Parameters can be seen as variables that get assigned a value and are passed from the server to the client. Some parameters start with the option keyword and some do not. Parameters that do not start with the option keyword are either parameters that control the behavior of the DHCP server or are parameters that are optional in the DHCP protocol.

The difference between “normal” parameters and “global” parameters lies purely in the scope of the parameters. If, for instance, the DNS is always the same, it is pointless to add a domain-name-servers parameter-definition statement to every network-definition statement. By assigning the domain-name-servers parameter a value at the beginning of the configuration file, the parameter becomes a global parameter and its value becomes the default value for that parameter.

The value of a global parameter can be overridden by assigning the parameter another value in subsequent sections.

A shared-network declaration is used if there are multiple subnets on the same physical network. Parameters that are the same for all the subnets belonging to the shared-network can be defined once above the subnet-declarations within the shared-network declaration that encompasses those subnet-declarations.

A subnet-declaration is used to define a network segment. Parameters that only apply to the subnet in question are defined within the subnet-declaration.

A subnet-declaration must contain a range statement that defines the IP-addresses the DHCP-server can give to clients on that subnet.

A group-declaration is used to group other declarations, including group-declarations, that have a number of properties in common so that the common properties only have to be be specified once in stead of for every declaration.

A host declaration is used to set properties for a specific client. The client identifies itself to the DHCP server by one of its unique properties such as its NIC address or its client-identifier.

Assuming that the IP range 21.31.x.x has been assigned to the company and that each department has its own network (determined by the highest four bits of the third octet), the subnets could be set up as follows:

The workstations on the company's network obtain their IP-address and the IP-addresses of the available network services from the company's DHCP-server via the department's DHCP-relay which also functions as a router.

The information needed to be able to build a configuration file has already been gathered in the previous sections when the network topology was devised.

In this section the actual configuration file /etc/dhcpd.conf will be filled with the necessary information.

# DNS
option domain-name-servers 21.31.0.2;
# SMTP
option smtp-server 21.31.0.3;
# POP3
option pop-server 21.31.0.4;
# NEWS
option nntp-server 21.31.0.5;
# NTP
option time-servers 21.31.0.6;
            

# DNS
option domain-name-servers dns.company.com;
# SMTP
option smtp-server smtp.company.com;
# POP3
option pop-server pop3.company.com;
# NEWS
option nntp-server news.company.com;
# NTP
option time-servers ntp.company.com;
            

21.31.16.0 - : | 0001 0101 | 0001 1111 | 0001 0000 | 0000 0000 | SALES
21.31.31.255 : | 0001 0101 | 0001 1111 | 0001 1111 | 1111 1111 | NETWORK

21.31.32.0 - : | 0001 0101 | 0001 1111 | 0010 0000 | 0000 0000 | ADMINISTRATION
21.31.47.255 : | 0001 0101 | 0001 1111 | 0010 1111 | 1111 1111 | NETWORK 

21.31.48.0 - : | 0001 0101 | 0001 1111 | 0011 0000 | 0000 0000 | ENGINEERING
21.31.63.255 : | 0001 0101 | 0001 1111 | 0011 1111 | 1111 1111 | NETWORK

21.31.64.0 - : | 0001 0101 | 0001 1111 | 0100 0000 | 0000 0000 | MANAGEMENT 
21.31.79.255 : | 0001 0101 | 0001 1111 | 0100 1111 | 1111 1111 | NETWORK

fixed-bits   : | 1111 1111 | 1111 1111 | 1111 0000 | 0000 0000 | NETMASK
                    255         255         240          0
            

21.31.57.105 : | 0001 0101 | 0001 1111 | 0011 1001 | 0110 1001 | IP-ADDRESS
255.255.240.0: | 1111 1111 | 1111 1111 | 1111 0000 | 0000 0000 | AND NETMASK
21.31.48.0:    | 0001 0101 | 0001 1111 | 0011 0000 | 0000 0000 | GIVES NETWORK
            

21.31.16.0 - : | 0001 0101 | 0001 1111 | 0001 0000 | 0000 0000 | SALES
0.0.15.255   : | 0000 0000 | 0000 0000 | 0000 1111 | 1111 1111 | OR INV NETMASK
21.31.31.255 : | 0001 0101 | 0001 1111 | 0001 1111 | 1111 1111 | GIVES BCAST

21.31.32.0 - : | 0001 0101 | 0001 1111 | 0010 0000 | 0000 0000 | ADMINISTRATION
0.0.15.255   : | 0000 0000 | 0000 0000 | 0000 1111 | 1111 1111 | OR INV NETMASK
21.31.47.255 : | 0001 0101 | 0001 1111 | 0010 1111 | 1111 1111 | GIVES BCAST

