the DHCP OFFER packet. The client may receive more than one offer if more than one DHCP server exists on the network. If the client is interested in the offer, it sends a DHCPREQUEST packet to the server. This indicates a formal request to lease the IP address offered by the server.
The DHCP server receives the DHCPREQUEST packet and leases an IP address to the client. The server sends a DHCPACK packet to the client. This is an official notification that the address has been granted and the client can start using the IP address and options in the lease.

7. Renewing an IP Address The IP address can either be permanent or timed
A permanent address is never reused for another client
Timed leases expire after a certain amount of time
Clients attempt to renew their lease based on the configured lease time
A DHCP server may either honor or reject a renew request

8. Renewing an IP Address (continued)

9. DHCP Server and Client
DHCP Server daemon (dhcpd) does not automatically get installed during a typical installation.
It can be installed during the OS installation or,
It can be installed via Yast or
It can be installed from the command line
DHCP client
2 DHCP Clients are available
dhcp client daemon (dhcpcd)
Internet Systems Consortium (ISC) dhcp-client
OpenSuse uses dhcpcd by default. It does not need a configuration file
ISC dhcp-client can be used for more complex situations and does require a configuration file (/etc/dhclient.conf)

10. DHCP Server Configuration /etc/dhcpd.conf file
The /etc/dhcpd.conf file contains configuration information for the DHCP Server.
It can be modified with Yast or with a text editor
If a text editor is used, check syntax and restart service
rcdhcpd check-syntax
rcdhcpd restart

11. Server Commands chkconfig rcdhcpd
chkconfig dhcpd -- to view the boot status of dhcpd
chkconfig dhcpd on – to configure dhcpcd to start at boot
chkconfig dhcpd off – to configure dhcpcd to not start at boot
rcdhcpd
rcdhcpd check-syntax – to check the syntax of /etc/dhcpd.conf
rcdhcpd status – displays the status of the dhcp server service
rcdhcpd start –starts the dhcp server
rcdhcpd stop – stops the dhcp server

12. Client Commands
Note: in each command, replace the pound sign (#) with the ethernet card number
ifconfig ethname -- to view the current IP address and IP settings of the ethname card
dhcpcd ethname -k -- forces dhcpcd to release the dhcp lease
dhcpcd ethname -n -- forces dhcpcd to try to renew the dhcp lease
ifdown ethname - to bring down the specified Ethernet card
ifup ethname - to bring up the specified Ethernet card
(Do not use ifup and ifdown to ‘renew’ dhcp settings.)

13. Files Server /etc/dhcpd.conf – server configuration file
/var/lib/dhcp/db/dhcpd.leases – stores client leases assigned by dhcpd
/etc/sysconfig/network/ifcfg- ethname - stores network card settings for ethname
/var/log/rc.dhcpd.log - log file for dhcpd
Client
/var/lib/dhcpcd/dhcpcd- ethname.info – contains dhcp settings assigned to client from server
/etc/resolv.conf – shows dns settings of machine. At client, this will show what settings have been received by dhcp server

14. Creating DHCP Fixed Addresses
Fixed Addresses (aka reservations) are used to hand out a specific IP address to a particular client
Useful when delivering IP addresses to devices that would normally use static addresses
Configure in Yast’s DHCP Server configuration / Host Management or by manually editing the dhcpd.conf file
Fixed Addresses are created based on MAC addresses
Commands that will display the MAC address of a card include:
ifconfig ethname
ifstatus ethname
ip link show ethname
Sample entry in dhcpd.conf file for fixed/static IP Address
host apex {
fixed-address ;
hardware ethernet 00:A0:78:8E:9E:AA; }

15. Creating DHCP “Exclusions”
No Exclusion entries exist for the dhcpd.conf file
To Exclude an IP address or range of IP addresses, specify the ranges around it.
For example on the network, to exclude through from being distributed by the DHCP server, use the ranges ( to ) and ( to )
Configure multiple ranges in Expert Settings within Yast’s DHCP Server configuration. The dhcpd.conf entry would look something like this:
subnet netmask {
range ;
range ; }

16. Configuring a DHCP Relay
DHCP packets cannot travel across a router
A relay agent is necessary in order to have a single DHCP server handle all leases on both network segments
The Linux DHCP Relay Agent (dhcrelay) allows you to relay DHCP and requests from a subnet with no DHCP server on it to one or more DHCP servers on other subnets.

17. Configuring a DHCP Relay
Relay agents receive broadcast DHCP packets and forward them to a DHCP server.
When a DHCP client requests information, the DHCP Relay Agent forwards the request to the list of DHCP servers specified when the DHCP Relay Agent is started.
dhcp-relay must be installed (Yast or zypper install)
To start the DHCP Relay Agent use the command dhcrelay server-ip.

18. Single Subnet One physical network segment
One logical subnet ( )
One DHCP Server
Single range is used to service all DHCP clients on Subnet A
dhcpd.conf entry:
subnet netmask {
range ; }

19. Shared-Network multiple subnets on same physical network (aka superscope)
dhcpd.conf entry:
shared-network shared {
subnet netmask {
range ; }
subnet netmask {
range ;
subnet netmask {
range ;
One physical network segment
Multiple logical subnets
Three separate subnets must be shared
One DHCP Server services all clients on Subnet A with an IP address from the shared network settings
Router configured with multiple addresses to allow packets to move from one logical network to another