Configuring DNS for Active Directory - MCTS Guide to Microsoft Windows Server 2008 Active Directory Configuration - page 356

The DNS Lookup Process Two different types of DNS lookup can be performed:
• Iterative query —When a DNS server receives an iterative query , it responds with the best
information it has to satisfy the query, such as the IP address of an A record it retrieves from
a local zone file or cache. If the DNS server doesn't have the specific information, it might
have the IP address of a name server that can satisfy the query; this type of response is called
a referral . If the server has no information, it sends a negative response that essentially says
“I can't help you.” DNS servers usually query each other by using iterative queries.
• Recursive query —A recursive query instructs the DNS server to process the query until it
responds with an address that satisfies the query or with an “I don't know” message. A
recursive query might require a DNS server to contact several other DNS servers before it
finally sends a response to the client. Queries made by DNS clients are recursive queries.

A typical DNS lookup made by a DNS client can involve both recursive and iterative queries.
A sample query demonstrating the hierarchical nature of DNS (see Figure 9-2) is outlined in the
following steps:
1. A user types www.microsoft.com in the Web browser's address bar. The computer running
the Web browser, called the DNS client or resolver , sends a recursive query to the address of
the DNS server in its IP configuration. Typically, this DNS server, called the local DNS server,
is maintained on the corporate network or at the ISP.
2. The local DNS server checks its local zone data. If the name isn't found locally, it sends an
iterative query to a DNS root server.
3. The root server sends a referral to the local DNS server with a list of addresses for the TLD
servers handling the com top-level domain.
4. The local DNS server sends another iterative query to a com TLD server.
5. The com TLD server responds with a referral to DNS servers responsible for the
microsoft.com domain.
6. The local DNS server then sends another iterative query to a microsoft.com DNS server.
7. The microsoft.com DNS server replies with the host record IP address for www.
microsoft.com.
8. The local DNS server responds to the client with the IP address for www.microsoft.com.

1) Recursive query for www.microsoft.com 2) Iterative query to root server

3) Referral - try one of these com TLD servers

8) The address is 207.46.19.190

DNS client Local

DNS server

Root
server

Microsoft.com
DNS server

com
TLD server

Figure 9-2

A DNS hierarchical lookup

Thankfully, the process depicted in Figure 9-2 doesn't occur with every DNS lookup.
Computers cache information they get from DNS, as you learned in Chapter 8. Furthermore, the
local DNS server also caches recent lookups. So the entire process shown in Figure 9-2 occurs
only when neither the computer doing the lookup nor the local DNS server has a cached copy
of the requested name resolution.