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Re: Needless DNS queries

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On Tue, 7 Jun 2022, Greg Wooledge wrote:

> On Tue, Jun 07, 2022 at 11:22:34AM -0400, Dan Ritter wrote:
>>
>> search		Search list for host-name lookup.  By default, the search
>>  [...]
>> 	This may be changed by listing the desired domain search
>> path following the search keyword with spaces or tabs separating the
>> names.  Resolver queries having fewer than ndots dots (default is
>> 1) in them will be attempted using each component of the search path in
>> turn until a match is found.
>
> I've read this paragraph a few times, and as far as I can tell, it's
> simply wrong.
>
Seems right to me:

$ cat /etc/resolv.conf
search home.woodall.me.uk
options ndots:3
nameserver 2001:8b0:bfcd:100:216:3eff:fee0:7102
nameserver 2001:8b0:bfcd:8100:216:3eff:fee1:7102

$ host ipv4.wlan.dirac
ipv4.wlan.dirac.home.woodall.me.uk has address 192.168.3.16
ipv4.wlan.dirac.home.woodall.me.uk has address 192.168.4.16

Change that 3 to a 2 and:

$ host ipv4.wlan.dirac
Host ipv4.wlan.dirac not found: 3(NXDOMAIN)


> If you go down farther in the page and look at:
>
>              ndots:n
>                     Sets a threshold for the number of dots which must appear
>                     in a name given to res_query(3) (see resolver(3))  before
>                     an  initial absolute query will be made.  The default for
>                     n is 1, meaning that if there are any dots in a name, the
>                     name  will  be tried first as an absolute name before any
>                     search list elements are appended to it.  The  value  for
>                     this option is silently capped to 15.
>
> This one says that it simply determines whether the name will be tried
> as is *before* appending the search domain(s) to it, or whether it just
> skips straight to appending the search domains.
>

Doesn't that say that, in my second example it will try ipv4.wlan.dirac.,
get NXDOMAIN, and pass that up the stack.

> My experience, and the OP's experience, suggests that the description in
> the ndots paragraph is correct, and the description in the search paragraph
> is not.
>
> To the best of my knowledge, there isn't any setting to *prevent* the
> appending of search domains to a name, no matter how many dots you put
> in the name.
>
>

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Thread

Needless DNS queries Dieter Rohlfing <dr-erc@gmx.net> - 2022-06-07 17:10 +0200
  Re: Needless DNS queries David Wright <deblis@lionunicorn.co.uk> - 2022-06-07 17:30 +0200
  Re: Needless DNS queries Dan Ritter <dsr@randomstring.org> - 2022-06-07 17:40 +0200
    Re: Needless DNS queries Tim Woodall <debianuser@woodall.me.uk> - 2022-06-07 18:10 +0200
    Re: Needless DNS queries Greg Wooledge <greg@wooledge.org> - 2022-06-07 19:00 +0200
      Re: Needless DNS queries Darac Marjal <mailinglist@darac.org.uk> - 2022-06-07 19:40 +0200
        Re: Needless DNS queries Lee <ler762@gmail.com> - 2022-06-08 00:10 +0200
      Re: Needless DNS queries Tim Woodall <debianuser@woodall.me.uk> - 2022-06-08 00:20 +0200
        Re: Needless DNS queries Lee <ler762@gmail.com> - 2022-06-08 03:00 +0200
          Re: Needless DNS queries Greg Wooledge <greg@wooledge.org> - 2022-06-08 03:10 +0200
            Re: Needless DNS queries Lee <ler762@gmail.com> - 2022-06-08 07:30 +0200
  Re: Needless DNS queries Dieter Rohlfing <dr-erc@gmx.net> - 2022-06-14 15:10 +0200

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