Hi Garth,

On Mar 7, 2013 12:44 AM, "Garthy D" <garthy_lmkltybr / entropicsoftware.com>
wrote:
>
>
> 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 week out-of-date because you are using an ISP that disregards TTLs, and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.
>

You may be right about the audience for the module being geared toward
shorter-lived applications. I did look through both the code and published
API, as well as trying some tests of my own. I did not find a solution to
this caching issue, which is why I looked to the list for insight. Should I
ask the same question on the core ruby list?

> 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.
>

Agreed. I appreciate you trying to help. At this point, however, I'd like
to figure out the best place to figure out what's going on with that gem
(core mailing list?).

> Cheers,
> Garth
>
>