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Reply-To: Brian DOT Inglis AT SystematicSw DOT ab DOT ca
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>
From: Brian Inglis <Brian DOT Inglis AT SystematicSw DOT ab DOT ca>
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Message-ID: <c69b1d0a-74e9-bfe0-7c7e-500499844da5@SystematicSw.ab.ca>
Date: Tue, 26 Jun 2018 08:18:51 -0600
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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.

-- 
Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada

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