[networking-guide] master: DHCP: Updates to Chapter (5e54f4a)
stephenw at fedoraproject.org
stephenw at fedoraproject.org
Sat Jul 26 13:10:45 UTC 2014
Repository : http://git.fedorahosted.org/cgit/docs/networking-guide.git
On branch : master
>---------------------------------------------------------------
commit 5e54f4a32b9efc7e7fd12f1b0719ffed4705df67
Author: Stephen Wadeley <swadeley at redhat.com>
Date: Sat Jul 26 15:02:01 2014 +0200
DHCP: Updates to Chapter
Thanks to ljozsa and jpopelka for their reviews.
>---------------------------------------------------------------
en-US/DHCP_Servers.xml | 134 ++++++++++++++++++++++++++----------------------
1 files changed, 72 insertions(+), 62 deletions(-)
diff --git a/en-US/DHCP_Servers.xml b/en-US/DHCP_Servers.xml
index 1116210..1e2dcc5 100644
--- a/en-US/DHCP_Servers.xml
+++ b/en-US/DHCP_Servers.xml
@@ -22,6 +22,10 @@
</indexterm>
<para><systemitem class="protocol">DHCP</systemitem> is useful for automatic configuration of client network interfaces. When configuring the client system, you can choose <systemitem class="protocol">DHCP</systemitem> instead of specifying an <systemitem class="protocol">IP</systemitem> address, netmask, gateway, or <systemitem class="protocol">DNS</systemitem> servers. The client retrieves this information from the <systemitem class="protocol">DHCP</systemitem> server. <systemitem class="protocol">DHCP</systemitem> is also useful if you want to change the <systemitem class="protocol">IP</systemitem> addresses of a large number of systems. Instead of reconfiguring all the systems, you can just edit one configuration file on the server for the new set of <systemitem class="protocol">IP</systemitem> addresses. If the <systemitem class="protocol">DNS</systemitem> servers for an organization changes, the changes happen on the <systemitem class="protocol">DHCP</systemitem> s
erver, not on the <systemitem class="protocol">DHCP</systemitem> clients. When you restart the network or reboot the clients, the changes go into effect.</para>
<para>If an organization has a functional <systemitem class="protocol">DHCP</systemitem> server correctly connected to a network, laptops and other mobile computer users can move these devices from office to office.</para>
+ <para>
+ Note that administrators of <systemitem class="protocol">DNS</systemitem> and <systemitem class="protocol">DHCP</systemitem> servers, as well as any provisioning applications, should agree on the host name format used in an organization. See <xref linkend="sec-Recommended_Naming_Practices" /> for more information on the format of host names.
+</para>
+
</section>
<section
id="sec-dhcp-configuring-server">
@@ -31,7 +35,7 @@
<secondary>server configuration</secondary>
</indexterm>
- <para>The <package>dhcp</package> package contains an <firstterm>Internet Systems Consortium</firstterm> (<acronym>ISC</acronym>) <systemitem class="protocol">DHCP</systemitem> server. First, install the package as the superuser:</para>
+ <para>The <package>dhcp</package> package contains an <firstterm>Internet Systems Consortium</firstterm> (<acronym>ISC</acronym>) <systemitem class="protocol">DHCP</systemitem> server. Install the package as <systemitem class="username">root</systemitem>:</para>
<screen>~]# <command>yum install dhcp</command>
</screen>
<para>Installing the <package>dhcp</package> package creates a file, <filename>/etc/dhcp/dhcpd.conf</filename>, which is merely an empty configuration file. As <systemitem class="username">root</systemitem>, issue the following command:</para>
@@ -54,7 +58,7 @@
<indexterm>
<primary>dhcpd.conf</primary>
</indexterm>
- <para>The first step in configuring a <systemitem class="protocol">DHCP</systemitem> server is to create the configuration file that stores the network information for the clients. Use this file to declare options and global options for client systems.</para>
+ <para>The first step in configuring a <systemitem class="protocol">DHCP</systemitem> server is to create the configuration file that stores the network information for the clients. Use this file to declare options for client systems.</para>
<para>The configuration file can contain extra tabs or blank lines for easier formatting. Keywords are case-insensitive and lines beginning with a hash sign (<literal>#</literal>) are considered comments.</para>
<para>There are two types of statements in the configuration file:</para>
<itemizedlist>
@@ -69,7 +73,7 @@
