Win 11 Cygwin dns-utils "dig" and "host": Option -6 causes command to timeout

Win 11 Cygwin dns-utils "dig" and "host": Option -6 causes command to timeout

[Jim Garrison]

Win 11 Cygwin "dig" and "host": Option -6 causes command to hang

TL;DR
-----

It appears "dig" and "host" both depend on the existence of 
/etc/resolv.conf, but only when the -6 option (send queries over IPv6) 
is specified.

/etc/resolv.conf does not exist on either my old Win 10 or relatively 
new Win 11 systems.

I have a vague recollection that at some point, maybe Windows 7 or even 
XP, there was a Cygwin /etc/resolv.conf that was a link to 
C:\windows\system32\drivers\etc\resolv.conf, but I don't think this has 
been true for quite a while if it ever was.  Also resolv.conf does not 
exist in the windows location either.

Details
-------

Sample:

     # Default query over IPv4
     $ host google.com
     google.com has address 74.125.197.102
     google.com has address 74.125.197.100
     google.com has address 74.125.197.138
     google.com has address 74.125.197.101
     google.com has address 74.125.197.139
     google.com has address 74.125.197.113
     google.com has IPv6 address 2607:f8b0:400e:c03::64
     google.com has IPv6 address 2607:f8b0:400e:c03::71
     google.com has IPv6 address 2607:f8b0:400e:c03::65
     google.com has IPv6 address 2607:f8b0:400e:c03::66
     google.com mail is handled by 10 smtp.google.com.


     # Query over IPv6
     $ host -6 google.com
     ;; connection timed out; no servers could be reached

     # Query over IPv6 with explicit DNS server
     $ host -6 google.com 2601:xxx:xxx:xxx:201:2eff:fe70:3bfe
     Using domain server:
     Name: 2601:xxx:xxx:xxx:201:2eff:fe70:3bfe
     Address: 2601:xxx:xxx:xxx:201:2eff:fe70:3bfe#53
     Aliases:

     google.com has address 74.125.197.139
     google.com has address 74.125.197.113
     google.com has address 74.125.197.102
     google.com has address 74.125.197.100
     google.com has address 74.125.197.138
     google.com has address 74.125.197.101
     google.com has IPv6 address 2607:f8b0:400e:c03::71
     google.com has IPv6 address 2607:f8b0:400e:c03::65
     google.com has IPv6 address 2607:f8b0:400e:c03::66
     google.com has IPv6 address 2607:f8b0:400e:c03::64
     google.com mail is handled by 10 smtp.google.com.

I ran the failing command above in strace and saw that it's trying to 
open /etc/resolv.conf

