As mentioned earlier in the chapter, the DNS system has some load-balancing features, namely, round-robin functionality. It may be helpful to distribute incoming connections equally between a few network hosts, such as web servers. If one web server is getting hit with lots of requests, it may be serving pages slowly or may even be rejecting or timing out some of the requests. In this case, one solution may be to set up a twin web server box and load-balance incoming traffic using DNS (this is also known as poor man's load balancing). You have to figure out a way of referencing two different servers using the same FQDN; if you create two separate "A" records pointing to different IP addresses, the DNS server will simply return the first record it matches during its zone search.
Round robin comes to the rescue. It issues replies for the same FQDN containing several IP addresses, which are ordered differently in each reply. This produces a cycling effect. Windows Server 2003 introduced the ability to control round-robin functionality based on record types—for instance, you may want to specifically turn off round robin in certain cases. The DoNotRoundRobinTypes key in HKEY_ LOCAL_MACHINE\System\CurrentControlSet\Services\DNS\Parameters helps achieve this. In addition, the Advanced tab of DNS Server Properties (shown in Figure 2-11) can be used to enable the netmask ordering feature. Netmask ordering forces the DNS server to issue the IP address closest to the requesting client from the pool of round-robin IP addresses, thereby reducing the length of the path that packets take from the client to the resource in question.
Note that round robin only works on authoritative servers. Non-authoritative servers cache the order in which IP addresses were returned by the authoritative server and fulfill subsequent requests from the cache.
FIGURE 2-1 1
Advanced tab of DNS server properties
Debug Logging Event Logging Monitoring Security
Interfaces Forwarders Advanced Root Hints
Server version number
¡Disable recursion (also disables forwarders} ¡"¡BIND secondaries ¡"¡Fail on load if bad zone data ¡ij Enable round robin ¡SjEnable netmask ordering l^jSecure cache against pollution
Name checking: Load zone data on startup: I? Enable automatic scavenging of stale records
From Active Directory and registry r
Reset to Default
One of the obvious shortcomings of round robin is its unawareness of the status of load-balanced servers, so in essence it will go on issuing load-balanced IP addresses, even if one of the load-balanced nodes goes down or gets overwhelmed by incoming requests. Another problem is evident on web servers that establish sessions, where round robin may forward requests to web servers that have no information about sessions previously established on other web servers in the pool. The Network Load Balancing (NLB or WLBS) and Failover Clustering (MSCS) features in Windows Server 2008 offer a more comprehensive solution.
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