<primary>DHCP</primary>
<secondary>options</secondary>
</indexterm>
- <para>The parameters that start with the keyword option are referred to as <firstterm>options</firstterm>. These options control DHCP options; whereas, parameters configure values that are not optional or control how the DHCP server behaves.</para>
+ <para>The parameters that start with the keyword option are referred to as <firstterm>options</firstterm>. These options control <systemitem class="protocol">DHCP</systemitem> options; whereas, parameters configure values that are not optional or control how the <systemitem class="protocol">DHCP</systemitem> server behaves.</para>
<indexterm>
<primary>DHCP</primary>
<secondary>global parameters</secondary>
@@ -84,13 +88,13 @@
<para>Instead of changing a <systemitem class="protocol">DHCP</systemitem> configuration file and restarting the service each time, using the <command>omshell</command> command provides an interactive way to connect to, query, and change the configuration of a <systemitem class="protocol">DHCP</systemitem> server. By using <command>omshell</command>, all changes can be made while the server is running. For more information on <command>omshell</command>, see the <command>omshell</command> man page.</para>
</note>
<para>In <xref
- linkend="subnet"/>, the <filename>routers</filename>, <filename>subnet-mask</filename>, <filename>domain-search</filename>, <filename>domain-name-servers</filename>, and <filename>time-offset</filename> options are used for any <filename>host</filename> statements declared below it.</para>
+ linkend="subnet"/>, the <command>routers</command>, <command>subnet-mask</command>, <command>domain-search</command>, <command>domain-name-servers</command>, and <command>time-offset</command> options are used for any <command>host</command> statements declared below it.</para>
<indexterm>
<primary>DHCP</primary>
<secondary>subnet</secondary>
</indexterm>
- <para>For every <filename>subnet</filename> which will be served, and for every <filename>subnet</filename> to which the <systemitem class="protocol">DHCP</systemitem> server is connected, there must be one <filename>subnet</filename> declaration, which tells the <systemitem class="protocol">DHCP</systemitem> daemon how to recognize that an address is on that <filename>subnet</filename>. A <filename>subnet</filename> declaration is required for each <filename>subnet</filename> even if no addresses will be dynamically allocated to that <filename>subnet</filename>.</para>
- <para>In this example, there are global options for every <systemitem class="protocol">DHCP</systemitem> client in the subnet and a <filename>range</filename> declared. Clients are assigned an <systemitem class="protocol">IP</systemitem> address within the <filename>range</filename>.</para>
+ <para>For every subnet which will be served, and for every subnet to which the <systemitem class="protocol">DHCP</systemitem> server is connected, there must be one <command>subnet</command> declaration, which tells the <systemitem class="protocol">DHCP</systemitem> daemon how to recognize that an address is on that subnet. A <command>subnet</command> declaration is required for each subnet even if no addresses will be dynamically allocated to that subnet.</para>
+ <para>In this example, there are global options for every <systemitem class="protocol">DHCP</systemitem> client in the subnet and a <command>range</command> declared. Clients are assigned an <systemitem class="protocol">IP</systemitem> address within the <command>range</command>.</para>
<example
id="subnet">
<title>Subnet Declaration</title>
@@ -103,8 +107,7 @@
range 192.168.1.10 192.168.1.100;
}</programlisting>
</example>
- <para>To configure a <systemitem class="protocol">DHCP</systemitem> server that leases a dynamic <systemitem class="protocol">IP</systemitem> address to a system within a subnet, modify <xref
- linkend="dynamic-ip"/> with your values. It declares a default lease time, maximum lease time, and network configuration values for the clients. This example assigns <systemitem class="protocol">IP</systemitem> addresses in the <filename>range</filename> <systemitem class="ipaddress">192.168.1.10</systemitem> and <systemitem class="ipaddress">192.168.1.100</systemitem> to client systems.</para>