     [...]
       206  515079 [main] host 2426 open: open(/etc/resolv.conf, 0x0)
        58  515137 [main] host 2426 normalize_posix_path: src 
/etc/resolv.conf
        59  515196 [main] host 2426 normalize_posix_path: 
/etc/resolv.conf = normalize_posix_path (/etc/resolv.conf)
        63  515259 [main] host 2426 mount_info::conv_to_win32_path: 
conv_to_win32_path (/etc/resolv.conf)
        68  515327 [main] host 2426 mount_info::cygdrive_win32_path: src 
'/etc/resolv.conf', dst ''
        61  515388 [main] host 2426 mount_info::conv_to_win32_path: 
src_path /etc/resolv.conf, dst C:\cygwin64\etc\resolv.conf, flags 
0x30008, rc 0
        81  515469 [main] host 2426 symlink_info::check: 0xC0000034 = 
NtCreateFile (\??\C:\cygwin64\etc\resolv.conf)
        82  515551 [main] host 2426 symlink_info::check: 0xC0000034 = 
NtQueryInformationFile (\??\C:\cygwin64\etc\resolv.conf)
        90  515641 [main] host 2426 symlink_info::check: 0xC0000034 = 
NtCreateFile (\??\C:\cygwin64\etc\resolv.conf.exe)
        81  515722 [main] host 2426 symlink_info::check: 0xC0000034 = 
NtQueryInformationFile (\??\C:\cygwin64\etc\resolv.conf.exe)
        90  515812 [main] host 2426 symlink_info::check: 0xC0000034 = 
NtCreateFile (\??\C:\cygwin64\etc\resolv.conf.lnk)
        81  515893 [main] host 2426 symlink_info::check: 0xC0000034 = 
NtQueryInformationFile (\??\C:\cygwin64\etc\resolv.conf.lnk)
        88  515981 [main] host 2426 symlink_info::check: 0xC0000034 = 
NtCreateFile (\??\C:\cygwin64\etc\resolv.conf.exe.lnk)
        82  516063 [main] host 2426 symlink_info::check: 0xC0000034 = 
NtQueryInformationFile (\??\C:\cygwin64\etc\resolv.conf.exe.lnk)
        81  516144 [main] host 2426 symlink_info::check: 0 = 
symlink.check(C:\cygwin64\etc\resolv.conf, 0x7FFFFB4B0) (mount_flags 
0x30008, path_flags 0x0)
        64  516208 [main] host 2426 mount_info::conv_to_win32_path: 
conv_to_win32_path (/etc)
        59  516267 [main] host 2426 mount_info::cygdrive_win32_path: src 
'/etc', dst ''
        60  516327 [main] host 2426 mount_info::conv_to_win32_path: 
src_path /etc, dst C:\cygwin64\etc, flags 0x30008, rc 0
        69  516396 [main] host 2426 symlink_info::check: 0x0 = 
NtCreateFile (\??\C:\cygwin64\etc)
       233  516629 [main] host 2426 symlink_info::check: not a symlink
        65  516694 [main] host 2426 symlink_info::check: 0 = 
symlink.check(C:\cygwin64\etc, 0x7FFFFB4B0) (mount_flags 0x30008, 
path_flags 0x0)
        62  516756 [main] host 2426 path_conv::check: 
this->path(C:\cygwin64\etc\resolv.conf), has_acls(1)
        61  516817 [main] host 2426 build_fh_pc: fh 0x80000BE18, dev 
000000C3
        60  516877 [main] host 2426 fhandler_base::open: 
(\??\C:\cygwin64\etc\resolv.conf, 0x0)
        89  516966 [main] host 2426 seterrno_from_nt_status: 
/cygdrive/d/a/scallywag/cygwin/cygwin-3.5.0-1.x86_64/src/newlib-cygwin/winsup/cygwin/fhandler/base.cc:717 
status 0xC0000034 -> windows error 2
        75  517041 [main] host 2426 geterrno_from_win_error: windows 
error 2 == errno 2
        59  517100 [main] host 2426 fhandler_base::open: 0xC0000034 = 
NtCreateFile (0x7FF84CA3A060, 0x80100000, 
\??\C:\cygwin64\etc\resolv.conf, io, NULL, 0x0, 0x7, 0x1, 0x4020, NULL, 0)
        85  517185 [main] host 2426 fhandler_base::open: 0 = 
fhandler_base::open(\??\C:\cygwin64\etc\resolv.conf, 0x0)
        90  517275 [main] host 2426 fhandler_base::open_fs: 0 = 
fhandler_disk_file::open(\??\C:\cygwin64\etc\resolv.conf, 0x0)
        90  517365 [main] host 2426 open: -1 = open(/etc/resolv.conf, 
0x0), errno 2
     [...]

I confirmed this was the problem by creating /etc/resolv.conf (within 
the Cygwin root, not a symlink) and included the search domain and IPv4 
and IPv6 dns servers, after which the -6 option no longer timed out.

-- 
Jim Garrison
jhg@acm.org

Re: Win 11 Cygwin dns-utils "dig" and "host": Option -6 causes command to timeout

[Brian Inglis]

On 2024-02-14 16:28, Jim Garrison via Cygwin wrote:
> Win 11 Cygwin "dig" and "host": Option -6 causes command to hang
> 
> TL;DR
> -----
> 
> It appears "dig" and "host" both depend on the existence of /etc/resolv.conf, 
> but only when the -6 option (send queries over IPv6) is specified.
> 
> /etc/resolv.conf does not exist on either my old Win 10 or relatively new Win 11 
> systems.
> 
> I have a vague recollection that at some point, maybe Windows 7 or even XP, 
> there was a Cygwin /etc/resolv.conf that was a link to 
> C:\windows\system32\drivers\etc\resolv.conf, but I don't think this has been 
> true for quite a while if it ever was.  Also resolv.conf does not exist in the 
> windows location either.
> 
> Details
> -------
> 
> Sample:
> 
>      # Default query over IPv4
>      $ host google.com
>      google.com has address 74.125.197.102
>      google.com has address 74.125.197.100
>      google.com has address 74.125.197.138
>      google.com has address 74.125.197.101
>      google.com has address 74.125.197.139
>      google.com has address 74.125.197.113
>      google.com has IPv6 address 2607:f8b0:400e:c03::64
>      google.com has IPv6 address 2607:f8b0:400e:c03::71
>      google.com has IPv6 address 2607:f8b0:400e:c03::65
>      google.com has IPv6 address 2607:f8b0:400e:c03::66
>      google.com mail is handled by 10 smtp.google.com.
> 
> 
>      # Query over IPv6
>      $ host -6 google.com
>      ;; connection timed out; no servers could be reached

