I Accidentally Used Php

I try to avoid PHP software when ever possible. But sometimes the best tool for the job is written in PHP. One of these tools is observium which is a network monitoring platform. And I can really recommend it. But sadly it's written in PHP. That is why I accidentally start debugging PHP code one evening.

But first things first. I want to add my RaspberryPi, which is my primary DNS server, to observium. I click on add device fill out the snmp infos and woops "Could not resolve $host". My first thought was well I forgot something, after I double checked everything it was still not working. This was the point where I was annoyed enough to debug PHP code.

After poking around in the source code I found this:

dns_get_record($host, DNS_A + DNS_AAAA)

This was my first WTF moment, I mean seriously DNS_A + DNS_AAAA what should that do. A grep later with no result, it was clear that it must be a function of PHP. And look: it's in the manual. Turns out the way they implement it, allows to do addition and subtraction with these constants since there are internally bit masks or something. Which is a smart idea but of course you don't find this in the manual, it's only in a user comment below.

Anyway the manual states what dns_get_record should return:

This function returns an array of associative arrays, or FALSE on failure.

Doesn't sound entirely wrong. A empty array on a failure might come in handy, why I show you in a second.

var_dump(dns_get_record($host, DNS_A));

array(1) {
  array(5) {
    string(14) "host.name.tdl"
    string(2) "IN"
    string(1) "A"
    string(12) ""

Like in the manual described a array is returned.

var_dump(dns_get_record($host, DNS_AAAA));

PHP Warning:  dns_get_record(): DNS Query failed in file.php on line 4

Like in the manual described it returns FALSE if the is no AAAA record found.

I guess at this point you can assume what happens when you combine these two requests.

var_dump(dns_get_record($host, DNS_A + DNS_AAAA));

PHP Warning:  dns_get_record(): DNS Query failed in file.php on line 4

It returns only FALSE in this case, even if there is a A record for this domain.

And the moral of this story

Deploy IPv6 everywhere to prevent this! Or maybe don't build software based on PHP. I personally recommend both things.

If you are a observium pro user it's fixed, according the mailing list in revision 6357 and
for everyone else with the next half yearly release.