21.31.48.0 - : | 0001 0101 | 0001 1111 | 0011 0000 | 0000 0000 | ENGINEERING
0.0.15.255   : | 0000 0000 | 0000 0000 | 0000 1111 | 1111 1111 | OR INV NETMASK
21.31.63.255 : | 0001 0101 | 0001 1111 | 0011 1111 | 1111 1111 | GIVES BCAST

21.31.64.0 - : | 0001 0101 | 0001 1111 | 0100 0000 | 0000 0000 | MANAGEMENT 
0.0.15.255   : | 0000 0000 | 0000 0000 | 0000 1111 | 1111 1111 | OR INV NETMASK
21.31.79.255 : | 0001 0101 | 0001 1111 | 0100 1111 | 1111 1111 | GIVES BCAST
            

21.31.57.105 : | 0001 0101 | 0001 1111 | 0011 1001 | 0110 1001 | IP-ADDRESS
0.0.15.255   : | 0000 0000 | 0000 0000 | 0000 1111 | 1111 1111 | OR INV NETMASK
21.31.63.255 : | 0001 0101 | 0001 1111 | 0011 1111 | 1111 1111 | GIVES BCAST
            

range 21.31.x.11 21.31.x.210
            

# The Sales network, floors 1-3
shared-network sales-net {
  # Sales-net specific parameters
  option routers 21.31.17.1;
  option lpr-servers 21.31.17.2, 21.31.18.2, 21.31.19.2, 21.31.18.3;
  option broadcast-address 21.31.31.255;
  subnet 21.31.17.0 netmask 255.255.240.0 {
    # Floor #1 specific parameters
    range 21.31.17.11 21.31.17.210;
  }
  subnet 21.31.18.0 netmask 255.255.240.0 {
    # Floor #2 specific parameters
    range 21.31.18.11 21.31.18.210;
  }
  subnet 21.31.19.0 netmask 255.255.240.0 {
    # Floor #3 specific parameters
    range 21.31.19.11 21.31.19.210;
  }
}

# The Administration network, floors 4-6
shared-network administration-net {
  # Administration-net specific parameters
  option routers 21.31.36.1;
  option lpr-servers 21.31.36.2, 21.31.37.2, 21.31.38.2, 21.31.37.3;
  option broadcast-address 21.31.47.255;
  subnet 21.31.36.0 netmask 255.255.240.0 {
    # Floor #4 specific parameters
    range 21.31.36.11 21.31.36.210;
  }
  subnet 21.31.18.0 netmask 255.255.240.0 {
    # Floor #5 specific parameters
    range 21.31.37.11 21.31.37.210;
  }
  subnet 21.31.19.0 netmask 255.255.240.0 {
    # Floor #6 specific parameters
    range 21.31.38.11 21.31.38.210;
  }
}

# The Engineering network, floors 7-9
shared-network engineering-net {
  # Engineering-net specific parameters
  option routers 21.31.55.1;
  option lpr-servers 21.31.55.2, 21.31.56.2, 21.31.57.2, 21.31.56.3;
  option broadcast-address 21.31.63.255;
  subnet 21.31.55.0 netmask 255.255.240.0 {
    # Floor #7 specific parameters
    range 21.31.55.11 21.31.55.210;
  }
  subnet 21.31.18.0 netmask 255.255.240.0 {
    # Floor #8 specific parameters
    range 21.31.56.11 21.31.56.210;
  }
  subnet 21.31.19.0 netmask 255.255.240.0 {
    # Floor #9 specific parameters
    range 21.31.57.11 21.31.57.210;
  }
}

# The Management network, floors 10-12
shared-network management-net {
  # Management-net specific parameters
  option routers 21.31.74.1;
  option lpr-servers 21.31.74.2, 21.31.75.2, 21.31.76.2, 21.31.75.3;
  option broadcast-address 21.31.79.255;
  subnet 21.31.74.0 netmask 255.255.240.0 {
    # Floor #10 specific parameters
    range 21.31.74.11 21.31.74.210;
  }
  subnet 21.31.18.0 netmask 255.255.240.0 {
    # Floor #11 specific parameters
    range 21.31.75.11 21.31.75.210;
  }
  subnet 21.31.19.0 netmask 255.255.240.0 {
    # Floor #12 specific parameters
    range 21.31.76.11 21.31.76.210;
  }
}
            

A static host is a host that always gets the same IP-address from the DHCP-server in opposite to dynamic hosts which get their IP-address from a range of IP-addresses.