Ditto!

Without IPv6 nameservers in /etc/resolv.conf I get:

$ host -6 google.com
;; connection timed out; no servers could be reached
Failing assertion due to probable leaked memory in context 0xa00020be0 ("dig") 
(stats[4].gets == 1).
mem.c:1121: INSIST(ctx->stats[i].gets == 0U) failed.

but with IPv6 nameservers in /etc/resolv.conf I get:

$ host -6 google.com
google.com has address 142.251.211.238
google.com has IPv6 address 2607:f8b0:400a:804::200e
google.com mail is handled by 10 smtp.google.com.

>      # Query over IPv6 with explicit DNS server
>      $ host -6 google.com 2601:xxx:xxx:xxx:201:2eff:fe70:3bfe
>      Using domain server:
>      Name: 2601:xxx:xxx:xxx:201:2eff:fe70:3bfe
>      Address: 2601:xxx:xxx:xxx:201:2eff:fe70:3bfe#53
>      Aliases:
> 
>      google.com has address 74.125.197.139
>      google.com has address 74.125.197.113
>      google.com has address 74.125.197.102
>      google.com has address 74.125.197.100
>      google.com has address 74.125.197.138
>      google.com has address 74.125.197.101
>      google.com has IPv6 address 2607:f8b0:400e:c03::71
>      google.com has IPv6 address 2607:f8b0:400e:c03::65
>      google.com has IPv6 address 2607:f8b0:400e:c03::66
>      google.com has IPv6 address 2607:f8b0:400e:c03::64
>      google.com mail is handled by 10 smtp.google.com.
> 
> I ran the failing command above in strace and saw that it's trying to open 
> /etc/resolv.conf
> 
>      [...]
>        206  515079 [main] host 2426 open: open(/etc/resolv.conf, 0x0)
>         58  515137 [main] host 2426 normalize_posix_path: src /etc/resolv.conf
>         59  515196 [main] host 2426 normalize_posix_path: /etc/resolv.conf = 
> normalize_posix_path (/etc/resolv.conf)
>         63  515259 [main] host 2426 mount_info::conv_to_win32_path: 
> conv_to_win32_path (/etc/resolv.conf)
>         68  515327 [main] host 2426 mount_info::cygdrive_win32_path: src 
> '/etc/resolv.conf', dst ''
>         61  515388 [main] host 2426 mount_info::conv_to_win32_path: src_path 
> /etc/resolv.conf, dst C:\cygwin64\etc\resolv.conf, flags 0x30008, rc 0
>         81  515469 [main] host 2426 symlink_info::check: 0xC0000034 = 
> NtCreateFile (\??\C:\cygwin64\etc\resolv.conf)
>         82  515551 [main] host 2426 symlink_info::check: 0xC0000034 = 
> NtQueryInformationFile (\??\C:\cygwin64\etc\resolv.conf)
>         90  515641 [main] host 2426 symlink_info::check: 0xC0000034 = 
> NtCreateFile (\??\C:\cygwin64\etc\resolv.conf.exe)
>         81  515722 [main] host 2426 symlink_info::check: 0xC0000034 = 
> NtQueryInformationFile (\??\C:\cygwin64\etc\resolv.conf.exe)
>         90  515812 [main] host 2426 symlink_info::check: 0xC0000034 = 
> NtCreateFile (\??\C:\cygwin64\etc\resolv.conf.lnk)
>         81  515893 [main] host 2426 symlink_info::check: 0xC0000034 = 
> NtQueryInformationFile (\??\C:\cygwin64\etc\resolv.conf.lnk)
>         88  515981 [main] host 2426 symlink_info::check: 0xC0000034 = 
> NtCreateFile (\??\C:\cygwin64\etc\resolv.conf.exe.lnk)
>         82  516063 [main] host 2426 symlink_info::check: 0xC0000034 = 
> NtQueryInformationFile (\??\C:\cygwin64\etc\resolv.conf.exe.lnk)
>         81  516144 [main] host 2426 symlink_info::check: 0 = 
