DOMAIN NAME SYSTEM in Software

Drawing Code 3 of 9 in Software DOMAIN NAME SYSTEM

higher than the hostid part In this way, a server serving the whole site is at a higher level than the servers serving each subnet This configuration makes the domain look inverted when compared to a generic or country domain To follow the convention of reading the domain labels from the bottom to the top, an IF address such as 1323445121 (a class B address with netid 13234) is read as 1214534132in-addr arpa See Figure 2511 for an illustration of the inverse domain configuration

DNS is designed as a client/server application A host that needs to map an address to a name or a name to an address calls a DNS client called a resolver The resolver accesses the closest DNS server with a mapping request If the server has the information, it satisfies the resolver; otherwise, it either refers the resolver to other servers or asks other servers to provide the infonnation After the resolver receives the mapping, it interprets the response to see if it is a real resolution or an error, and finally delivers the result to the process that requested it

Most of the time, the resolver gives a domain name to the server and asks for the corresponding address In this case, the server checks the generic domains or the country domains to find the mapping If the domain name is from the generic domains section, the resolver receives a domain name such as "chalatcjhdaedu" The query is sent by the resolver to the local DNS server for resolution If the local server cannot resolve the query, it either refers the resolver to other servers or asks other servers directly If the domain name is from the country domains section, the resolver receives a domain name such as "chjhdacucaus" The procedure is the same

A client can send an IP address to a server to be mapped to a domain name As mentioned before, this is called a PTR query To answer queries of this kind, DNS uses the inverse domain However, in the request, the IP address is reversed and the two labels in-addr and arpa are appended to create a domain acceptable by the inverse domain section For example, if the resolver receives the IF address 1323445121, the resolver first inverts the address and then adds the two labels before sending The domain name sent is "1214534132in-addrarpa" which is received by the local DNS and resolved

The client (resolver) can ask for a recursive answer from a name server This means that the resolver expects the server to supply the final answer If the server is the authority for the domain name, it checks its database and responds If the server is not the authority, it sends the request to another server (the parent usually) and waits for the response If the parent is the authority, it responds; otherwise, it sends the query to yet another server When the query is finally resolved, the response travels back until it finally reaches the requesting client This is called recursive resolution and is shown in Figure 2512 Figure 2512 Recursive resolution

If the client does not ask for a recursive answer, the mapping can be done iteratively If the server is an authority for the name, it sends the answer If it is not, it returns (to the client) the IP address of the server that it thinks can resolve the query The client is responsible for repeating the query to this second server If the newly addressed server can resolve the problem, it answers the query with the IP address; otherwise, it returns the IP address of a new server to the client Now the client must repeat the query to the third server This process is called iterative resolution because the client repeats the same query to multiple servers In Figure 2513 the client queries four servers before it gets an answer from the mcgrawcom server