From: "znz (Kazuhiro NISHIYAMA) via ruby-core" <ruby-core@...>
Date: 2026-11-04T23:59:40+00:00
Subject: [ruby-core:127338] [Ruby Bug#22508] `Resolv.new(use_ipv6: ...)` is silently ignored since resolv 0.6.0 (Ruby 3.4)

Issue #22508 has been reported by znz (Kazuhiro NISHIYAMA).

----------------------------------------
Bug #22508: `Resolv.new(use_ipv6: ...)` is silently ignored since resolv 0.6.0 (Ruby 3.4)
https://bugs.ruby-lang.org/issues/22508

* Author: znz (Kazuhiro NISHIYAMA)
* Status: Open
* ruby -v: ruby 3.4.12 (2026-10-23 revision 366f5fdc00) +PRISM [x86_64-linux]
* Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN
----------------------------------------
Since resolv 0.6.0 (bundled with Ruby 3.4.0), the `use_ipv6:` keyword argument of `Resolv.new` is silently ignored when no positional argument is given. In resolv 0.3.0 (Ruby 3.3) it set `use_ipv6` of the DNS resolver. Is this intended?

```ruby
require 'resolv'
puts "ruby #{RUBY_VERSION}, resolv #{Resolv::VERSION}"

def dns_use_ipv6(r)
  dns = r.instance_variable_get(:@resolvers).find { |x| x.is_a?(Resolv::DNS) }
  cfg = dns.instance_variable_get(:@config)
  cfg.lazy_initialize
  cfg.use_ipv6?
end

p dns_use_ipv6(Resolv.new(use_ipv6: false))
p dns_use_ipv6(Resolv.new(use_ipv6: true))
p dns_use_ipv6(Resolv.new({use_ipv6: false}))
```

```
ruby 3.3.0, resolv 0.3.0
false
true
(NoMethodError: a Hash is not accepted as resolvers in 0.3.0)

ruby 3.4.0, resolv 0.6.0
nil
nil
false

ruby 3.4.12, resolv 0.7.2
nil
nil
false
```

The same `nil` / `nil` / `false` is printed on Ruby 4.0.9 (resolv 0.7.2) and 4.1.0dev (resolv 0.8.1). No warning is printed; the deprecation warning only appears when a positional argument and the keyword are given together.

When `Resolv::DNS::Config#use_ipv6?` is `nil`, `Resolv::DNS#use_ipv6?` falls back to checking `Socket.ip_address_list` for a global IPv6 address, so `Resolv.new(use_ipv6: false).getaddresses(name)` queries AAAA records on an IPv6-capable host, which is what the keyword was meant to turn off.

Background:

- The keyword was added in resolv 0.3.0 ([ruby/resolv#1](https://github.com/ruby/resolv/pull/1), merged 2023-11-24; v0.3.0 was tagged 2023-12-13 and bundled with Ruby 3.3.0): `DNS.new(DNS::Config.default_config_hash.merge(use_ipv6: use_ipv6))`.
- [ruby/resolv#43](https://github.com/ruby/resolv/pull/43) (opened 2023-11-25, merged 2024-12-13, released as resolv 0.6.0 with Ruby 3.4.0) changed `Resolv#initialize` to accept a Hash as `resolvers` and merge it into the DNS config. Since then `use_ipv6:` is only used to decide whether to print the deprecation warning, and only when a positional argument is also given; its value is never passed to `Resolv::DNS`. The PR description says "This is backwards compatible, because there has not yet been a release with the use_ipv6 keyword supported", but 0.3.0 had been released a year before the merge.
- The warning text ("Support for separate use_ipv6 keyword is deprecated, as it is ignored if an argument is provided. Do not provide a positional argument if using the use_ipv6 keyword argument.") implies that the keyword alone still works. The RDoc of `Resolv.new` in 0.6.0 and later does not mention `use_ipv6:` at all.

Questions:

1. Should `Resolv.new(use_ipv6: v)` without a positional argument be honored again (for example by merging `use_ipv6: v` into the DNS config when `resolvers` is `nil`), as the warning text suggests?
2. If the keyword is meant to go away, should it warn (or raise `ArgumentError`) whenever it is given, so that existing `Resolv.new(use_ipv6: false)` callers notice, and should the RDoc of `Resolv.new` mention `Resolv.new({use_ipv6: false})` as the replacement?

I am asking because the Japanese reference manual currently documents the behavior as is (https://github.com/rurema/doctree/pull/3698) and I would like to know which behavior to describe.

Reproduced with the `ghcr.io/ruby/all-ruby` (3.3.0, 3.4.0, 4.0.0), `ghcr.io/ruby/ruby:3.4` (3.4.12), `ghcr.io/ruby/ruby:4.0` (4.0.9) and `ghcr.io/ruby/ruby:master` Docker images (x86_64-linux).



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