> symlink.check(C:\cygwin64\etc\resolv.conf, 0x7FFFFB4B0) (mount_flags 0x30008, 
> path_flags 0x0)
>         64  516208 [main] host 2426 mount_info::conv_to_win32_path: 
> conv_to_win32_path (/etc)
>         59  516267 [main] host 2426 mount_info::cygdrive_win32_path: src '/etc', 
> dst ''
>         60  516327 [main] host 2426 mount_info::conv_to_win32_path: src_path 
> /etc, dst C:\cygwin64\etc, flags 0x30008, rc 0
>         69  516396 [main] host 2426 symlink_info::check: 0x0 = NtCreateFile 
> (\??\C:\cygwin64\etc)
>        233  516629 [main] host 2426 symlink_info::check: not a symlink
>         65  516694 [main] host 2426 symlink_info::check: 0 = 
> symlink.check(C:\cygwin64\etc, 0x7FFFFB4B0) (mount_flags 0x30008, path_flags 0x0)
>         62  516756 [main] host 2426 path_conv::check: 
> this->path(C:\cygwin64\etc\resolv.conf), has_acls(1)
>         61  516817 [main] host 2426 build_fh_pc: fh 0x80000BE18, dev 000000C3
>         60  516877 [main] host 2426 fhandler_base::open: 
> (\??\C:\cygwin64\etc\resolv.conf, 0x0)
>         89  516966 [main] host 2426 seterrno_from_nt_status: 
> /cygdrive/d/a/scallywag/cygwin/cygwin-3.5.0-1.x86_64/src/newlib-cygwin/winsup/cygwin/fhandler/base.cc:717 status 0xC0000034 -> windows error 2
>         75  517041 [main] host 2426 geterrno_from_win_error: windows error 2 == 
> errno 2
>         59  517100 [main] host 2426 fhandler_base::open: 0xC0000034 = 
> NtCreateFile (0x7FF84CA3A060, 0x80100000, \??\C:\cygwin64\etc\resolv.conf, io, 
> NULL, 0x0, 0x7, 0x1, 0x4020, NULL, 0)
>         85  517185 [main] host 2426 fhandler_base::open: 0 = 
> fhandler_base::open(\??\C:\cygwin64\etc\resolv.conf, 0x0)
>         90  517275 [main] host 2426 fhandler_base::open_fs: 0 = 
> fhandler_disk_file::open(\??\C:\cygwin64\etc\resolv.conf, 0x0)
>         90  517365 [main] host 2426 open: -1 = open(/etc/resolv.conf, 0x0), errno 2
>      [...]
> 
> I confirmed this was the problem by creating /etc/resolv.conf (within the Cygwin 
> root, not a symlink) and included the search domain and IPv4 and IPv6 dns 
> servers, after which the -6 option no longer timed out.

I added a Cygwin postinstall script:

	/etc/postinstall/0p_l_etc_resolv_conf.dash

to create resolv.conf from `ipconfig /all` DNS servers, an internal list of open 
public DNS servers, and search domains, update:

	/var/run/resolvconf/resolv.conf

when changed, and `ln -frsvt /etc/`, also run from a Scheduled Task on System 
Restart.

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

La perfection est atteinte                   Perfection is achieved
non pas lorsqu'il n'y a plus rien à ajouter  not when there is no more to add
mais lorsqu'il n'y a plus rien à retirer     but when there is no more to cut
                                 -- Antoine de Saint-Exupéry

RE: [EXTERNAL] Re: Win 11 Cygwin dns-utils "dig" and "host": Option -6 causes command to timeout

[Lavrentiev, Anton (NIH/NLM/NCBI) [C]]

> $ host -6 google.com
> ;; connection timed out; no servers could be reached

FWIW, this hangs just the same on Linux when no IPv6 nameservers configured in /etc/resolv.conf

What it tries to do is to inquire the nameserver at [::1]:53 (which is the local host),
and then, since bind is not running on the host, that times out.

