Resolving DNS Queries

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Resolving DNS Queries

A client querying a DNS Server is called a resolver, while a DNS Server is generically called a name server. DNS works at the Application layer of the OSI model, which is the top or seventh layer. By working at this layer, DNS can more easily communicate with the client applications needing to resolve a host name. DNS can use either UDP or TCP for its communications. DNS tries to use UDP—which is more efficient—for better performance, but DNS resorts to TCP if can’t communicate properly through UDP. TCP and UDP are discussed more completely in Chapter 2.

Three types of queries can be made to a DNS Server: recursive, iterative, and inverse. Some examples of queries include a web browser—such as Internet Explorer—requesting the IP address for a web site, a Microsoft client requesting a browse list, another DNS Server requesting a name query, or a WINS server unable to resolve a name from its own database.

A recursive query forces the DNS Server to respond to the request with either a failure or a successful response that includes the TCP/IP address for the domain name requested. Resolvers typically make recursive queries. With a recursive query, the DNS Server must contact any other DNS Servers it needs to resolve the request. When it receives a response from the other DNS Server(s), it then sends a response to the client. With a recursive query, the DNS Server is not allowed to pass the buck by simply giving the client the address of another DNS Server that might be able to handle the request. This type of query is made from a resolver to a name server, and also from a name server to its forwarder (another name server configured to handle requests forwarded to it).

An iterative query is one in which the name server is expected to provide the best information based on what the server knows from local zone files or from caching. If the name server doesn’t have any information to answer the query, it simply sends a negative response. This is like playing the game, Go Fish. A player asks another player for a certain card: “Do you have any Jacks?” The player either answers yes and supplies the requested information (Jacks) or answers no and says, “Go Fish.” In other words, I don’t have what you’re looking for; go try someone else. A forwarder makes this type of query as it tries to find names outside its local domain. It may have to query a number of outside DNS Servers in an attempt to resolve the name.

Figure 12.6 shows the entire query process, with a DNS client making an initial query of a DNS Server to resolve the name www.erudite.com. The client makes a recursive query; it expects to receive an answer without being referred to another server. The DNS Server receiving the query can’t resolve the host name with its own information (cached or from zone files), so it makes an iterative query to a root name server.

The root server sends back the address of the name server for the com domain. The DNS Server then sends an iterative query to the com name server. This server sends back the address of the name server authoritative for the erudite.com domain. The DNS Server then sends a query for www.erudite.com to this server, and the erudite.com name server finds a resolution for www and returns a reply. The local DNS Server can finally respond to the client that made the original request for the name resolution. The client was kept on hold while the DNS Server worked to find a response. Because the client sent a recursive query, the DNS Server was forced to go to this extra work until it could obtain an answer.

The third type of query , an inverse query, is used when the client wants to know the host name of a specified TCP/IP address. A special domain in the DNS name space resolves this type of query. Otherwise, a DNS Server would have to completely search all the DNS domains to make sure it found the correct name. This special domain is called in-addr.arpa. Nodes in this space are named after TCP/IP addresses rather than alphabetic host names. However, these node names have the TCP/IP names in reverse order.

Remember that TCP/IP addresses move from general to specific. The first octet(s) refers to the network; additional octets or portions of octets may be dedicated to defining a subnet as specified by the subnet mask; the remaining octets or parts of octets specify the host address of a specific computer. With DNS, however, host names are read from right to left, with the name of the domain on the right, the name of any subdomains moving from right to left, and finally the name of the host on the leftmost part of the fully qualified name. In order to make the node names of the inverse lookup zones compatible with DNS, the zone files are named with IP addresses, but the addresses are written in reverse order.

Inverse lookup queries are used when a client requests a service that only specified host names have been given permission to use. The server receiving the request only knows the IP address of the client, so the server must find out the host name to see whether the client is on the approved list. In this case, the server issues an inverse lookup query to find the host name matching the IP address of the client that requested the service.

A number of DNS Servers also have zones for inverse lookups. The highest levels of zone files, for Class A, B, and C networks, are maintained by InterNIC. Then individual network address owners can have zone files for subnets on their own networks.

A name server can return three types of responses to a query: a successful response with the IP address for the requested host name (or the host name for an inverse lookup), a pointer to another name server (only in an iterative query), or a failure message.

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