DHCP is an acronym for Dynamic Host Configuration Protocol. It’s part of the TCP/IP protocol suite and provides a dynamic configuration. It provides IP address, subnet mask, gateway, IP number of DNS servers, WINS servers, among other predefined settings for the hosts.
This makes much more agile network configuration, because the administrator does not need to enter all manually data. Just configure the DHCP server once.
See also: original publication.
HOW BEGAN THE DHCP?
DHCP for IPv4 was considered a standard from October 1993, updated in March 1997 by the technical specifications of RFC 2131. More recently, in July 2003, received a new specification for cataloging version 6 (DHCPv6), referred to IPv6.
DHCP works as follows: a client (named host) sends a UDP packet over the network in broadcast mode (for all nodes) with a DHCP request over port 67. The DHCP servers that capture this package, answer the door 68 with a package with all pre-defined by the administrator settings.
RULES WHEN IT GIVES AN IP
You can assign an IP via DHCP by three ways, depending on the implementation:
1. Manual Attribution: In this form of attribution there’s a table to associate the client’s MAC address and the IP address that will be provided. This association is done manually by the network administrator, which will send the DHCP settings only to registered MAC addresses in the list.
2. Auto Assign: Almost identical to the manual assignment, but in this case there’s no allocation by MAC address. This means that any host that connect this network will receive the settings package, since there’s still available IP addresses in the range.
3. Dynamic Attribution: Recycles the IP addresses between the hosts. The administrator uses this form of attribution when you want the hosts requesting DHCP configuration package when you boot the machine on the network. Also, while the machine is connected to the network, requesting recycling will address as the expiration time set by the administrator. This is the form most commonly used in corporate networks.
STEP-BY-STEP OF A DHCP REQUEST
When connecting to a network and be configured for DHCP, a client must go through the following steps:
1. The Discovery of DHCP
The client sends a broadcast message on the subnet to check for DHCP servers configured.
2. The Offer of DHCP
When the DHCP server received the request for client IP lease, reserve an IP for that client and provides an IP lease through a message known as DHCPOFFER. This message contains the MAC address of that client, the IP available to that host, the subnet mask, network and other settings already mentioned.
3. The Request of DHCP
To answer the server, the client sends a message called DHCPREQUEST by broadcast to all DHCP servers (if more than one in the same network) can see the message. The message notifies that the customer accepted the offer IP granted.
4. The Acknowloedge from DHCP Server
When the server receives the client’s message named DHCPREQUEST, it sends the message, known as DHCPPACK again. In this package, there is a concession and the other configurations requested by the client. Here, all the DHCP process is completed.
That’s all, folks!
Wikipedia EN (DHCP)
Computer Networks: A Systems Approach, por Larry L. Peterson,Bruce S. Davie
Wikipédia (SAMS Publishing)
Site Iana: “DHCP Parameters”
Site Tools: “RFC 2132”
Site Tools: “Failover”
Site Tools: “RFC 3046”
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