+ <para>To configure a <systemitem class="protocol">DHCP</systemitem> server that leases a dynamic <systemitem class="protocol">IP</systemitem> address to a system within a subnet, modify the example values from <xref linkend="dynamic-ip"/>. It declares a default lease time, maximum lease time, and network configuration values for the clients. This example assigns <systemitem class="protocol">IP</systemitem> addresses in the <command>range</command> <systemitem class="ipaddress">192.168.1.10</systemitem> and <systemitem class="ipaddress">192.168.1.100</systemitem> to client systems.</para>
<example
id="dynamic-ip">
<title>Range Parameter</title>
@@ -119,9 +122,9 @@ subnet 192.168.1.0 netmask 255.255.255.0 {
range 192.168.1.10 192.168.1.100;
}</programlisting>
</example>
- <para>To assign an <systemitem class="protocol">IP</systemitem> address to a client based on the MAC address of the network interface card, use the <filename>hardware ethernet</filename> parameter within a <filename>host</filename> declaration. As demonstrated in <xref
- linkend="static-ip"/>, the <filename>host apex</filename> declaration specifies that the network interface card with the MAC address <systemitem class="etheraddress">00:A0:78:8E:9E:AA</systemitem> always receives the <systemitem class="protocol">IP</systemitem> address <systemitem class="ipaddress">192.168.1.4</systemitem>.</para>
- <para>Note that you can also use the optional parameter <filename>host-name</filename> to assign a host name to the client.</para>
+ <para>To assign an <systemitem class="protocol">IP</systemitem> address to a client based on the MAC address of the network interface card, use the <command>hardware ethernet</command> parameter within a <command>host</command> declaration. As demonstrated in <xref
+ linkend="static-ip"/>, the <command>host apex</command> declaration specifies that the network interface card with the MAC address <systemitem class="etheraddress">00:A0:78:8E:9E:AA</systemitem> always receives the <systemitem class="protocol">IP</systemitem> address <systemitem class="ipaddress">192.168.1.4</systemitem>.</para>
+ <para>Note that you can also use the optional parameter <option>host-name</option> to assign a host name to the client.</para>
<example
id="static-ip">
<title>Static IP Address Using DHCP</title>
@@ -133,12 +136,10 @@ subnet 192.168.1.0 netmask 255.255.255.0 {
</example>
<indexterm>
<primary>DHCP</primary>
- <secondary>
- <filename>shared-network</filename>
- </secondary>
+ <secondary><command>shared-network</command></secondary>
</indexterm>
- <para>All subnets that share the same physical network should be declared within a <filename>shared-network</filename> declaration as shown in <xref
- linkend="shared-network"/>. Parameters within the <filename>shared-network</filename>, but outside the enclosed <filename>subnet</filename> declarations, are considered to be global parameters. The name of the <filename>shared-network</filename> must be a descriptive title for the network, such as using the title 'test-lab' to describe all the subnets in a test lab environment.</para>
+ <para>All subnets that share the same physical network should be declared within a <command>shared-network</command> declaration as shown in <xref
+ linkend="shared-network"/>. Parameters within the <command>shared-network</command>, but outside the enclosed subnet declarations, are considered to be global parameters. The name assigned to <command>shared-network</command> must be a descriptive title for the network, such as using the title <quote>test-lab</quote> to describe all the subnets in a test lab environment.</para>
<example
id="shared-network">
<title>Shared-network Declaration</title>
@@ -146,13 +147,13 @@ subnet 192.168.1.0 netmask 255.255.255.0 {
option domain-search "test.redhat.com";
option domain-name-servers ns1.redhat.com, ns2.redhat.com;
option routers 192.168.0.254;
- more parameters for EXAMPLE shared-network
+ #more parameters for EXAMPLE shared-network
subnet 192.168.1.0 netmask 255.255.252.0 {
- parameters for subnet
+ #parameters for subnet
range 192.168.1.1 192.168.1.254;
}
subnet 192.168.2.0 netmask 255.255.252.0 {
- parameters for subnet
+ #parameters for subnet
range 192.168.2.1 192.168.2.254;
}
}</programlisting>
@@ -162,7 +163,7 @@ subnet 192.168.1.0 netmask 255.255.255.0 {
<secondary>group</secondary>
</indexterm>
<para>As demonstrated in <xref
- linkend="group"/>, the <filename>group</filename> declaration is used to apply global parameters to a group of declarations. For example, shared networks, subnets, and hosts can be grouped.</para>
