101 lines
7.1 KiB
HTML
101 lines
7.1 KiB
HTML
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<meta name="DC.Title" content="DHCP client-server interaction" />
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<meta name="abstract" content="A client gets the DHCP information from the server, and specific messages are sent between the client and the server. DHCP obtains and returns leases." />
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<title>DHCP client-server interaction</title>
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</head>
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<body id="rzakgconceptinteract"><a name="rzakgconceptinteract"><!-- --></a>
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<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">DHCP client-server interaction</h1>
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<div><p>A client gets the DHCP information from the server, and specific
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messages are sent between the client and the server. DHCP obtains and returns
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leases.</p>
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<div class="section"><p>DHCP provides an automated method for dynamic
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client configuration. Clients that are DHCP enabled automatically obtain their
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own IP address and configuration parameters from the server. This process
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occurs through a series of steps, illustrated in the following figure.</p>
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<div class="fignone"><span class="figcap">Figure 1. DHCP client-server interaction</span><br /><img src="rzakg508.gif" alt="DHCP client-server interaction." /><br /></div>
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<dl><dt class="dlterm">Client requests DHCP information: DHCPDISCOVER</dt>
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<dd>First, the client sends out a DISCOVER message requesting an IP address.
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The DISCOVER message contains an identifier unique to the client (typically
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the MAC address). The message might also contain other requests, such as requested
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options (for example, subnet mask, domain name server, domain name, or static
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route). The message is sent out as a broadcast. If the network contains routers,
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those routers can be configured to forward DISCOVER packets to DHCP servers
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on attached networks.</dd>
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<dt class="dlterm">DHCP server offers information to client: DHCPOFFER</dt>
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<dd>Any DHCP server that receives the DISCOVER message might send an OFFER
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message in response. The DHCP server might not send an OFFER message back
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to the client for multiple reasons: the most common reasons are
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that all available addresses are currently leased, the subnet is not configured,
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or the client is not supported. If the DHCP server sends an OFFER message
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in response, the DHCPOFFER will contain an available IP address and any other
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configuration information that is defined in the DHCP setup.</dd>
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<dt class="dlterm">Client accepts DHCP server offer: DHCPREQUEST</dt>
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<dd>The client receives OFFER messages from the DHCP servers that responded
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to the DISCOVER. The client compares the offers with the settings that it
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requested, and then selects the server that it wants to use. It sends a REQUEST
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message to accept the offer, indicating which server it selected. This message
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is broadcast to the entire network to let all DHCP servers know which server
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was selected.</dd>
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<dt class="dlterm">DHCP server acknowledges the client and leases the IP address: DHCPACK</dt>
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<dd>If a server receives a REQUEST message, the server marks the address as
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leased. Servers that are not selected will return offered addresses to their
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available pool. The selected server sends the client an acknowledgment (DHCPACK),
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which contains additional configuration information.<p>The client might now
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use the IP address and configuration parameters. It will use these settings
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until its lease expires or until the client sends a DHCPRELEASE message to
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the server to end the lease.</p>
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</dd>
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<dt class="dlterm">Client attempts to renew the lease: DHCPREQUEST, DHCPACK</dt>
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<dd>The client starts to renew a lease when half of the lease time has passed.
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The client requests the renewal by sending a REQUEST message to the server.
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If the server accepts the request, it will send a DHCPACK message back to
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the client. If the server does not respond to the request, the client might
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continue to use the IP address and configuration information until the lease
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expires. As long as the lease is still active, the client and server do not
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need to go through the DHCPDISCOVER and DHCPREQUEST process. When the lease
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has expired, the client must start over with the DHCPDISCOVER process.</dd>
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<dt class="dlterm">Client ends the lease: DHCPRELEASE</dt>
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<dd>The client ends the lease by sending a RELEASE message to the DHCP server.
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The server will then return the client's IP address to the available address
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pool.</dd>
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</dl>
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</div>
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</div>
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<div>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="rzakgconceptparent.htm" title="DHCP interacts with clients and works in a network.">DHCP concepts</a></div>
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</div>
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<div class="relconcepts"><strong>Related concepts</strong><br />
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<div><a href="rzakgconceptrelayrouter.htm" title="You might need to use a DHCP relay agent in your network; sometimes a router is sufficient. You can use both a DHCP relay agent and a router to efficiently and securely transfer data throughout the network.">Relay agents and routers</a></div>
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<div><a href="rzakgconceptleases.htm" title="You can find what DHCP leases are and some questions are provided for you to consider when determining the lease time for your DHCP clients.">Leases</a></div>
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<div><a href="rzakgproblemnoip.htm" title="Problems might occur if the clients cannot receive an IP address or the configuration information. An IP address is leased to a client through a four-step process between the client and the DHCP server.">Problem: Clients are not receiving an IP address or their configuration information</a></div>
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</div>
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</div>
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</body>
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