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Check out this case study of the Australian Academic and Research Network that illustrates the practical uses of forwarding DNS queries.
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Australian Academic and Research Network

To learn more about why forwarding your DNS queries is important, check out this rather extreme case study of the Australian Academic and Research Network, or AARNet2. The network is shared among many institutions in Australia, and they all share one Internet link to the United States. The link is overloaded, of course, in addition to being very expensive—and even more expensive to upgrade to a faster speed. To help discourage unneeded traffic to the U.S. and to keep the link usable, its use is metered and paid for by megabyte. It is quite expensive for an FTP archive inside AARNet2 to fetch a 700MB CD-ROM image. Due to the cost, the Australians have been very organized about how they give access to the Internet. All manner of traffic is concentrated in a hierarchy, and only very few DNS servers inside AARNet2 have access to the U.S. link. The same goes for Usenet and other traffic that can be organized in this way. In addition, there are large software mirrors on AARNet2, so people tend to get software from them instead of from other sources elsewhere in the world.

The congestion of the U.S. link also adds two other factors. First, UDP packets—which DNS uses—are often discarded by routers when networks are congested. So, the DNS query may not get through to the outside. This causes retries, which adds to the traffic. If multiple servers are doing retries, the traffic will raise proportionally. Second, the network delay—both due to the congestion and to the sheer length of the network link—combined with the limited speed of light causes a significant delay, even when queries and answers do get through. When the delay is longer than the timeout in BIND, this also causes retries. AARNet2 clearly do not want every other server to have its own BIND querying DNS across the U.S. link.

A typical setup in Australia is that an AARNet2 member concentrates all its DNS traffic to one DNS server. This server has access to a regional DNS server, which then forwards to the national DNS service, which in turn puts the query on the U.S. link. Each level of forwarding concentrates the traffic and increases the likeliness that a cached answer is found. The structure of AARNet2 and DNS forwarding is shown in Figure 1.

Figure 1

Within each region, the members connect to the regional hub, which in turn connects to the national hub. The DNS forwarding structure mirrors this, with queries being forwarded through each level until a cached answer is found or the national hub resolves it.

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