+ linkend="group"/>, the <command>group</command> declaration is used to apply global parameters to a group of declarations. For example, shared networks, subnets, and hosts can be grouped.</para>
<example
id="group">
<title>Group Declaration</title>
@@ -186,19 +187,19 @@ subnet 192.168.1.0 netmask 255.255.255.0 {
</example>
<note>
<title>Using the Example Configuration File</title>
- <para>The example configuration file provided can be used as a starting point and custom configuration options can be added to it. To copy it to the proper location, use the following command:</para>
-<screen><command>cp /usr/share/doc/dhcp/dhcpd.conf.example /etc/dhcp/dhcpd.conf</command></screen>
-
+ <para>You can use the provided example configuration file as a starting point and add custom configuration options to it. To copy this file to the proper location, use the following command as <systemitem class="username">root</systemitem>:</para>
+ <screen>~]# <command>cp /usr/share/doc/dhcp-<replaceable><version_number></replaceable>/dhcpd.conf.example /etc/dhcp/dhcpd.conf</command></screen>
+ <para>... where <replaceable><version_number></replaceable> is the <systemitem class="protocol">DHCP</systemitem> version number.</para>
</note>
- <para>For a complete list of option statements and what they do, refer to the <filename>dhcp-options</filename> man page.</para>
+ <para>For a complete list of option statements and what they do, see the <filename>dhcp-options(5)</filename> man page.</para>
</section>
<section
id="lease-database">
<title>Lease Database</title>
- <para>On the DHCP server, the file <filename>/var/lib/dhcpd/dhcpd.leases</filename> stores the DHCP client lease database. Do not change this file. DHCP lease information for each recently assigned IP address is automatically stored in the lease database. The information includes the length of the lease, to whom the IP address has been assigned, the start and end dates for the lease, and the MAC address of the network interface card that was used to retrieve the lease.</para>
+ <para>On the <systemitem class="protocol">DHCP</systemitem> server, the file <filename>/var/lib/dhcpd/dhcpd.leases</filename> stores the <systemitem class="protocol">DHCP</systemitem> client lease database. Do not change this file. <systemitem class="protocol">DHCP</systemitem> lease information for each recently assigned <systemitem class="protocol">IP</systemitem> address is automatically stored in the lease database. The information includes the length of the lease, to whom the <systemitem class="protocol">IP</systemitem> address has been assigned, the start and end dates for the lease, and the MAC address of the network interface card that was used to retrieve the lease.</para>
<para>All times in the lease database are in Coordinated Universal Time (UTC), not local time.</para>
<para>The lease database is recreated from time to time so that it is not too large. First, all known leases are saved in a temporary lease database. The <filename>dhcpd.leases</filename> file is renamed <filename>dhcpd.leases~</filename> and the temporary lease database is written to <filename>dhcpd.leases</filename>.</para>
- <para>The DHCP daemon could be killed or the system could crash after the lease database has been renamed to the backup file but before the new file has been written. If this happens, the <filename>dhcpd.leases</filename> file does not exist, but it is required to start the service. Do not create a new lease file. If you do, all old leases are lost which causes many problems. The correct solution is to rename the <filename>dhcpd.leases~</filename> backup file to <filename>dhcpd.leases</filename> and then start the daemon.</para>
+ <para>The <systemitem class="protocol">DHCP</systemitem> daemon could be killed or the system could crash after the lease database has been renamed to the backup file but before the new file has been written. If this happens, the <filename>dhcpd.leases</filename> file does not exist, but it is required to start the service. Do not create a new lease file. If you do, all old leases are lost which causes many problems. The correct solution is to rename the <filename>dhcpd.leases~</filename> backup file to <filename>dhcpd.leases</filename> and then start the daemon.</para>
</section>
<section>
<title>Starting and Stopping the Server</title>
@@ -212,21 +213,21 @@ subnet 192.168.1.0 netmask 255.255.255.0 {
</indexterm>
<indexterm>
<primary>DHCP</primary>
