DHCP - Dynamic Host Configuration Protocol
 

In order for dhcpd to work correctly with picky DHCP clients (e.g., Windows 95), it must be able to send packets with an IP destination address of 255.255.255.255. Unfortunately, Linux insists on changing 255.255.255.255 into the local subnet broadcast address. This results in a DHCP protocol violation, and while many DHCP clients don't notice the problem, some (specifically, all Microsoft DHCP clients) do. Clients that have this problem will appear not to see DHCPOFFER messages from the server.
 

# route add -host 255.255.255.255 dev eth0  

255.255.255.255 0.0.0.0 255.255.255.255 eth0  

The DHCP server configuration file is /etc/dhcpd.conf. The file essentially consists of a list of statements. The statements fall into two broad categories - parameters and declarations. Parameter statements either say how to do something (e.g., how long a lease to offer), whether to do something (e.g., should dhcpd provide addresses to unknown clients), or what parameters to provide to the client (e.g., use gateway 192.168.0.1).
 
Declarations are used to describe the topology of the network, to describe clients on the network, to provide addresses that can be assigned to clients, or to apply a group of parameters to a group of declarations. In any group of parameters and declarations, all parameters must be specified before any declarations which depend on those parameters may be specified.
 
Declarations about network topology include the shared-network and the subnet declarations. If clients on a subnet are to be assigned addresses dynamically, a range declaration must appear within the subnet declaration. For clients with statically assigned addresses, or for installations where only known clients will be served, each such client must have a host declaration. If parameters are to be applied to a group of declarations which are not related strictly on a per-subnet basis, the group declaration can be used.
 
For every subnet which will be served, and for every subnet to which the dhcp server is connected, there must be one subnet declaration, which tells dhcpd how to recognize that an address is on that subnet. A subnet declaration is required for each subnet even if no addresses will be dynamically allocated on that subnet.
 
Some installations have physical networks on which more than one IP subnet operates. For example, if there is a site-wide requirement that 8-bit subnet masks be used, but a department with a single physical ethernet network expands to the point where it has more than 254 nodes, it may be necessary to run two 8-bit subnets on the same ethernet until such time as a new physical network can be added. In this case, the subnet declarations for these two networks may be enclosed in a shared-network declaration.
 
Some sites may have departments which have clients on more than one subnet, but it may be desirable to offer those clients a uniform set of parameters which are different than what would be offered to clients from other departments on the same subnet. For clients which will be declared explicitly with host declarations, these declarations can be enclosed in a group declaration along with the parameters which are common to that department. For clients whose addresses will be dynamically assigned, there is currently no way to group parameter assignments other than by network topology.
 
When a client is to be booted, its boot parameters are determined by first consulting that client's host declaration (if any), then consulting the group declaration (if any) which enclosed that host declaration, then consulting the subnet declaration for the subnet on which the client is booting, then consulting the shared-network declaration (if any) containing that subnet, and finally consulting the top-level parameters which may be specified outside of any declaration.
 
When dhcpd tries to find a host declaration for a client, it first looks for a host declaration which has a fixed address parameter which matches the subnet or shared network on which the client is booting. If it doesn't find any such entry, it then tries to find an entry which has no fixed-address parameter. If no such entry is found, then dhcpd acts as if there is no entry in the /etc/dhcpd.conf file for that client, even if there is an entry for that client on a different subnet or shared network.
 

Let's see some examples. A typical /etc/dhcpd.conf file will look something like this:
 

global parameters...  

 

shared-network SIMPSONS {  
  shared-network-specific parameters...  

 

  subnet 192.168.0.0 netmask 255.255.255.224 {  
    subnet-specific parameters...  

 

    range 192.168.0.10 192.168.0.30;  
  }  

 

  subnet 192.168.0.32 netmask 255.255.255.224 {  
    subnet-specific parameters...  

 

    range 192.168.0.42 192.168.0.62;  
  }  
}  

 

subnet 192.168.0.64 netmask 255.255.255.224 {  
  subnet-specific parameters...  

 

  range 192.168.0.74 192.168.0.94;  
}  

 

group {  
  group-specific parameters...  

 

  host bart.children.simpson.com {  
    host-specific parameters...  
  }  
  host lisa.chilren.simpson.com {  
    host-specific parameters...  
  }  
  host maggie.children.simpson.com {  
    host-specific parameters...  
  }  
}  

option domain-name "simpsons.com";  
option domain-name-servers ns1.simpsons.com, ns2.simpsons.com;  

option domain-name "accounting.simpsons.com";  

option routers 192.168.0.1;  

option domain-name "children.simpsons.com";  

max-lease-time 120;  
default-lease-time 120;  

We basically talked about the structure of the configuration file here, while missing an explanation of the actual options.
 
Please refer to the man page dhcpd.conf(5) for a list of all possible options and their meanings. The example /etc/dhcpd.conf installed with the dhcp package is also a good reference. It shows a reasonable setup and is extensively commented. There is also a mini-howto on DHCP. You can find in the directory /usr/doc/howto/en/mini, the filename is DHCP.gz. Use the command less DHCP.gz to open it.
 
A list of frequently asked questions (FAQ) on DHCP is available online at the URL http://web.syr.edu/~jmwobus/comfaqs/dhcp.faq.html.