Issue #17748 has been updated by rhenium (Kazuki Yamaguchi).


I was able to reproduce the issue on my Linux box and was digging into this. There actually seem to be two issues behind.

 - If a search list is not given to Resolv::DNS and also the local domain name of the system is not set (Socket.gethostname is not fully qualified), the same query for the domain name is generated twice.

    https://github.com/ruby/resolv/pull/8

 - The change in [Bug #12838] didn't fully resolve the root issue.

    https://github.com/ruby/resolv/pull/9

----------------------------------------
Bug #17748: Ruby 3.0 takes a long time to resolv DNS of nonexistent domains
https://bugs.ruby-lang.org/issues/17748#change-91100

* Author: xdmx (Eric Bloom)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [x86_64-linux]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------
I'm running into a subtle bug when trying to resolv a nonexistent domain.
```
$ ruby -v          
ruby 2.7.2p137 (2020-10-01 revision 5445e04352) [x86_64-linux]
$ irb
require 'resolv'
=> true
t = Time.now ; Resolv::DNS.open { |dns| dns.getresources('thisisaninvaliddomain.com', Resolv::DNS::Resource::IN::A) } ; Time.now - t
=> 0.051897333
```

This works fine and it's fast, but as soon as I try to run this on 3.0:
```
$ rbenv local 3.0.0
$ ruby -v
ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [x86_64-linux]
$ irb
require 'resolv'
=> true
t = Time.now ; Resolv::DNS.open { |dns| dns.getresources('thisisaninvaliddomain.com', Resolv::DNS::Resource::IN::A) } ; Time.now - t
=> 76.314624548
t = Time.now ; Resolv::DNS.open { |dns| dns.getresources('domain.com', Resolv::DNS::Resource::IN::A) } ; Time.now - t
=> 0.081165397
```

In this case the resolver takes a very long time to return a result, and it only happens with nonexistent domains. For those that exist it's running fast.

What's weird is that the only change I do is switching from 2.7 to 3.0, and then I suddenly have this problem.

The above code was run on my local machine (Arch Linux) with Ruby installed through rbenv. I've also double checked it on a completely different machine (server running Ubuntu 18.04) that is connected from a different country and I'm having the exact same problem. In that case Ruby was compiled directly from the source and installed on the server.

So this should potentially exclude any connection issue (as locally with 2.7 is fast, and it was tried in 2 different places), the way it was installed (rbenv vs source) and the distro (Arch vs Ubuntu). I asked someone running macos and ruby 3.0 and it seems that it was fast there, so maybe it's only a linux related bug, I wasn't able to get other people to try.

Resolving the domain with `resolvctl` is also running fast:
```
$ time resolvectl query thisisaninvaliddomain.com
thisisaninvaliddomain.com: resolve call failed: 'thisisaninvaliddomain.com' not found
resolvectl query thisisaninvaliddomain.com  0.00s user 0.01s system 71% cpu 0.009 total
```



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