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DHCP ("Dynamic Host Configuration Protocol" (Used to configure…
DHCP
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Operation
Employs a connectionless service model, using the UDP
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Phases
Server discovery
DHCP client broadcasts a DHCPDISCOVER message on the network subnet using the destination address 255.255.255.255 or the specific subnet broadcast address
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If the client remains connected to the same network, the server may grant the request.
Otherwise, it depends whether the server is set up as authoritative or not
IP lease offer
When a DHCP server receives a DHCPDISCOVER message from a client, which is an IP address lease request containing:
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IP lease request
In response to the DHCP offer, the client replies with a DHCPREQUEST message, broadcast to the server, requesting the offered address
A client can receive DHCP offers from multiple servers, but will accept only one DHCP offer
IP lease aknowledgement
When the DHCP server receives the DHCPREQUEST message from the client, the configuration process enters its final phase
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DHCP releasing
Client sends a request to the DHCP server to release the DHCP information and the client deactivates its IP address
As client devices usually do not know when users may unplug them from the network, the protocol does not mandate the sending of DHCP Release.
Relaying
In small networks, where only one IP subnet is being managed, DHCP clients communicate directly with DHCP servers
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a DHCP client that has not yet acquired an IP address cannot communicate directly with the DHCP server using IP routing
In order to allow DHCP clients on subnets not directly served by DHCP servers to communicate with DHCP servers, DHCP relay agents can be installed on these subnets
The DHCP client broadcasts on the local link; the relay agent receives the broadcast and transmits it to one or more DHCP servers using unicast
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Security
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Because the DHCP server has no secure mechanism for authenticating the client, clients can gain unauthorized access to IP addresses by presenting credentials
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IP allocation addresses
Automatic allocation
DHCP server permanently assigns an IP address to a requesting client from the range defined by the administrator
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Manual allocation
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If no match for the client's MAC address, the server may or may not optionally fall back to either Dynamic of Automatic allocation
Dynamic allocation
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Each DHCP client on the LAN is configured to request an IP address from the DHCP server during network initialization
Reliability
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If the DHCP server is unreachable for an extended period of time, the DHCP client will attempt to rebind
In order for rebinding to work, when the client successfully contacts a backup DHCP server, that server must have accurate information about the client's binding
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A proposal for implementing fault-tolerant DHCP servers was submitted to the Internet Engineering Task Force, but never formalized