Hi Timur,

On 07/03/13 18:15, Timur Alperovich wrote:
> Thanks for taking the time to reply!

Not a problem.

> On Wed, Mar 6, 2013 at 6:39 PM, Garthy D
> <garthy_lmkltybr / entropicsoftware.com>  wrote:
>> If you're working with DNS you're going to run into all sorts of caching fun
>> in general, so it's a good idea to be prepared for it. Applications (eg.
>> browsers), operating systems, ISPs, and nameservers all do their own caching
>> of DNS results. Did you know, for example, that the data could be over a
>> week out-of-date because you are using an ISP that disregards TTLs, and your
>> browser has been caching previous results to remain responsive?
>
> Yes, I do realize that propagating updates can take a significant
> amount of time. However, eventually, they should be propagated -- the
> behavior you're describing sounds like a bug.

The browser behaviour is an oddity- it's what you actually want most of 
the time, especially if the operating system lookup is sluggish (might 
be true on Windows? I'm not sure), but if you're doing something 
involving messing about with hosts, it's positively painful to work with.

For the ISP case, it's more poor behaviour on the part of an ISP rather 
than a bug. :} None *should* ignore TTL, because used properly it is 
incredibly useful, especially when migrating hosts. But some do. It 
drove me crazy when I had to deal with it- lazy behaviour on the part of 
ISPs of which you aren't even a customer can have nasty effects if your 
*clients* are using that ISP.

>> I can't speak for the Resolv class myself- I've never used it- but the
>> rationale is probably similar. If you've got a hard requirement such as
>> checking /etc/hosts for changes, you might want to consider a wrapper or
>> proxy class that watches /etc/hosts for changes in its timestamp, and then
>> does whatever you need- restarting OS-level resolvers, perhaps dropping and
>> recreating the Resolv object, or somehow forcing it to drop its cache.
>> Exactly what needs to be done will depend on the problem you are trying to
>> solve.
>
> As I mentioned, one workaround is to call getaddrinfo, since glibc
> does not cache getaddrinfo responses.
>
> However, I was trying to point out the larger issue here: the Resolv
> class (and the Dnsruby gem), which expose these operations through
> class methods, would never recover in case of /etc/hosts being
> updated. This is not a delay in propagation, but actually incorrect
> behavior -- through looking at the source, I didn't see anything that
> would cause the cache to be either invalidated or updated. That in
> itself appears like a bug, so I wanted to see if this was a conscious
> decision and whether there are any plans to address it. Maybe "talk"
> is not the best venue for it?
>
> You are correct, however, to point out that one can work around it by
> reloading the class on every query, but that seems like overkill?

Oh yes, it's definitely overkill. I was just saying how I'd work around 
it if I was faced with a similar problem and needed a solution in the 
face of such a shortcoming. :)

I'd definitely agree that if there is no way to clear the cache short of 
dropping the whole object, then there is an issue/shortcoming/bug.

Another thing to bear in mind is that the fault might not be in Resolv 
(it might be, I'm not familiar with it). On Linux, for example, 
gethostbyname (apparently) goes through nscd, which is providing its own 
caching. I know there have been times in the past when I've disabled 
that, where the caching has caused more problems than it solved. In this 
case, even being able to drop the Resolv cache wouldn't be enough, 
because it is layered on something else that is providing caching. Now 
this is all thoeretical, but I'm just bringing up something that might 
be happening. Is the fault actually with Resolv? If not, you're going to 
find where the problem occurs, and find a way around that issue. If yes, 
it does seem like the lack of ability to drop the cache is either a 
shortcoming or bug. But in either case, what to do? Suppose no immediate 
fix is available, or you need something that works generally. You might 
need to add some logic to your app to handle the additional hard 
requirement you have (immediate update if /etc/hosts changes), to ensure 
that no matter the details of the underlying implementation, your app 
behaves as it should. The audience for Resolv might be more geared 
toward simpler use cases employed by browsers and net apps, where 
ongoing indefinite caching is sufficient.

Having said that- I definitely don't want to derail any intended 
discussion on any shortcomings of the Resolv class. Please don't take it 
that way. :) I'm just running through some possible concerns and 
solutions. I'm not suggesting that Resolv is completely fine and 
shouldn't be changed. From what you've described, it sounds like there 
is an issue there in there that needs to be addressed- being able to 
drop the cache at a minimum, and detecting source changes (eg. 
/etc/hosts) at best.

Cheers,
Garth