This, however, is looks like a bug to me:

> Failing assertion due to probable leaked memory in context 0xa00020be0 ("dig")
> (stats[4].gets == 1).
> mem.c:1121: INSIST(ctx->stats[i].gets == 0U) failed.

and I can't see the same on Linux.

Anton Lavrentiev
Contractor NIH/NLM/NCBI

Re: Win 11 Cygwin dns-utils "dig" and "host": Option -6 causes command to timeout

[Jim Garrison]

On 2/14/2024 17:08, Brian Inglis via Cygwin wrote:
> On 2024-02-14 16:28, Jim Garrison via Cygwin wrote:
>> Win 11 Cygwin "dig" and "host": Option -6 causes command to hang
>>

[snip]


> Ditto!
> 
> Without IPv6 nameservers in /etc/resolv.conf I get:
> 
> $ host -6 google.com
> ;; connection timed out; no servers could be reached
> Failing assertion due to probable leaked memory in context 0xa00020be0 
> ("dig") (stats[4].gets == 1).
> mem.c:1121: INSIST(ctx->stats[i].gets == 0U) failed.
> 
> but with IPv6 nameservers in /etc/resolv.conf I get:
> 
> $ host -6 google.com
> google.com has address 142.251.211.238
> google.com has IPv6 address 2607:f8b0:400a:804::200e
> google.com mail is handled by 10 smtp.google.com.
> 
[snip]
> 
> I added a Cygwin postinstall script:
> 
>      /etc/postinstall/0p_l_etc_resolv_conf.dash
> 
> to create resolv.conf from `ipconfig /all` DNS servers, an internal list 
> of open public DNS servers, and search domains, update:
> 
>      /var/run/resolvconf/resolv.conf
> 
> when changed, and `ln -frsvt /etc/`, also run from a Scheduled Task on 
> System Restart.

Unfortunately, this is only a partial solution because your delegated 
prefix can change if your ISP changes it.  The firewall knows how to 
track this, and will issue a new LAN IP address at the next lease 
expiration.  But, the firewall is also providing name resolution, and 
after its LAN prefix changes (and the grace period expires) Cygwin's 
name resolultion would be left pointing to a non-existent address.

Also, IPv4 resolution does not seem to depend on resolv.conf, and 
Windows clearly does not depend on resolv.conf for either IPv4 or IPv6. 
Maybe Cygwin should just ask Windows for the name servers?

As a counterpoint I note that on dual stack Linux one DOES need an extra 
daemon (resolvconf or systemd-resolved) to manage merging of IPv4 and 
IPv6 DHCP configurations.  Without one of those, the IPv6 client 
overwrites the resolv.conf created by the IPv4 client.

Maybe Cygwin needs the equivalent of resolvconf?  I found references to 
a file called resolvconf in the sources/devel packages for unbound, but 
unbound itself doesn't seem to contain anything with that name.  Also 
unbound seems to be currently unmaintained.

-- 
Jim Garrison
jhg@acm.org

RE: [EXTERNAL] Re: Win 11 Cygwin dns-utils "dig" and "host": Option -6 causes command to timeout

[Lavrentiev, Anton (NIH/NLM/NCBI) [C]]

> Maybe Cygwin should just ask Windows for the name servers?

Cygwin does ask Windows, by default, when gethostbyname() or getnameinfo() are used (which most applications do).
The lookup does not depend on /etc/resolv.conf unless you configured it to do so in "options" in there.
(That would be "options osquery".)  So generally, it's Windows which actually sends a DNS request
and processes the result back (Cygwin then uses that information).

"host" and "dig" are special beasts, in that respect, as they don't use the Windows DNS query API
but form and send DNS requests on their own, which is why they need a properly configured
/etc/resolv.conf file, to know what servers to use.

HTH,

Anton Lavrentiev
Contractor NIH/NLM/NCBI

RE: [EXTERNAL] Re: Win 11 Cygwin dns-utils "dig" and "host": Option -6 causes command to timeout

[Lavrentiev, Anton (NIH/NLM/NCBI) [C]]

Correction:

> (That would be "options osquery".)

Sorry, I have forgotten a few pieces since last time I worked with that code.

So in the absence of "/etc/resolv.conf", Cygwin uses OS (Windows DNS Query) API.

If /etc/resolv.conf is present, then "options osquery" tells Cygwin to use
the Windows DNS API (and conversely, if that option is not there, Cygwin
resolver sends and processes DNS queries on its own).

