Книга: Embedded Linux Primer: A Practical, Real-World Approach

12.3.2. BOOTP/DHCP Server

12.3.2. BOOTP/DHCP Server

Having a DHCP server on your development host simplifies the configuration management for your embedded target. We have already established the reasons why an Ethernet interface on your target hardware is a good idea. When Linux boots on your target board, it needs to configure the Ethernet interface before the interface will be useful. Moreover, if you are using an NFS root mount configuration on your target board, Linux needs to configure your target's Ethernet interface before the boot process can complete. We covered NFS in detail in Chapter 9, "File Systems."

In general, Linux can use two methods to initialize its Ethernet/IP interface during boot:

• Hard-code the Ethernet interface parameters either on the Linux kernel command line or in the default configuration

• Configure the kernel to automatically detect the network settings at boot time

For obvious reasons, the latter choice is the most flexible. DHCP or BOOTP is the protocol your target and server use to accomplish the automatic detection of network settings. For details of the DHCP or BOOTP protocols, see Section 12.4.1 at the end of this chapter.

A DHCP server controls the IP address assignments for IP subnets for which it has been configured, and for DHCP or BOOTP clients that have been configured to participate. A DHCP server listens for requests from a DHCP client (such as your target board), and assigns addresses and other pertinent information to the client as part of the boot process. A typical DHCP exchange (see Listing 12-5) can be examined by starting your DHCP server with the -d debug switch and observing the output when a target machine requests configuration.

Listing 12-5. Typical DHCP Exchange

tgt> DHCPDISCOVER from 00:09:5b:65:1d:d5 via eth0
svr> DHCPOFFER on 192.168.0.9 to 00:09:5b:65:1d:d5 via eth0
tgt> DHCPREQUEST for 192.168.0.9 (192.168.0.1) from
  00:09:5b:65:1d:d5 via eth0
svr> DHCPACK on 192.168.0.9 to 00:09:5b:65:1d:d5 via eth0

The sequence starts with the client (target) transmitting a broadcast frame attempting to discover a DHCP server. This is shown by the DHCPDISCOVER message shown. The server responds (if it has been so configured and enabled) by offering an IP address for the client. This is evidenced by the DHCPOFFER message. The client then responds by testing this IP address locally. The testing includes sending the DHCPREQUEST packet to the DHCP server, as shown. Finally, the server responds by acknowledging the IP address assignment to the client, thus completing the automatic target configuration.

It is interesting to note that a properly configured client will remember the last address it was assigned by a DHCP server. The next time it boots, it will skip the DHCPDISCOVER stage and proceed directly to the DHCPREQUEST stage, assuming that it can reuse the same IP address that the server previously assigned. A booting Linux kernel does not have this capability and emits the same sequence every time it boots.

Configuration of your host's DHCP server is not difficult. As usual, our advice is to consult the documentation that came with your desktop Linux distribution. On a Red Hat or Fedora Core distribution, the configuration entry for a single target might look like Listing 12-6.

Listing 12-6. Example DHCP Server Configuration

# Example DHCP Server configuration
allow bootp;
subnet 192.168.1.0 netmask 255.255.255.0 {
 default-lease-time 1209600; # two weeks
 option routers 192.168.1.1;
 option domain-name-servers 1.2.3.4;
 group {
  host pdna1 {
   hardware ethernet 00:30:bd:2a:26:1f;
   fixed-address 192.168.1.68;
   filename "uImage-pdna";
   option root-path "/home/chris/sandbox/pdna-target";
  }
 }
} 

This is a simple example, meant only to show the kind of information you can pass to your target system. There is a one-to-one mapping of the target MAC address to its assigned IP address. In addition to its fixed IP address, you can pass other information to your target. In this example, the default router and DNS server addresses are passed to your target, along with the filename of a file of your choice, and a root path for your kernel to mount an NFS root mount from. The filename might be used by your bootloader to load a kernel image from your TFTP server. You can also configure your DHCP server to hand out IP addresses from a predefined range, but it is very convenient to use a fixed address such as that shown in Listing 12-6.

You must enable the DHCP server on your Linux development workstation. This is typically done through your main menu or via the command line. Consult the documentation for your own Linux distribution for details suitable for your environment. For example, to enable the DHCP server on a Fedora Core 2 Linux distribution, simply type the following command from a root command prompt:

$ /etc/init.d/dhcpd start (or restart)

You must do this each time you start your development workstation, unless you configure it to start automatically.

Many nuances are involved with installing a DHCP server, so unless your server is on a private network, it is advisable to check with your system administrator before going live with your own. If you coexist with a corporate LAN, it is very possible that you will interfere with its own DHCP service.

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