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

Eli Rodriguez riesgo at gmail.com
Tue Aug 27 13:34:17 EDT 2013


Hi Brian,

You can override the resolv.conf search set by the dhclient (at least in
RedHat variants) by adding SEARCH="example.com example2.com etc.com" to
your /etc/sysconfig/network config.

thanks,
--Eli


On Tue, Aug 27, 2013 at 1:23 PM, Eli Rodriguez <riesgo at gmail.com> wrote:

> 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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>


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