Hello:
I am attaching with this email a wireshark capture file
"failure01.pcapng" (size: 80KB). I don't know if I am allowed to attach
files in a mailing list and if you will receive it on your end.
What I did in this experiment:
- I am running my experiment on my laptop that has windows 7
- My laptop (IP 192.168.1.109) is connected wirelessly to my router
(IP 192.168.1.1)
- On my laptop I am running a small program that runs in a loop to
use libcares to resolve www.google.com to its IP address. The loop
has 100 iterations. Each request to libcares to resolve the hostname
to IP has a timeout of 10 seconds.
- On the 95th iteration libcares timesout and fails to resolve
www.google.com to its IP address.
- While performing the experiment I was capturing the traffic with
wireshark. I used a filter of "arp || dns"
- What I see is that on the 95th iteration I get an ICMP error
"Destination unreachable".
- I think it is quite possible to get such an error. Afterall I am
running libcares using UDP and my setup is wireless.
- Am I using libcares wrong ? What is the proper way to use libcares
? I am asking libcares to perform a hostname to IP translation
with a timeout of 10 seconds. Should libcares not try again within
that 10 seconds if it gets an ICMP error while trying to reach the
nameserver ?
Thanks
Richard
On Wed, May 27, 2015 at 3:28 PM, Ben Greear <greearb_at_candelatech.com> wrote:
> On 05/27/2015 12:22 PM, Richard wrote:
> > Thanks for the response.
> >
> > I am using the latest stable release which is: c-ares-1.10.0
> >
> > Is that what I should be using ? Or should I be using the development
> version from github.
>
> Looks like you have a good test case to try it out...so I would try latest
> as well.
>
> If it is still broken, at least it's not some bug that is already fixed
> and it would be worth more time trying to find the issue.
>
> Thanks,
> Ben
>
> >
> > Regards
> > Richard
> >
> > On Wed, May 27, 2015 at 3:07 PM, Ben Greear <greearb_at_candelatech.com
> <mailto:greearb_at_candelatech.com>> wrote:
> >
> > On 05/27/2015 12:02 PM, Richard wrote:
> > > Hello:
> > >
> > > I am sorry if this message is a duplicate. I tried to post this
> same
> > > message a few weeks ago through gmane but I never got a response.
> I am
> > > trying to post again directly via the c-ares mailing list.
> > >
> > > I am a recent libcares user. I hope this is the right forum to
> post this
> > > question. If not I apologize and please let me know what is the
> correct
> > > forum or mailing list to use.
> > >
> > > The project I am working on recently switched to use libcares for
> > > DNS queries. Prior to that we were using the standard
> getaddrinfo()
> > > (see "man 3 getaddrinfo") for name to ip resolution. With
> getaddrinfo()
> > > we could not specify a timeout value to perform the DNS query.
> Hence we
> > > switched to use libcares.
> > >
> > > Our use of libcares is getting some stability issues especially if
> > > we use wireless network. libcares will sometimes fail to resolve
> the
> > > hostname. Libcares will fail to resolve the hostname within our
> timeout
> > > of 60 seconds.
> > >
> > > Let me elaborate:
> > >
> > > - If I do a few DNS queries using libcares then everything is fine.
> > >
> > > - I wrote a profiling tool that does many DNS queries in a loop.
> For
> > > example, we resolve "www.google.com <http://www.google.com> <
> http://www.google.com>" and we do 100 iterations of that
> > > in a loop.
> > >
> > > - If I run that with my laptop that has a wired connection to my
> router
> > > then everything is fine. Name resolution with libcares is rock
> solid
> > > and it does not fail to resolve www.google.com <
> http://www.google.com> <http://www.google.com> to its IP address for
> > > every iteration
> > >
> > > - If I run that with my laptop that has a wireless connection to my
> > > router. Then maybe at the 60th iteration libcares will timeout and
> not
> > > be able to resolve "www.google.com <http://www.google.com> <
> http://www.google.com>"
> > >
> > > There must be something I am doing wrong. I am probably using
> libcares
> > > in a way that it is not supposed to be used. I am wondering how
> other
> > > projects such as curl are using libcares.
> > >
> > > A few things come to my mind:
> > >
> > > [1] By default libcares makes DNS queries using UDP. Should I use
> the
> > > ARES_FLAG_USEVC to specify libcares to use TCP instead ?
> > >
> > > [2] Should I implement caching ? At my application level, should I
> > > cache the name to ip resolution and don't invoke libcares to make
> a DNS
> > > query. Then this leads to the next question of what timeout should
> I
> > > use to evict old entries from my cache.
> > >
> > > Once again, I am probably using libcares in a wrong way. So your
> insight
> > > is much appreciated.
> >
> > I doubt using TCP is a good idea. Maybe cares has issues dealing
> with
> > retransmitting frames, or something like that.
> >
> > First, make sure you are using the latest libcares.
> >
> > Then, maybe a network capture that involves a failure would be
> helpful.
> >
> > You might also try using 'netem' or some other network impairment
> tool to
> > drop frames (and/or add delay) to reproduce this in a more
> controlled environment.
> >
> > Thanks,
> > Ben
> >
> >
> > --
> > Ben Greear <greearb_at_candelatech.com <mailto:greearb_at_candelatech.com
> >>
> > Candela Technologies Inc http://www.candelatech.com
> >
> >
>
>
> --
> Ben Greear <greearb_at_candelatech.com>
> Candela Technologies Inc http://www.candelatech.com
>
>