- <secondary>
- <filename>dhcpd.leases</filename>
- </secondary>
+ <secondary><filename>dhcpd.leases</filename></secondary>
</indexterm>
<indexterm>
<primary>dhcpd.leases</primary>
</indexterm>
<important>
<title>Starting the DHCP Server for the First Time</title>
- <para>When the <systemitem class="protocol">DHCP</systemitem> server is started for the first time, it fails unless the <filename>dhcpd.leases</filename> file exists. Use the command <command>touch /var/lib/dhcpd/dhcpd.leases</command> to create the file if it does not exist.</para>
- <para>If the same server is also running BIND as a <systemitem class="protocol">DNS</systemitem> server, this step is not necessary, as starting the <command>named</command> service automatically checks for a <filename>dhcpd.leases</filename> file.</para>
+ <para>When the <systemitem class="protocol">DHCP</systemitem> server is started for the first time, it fails unless the <filename>dhcpd.leases</filename> file exists. You can use the command <command>touch /var/lib/dhcpd/dhcpd.leases</command> to create the file if it does not exist. If the same server is also running BIND as a <systemitem class="protocol">DNS</systemitem> server, this step is not necessary, as starting the <command>named</command> service automatically checks for a <filename>dhcpd.leases</filename> file.</para>
+ <para>
+ Do not create a new lease file on a system that was previously running. If you do, all old leases are lost which causes many problems. The correct solution is to rename the <filename>dhcpd.leases~</filename> backup file to <filename>dhcpd.leases</filename> and then start the daemon.
+ </para>
</important>
- <para>To start the DHCP service, use the following command:</para>
+ <para>To start the <systemitem class="protocol">DHCP</systemitem> service, use the following command:</para>
<screen><command>systemctl start dhcpd.service</command></screen>
- <para>To stop the DHCP server, type:</para>
+ <para>To stop the <systemitem class="protocol">DHCP</systemitem> server, type:</para>
<screen><command>systemctl stop dhcpd.service</command></screen>
<para>By default, the DHCP service does not start at boot time. To configure the daemon to start automatically at boot time, run:</para>
<screen><command>systemctl enable dhcpd.service</command></screen>
@@ -242,17 +243,23 @@ subnet 192.168.1.0 netmask 255.255.255.0 {
<primary>DHCP</primary>
<secondary>command line options</secondary>
</indexterm>
- <para>If more than one network interface is attached to the system, but the DHCP server should only be started on one of the interfaces, configure the DHCP server to start only on that device. In <filename>/etc/sysconfig/dhcpd</filename>, add the name of the interface to the list of <command>DHCPDARGS</command>:</para>
- <programlisting># Command line options here
-DHCPDARGS=eth0</programlisting>
- <para>This is useful for a firewall machine with two network cards. One network card can be configured as a DHCP client to retrieve an IP address to the Internet. The other network card can be used as a DHCP server for the internal network behind the firewall. Specifying only the network card connected to the internal network makes the system more secure because users can not connect to the daemon via the Internet.</para>
- <para>Other command line options that can be specified in <filename>/etc/sysconfig/dhcpd</filename> include:</para>
+ <para>If more than one network interface is attached to the system, but the <systemitem class="protocol">DHCP</systemitem> server should only listen for <systemitem class="protocol">DHCP</systemitem> requests on one of the interfaces, configure the <systemitem class="protocol">DHCP</systemitem> server to listen only on that device. The <systemitem class="protocol">DHCP</systemitem> daemon will only listen on interfaces for which it finds a subnet declaration in the <filename>/etc/dhcp/dhcpd.conf</filename> file.</para>
+ <para>This is useful for a firewall machine with two network cards. One network card can be configured as a <systemitem class="protocol">DHCP</systemitem> client to retrieve an <systemitem class="protocol">IP</systemitem> address to the Internet. The other network card can be used as a <systemitem class="protocol">DHCP</systemitem> server for the internal network behind the firewall. Specifying only the network card connected to the internal network makes the system more secure because users can not connect to the daemon via the Internet.</para>
