Bug #22508: `Resolv.new(use_ipv6: ...)` is silently ignored since resolv 0.6.0 (Ruby 3.4)

• Status: Open
• Assignee: -
• Target version: -
• 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

[znz (Kazuhiro NISHIYAMA)]

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?

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, 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 (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).