DNS failures can be the result of some fairly innocuous issues that arise due to fundamental configuration flaws or recent changes (and updates) to a system. This type of event can happen and, for this reason, it is always useful to run through a checklist of sanity checks before you hit the panic button.
So, by starting with a basic tool such as
dig, you can begin testing for areas of concern. For example, you can use
telnet like this:
# telnet <remote-server-address> 53
telnet command is a nice and easy tool to use, and if the connection is refused or takes too long, then you can rule out the possibility of RDNS errors by simply renaming the reverse DNS file and trying again.