[ruby-core:102614] [Ruby master Bug#17658] DNS resolution failure with multiple named resolvers

[mcarpenter@free.fr]

Issue #17658 has been reported by mcarpenter (Martin Carpenter).

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Bug #17658: DNS resolution failure with multiple named resolvers
https://bugs.ruby-lang.org/issues/17658

* Author: mcarpenter (Martin Carpenter)
* Status: Open
* Priority: Normal
* ruby -v: ruby 2.7.2p137 (2020-10-01 revision 5445e04352) [x86_64-linux]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------
## Description

I created a `Resolv::DNS` resolver with two nameservers described using their domain names. Calling `#getresources` on this for an `A` or `NS` (and possibly other resource types) of domain `example.com` returns an empty list (no results, no error). I expected it to return the corresponding resource record (IP of `example.com`).

Instead if the `Resolv::DNS` instance is created with one resolver domain name or two resolvers' IP addresses then correct results are obtained. The error occurs only when more than two nameservers [or possibly more] are specified using their domain names.

## Testcases

I used two well-known public DNS servers to test (they have funky but legitimate TLDs):
 * dns.google (8.8.4.4, 8.8.8.8)
 * one.one.one.one (1.1.1.1, 1.0.0.1)

### ✗ Failing: two nameservers by name
```ruby
require 'resolv'
Resolv::DNS.new({nameserver: ['dns.google', 'one.one.one.one']}).getresources('example.com', Resolv::DNS::Resource::IN::A)
=> []
```

### ✓ Successful: one nameserver by name
(either as a single-item list or a string)
```ruby
Resolv::DNS.new({nameserver: 'dns.google'}).getresources('example.com', Resolv::DNS::Resource::IN::A)
=> [#<Resolv::DNS::Resource::IN::A:0x000055ee95df9ad8 @address=#<Resolv::IPv4 93.184.216.34>, @ttl=16840>]
Resolv::DNS.new({nameserver: ['one.one.one.one']}).getresources('example.com', Resolv::DNS::Resource::IN::A)
=> [#<Resolv::DNS::Resource::IN::A:0x000055ee95517c80 @address=#<Resolv::IPv4 93.184.216.34>, @ttl=77160>]
```

### ✓ Successful: one nameserver by IP
(either as a single-item list or a string)
```ruby
Resolv::DNS.new({nameserver: ['8.8.4.4']}).getresources('example.com', Resolv::DNS::Resource::IN::A) 
=> [#<Resolv::DNS::Resource::IN::A:0x000055ee95e28d38 @address=#<Resolv::IPv4 93.184.216.34>, @ttl=20931>]
Resolv::DNS.new({nameserver: '1.1.1.1'}).getresources('example.com', Resolv::DNS::Resource::IN::A)
=> [#<Resolv::DNS::Resource::IN::A:0x000055ee95e41360 @address=#<Resolv::IPv4 93.184.216.34>, @ttl=66081>]
```

### ✓ Successful: two nameservers by IP
```ruby
Resolv::DNS.new({nameserver: ['8.8.4.4', '1.1.1.1']}).getresources('example.com', Resolv::DNS::Resource::IN::A) 
=> [#<Resolv::DNS::Resource::IN::A:0x000055ee95d62ea8 @address=#<Resolv::IPv4 93.184.216.34>, @ttl=20894>]
```

## A little analysis

I didn't get to the bottom of this but this may help:
 * When only one nameserver is used then a `Resolv::DNS::Requester::ConnectedUDP::Sender` is used; when two nameservers are used an `UnconnectedUDP::Sender` is used.
 * The code appears to be timing out and then retried even though correct requests are made and responses received.
 * In particular, if I packet trace 53/udp then the conversation is exactly as I would expect and near identical in both failing and successful cases. That is:
   * Request A record for first nameserver from local resolver
   * Receive IP of first nameserver
   * Request A record of `example.com` from first nameserver
   * Receive IP of `example.com`
   In the successful case, the conversation ends here. In the failing case, the code round-robins between the two nameservers until it finally returns `[]`.


## Test environment

Tested ruby 2.5.0p0 on Red Hat 6 and rubies 2.7.1p83, 2.7.2p137 on Ubuntu 20 from two different networks with identical results.



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