The code can be looked at here:

https://www.cygwin.com/cgit/newlib-cygwin/tree/winsup/cygwin/libc/minires.c

In particular, you can use the following options to debug what Cygwin is doing in the resolver:

options debug          # Cygwin own resolver

or

options debug osquery  # Cygwin piggy-backs on Windows

And that's how it is done:
https://www.cygwin.com/cgit/newlib-cygwin/tree/winsup/cygwin/libc/minires-os-if.c

HTH,

Anton Lavrentiev
Contractor NIH/NLM/NCBI

Re: Win 11 Cygwin dns-utils "dig" and "host": Option -6 causes command to timeout

[Brian Inglis]

[-- Attachment #1: Type: text/plain, Size: 3817 bytes --]

On 2024-02-15 10:56, Jim Garrison via Cygwin wrote:
> On 2/14/2024 17:08, Brian Inglis via Cygwin wrote:
>> On 2024-02-14 16:28, Jim Garrison via Cygwin wrote:
>>> Win 11 Cygwin "dig" and "host": Option -6 causes command to hang

>> Ditto!
>>
>> Without IPv6 nameservers in /etc/resolv.conf I get:
>>
>> $ host -6 google.com
>> ;; connection timed out; no servers could be reached
>> Failing assertion due to probable leaked memory in context 0xa00020be0 ("dig") 
>> (stats[4].gets == 1).
>> mem.c:1121: INSIST(ctx->stats[i].gets == 0U) failed.
>>
>> but with IPv6 nameservers in /etc/resolv.conf I get:
>>
>> $ host -6 google.com
>> google.com has address 142.251.211.238
>> google.com has IPv6 address 2607:f8b0:400a:804::200e
>> google.com mail is handled by 10 smtp.google.com.

>> I added a Cygwin postinstall script:
>>
>>      /etc/postinstall/0p_l_etc_resolv_conf.dash
>>
>> to create resolv.conf from `ipconfig /all` DNS servers, an internal list of 
>> open public DNS servers, and search domains, update:
>>
>>      /var/run/resolvconf/resolv.conf
>>
>> when changed, and `ln -frsvt /etc/`, also run from a Scheduled Task on System 
>> Restart.

> Unfortunately, this is only a partial solution because your delegated prefix can 
> change if your ISP changes it.  The firewall knows how to track this, and will 
> issue a new LAN IP address at the next lease expiration.  But, the firewall is 
> also providing name resolution, and after its LAN prefix changes (and the grace 
> period expires) Cygwin's name resolultion would be left pointing to a 
> non-existent address.

That is why I run the resolv.conf postinstall script to check and change 
/etc/resolv.conf near the start of Cygwin Setup postinstall, and after Windows 
restart.
You can set up the cron daemon or Scheduled Tasks to do this whenever you need, 
or if you run a change detection script, it could do so: it only changes the 
installed file if the generated file contents differ.

> Also, IPv4 resolution does not seem to depend on resolv.conf, and Windows 
> clearly does not depend on resolv.conf for either IPv4 or IPv6. Maybe Cygwin 
> should just ask Windows for the name servers?

That's what my resolv.conf setup script does with Windows `ipconfig /all` output.

> As a counterpoint I note that on dual stack Linux one DOES need an extra daemon 
> (resolvconf or systemd-resolved) to manage merging of IPv4 and IPv6 DHCP 
> configurations.  Without one of those, the IPv6 client overwrites the 
> resolv.conf created by the IPv4 client.

That's why I create /var/run/resolvconf/resolv.conf for compatibility then 
symlink to /etc/resolv.conf (and my /var/run is a compatibility symlink to /run/).

> Maybe Cygwin needs the equivalent of resolvconf?  I found references to a file 
> called resolvconf in the sources/devel packages for unbound, but unbound itself 
> doesn't seem to contain anything with that name. Also unbound seems to be 
> currently unmaintained.

Windows does not discriminate between IP v4 and v6 DNS name servers, but 
unfortunately Cygwin does not support a/hosts/v4/v6 enumeration.

Have a look at the attached scripts, try them out on your system, and adapt them 
to do what you want with them: treat them as Public Domain; they have some 
tweaks to support some open public DNS name servers as backups for my ISP's, and 
tweaks for that ISP and my preferences, but unsure if "search" list suffixes 
work or affect anything?

