Although it's true that some recursive-nameserver configurations are (sloppily) referred to as "caching", e.g., by RHEL/Fedora/CentOS, that's a really bad name for that function -- because caching is orthogonal to recursion.
Theoretically, you could write a nameserver that does recursive service but doesn't cache its results. (That would be a bit perverse, and I don't know of any.) Conversely, nameserver packages that cache but know nothing about how to recurse and instead do less-helpful alternative iterative service are common: dnsmasq, pdnsd, etc. ... ...
Above text source: http://linuxgazette.net/170/lan.html
Please explain what does the author means by "caching is orthogonal to recursion" ?
caching is orthogonal to recursion?
Caching doesn't require/imply recursion.
The term "orthogonal" is meant to be interpreted from a mathematical sense loosely has "the things have nothing in common i.e. separate concepts".