Глава 12. DHCP

Содержание

12.1. Configuring a DHCP Server with YaST
12.2. DHCP Software Packages
12.3. The DHCP Server dhcpd
12.4. For More Information

Аннотация

The purpose of the Dynamic Host Configuration Protocol (DHCP) is to assign network settings centrally (from a server) rather than configuring them locally on each and every workstation. A host configured to use DHCP does not have control over its own static address. It is enabled to configure itself completely and automatically according to directions from the server. If you use the NetworkManager on the client side, you do not need to configure the client at all. This is useful if you have changing environments and only one interface active at a time. Never use NetworkManager on a machine that runs a DHCP server.

One way to configure a DHCP server is to identify each client using the hardware address of its network card (which should be fixed in most cases), then supply that client with identical settings each time it connects to the server. DHCP can also be configured to assign addresses to each relevant client dynamically from an address pool set up for this purpose. In the latter case, the DHCP server tries to assign the same address to the client each time it receives a request, even over extended periods. This works only if the network does not have more clients than addresses.

DHCP makes life easier for system administrators. Any changes, even bigger ones, related to addresses and the network configuration in general can be implemented centrally by editing the server's configuration file. This is much more convenient than reconfiguring numerous workstations. It is also much easier to integrate machines, particularly new machines, into the network, because they can be given an IP address from the pool. Retrieving the appropriate network settings from a DHCP server is especially useful in case of laptops regularly used in different networks.

In this chapter, the DHCP server will run in the same subnet as the workstations, 192.168.2.0/24 with 192.168.2.1 as gateway. It has the fixed IP address 192.168.2.254 and serves two address ranges, 192.168.2.10 to 192.168.2.20 and 192.168.2.100 192.168.2.200.

A DHCP server supplies not only the IP address and the netmask, but also the hostname, domain name, gateway, and name server addresses for the client to use. In addition to that, DHCP allows a number of other parameters to be configured in a centralized way, for example, a time server from which clients may poll the current time or even a print server.

12.1. Configuring a DHCP Server with YaST

To install a DHCP server, start YaST and select Software+Software Management. Choose Filter+Patterns and select DHCP and DNS Server. Confirm the installation of the dependent packages to finish the installation process.

[Important]LDAP Support

The YaST DHCP module can be set up to store the server configuration locally (on the host that runs the DHCP server) or to have its configuration data managed by an LDAP server. If you want to use LDAP, set up your LDAP environment before configuring the DHCP server.

For more information about LDAP, see Глава 4, LDAP — Сервис директорий (↑Руководство по безопасности).

The YaST DHCP module (yast2-dhcp-server) allows you to set up your own DHCP server for the local network. The module can run in wizard mode or expert configuration mode.

12.1.1. Initial Configuration (Wizard)

When the module is started for the first time, a wizard starts, prompting you to make a few basic decisions concerning server administration. Completing this initial setup produces a very basic server configuration that should function in its essential aspects. The expert mode can be used to deal with more advanced configuration tasks.

Card Selection

In the first step, YaST looks for the network interfaces available on your system and displays them in a list. From the list, select the interface to which the DHCP server should listen and click Select. After this, select Open Firewall for Selected Interfaces to open the firewall for this interface, and click Next. See Рисунок 12.1, «DHCP Server: Card Selection».

Рисунок 12.1. DHCP Server: Card Selection

DHCP Server: Card Selection

Global Settings

Use the check box to determine whether your DHCP settings should be automatically stored by an LDAP server. In the entry fields, provide the network specifics for all clients the DHCP server should manage. These specifics are the domain name, address of a time server, addresses of the primary and secondary name server, addresses of a print and a WINS server (for a mixed network with both Windows and Linux clients), gateway address, and lease time. See Рисунок 12.2, «DHCP Server: Global Settings».