+ <para>
+ To specify command line options, copy and then edit the <filename>dhcpd.service</filename> file as the <systemitem class="username">root</systemitem> user. For example, as follows:
+ <screen>~]# <command>cp /usr/lib/systemd/system/dhcpd.service /etc/systemd/system/</command>
+~]# <command>vi /etc/systemd/system/dhcpd.service</command></screen>
+Edit the line under section [Service]:
+<synopsis>ExecStart=/usr/sbin/dhcpd -f -cf /etc/dhcp/dhcpd.conf -user dhcpd -group dhcpd --no-pid <replaceable>your_interface_name(s)</replaceable></synopsis>
+Then, as the <systemitem class="username">root</systemitem> user, restart the service:
+<screen>~]# <command>systemctl --system daemon-reload</command>
+~]# <command>systemctl restart dhcpd</command></screen>
+ </para>
+ <para>Command line options can be appended to <command>ExecStart=/usr/sbin/dhcpd</command> in the <filename>/etc/systemd/system/dhcpd.service</filename> unit file under section [Service]. They include:</para>
<itemizedlist>
<listitem>
<para>
- <command>-p <replaceable>portnum</replaceable>
- </command> — Specifies the UDP port number on which <command>dhcpd</command> should listen. The default is port 67. The DHCP server transmits responses to the DHCP clients at a port number one greater than the UDP port specified. For example, if the default port 67 is used, the server listens on port 67 for requests and responses to the client on port 68. If a port is specified here and the DHCP relay agent is used, the same port on which the DHCP relay agent should listen must be specified. Refer to <xref
- linkend="dhcp-relay-agent"/> for details.</para>
+ <command>-p <replaceable><portnum></replaceable></command> — Specifies the UDP port number on which <command>dhcpd</command> should listen. The default is port 67. The <systemitem class="protocol">DHCP</systemitem> server transmits responses to the <systemitem class="protocol">DHCP</systemitem> clients at a port number one greater than the UDP port specified. For example, if the default port 67 is used, the server listens on port 67 for requests and responds to the client on port 68. If a port is specified here and the <systemitem class="protocol">DHCP</systemitem> relay agent is used, the same port on which the <systemitem class="protocol">DHCP</systemitem> relay agent should listen must be specified. See <xref linkend="dhcp-relay-agent" /> for details.</para>
</listitem>
<listitem>
<para>
@@ -270,7 +277,7 @@ DHCPDARGS=eth0</programlisting>
<listitem>
<para>
<command>-lf <replaceable>filename</replaceable>
- </command> — Specifies the location of the lease database file. If a lease database file already exists, it is very important that the same file be used every time the DHCP server is started. It is strongly recommended that this option only be used for debugging purposes on non-production machines. The default location is <filename>/var/lib/dhcpd/dhcpd.leases</filename>.</para>
+ </command> — Specifies the location of the lease database file. If a lease database file already exists, it is very important that the same file be used every time the <systemitem class="protocol">DHCP</systemitem> server is started. It is strongly recommended that this option only be used for debugging purposes on non-production machines. The default location is <filename>/var/lib/dhcpd/dhcpd.leases</filename>.</para>
</listitem>
<listitem>
<para>
@@ -287,18 +294,21 @@ DHCPDARGS=eth0</programlisting>
</indexterm>
<indexterm>
<primary>DHCP</primary>
- <secondary>
- dhcrelay
- </secondary>
+ <secondary>dhcrelay</secondary>
</indexterm>
<indexterm>
- <primary>
- dhcrelay
- </primary>
+ <primary>dhcrelay</primary>
</indexterm>
- <para>The DHCP Relay Agent (<command>dhcrelay</command>) allows for the relay of DHCP and BOOTP requests from a subnet with no DHCP server on it to one or more DHCP servers on other subnets.</para>
- <para>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. When a DHCP server returns a reply, the reply is broadcast or unicast on the network that sent the original request.</para>
+ <para>
+ The DHCP Relay Agent (<application>dhcrelay</application>) enables the relay of <systemitem class="protocol">DHCP</systemitem> and <systemitem class="protocol">BOOTP</systemitem> requests from a subnet with no <systemitem class="protocol">DHCP</systemitem> server on it to one or more <systemitem class="protocol">DHCP</systemitem> servers on other subnets.
