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Subject:	Re: Automatically populate resolv.conf when new DNS is acquired
To:	cygwin AT cygwin DOT com
References:	<65242f5d-cc15-0024-47a8-0ce6be81aced AT mail DOT com> <c69b1d0a-74e9-bfe0-7c7e-500499844da5 AT SystematicSw DOT ab DOT ca>
From:	john doe <johndoe65534 AT mail DOT com>
Message-ID:	<7b7e40f8-3f84-4080-8ffa-9ac92b973435@mail.com>
Date:	Wed, 27 Jun 2018 11:31:21 +0200
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Good morning  Brian, bottom posting.

On 6/26/2018 4:18 PM, Brian Inglis wrote:
> On 2018-06-26 04:33, john doe wrote:
>> In gnupg2 the use of dirmngr utility is required to interact with a keyserver.
>> Dirmngr requires that '/etc/resolv.conf' be populated with my name servers.
>> That means that everytime the dns changes (new network ...) I need to manually
>> edit that file.
>> How can I let Cygwin update that file whenever the DNS is changed?
> 
> Attached an awk script to generate resolv.conf from Windows ipconfig /all
> output, run from .cygwin_profile (sourced under Cygwin from login .bash_profile)
> using the stanza below: it only replaces an existing writable /etc/resolv.conf
> when the content changes - touch, chown, chmod /etc/resolv.conf to enable.
> 
> # update /etc/resolv.conf if changed
> c=/etc/resolv.conf
> test -w $c					&& \
> i=$(/usr/bin/which -- ipconfig)			&& \
> r=$(/usr/bin/which -- resolv.awk)		&& \
> t=$(/bin/mktemp -t -- resolv.conf.$$.XXXXXXXX)  && \
> if $i /all | $r      >  $t; then
>      /usr/bin/cmp -s -- $t $c	|| \
>      /bin/cp -fv     -- $t $c
>      /bin/rm -f	    -- $t
> fi
> 
> unset c i r t
> 
> This could be used in a bash script run from a Windows scheduled task when a
> relevant DHCP event occurs: you can find DHCP events by checking Windows Admin
> Tools/Event Viewer/Window Logs/System/Filter Current Log/Event
> Sources/Dhcp-Client,DHCPv6-Client, or a similar PowerShell script.
> 

Thanks for the awk script and the explanation on how to use it! :)

By Windows Admin you mean "Windows Admin Center"?
I'm using Cygwin on a laptop (win 7 pro) and sadly, if I'm not 
mistaking, "Windows Admin" is not available on non-server platform.

If I can't find a way to determine when my DNS changes I can clearly 
emulate an hourly cron job by using "task scheduler".

Many thanks for the task scheduler hint and for your help.

-- 
John Doe

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