[sf-lug] resolver problem

Bobbie Sellers bliss-sf4ever at dslextreme.com
Fri Apr 8 08:11:15 PDT 2016

On 04/08/2016 07:59 AM, Alex Kleider wrote:
> I want to publicly thank both Michael and Rick for all their 
> suggestions, and especially for hammering home the 'best way to answer 
> questions,' and yes, I am very familiar with the paper co-authored by
> Moen and Raymond.  Your  suggestions in this thread have given me 
> greater insight into how better to comply!  Michael's suggested 
> 'high-level summary table' is to my mind a keeper.
> I'm inclined to agree with Rick's suggestion that it might be best to 
> simply reinstall the OS and be done with it but that would mean never 
> knowing what went wrong.  So my question is: In your minds, is the 
> problem of sufficient interest to pursue?  I'll keep working on it 
> following your suggestions if you deem 'yes.'
> Sincerely,
> Alex
     Well since you are talking re-install then I will toss in my 2 
cents worth.
     Have you tried un-installing and re-installing Firefox?

> On 2016-04-08 02:42, Rick Moen wrote:
>> Quoting Michael Paoli (Michael.Paoli at cal.berkeley.edu):
>>> I'll also mention, with modern webservers and virtual name hosting and
>>> SNI, etc., hitting an http or https URL by IP address, rather than
>>> hostname, may not get same results, even if the site is working
>>> perfectly and as expected.  However in both cases, one should at least
>>> still be able to connect to the site (open TCP connection to the IP
>>> address and port - by name or IP address).
>> Yeah, I just didn't want to get into that fine point.  I figured if Alex
>> test-connected by IP rather than FQDN and reported back 'I got a page
>> back but not what I expected', I'd explain 'That means success, but
>> the NameVirtualHost complication meant the HTTPd didn't return the exact
>> requested site HTML', and such.
>> Sometimes, it's actively bad to cover every little detail, possible edge
>> case, etc., because the resulting verbosity stands in the way of clarity
>> on the essential message.
>>> I'd also be very interested in seeing
>>> tcpdump capture of UDP and TCP traffic to port 53,
>>> while strace is run on each of these commands:
>>> $ curl -I https://github.com/
>>> $ ping github.com
>>> ... each captured and saved separately for those two commands.
>> Alex is amazing, but you're asking for some pretty hard-core stuff from
>> him.  Could happen.  ;->
>> Me, I'd just see if the DNS-related badness observed in the installed
>> distro occurs identically if running the exact same Ubuntu release from
>> live-CD media.  If (as expected) no, then assume the installed OS is
>> screwed up.
>> If the host's installed OS is screwed up, then it's either user dotfiles
>> in homedirs, system conffiles in /etc, or system binaries/libs.
>> Problems with user dotfiles can be cross-checked by testing using a new
>> scratch user.  But if the problem's one of the other two things, then
>> IMO the sysadmin needs to replace the blown system and, in the future,
>> be a great deal more careful what he/she does with root authority
>> (including sudo superuser, etc.).
>> IMO, the key tool to use is simple logic, as usual.  Tally up suspects,
>> attempt to see which can be ruled out, use standard methods for dividing
>> one large problem into multiple smaller problems, and all the usual ways
>> of figuring things out.
>>> Oh, and of course also, good to keep a log of what you observe, try,
>>> change, etc.
>> I don't want to dump on Alex, whom I hold in high regard, but the
>> leading suspect in any system-level damage to a *ix box is the sysadmin
>> -- i.e., actions a superuser took that changed /etc -tree contents or
>> system binaries.  Anyone who doesn't believe that, try absent-mindedly
>> messing around with /etc/pam.d/* or /etc/nsswitch.conf using a text
>> editor and root (or, by implication, sudo superuser) authority. For
>> extra credit, don't keep track of what you change.
>> It's not for nothing that sysadmins adopted configuration management.
>>> So, yes, I find it odd that ping works, but curl fails with an apparent
>>> resolver error - how might it be that it seems one resolves, but the
>>> other doesn't?  That does seem quite odd.  But there's an answer down
>>> there somewhere.
>> Am betting that ping and curl use different syscalls for DNS lookup.
>> Like maybe, one uses the local glibc-provided resolver (DNS client), the
>> other uses a nameserver from /etc/resolv.conf .  Or something like that.
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