[sf-lug] resolver problem

Michael Paoli Michael.Paoli at cal.berkeley.edu
Thu Apr 7 00:26:40 PDT 2016

A few more divide & conquer things you could try ...

So ... works with one browser, not another, works with ping,
but not git?  Efficient divide & conquer typically involves
devising series of sufficiently easy/feasible tests, that
effectively divide possible causes into two sets of
roughly approximate probability, and results of test
rule one set in, and the other out - or at least
significantly assist in determining probability - after test -
between the two sets.  One can also think of whittling it down
to the simplest test that can show the "works" vs. "doesn't work" -
at which point it's generally obvious what the problem is, or at least
exactly where the problem is.

So ... "browsers" ... & http & https.

What about wget and/or curl?

$ curl -I https://www.google.com/
$ curl -I https://github.com/
$ wget -q -O - https://www.google.com/ | head -c 256; echo
$ wget -q -O - https://github.com/ | head -c 256; echo

> From: "Michael Paoli" <Michael.Paoli at cal.berkeley.edu>
> Subject: Re: [sf-lug] resolver problem
> Date: Wed, 06 Apr 2016 15:11:16 -0700

> So ... continue divide & conquer ...
> What about if the misbehaving browser is launched fresh under another
> user on the host - one that's not launched that browser before, or
> has had all user configuration saved state stuff from the browser
> totally removed first?  Does that other user experience the same
> problem, or not?
> If that user doesn't get same problem, then it's likely limited to
> user state/configuration information stored by the browser.
> If other user gets same problem, and it's continuing to work in
> one browser, but not the other, how 'bout use of tcpdump to look at
> DNS traffic, http traffic, and https traffic.  Most notably when
> the browser attempts are made, how to the two differ in what they
> do/don't attempt to retrieve from DNS (and other resolver bits before
> that), and what they do and don't attempt to connect to (or if they
> fail prior to that due to failure to resolve).
> What about the git command that's failing?  Same failure if run by
> another user on the same host?
> Also, when you boot from the (I presume it's actually DVD) Ubuntu
> 14.04[.x] ISO, what does it have for configuration data in /etc/resolv.conf
> and what does it have for hosts in /etc/nsswitch.conf ?
> Also, check on the host, in recent system logs - anything particularly
> unusual or indicative of a problem?  Likewise look at the output of
> dmesg - most notably at/towards the tail end of it, for anything that
> may be indicative of a problem.
> If it works with one user, and not the other, one may also want to inspect
> various environment and configuration settings. especially if the difference
> between working and not extends beyond per-user saved browser state file
> data - e.g. if also git fails for one user, but works for the other user.
> https://en.wikipedia.org/wiki/Divide_and_conquer_algorithms
>> From: "Alex Kleider" <akleider at sonic.net>
>> Subject: Re: [sf-lug] resolver problem
>> Date: Wed, 06 Apr 2016 16:22:05 +0000
>> I shutdown, removed the hard drive and booted from the ubuntu CD:
>> everything works as expected-
>> I assume it is safe to say that what ever the problem is, it's to do with
>> something wrong on my hard drive- presumable some configuration somewhere,
>> not a hardware issue.
>> Before doing the above, I checked firefox vs chromium again
>> and the exact same urls complete with he protocol part at the beginning
>> were resolved by the latter, not by the former.

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