barcodelib.barcode.winforms.dll download FIGURE 19-6 The DNS forward lookup zone for the internal network in VS .NET

Draw Data Matrix 2d barcode in VS .NET FIGURE 19-6 The DNS forward lookup zone for the internal network

FIGURE 19-6 The DNS forward lookup zone for the internal network
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Select New Host (A Or AAAA) from the Action menu to open the New Host dialog box shown in Figure 19-7.
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FIGURE 19-7 Adding a new host record for the printer
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Fill in the host name and IP address for the new DNS record and select the Create Associated Pointer (PTR) Record check box to also create a reverse lookup record for the device. Click Add Host and then click OK in the DNS message dialog box to create the record. Click Done to return to the DNS Manager console.
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If you later delete the A record, the associated PTR record is not automatically
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deleted. You must manually delete it from the reverse lookup zone.
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When a client makes a DNS query of a DNS server, and the server doesn t have the information either in its own records or in its cache of known IP addresses from previous queries, there are three possible options for the server: Return record not found Forward the query to a nearby server that might have the information Forward the query to one of the Internet s root DNS servers Obviously the rst option isn t terribly useful, unless you re creating a very private test network and you don t want any queries going outside it. The second option, DNS forwarding, was the default behavior for SBS 2003 and is still the default if you migrate from SBS 2003 to SBS 2008. In SBS 2003, the DNS server was con gured to automatically forward DNS requests that it didn t have the answer for to the DNS server of your Internet service provider (ISP). This was ef cient because the ISP s DNS servers were
Managing Connectivity
usually no more than a hop or two away, and the answer was quickly returned. This is a good idea if you trust your ISP to have accurate and safe DNS servers. The third option is for the server to forward any DNS query for which it doesn t have the answer to the Internet s DNS root servers. This option, which uses root hints, is somewhat slower than querying the ISP s servers that are a lot closer, but it does ensure an accurate answer.
REAL WORLD
DNS Poisoning Attacks
he standard setup for most internal DNS servers, including SBS 2003, was to set up your DNS server as a primary zone and then con gure it to forward all
other requests to your ISP s designated DNS servers. This resulted in fast and private support for internal name resolution while providing the fastest resolution of names outside your private network and reducing overall traf c for your ISP and the Internet as a whole. Unfortunately, this exposes your network to DNS poisoning attacks such as the widespread cache corruption attack that affects all versions of BIND before version 9. If some malicious program manages to subvert the DNS servers maintained by your ISP because your ISP hasn t gotten around to updating them, your DNS server will pass that problem on to your internal clients. The problem is especially a concern if your ISP is somewhat slow to apply patches to its DNS servers, as seems to be the case for many ISPs, both large and small. BIND is the most common DNS server software used by ISPs, and several vulnerabilities have been identi ed against BIND, especially versions before BIND 9. Patches to correct these vulnerabilities are available, but if your ISP is slow to apply the patch, you could be exposed. If you don t specify a server to forward to, your DNS server will use root hints to directly resolve the address. This might be somewhat slower, and it certainly increases the overall traf c on the Internet, but if the root servers are poisoned, we re all in trouble. If you trust your ISP to maintain its servers adequately, go ahead and forward to their servers. Personally, we ve stopped doing so, and we re really glad that SBS 2008 doesn t do this either.
If you do trust your ISP, and you want to con gure SBS 2008 for DNS forwarding, use the following steps:
1. 2. 3.
Open the DNS Manager console if it isn t already open. Right-click the SBS server in the left pane and select Properties. Click the Forwarders tab, as shown in Figure 19-8.
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