[TriLUG] How ssh et al resolves domain names...

[Eli Rodriguez]

Hi Brian,

I think Igor gave you the answer.   Update your search in resolv.conf to
include your local domain.

Thanks,
--Eli
On Aug 27, 2013 1:03 PM, "Brian Henning" <bhenning at pineinst.com> wrote:

> Aaron,
>
> Maybe I'm just daft, but I didn't learn anything from that exercise.
> Specifically, you suggested I look for things that ping does that host
> doesn't, but the fact is, ping does less than host does.  Did you mean I
> should be observing what host does that ping doesn't do?
>
> Anyway, host opens lots of libraries, including libdns, libbind9,
> libcrypto,
> locale stuff, etc etc etc.  After all that, it does this:
>
> (...)
> open("/lib64/libresolv.so.2", O_RDONLY) = 3
> (...)
> open("/etc/gai.conf", O_RDONLY)         = -1 ENOENT (No such file or
> directory)
> open("/etc/nsswitch.conf", O_RDONLY)    = 6
> open("/etc/host.conf", O_RDONLY)        = 6
> open("/etc/resolv.conf", O_RDONLY)      = 6
> open("/lib64/libnss_files.so.2", O_RDONLY) = 6
> open("/etc/hosts", O_RDONLY)            = 6
> open("/etc/hosts", O_RDONLY)            = 6
> open("/lib64/libnss_dns.so.2", O_RDONLY) = 6
>
> Ping, on the other hand, does only this:
> open("/lib64/libresolv.so.2", O_RDONLY) = 3
> open("/lib64/libc.so.6", O_RDONLY)      = 3
> open("/etc/resolv.conf", O_RDONLY)      = 3
> open("/etc/nsswitch.conf", O_RDONLY)    = 3
> open("/lib64/libnss_files.so.2", O_RDONLY) = 3
> open("/etc/host.conf", O_RDONLY)        = 3
> open("/etc/hosts", O_RDONLY)            = 3
> open("/lib64/libnss_dns.so.2", O_RDONLY) = 3
>
> (note that I've removed references to ld.so.cache)
>
> Aside from some minor differences in the order of reading config files,
> they
> look strikingly similar.  Both link to libnss_files, then libnss_dns, which
> I presume corresponds to this line in nsswitch.conf:
>
> hosts: files dns
>
> What I seem to observe is that both commands finish their search for a host
> by invoking the methods in libnss_dns.  That leaves me where I started,
> thinking "these should be finding their DNS info the same way but are
> getting different results."
>
> I read up on what could be in /etc/host.conf (mine is an empty file), and
> none of the options described seemed like they would cause this by being
> left to default, or remedy it by being set.
>
> So, sadly, the penny has yet to drop...  I can haz another clue?
>
> Thanks,
> ~Brian
>
>
> -----Original Message-----
> From: trilug-bounces at trilug.org [mailto:trilug-bounces at trilug.org] On
> Behalf
> Of Aaron Joyner
> Sent: Tuesday, August 27, 2013 10:18 AM
> To: Triangle Linux Users Group General Discussion
> Subject: Re: [TriLUG] How ssh et al resolves domain names...
>
> As an exercise, try the following commands, and see what you can glean
> about what those commands are doing:
>
> strace host mirawall -c1 2>&1 | grep open
> strace ping mirawall -c1 2>&1 | grep open
>
> Some of the things that show up in the latter, that don't show up in the
> former, will be of interest to you.
>
> Please do write back and summarize what you learn,
> Aaron S. Joyner
>
>
>
>
> --
> This message was sent to: Eli Rodriguez <riesgo at gmail.com>
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