Mapping a domain name to a physical IP address or an IP address to a domain name is called name-address resolution.
Resolver
DNS is designed as a client-server application. A host needs to map an address to a name or a name to an address calls a DNS client called 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 information.
After the resolver recieves 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.
Mapping Domain names to Addresses
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 genric domains section, the resolver receives a domain name such as "chal.atc.fhda.edu.". The query is sent by the resolver to 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 "ch.fhda.cu.ca.us.". The procedure is the same.
Mapping Addresses to Names
A client can send an IP address to a server to be mappedto 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 should be reversed and two labels, in-addr and arpa, should be appended to create a domain acceptable by inverse domain section. For example, If the resolver receives the Ip address 132.34.45.121, the resolver first inverts the address and then adds the two labels before sending. The domain name sent is "121.45.34.132.in-addr.arpa." , which is received by local DNS and resolved.
Recursive Resolution
The client (resolver) an 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 yet another server. When the query is finally resolved, the response levels back until itfinally reaches the requesting client.
Iterative Resolution
If the client does not ask for a recursiveanswer, 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; otherwiese, it returns the IP address of a new server to the client. Now the client must repeat the query to the third server. thi sprocess is called iterative because theclient repeats the same query to multiple servers.
Domain Name Caching (Cached Version of Page)
Each time a server recieves a query from a name that is not in its domain, it needs to search its database for a server IP address. Reduction of this search time would increase efficiency. DNS handles this with a mechanism called caching. When a server asks for a mapping from another server and receives the response, it stores this information in its cache memory before sending it to the client. If the same or another client asks for the same mapping, it can check its cache memory and resolve the problem. However,to inform the client that response is coming from the cache memory and not from an authitative source, the server marks the response as unauthoritative.
Caching speeds up resolution, but it can also be problematic. If a server cache a mapping for a long time, it may send an outdated mapping to the client. To counter this, two techniques are used. First, the authoritative server always adds a piece of information to the mapping called time-to-live (TTL). It defines the time in seconds that the reciever server can cache the information. After that time, the mapping is invalid and any query must be sent again to the authoritative server. Second, DNS requires that each server keeps a TTL counter for each mapping it caches. The cache memory must be searched periodically and those mappings with an expired TTL must be purged.
That was the discussion on Name-address resolution in DNS, Iterative and recursive resolution of domain addresses. We also learned about Caching and Domain name mappings, & TTL. Hope you have enjoyed reading the article.
Related Post
Resolver
DNS is designed as a client-server application. A host needs to map an address to a name or a name to an address calls a DNS client called 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 information.
After the resolver recieves 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.
Mapping Domain names to Addresses
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 genric domains section, the resolver receives a domain name such as "chal.atc.fhda.edu.". The query is sent by the resolver to 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 "ch.fhda.cu.ca.us.". The procedure is the same.
Mapping Addresses to Names
A client can send an IP address to a server to be mappedto 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 should be reversed and two labels, in-addr and arpa, should be appended to create a domain acceptable by inverse domain section. For example, If the resolver receives the Ip address 132.34.45.121, the resolver first inverts the address and then adds the two labels before sending. The domain name sent is "121.45.34.132.in-addr.arpa." , which is received by local DNS and resolved.
Recursive Resolution
The client (resolver) an 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 yet another server. When the query is finally resolved, the response levels back until itfinally reaches the requesting client.
Iterative Resolution
If the client does not ask for a recursiveanswer, 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; otherwiese, it returns the IP address of a new server to the client. Now the client must repeat the query to the third server. thi sprocess is called iterative because theclient repeats the same query to multiple servers.
Domain Name Caching (Cached Version of Page)
Each time a server recieves a query from a name that is not in its domain, it needs to search its database for a server IP address. Reduction of this search time would increase efficiency. DNS handles this with a mechanism called caching. When a server asks for a mapping from another server and receives the response, it stores this information in its cache memory before sending it to the client. If the same or another client asks for the same mapping, it can check its cache memory and resolve the problem. However,to inform the client that response is coming from the cache memory and not from an authitative source, the server marks the response as unauthoritative.
Caching speeds up resolution, but it can also be problematic. If a server cache a mapping for a long time, it may send an outdated mapping to the client. To counter this, two techniques are used. First, the authoritative server always adds a piece of information to the mapping called time-to-live (TTL). It defines the time in seconds that the reciever server can cache the information. After that time, the mapping is invalid and any query must be sent again to the authoritative server. Second, DNS requires that each server keeps a TTL counter for each mapping it caches. The cache memory must be searched periodically and those mappings with an expired TTL must be purged.
That was the discussion on Name-address resolution in DNS, Iterative and recursive resolution of domain addresses. We also learned about Caching and Domain name mappings, & TTL. Hope you have enjoyed reading the article.
Related Post
- DNS Messages and Their Format
- DNS In the Internet.
- Domain Name Space.
- Domain Name Systen (DNS)
Well ! DNS means Domain Name System or Domain name server which helps to transfer ip address to domain name and domain name to ip addresses.It plays vital role in internet technology.For example,if you provide domain name like Google.com it convert the domain name into particular ip address.If you provide the ip address like 74.125.239.9 the DNS search for the particular website and display on your browser.DNS consist of many resource records.Some of them are A record,CNAME record,DNAME record etc., You can check domain to ip ,Ip to domain and DNS query using the site Whoisxy.com .
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