Autamatically populate resolv.conf when new DNS is acquired

Autamatically populate resolv.conf when new DNS is acquired

[john doe]

Hi,

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?

-- 
John Doe

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

[Brian Inglis]

[-- Attachment #1: Type: text/plain, Size: 1372 bytes --]

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

[-- Attachment #2: resolv.awk --]
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#!/usr/bin/awk -f
# resolv.awk - create Windows resolv.conf from ipconfig /all output

				{ sub( /\r/, "", $NF) }	# trim \r

# collect DNS domain suffixes
/D[Nn][Ss]\sSuffix[^:]*:\s\S/	{ domain[$NF] = $NF }

# collect DNS search suffixes
/Search\sList[^:]*:\s\S/	{ search[$NF] = $NF }

# collect DNS server IP addresses
/DNS\sServers[^:]*:\s\S/	{ dns = 1 }			# enable

dns && $NF ~ /^[0-9.]+$/	{ nameserver[++ns] = $NF }	# collect

dns && $NF !~ /^[0-9.]+$/	{ dns = 0 }			# disable

# output unique resolv.conf entries
END {
    for (n = 1; n <= ns; ++n)	{ print "nameserver", nameserver[n] }

    for (d in domain)		{ print "domain", domain[d] }

    p = "search"

    for (s in search)		{
	printf "%s %s", p, search[s]
	p = ""
    }

    if (!p)			{ print p }
}


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

[john doe]

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

[Brian Inglis]

On 2018-06-27 03:31, john doe wrote:
> Good morning  Brian, bottom posting.
> By Windows Admin you mean "Windows Admin Center"?

Line wrapped - Windows Admin Tools - on W10 - or search for event (log) viewer

> 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.

May be Administrative Tools on W7

> 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".

If you're using DHCP it could potentially change when your lease expires,
sometimes a couple of days, (depends on your router settings or ISP: my external
IP changes every few months, internal and DNS never) or if on WiFi, when you
connect to a new AP.

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

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

[Andrey Repin]

Greetings, john doe!

> 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.

Looks like an upstream bug. /etc/resolv.conf is not a required file for DNS
resolution.

> 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?



-- 
With best regards,
Andrey Repin
Wednesday, June 27, 2018 10:44:06

Sorry for my terrible english...


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

[john doe]

On 6/27/2018 9:45 AM, Andrey Repin wrote:
> Greetings, john doe!
> 
>> 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.
> 
> Looks like an upstream bug. /etc/resolv.conf is not a required file for DNS
> resolution.
> 

Yes, I have also contacted the dirmngr developer regarding that issue.
But I have to find a work around in the meantime! :)

-- 
John Doe

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