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

La perfection est atteinte                   Perfection is achieved
non pas lorsqu'il n'y a plus rien à ajouter  not when there is no more to add
mais lorsqu'il n'y a plus rien à retirer     but when there is no more to cut
                                 -- Antoine de Saint-Exupéry

[-- Attachment #2: cyg-resolv.awk --]
[-- Type: text/plain, Size: 5449 bytes --]

#!/usr/bin/awk -f
# cyg-resolv.awk - create Cygwin resolv.conf from Windows ipconfig /all output

BEGIN {
# ISP domains
    isp_domains	= "shaw[^.]*[.]|sjrb[.]"
    isp		= "shaw.ca."
# suffixes
    SUF  = "ca. uk. org. com. net. edu."
# public name servers Quad 9, Cloudflare, Open DNS, Google, Comodo
    NS		= "dns.quad9.net. one.one.one.one. "			\
		"resolver2.opendns.com. resolver1.opendns.com. "	\
		"google-public-dns-b.google.com. google-public-dns-a.google.com. " \
		"ns2.recursive.dnsbycomodo.com. ns1.recursive.dnsbycomodo.com. "   \
		""
# CleanBrowsing
#	    "security-filter-dns.cleanbrowsing.org. "			\
#	    "adult-filter-dns.cleanbrowsing.org. "			\
#	    "family-filter-dns.cleanbrowsing.org. "			\
# CIRA Canadian Shield Protected+malware+phishing
#	    "CAshieldProtected"						\
# resolv.conf comments
    CFHD = "# /etc/resolv.conf"
    CFHD = CFHD " - Internet Domain Name System resolver configuration file"
    CDS  = "# domain suffix"
    CSSL = "# suffix search list"
# hosts command
    hosts = "/usr/bin/getent ahosts"
# IP v4
    # digit patterns: [0-9], [1-9], [6-9], [0-4], [0-5], [0[0]]0-255, .[0[0]]0-255, .[0]16-31
    z9		= "[0-9]"
    o9		= "[1-9]"
    s9		= "[6-9]"
    z4		= "[0-4]"
    z5		= "[0-5]"
    p0_255	= "(0{0,2}" z9 "|0{0,1}" o9 z9 "|1" z9 z9 "|2(" z4 z9 "|5" z5 "))"
    d0_255	= "(." p0_255 ")"
    d16_31	= ".0?(1" s9 "|2" z9 "|3[01])"
    ipv4	= "^" p0_255 d0_255 "{3}$"
# private subnets
    # 10.0.0.0-.255.255.255
    p10		= "^10" d0_255 "{3}$"
    # 169.254.0.0-.255.255 Zeroconf Link-Local (IPv4LL) Automatic Private IP Addressing (APIPA) 
    p169	= "^169.254" d0_255 "{2}$"
    # 172.16.0.0-.31.255.255
    p172	= "^172" d16_31 d0_255 "{2}$"
    # 192.168.0.0-.255.255
    p192	= "^192.168" d0_255 "{2}$"
# IP v6
    # hex, id
    px		= "[0-9A-Fa-f]"
    gx		= px "{0,4}"
    cx		= ":" gx
    id		= "[0-9A-Za-z]"
    ipv6	= "^" gx "(" cx "){1,7}" "(%" id "+)?$"
# private subnets
    # fc00::/7	fc-fe + ff bit 7 local
    pfc00	= "^[Ff][C-Fc-f]" px "{2}:"
# seen to avoid duplication
    used["127.0.0.1"] = "127.0.0.1"
}


function nextaddr( addr, name, ns, nameserveraddr, nameserverhostname	,private,n) {
    if (DEBUG) print addr, name, ns > DEBUG

