I encountered a strange behaviour on the dns.reverse() method. e.g:

If you resolve the domain name www.fagron.com to an IP address:

dns.resolve4('www.fagron.com', function (e, addresses) {
    console.log(addresses); // [ '87.233.241.210' ]
});

If you now re-use this IPv4 address and do a reverse DNS resolution:

dns.reverse('87.233.241.210', function (e, hostnames) {
    console.log(hostnames); // [ '-' ]
});

It seems fairly strange that the returned value is a - dash character (45) since you would expect that:

• if an error occurred, the error is returned and the hostnames argument is undefined
• if no hostname is found at all, the hostnames argument is an empty array []

Node.js version is: v4.2.2
Operating system version is: Darwin myhostname.local 14.5.0 Darwin Kernel Version 14.5.0: Wed Jul 29 02:26:53 PDT 2015; root:xnu-2782.40.9~1/RELEASE_X86_64 x86_64