In web terms a redirect hack is an umbrella concept that refers to various strategies to redirect visitors from one web page to another. There are several legitimate reasons a site may use this technology. These are generally necessary conditions where, for example, a site has migrated to another, or a page needs to temporarily send its visitors to another location.
Try http://www.cnnfn.com/ and notice how your browser is redirected to http://money.cnn.com/. When CNNfn shut down its doors it moved its operations to this new site, and used a redirect to take its faithful readers to its new abode. Same is true with Microsoft when it acquired Great Plains Software and renamed the business to Microsoft Dynamics. The URL http://www.greatplains.com/ now redirects to http://www.microsoft.com/dynamics/. These types of redirects are known as 301 or permanent redirects, referring to the code sent by a web server to a browser asking it to redirect to a new URL. The other type of redirect is known as 302 or temporary. It is used to announce a temporary relocation of a page or a site and results in the same type of redirection. The difference is mainly in how a browser or a search engine is supposed to treat the redirection.
Since temporary redirects are supposed to be, well, temporary, visiting programs are supposed to treat the original URLs as valid, like any other legit web page. For example, per rules, a search engine should continue to keep the original URL in its index and give the referee the same weight and value as the original URL.
Soon enough, unethical search engine optimization (SEO) guys discovered this rule (or loophole) and started gaming search engines by placing links on various sites that bounced off certain URLs from well-known web sites and redirected visitors to their own sites. When search engines encountered such links, they would give the referenced sites high rankings because the links were from trusted and well-regarded sites.
Take a look at this sample URL (it still works as of this writing):
It appears that AOL has redirected one of its own pages to example.com. Most likely AOL uses this page for its own site, but as is, it can't stop others from using it too. In reality you can replace example.com with any page's URL and any search engine (or your browser) would be faithfully ushered there, courtesy of AOL servers. As a benefit, the search engine would bestow a high ranking on that final URL by proxy, because that page is believed to be somehow associated with AOL.
A High ranking means more frequent appearances on or near the top of search engine results pages, which means that gamers can garner a lot of traffic on their sites for little work, and traffic translates to money. In simple cases visitors are greeted with pay-per-click or other types of ads, stock pump-and-dump schemes, or they might be scammed into buying stuff. In more insidious cases visitors might be tricked into installing spyware or malware on their computers to track their activities and annoy them with incessant popup ads, or worse, ship their private information to the bad guys waiting to wipe clean bank and trading accounts.
Fortunately major search engines have now optimized their systems to forestall redirect hacks and most gamers don't see much (if any) gain from employing this tactic. But don't count out redirect hacks just yet. A more nefarious redirect usage involving phishing is alive and well, and still thriving. More on that in the next blog entry.