Рисунок 12.2. DHCP Server: Global Settings

DHCP Server: Global Settings

Dynamic DHCP

In this step, configure how dynamic IP addresses should be assigned to clients. To do so, specify an IP range from which the server can assign addresses to DHCP clients. All these addresses must be covered by the same netmask. Also specify the lease time during which a client may keep its IP address without needing to request an extension of the lease. Optionally, specify the maximum lease time—the period during which the server reserves an IP address for a particular client. See Рисунок 12.3, «DHCP Server: Dynamic DHCP».

Рисунок 12.3. DHCP Server: Dynamic DHCP

DHCP Server: Dynamic DHCP

Finishing the Configuration and Setting the Start Mode

After the third part of the configuration wizard, a last dialog is shown in which you can define how the DHCP server should be started. Here, specify whether to start the DHCP server automatically when the system is booted or manually when needed (for example, for testing purposes). Click Finish to complete the configuration of the server. See Рисунок 12.4, «DHCP Server: Start-Up».

Рисунок 12.4. DHCP Server: Start-Up

DHCP Server: Start-Up

12.2. DHCP Software Packages

Both the DHCP server and the DHCP clients are available for openSUSE. The DHCP server available is dhcpd (published by the Internet Systems Consortium). On the client side, choose between two different DHCP client programs: dhcp-client (also from ISC) and the DHCP client daemon in the dhcpcd package.

openSUSE installs dhcpcd by default. The program is very easy to handle and is launched automatically on each system boot to watch for a DHCP server. It does not need a configuration file to do its job and works out of the box in most standard setups. For more complex situations, use the ISC dhcp-client, which is controlled by means of the configuration file /etc/dhclient.conf.

12.3. The DHCP Server dhcpd

The core of any DHCP system is the dynamic host configuration protocol daemon. This server leases addresses and watches how they are used, according to the settings defined in the configuration file /etc/dhcpd.conf. By changing the parameters and values in this file, a system administrator can influence the program's behavior in numerous ways. Look at the basic sample /etc/dhcpd.conf file in Пример 12.1, «The Configuration File /etc/dhcpd.conf».

Пример 12.1. The Configuration File /etc/dhcpd.conf

default-lease-time 600;         # 10 minutes
max-lease-time 7200;            # 2  hours

option domain-name "example.com";
option domain-name-servers 192.168.1.116;
option broadcast-address 192.168.2.255;
option routers 192.168.2.1;
option subnet-mask 255.255.255.0;

subnet 192.168.2.0 netmask 255.255.255.0
 {
  range 192.168.2.10 192.168.2.20;
  range 192.168.2.100 192.168.2.200;
 }

This simple configuration file should be sufficient to get the DHCP server to assign IP addresses in the network. Make sure that a semicolon is inserted at the end of each line, because otherwise dhcpd is not started.

The sample file can be divided into three sections. The first one defines how many seconds an IP address is leased to a requesting client by default (default-lease-time) before it should apply for renewal. This section also includes a statement of the maximum period for which a machine may keep an IP address assigned by the DHCP server without applying for renewal (max-lease-time).

In the second part, some basic network parameters are defined on a global level:

  • The line option domain-name defines the default domain of your network.

  • With the entry option domain-name-servers, specify up to three values for the DNS servers used to resolve IP addresses into hostnames and vice versa. Ideally, configure a name server on your machine or somewhere else in your network before setting up DHCP. That name server should also define a hostname for each dynamic address and vice versa. To learn how to configure your own name server, read Глава 11, The Domain Name System.

  • The line option broadcast-address defines the broadcast address the requesting client should use.

  • With option routers, set where the server should send data packets that cannot be delivered to a host on the local network (according to the source and target host address and the subnet mask provided). In most cases, especially in smaller networks, this router is identical to the Internet gateway.

  • With option subnet-mask, specify the netmask assigned to clients.

The last section of the file defines a network, including a subnet mask. To finish, specify the address range that the DHCP daemon should use to assign IP addresses to interested clients. In Пример 12.1, «The Configuration File /etc/dhcpd.conf», clients may be given any address between 192.168.2.10 and 192.168.2.20 as well as 192.168.2.100 and 192.168.2.200.