+ </para>
+
+ <para>
+ When a <systemitem class="protocol">DHCP</systemitem> client requests information, the DHCP Relay Agent forwards the request to the list of <systemitem class="protocol">DHCP</systemitem> servers specified when the DHCP Relay Agent is started. When a <systemitem class="protocol">DHCP</systemitem> server returns a reply, the reply is broadcast or unicast on the network that sent the original request.
+ </para>
+
<para>The DHCP Relay Agent listens for DHCP requests on all interfaces unless the interfaces are specified in <filename>/etc/sysconfig/dhcrelay</filename> with the <computeroutput>INTERFACES</computeroutput> directive.</para>
+
<para>To start the DHCP Relay Agent, use the following command:</para>
<screen><command>systemctl start dhcrelay.service</command></screen>
</section>
@@ -311,13 +321,12 @@ DHCPDARGS=eth0</programlisting>
<primary>Multihomed DHCP</primary>
<secondary>server configuration</secondary>
</indexterm>
- <para>A multihomed DHCP server serves multiple networks, that is, multiple subnets. The examples in these sections detail how to configure a DHCP server to serve multiple networks, select which network interfaces to listen on, and how to define network settings for systems that move networks.</para>
- <para>Before making any changes, back up the existing <filename>/etc/sysconfig/dhcpd</filename> and <filename>/etc/dhcp/dhcpd.conf</filename> files.</para>
- <para>The DHCP daemon listens on all network interfaces unless otherwise specified. Use the <filename>/etc/sysconfig/dhcpd</filename> file to specify which network interfaces the DHCP daemon listens on. The following <filename>/etc/sysconfig/dhcpd</filename> example specifies that the DHCP daemon listens on the <filename>eth0</filename> and <filename>eth1</filename> interfaces:</para>
- <programlisting>DHCPDARGS="eth0 eth1";</programlisting>
- <para>If a system has three network interfaces cards -- <filename>eth0</filename>, <filename>eth1</filename>, and <filename>eth2</filename> -- and it is only desired that the DHCP daemon listens on <filename>eth0</filename>, then only specify <computeroutput>eth0</computeroutput> in <filename>/etc/sysconfig/dhcpd</filename>:</para>
- <programlisting>DHCPDARGS="eth0";</programlisting>
- <para>The following is a basic <filename>/etc/dhcp/dhcpd.conf</filename> file, for a server that has two network interfaces, <filename>eth0</filename> in a 10.0.0.0/24 network, and <filename>eth1</filename> in a 172.16.0.0/24 network. Multiple <computeroutput>subnet</computeroutput> declarations allow different settings to be defined for multiple networks:</para>
+ <para>A multihomed <systemitem class="protocol">DHCP</systemitem> server serves multiple networks, that is, multiple subnets. The examples in these sections detail how to configure a <systemitem class="protocol">DHCP</systemitem> server to serve multiple networks, select which network interfaces to listen on, and how to define network settings for systems that move networks.</para>
+ <para>Before making any changes, back up the existing <filename>/etc/dhcp/dhcpd.conf</filename> file.</para>
+ <para>The <systemitem class="protocol">DHCP</systemitem> daemon will only listen on interfaces for which it finds a subnet declaration in the <filename>/etc/dhcp/dhcpd.conf</filename> file.</para>
