Dynamic Host Configuration Protocol Domain Name System & Dynamic Host Configuration Protocol

DNS - History The Domain Name System, was born because of the necesity of remember easily the name of every server conected to the Internet. The Stanford Research Institute (now SRI International) maintained a text file named HOSTS.TXT that mapped host names to the numerical addresses of computers on the ARPANET.  Maintenance of numerical addresses, was handled by Jon Postel at the University of Southern California's Information Sciences Institute(ISI). By the early 1980s, there were huge amounts of hosts and the emerging network required an automated naming system to address technical and personnel issues. Postel directed the task of forging a compromise between five competing proposals of solutions to Paul Mockapetris, and instead of that, he created the Domain Name System.

DNS – What is it? The Domain Name System (DNS) is a hierarchical decentralized naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities. It translates more readily memorized domain names to the numerical IP addresses needed for the purpose of locating and identifying computer services and devices with the underlying network protocols. The Domain Name System is an essential component of the functionality of the Internet.

en.example.com DNS – Domain Name parts Top Level Domain: Label on the right side. Subdomain: Label on the center. Hostname: Label on the left side. en.example.com Hostname Top Level Domain Subdomain

DNS – Configuration Types There are several Configuration Types: Master (a.k.a. Primary) DNS Server Slave (a.k.a Secondary) DNS Server Caching (a.k.a. resolver cache) DNS Server

DNS – Master Server A Master DNS defines one or more zone files for which this DNS is Authoritative ('typemaster'). The zone has been delegated (via an NS Resource Record) to this DNS.

DNS – Slave Server A Slave DNS gets its zone data using a zone transfer operation (typically from a zone master) and it will respond as authoritative for those zones for which it is defined to be a 'slave' and for which it has a currently valid zone configuration. It is impossible to determine from a query result that it came from a zone master or slave.

DNS – Caching Server A DNS Caching Server (sometimes called a DNS resolver cache) is a temporary database, maintained by a computer's operating system, that contains records of all recent visits and attempted visits to Web sites and other Internet domains.

DNS – DNS Provider API (Windows) The DNS Provider is written in C# and runs using the System.Management namespace of the .NET 1.1 Framework to communicate with the Microsoft Windows Management Instrumentation (WMI) DNS Provider on the Microsoft Windows ServerT 2003 operating system. The Remoting Server object is implemented as a server-activated, by-reference, single-call object. Also, the DNS Provider assembly has an associated configuration file that the service provider can edit to set the port, activation type, and protocol. The configuration defaults to port 80, activation type binary, and protocol HTTP.

DNS – DNS Provider API (Windows) The DNS Provider allows for the configuration of DNS servers using methods that: Create, modify, pause, resume, reload, refresh, and delete zones. Establish zone delegations and rescind them. Build zone files. Add, examine, and remove resource records and other pertinent data.

DHCP - History In 1984, the Reverse Address Resolution Protocol (RARP), was introduced to allow simple devices such as diskless workstations to dynamically obtain a suitable IP address. It made implementation difficult on many server platforms, and also required that a server be present on each individual network link. RARP was superseded by the Bootstrap Protocol (BOOTP) in 1985 because one central BOOTP server was allowed to serve hosts on many IP subnets, while RARP not. DHCP was defined in 1993, and is based on BOOTP but can dynamically allocate IP addresses from a pool and reclaim them when they are no longer in use.

DHCP – What is it? The Dynamic Host Configuration Protocol (DHCP) is a standardized network protocol used on Internet Protocol (IP) networks. A router or a residential gateway can be enabled to act as a DHCP server. A DHCP server enables computers to request IP addresses and networking parameters automatically, reducing the need for a network administrator or a user to configure these settings manually. In the absence of a DHCP server, each computer or other device (eg., a printer) on the network needs to be statically (manually) assigned to an IP address.

DHCP – Allocation Methods Depending on implementation, the DHCP server may have three methods of allocating IP addresses: Dynamic Allocation Automatic Allocation Static Allocation (Manual Allocation)

DHCP – Dynamic Allocation A network administrator reserves a range of IP addresses for DHCP, and each DHCP client on the LAN is configured to request an IP address from the DHCP server during network initialization. The request-and-grant process uses a lease concept with a controllable time period, allowing the DHCP server to reclaim (and then reallocate) IP addresses that are not renewed.

DHCP – Automatic Allocation The DHCP server permanently assigns an IP address to a requesting client from the range defined by the administrator. This is like dynamic allocation, but the DHCP server keeps a table of past IP address assignments, so that it can preferentially assign to a client the same IP address that the client previously had.

DHCP – Static Allocation Is a useful feature which makes the DHCP server on your router always assign the same IP address to a specific computer on your LAN. To be more specific, the DHCP server assigns this static IP to a unique MAC address assigned to each NIC on your LAN. Your computer boots and requests its IP from the router's DHCP server. The DHCP server recognizes the MAC address of your device's NIC and assigns the static IP address to it. 

DHCP – Messages There are different messages used in the process: DHCPDISCOVER: Marks the beginning of a DHCP interaction between client and server. This message is sent by a client that is connected to a local subnet. It’s a broadcast message that uses 255.255.255.255 as destination IP address while the source IP address is 0.0.0.0 DHCPOFFER: Is sent in response to DHCPDISCOVER by a DHCP server to DHCP client. This message contains the network configuration settings for the client that sent the DHCPDISCOVER message. DHCPREQUEST: This DHCP message is sent in response to DHCPOFFER indicating that the client has accepted the network configuration sent in DHCPOFFER message from the server. ACKNOWLEDGEMENT: When the DHCP server receives the DHCPREQUEST message from the client, the configuration process enters its final phase. The acknowledgement phase involves sending a DHCPACK packet to the client. This packet includes the lease duration and any other configuration information that the client might have requested. At this point, the IP configuration process is completed.

DHCP – Client API (Windows) The Dynamic Host Configuration Protocol (DHCP) Application Programming Interface (API), also referred to as DHCP Client Options, enables Microsoft Windows clients to query specific options from DHCP servers. This capability enables vendor-specific options exposed through DHCP servers to be queried by Windows clients. The following examples illustrate two uses of the DHCP Client API: Example 1 illustrates how to use the DhcpRequestParams function to retrieve a host name. Example 2 shows how the DhcpRegisterParamChange function can be used to keep track of host name changes.

DHCP – Using the DhcpRequestParams function

DHCP – Using the DhcpRequestParams function

DHCP – Using the DhcpRegisterParamChange function

Author: Sergio Gómez García