After editing these few lines, you should be able to activate the DHCP daemon with the command rcdhcpd start. It will be ready for use immediately. Use the command rcdhcpd check-syntax to perform a brief syntax check. If you encounter any unexpected problems with your configuration (the server aborts with an error or does not return done on start), you should be able to find out what has gone wrong by looking for information either in the main system log /var/log/messages or on console 10 (Ctrl+Alt+F10).

On a default openSUSE system, the DHCP daemon is started in a chroot environment for security reasons. The configuration files must be copied to the chroot environment so the daemon can find them. Normally, there is no need to worry about this because the command rcdhcpd start automatically copies the files.

12.3.1. Clients with Fixed IP Addresses

DHCP can also be used to assign a predefined, static address to a specific client. Addresses assigned explicitly always take priority over dynamic addresses from the pool. A static address never expires in the way a dynamic address would, for example, if there were not enough addresses available and the server needed to redistribute them among clients.

To identify a client configured with a static address, dhcpd uses the hardware address (which is a globally unique, fixed numerical code consisting of six octet pairs) for the identification of all network devices (for example, 00:30:6E:08:EC:80). If the respective lines, like the ones in Пример 12.2, «Additions to the Configuration File», are added to the configuration file of Пример 12.1, «The Configuration File /etc/dhcpd.conf», the DHCP daemon always assigns the same set of data to the corresponding client.

Пример 12.2. Additions to the Configuration File

host jupiter {
hardware ethernet 00:30:6E:08:EC:80;
fixed-address 192.168.2.100;
}

The name of the respective client (host hostname, here jupiter) is entered in the first line and the MAC address in the second line. On Linux hosts, find the MAC address with the command ip link show followed by the network device (for example, eth0). The output should contain something like

link/ether 00:30:6E:08:EC:80

In the preceding example, a client with a network card having the MAC address 00:30:6E:08:EC:80 is assigned the IP address 192.168.2.100 and the hostname jupiter automatically. The type of hardware to enter is ethernet in nearly all cases, although token-ring, which is often found on IBM systems, is also supported.

12.3.2. The openSUSE Version

To improve security, the openSUSE version of the ISC's DHCP server comes with the non-root/chroot patch by Ari Edelkind applied. This enables dhcpd to run with the user ID nobody and run in a chroot environment (/var/lib/dhcp). To make this possible, the configuration file dhcpd.conf must be located in /var/lib/dhcp/etc. The init script automatically copies the file to this directory when starting.

Control the server's behavior regarding this feature by means of entries in the file /etc/sysconfig/dhcpd. To run dhcpd without the chroot environment, set the variable DHCPD_RUN_CHROOTED in /etc/sysconfig/dhcpd to «no».

To enable dhcpd to resolve hostnames even from within the chroot environment, some other configuration files must be copied as well:

  • /etc/localtime

  • /etc/host.conf

  • /etc/hosts

  • /etc/resolv.conf

These files are copied to /var/lib/dhcp/etc/ when starting the init script. Take these copies into account for any changes that they require if they are dynamically modified by scripts like /etc/ppp/ip-up. However, there should be no need to worry about this if the configuration file only specifies IP addresses (instead of hostnames).

If your configuration includes additional files that should be copied into the chroot environment, set these under the variable DHCPD_CONF_INCLUDE_FILES in the file /etc/sysconfig/dhcpd. To ensure that the DHCP logging facility keeps working even after a restart of the syslog-ng daemon, there is an additional entry SYSLOGD_ADDITIONAL_SOCKET_DHCP in the file /etc/sysconfig/syslog.

12.4. For More Information

More information about DHCP is available at the Web site of the Internet Systems Consortium (http://www.isc.org/products/DHCP/). Information is also available in the dhcpd, dhcpd.conf, dhcpd.leases, and dhcp-options man pages.