Obviously, the DHCP-server must be able recognize the host to be able to conclude that the host has been defined as a static one in the DHCP-server's configuration file. This can be done by using the dhcp-client-identifier option or by using the hardware ethernet option.

The dhcp-client-identifier is send to the server by the client (host) and must uniquely identify that client. This is not safe because there is no way to be sure that there isn't a second client that uses the same identifier.

The hardware ethernet option causes the match to be done on the client's NIC-address which is world-wide unique.

If the client does not send a dhcp-client-identifier, then the NIC-address is used to identify the client.

There are two designers, working for the Engineering department, that come to the office sometimes to get a hardcopy of their designs in colour. These designers are called “luke” and “leah” and they bring their laptops and connect them to the Engineering-network. The host names of their machines will be “luke” and “leah”.

To make this so, the administrator has added the following lines to the DHCP-server's configuration file:

group {
      # options that apply to all the static hosts
      option routers 21.31.55.1;
      option lpr-servers 21.31.56.3;
      option broadcast-address 21.31.63.255;
      netmask 255.255.240.0;
      host luke {
              # specific for luke
              hardware ethernet AA:88:54:72:7F:92;
              fixed-address 21.31.55.211;
              option host-name "luke";
      }

      host leah {
              # specific for leah
              hardware ethernet CC:88:54:72:84:4F;
              fixed-address 21.31.55.212;
              option host-name "leah";
      }
}
          

This is a special static host. If luke and leah's laptops were BOOTP-clients, the administrator could have added the following lines to the DHCP-server's configuration file:

group {
      # options that apply to all the static hosts
      option routers 21.31.55.1;
      option lpr-servers 21.31.56.3;
      option broadcast-address 21.31.63.255;
      netmask 255.255.240.0;
      host luke {
              # specific for luke
              filename "lukes-boot-file";
              server-name "server name to send to luke";
              next-server <address of server to load boot-file from>;
              hardware ethernet AA:88:54:72:7F:92;
              fixed-address 21.31.55.211;
              option host-name "luke";
      }

      host leah {
              # specific for leah
              filename "leahs-boot-file";
              server-name "server name to send to leah";
              next-server <address of server to load boot-file from>;
              hardware ethernet CC:88:54:72:84:4F;
              fixed-address 21.31.55.212;
              option host-name "leah";
      }
}
          

The filename option states the name of the file to get from the server defined in the next-server option. If the next-server is omitted, the server to get the file from is the DHCP-server. The server-name can be used to send the name of the server the client is booting from to the client.

For information on the valid options, consult the dhcp-options man page (man dhcp-options) and the dhcpd.conf man page (man dhcpd.conf).

A lease is the amount of time a client may use the IP-address it got from the DHCP-server. The client must refresh the lease periodically because the IP-address can be given to another client if the lease is expired. Normally, a client will be given the same IP-address if the lease is refreshed before it expired.

The option max-lease-time is used to specify the maximum amount of time in seconds that will be assigned to a lease if the client asks for a specific expiration time.

The option default-lease-time is used to specify the amount of time in seconds that will be assigned to a lease if a client does not ask for a specific expiration time.

The DHCP-server keeps a database of the leases it has issued in the file /var/dhcp/dhcpd.leases. If this file is empty, this is probably caused by the fact that you have only defined static hosts in the DHCP-server's configuration file and you haven't used any range statements.

Unless you specify otherwise, dhcpd will listen on all interfaces for a dhcp request. If you only want to serve requests on eth0 for instance, you can tell this to the daemon by including the parameter on the command line that starts the daemon.

This is done as follows:

# /etc/init.d/dhcp restart
          

This will stop the running daemon, wait two seconds, then start a new daemon which causes /etc/dhcpd.conf to be read again.

In our earlier example there is one DHCP-server for the whole network and there are routers between the clients and that server.

If a client would be able to connect to the DHCP-server through a router, the DHCP-server would not see the NIC-address of the client but the NIC-address of the router. This would mean that a static host for instance, could not be recognized by its hardware address.

A DHCP-relay agent, such as dhcrelay provides a means for relaying DHCP and BOOTP requests from one of the subnets to the company's DHCP-server.

If you need more information, consult The Internet Consortium DHCP Homepage.

If not specified otherwise on the command line, the DHCP-relay agent listens for DHCP requests on all interfaces and forwards them to the DHCP-servers specified on the command line. A reply is only sent to the network the request came from.

Please consult the man page (man dhcrelay) for further details.