[RndTbl] programatically determine if DNS is down or blocked?

Trevor Cordes trevor at tecnopolis.ca
Wed Apr 6 14:43:11 CDT 2016

I have a program that needs to do DNS lookups, etc.  Things will go Really 
Wonky if there's a problem with DNS (from a client perspective).  Thus, 
I'd like the program to be able to check if DNS isn't working.  In this 
particular setup, the box runs its own named with a view setup for 

In particular, I want to test that:
1. named is running and answering calls to its port
2. named is getting real answers from the net (i.e. doing its recursive 
resolution properly; and port 53 to outside NS's isn't being blocked)
3. (rare) root NS's aren't getting DDoS'd / whole world's DNS is down

I want to do this inside a perl or php program, but any language 
pseudo-code will be fine as a template.  I don't want to run a full-blown 
monitoring program separately.

Ideally, I'll have some php like:
function IsDnsOk() {
  check named
  check resolution ok

To be used possibly like:
while (!IsDnsOk()) {
  sleep 10;

But I want any check to be fairly lightweight.  I don't want to fire off 
useless DNS lookups to root name servers at a potential rate of thousands 
a minute.  Plus, I guess I have to worry about cached results making 
things look ok when they are really not.  Any solution I implement will 
probably be designed to only run these tests once a minute or something, 
which won't catch the instant DNS goes down, but will mitigate the damage 

I can think of a number of fairly simple tests I can try, but they feel 
kind of kludgy.  Is there a better way?

Ideally, the PHP calls, like gethostbyname would be able to return a 
"down" status, but they decided to lump in to the same return value 
failure due to no dns record and failure due to any other reason.


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