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Re: Automatically populate resolv.conf when new DNS is acquired

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
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

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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