+ <para>The following is a basic <filename>/etc/dhcp/dhcpd.conf</filename> file, for a server that has two network interfaces, <interface>eth0</interface> in a <systemitem class="ipaddress">10.0.0.0/24</systemitem> network, and <interface>eth1</interface> in a <systemitem class="ipaddress">172.16.0.0/24</systemitem> network. Multiple <computeroutput>subnet</computeroutput> declarations allow you to define different settings for multiple networks:</para>
+
+
<programlisting>default-lease-time <replaceable>600</replaceable>;
max-lease-time <replaceable>7200</replaceable>;
subnet 10.0.0.0 netmask 255.255.255.0 {
@@ -336,8 +345,8 @@ subnet 172.16.0.0 netmask 255.255.255.0 {
<computeroutput>subnet <replaceable>10.0.0.0</replaceable> netmask <replaceable>255.255.255.0</replaceable>;</computeroutput>
</term>
<listitem>
- <para>A <computeroutput>subnet</computeroutput> declaration is required for every network your DHCP server is serving. Multiple subnets require multiple <computeroutput>subnet</computeroutput> declarations. If the DHCP server does not have a network interface in a range of a <computeroutput>subnet</computeroutput> declaration, the DHCP server does not serve that network.</para>
- <para>If there is only one <computeroutput>subnet</computeroutput> declaration, and no network interfaces are in the range of that subnet, the DHCP daemon fails to start, and an error such as the following is logged to <filename>/var/log/messages</filename>:</para>
+ <para>A <computeroutput>subnet</computeroutput> declaration is required for every network your <systemitem class="protocol">DHCP</systemitem> server is serving. Multiple subnets require multiple <computeroutput>subnet</computeroutput> declarations. If the <systemitem class="protocol">DHCP</systemitem> server does not have a network interface in a range of a <computeroutput>subnet</computeroutput> declaration, the <systemitem class="protocol">DHCP</systemitem> server does not serve that network.</para>
+ <para>If there is only one <computeroutput>subnet</computeroutput> declaration, and no network interfaces are in the range of that subnet, the <systemitem class="protocol">DHCP</systemitem> daemon fails to start, and an error such as the following is logged to <filename>/var/log/messages</filename>:</para>
<programlisting>dhcpd: No subnet declaration for eth0 (0.0.0.0).
dhcpd: ** Ignoring requests on eth0. If this is not what
dhcpd: you want, please write a subnet declaration
@@ -373,8 +382,9 @@ dhcpd: Not configured to listen on any interfaces!</programlisting>
</listitem>
</varlistentry>
</variablelist>
- <para>For further information, see the <computeroutput>dhcpd.conf(5)</computeroutput> man page.</para>
- <section
+ <para>For further information, see the <filename>dhcpd.conf(5)</filename> man page.</para>
+
+ <section
id="sec-dns_Host_Configuration">
<title>Host Configuration</title>
<indexterm>
@@ -464,7 +474,7 @@ host interface1 {
<para>make the name in the <computeroutput>host</computeroutput> declaration unique.</para>
</listitem>
</itemizedlist>
- <para>When a name given in a <computeroutput>host</computeroutput> declaration is not unique, the DHCP daemon fails to start, and an error such as the following is logged to <filename>/var/log/messages</filename>:</para>
+ <para>When a name given in a <computeroutput>host</computeroutput> declaration is not unique, the <systemitem class="protocol">DHCP</systemitem> daemon fails to start, and an error such as the following is logged to <filename>/var/log/messages</filename>:</para>
<programlisting>dhcpd: /etc/dhcp/dhcpd.conf line 31: host interface0: already exists
dhcpd: }
dhcpd: ^
More information about the docs-commits
mailing list