    # private subnets
    if (!(addr in used || addr ~ p10 || addr ~ p169 || addr ~ p172 || addr ~ p192 || addr ~ pfc00)) {
	n = ++ns

	if (addr)   { nameserveraddr[n]	= addr }

	if (name)   { nameserverhostname[n] = name }

	if (DEBUG) print addr, name, ns > DEBUG

	used[addr] = addr
    }

    return ns
}


function addrs( NS, nameserveraddr, ns	,c) {
# $ getent ahosts $NS
# 1.0.0.1         STREAM      one.one.one.one
# 1.0.0.1         DGRAM       one.one.one.one
# 1.1.1.1         STREAM
# 1.1.1.1         DGRAM
# 149.112.112.112 0      dns.quad9.net
# 9.9.9.9         0
# 208.67.220.220  0      resolver2.opendns.com
# 208.67.222.222  0      resolver1.opendns.com
# 8.8.4.4         0      google-public-dns-b.google.com
# 8.8.8.8         0      google-public-dns-a.google.com
# 149.112.122.20  0      CAshieldProtected      
# 149.112.121.20  0
    c = hosts " " NS

    while ((c | getline) > 0) {
	    ns = nextaddr( $1, $3, ns, nameserveraddr, nameserverhostname)
    }

    close( c )

    return ns
}


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


# collect DNS domain suffixes
/D[Nn][Ss]\sSuffix[^:]*:\s\S/	{
    last = $NF
    if (last ~ /\./ && last !~ /\.$/)	last = last ".";	# add root dot

    if (!(last in domain)) {
	domain[last] = last
	domains = domains " " last
    }

    while (last ~ /\..+\..+/) {	# strip labels if more than two for domain
	sub(/^[^.]+./, "", last)
	if (last ~ /\./ && last !~ /\.$/)	last = last "."; # add root dot

	if (!(last in domain)) {
	    domain[last] = last
	    domains = domains " " last
	}
    }
}


# collect DNS search suffixes
/Search\sList[^:]*:\s\S/	{
    for (d in domain) {
	if (!(d in search))	search[d] = d

	if (d ~ isp_domains) {
	    last = isp

	    if (!(last in search)) {
		search[last] = last
		domains = domains " " last
	    }
	}
    }

    last = $NF
    if (last ~ /\./ && last !~ /\.$/)	last = last ".";	# add root dot

    if (!(last in search)) {
	search[last] = last
	domains = domains " " last
    }

    while (last ~ /\..+\..+/) {	# strip labels if more than two for domain
	sub(/^[^.]+./, "", last)
	if (last ~ /\./ && last !~ /\.$/)	last = last "."; # add root dot

	if (!(last in search)) {
	    search[last] = last
	    domains = domains " " last
	}
    }

    ns = split( SUF, sa)

    for (s = 1; s <= ns; ++s) {
	last = sa[s]
	if (last ~ /\./ && last !~ /\.$/)	last = last "."; # add root dot

	if (!(last in search)) {
	    search[last] = last
	    domains = domains " " last
	}
    }
}


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


dns && ($NF ~ ipv4 || $NF ~ ipv6)	{		# collect IP addrs
    ns = nextaddr( $NF, last, ns, nameserveraddr, nameserverhostname)
    last = ""
}


dns && $NF !~ ipv6 && $NF !~ ipv4	{ dns = 0 }	# non-IP disable


# output unique resolv.conf entries
END {
    print CFHD

    ns = addrs( NS, nameserveraddr, ns)

    for (n = 0; n <= ns; ++n)	{
	if (n in nameserverhostname && nameserverhostname[n]) {
	    print "#", nameserverhostname[n]
	}

	if (n in nameserveraddr && nameserveraddr[n]) {
	    print "nameserver", nameserveraddr[n]
	}
    }

    print CDS

    for (d in domain)		{ print "domain " d }

    print CSSL
    if (domains)	print "search" domains
}


[-- Attachment #3: 0p_l_etc_resolv_conf.dash --]
[-- Type: text/plain, Size: 527 bytes --]

#!/bin/dash
# 0p_l_etc_resolv_conf.dash - update /etc/resolv.conf if changed

r=resolv
cr=/usr/local/bin/cyg-$r.awk
run=/var/run/${r}conf
rrc=$run/$r.conf
SYSCONFDIR=${SYSCONFDIR:-/etc}
conf=$SYSCONFDIR/$r.conf

/bin/mkdir -pv		-- $run/		&& \
ip=$(/usr/bin/which	-- ipconfig)		&& \
tmp=$(/bin/mktemp -t	-- .XXXXXXXX.)		&& \
$ip /all | $cr	 > $tmp				&& \
[ -s $tmp ]					&& \
[ -w $rrc ]	|| : > $rrc
if ! /usr/bin/cmp -s	-- $tmp $rrc; then
    /bin/cp -fv		-- $tmp $rrc
    /bin/ln -frsTv	-- $rrc $conf
fi

/bin/rm -f		-- $tmp