[DNS] Reverse DNS resolution returns - (dash) character

[Mickael-van-der-Beek]

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

[ChALkeR]

$ dig +noall +answer -x 87.233.241.210
210.241.233.87.in-addr.arpa. 86366 IN   PTR     -.

That's what DNS actually returns. There is nothing that Node.js could or should do here.

[Mickael-van-der-Beek]

@ChALkeR Thanks for the quick answer.

Is this part of the DNS protocol ? I can't find information anywhere about a possible - dash character as a return value even after looking through the Node.js and C-Ares source codes.

[bnoordhuis]

It's technically legal, the RDATA field in PTR records is free-form. Suggest close?

[Mickael-van-der-Beek]

It might be interesting to mention this in the DNS core module documentation if that's the case because otherwise anyone that does not validate the hostnames returned will encounter unnecessary errors further down the pipeline.

Yup, the issue can be closed. Thanks for the information!