[ruby-core:120876] [Ruby master Bug#21104] Net::HTTP connections failing in Ruby >= 3.4.0 on macOS with Happy Eyeballs enabled

Issue #21104 has been updated by shioimm (Misaki Shioi).


@mjt58
Thank you for your reply. Since you're seeing this issue across different environments, I'm starting to think it's more likely influenced by software running on the host rather than a problem with network intermediaries.
Could you also check the following?

- Do you get any errors when running `TCPSocket.new` directly without using net/http? For example, try executing a script like this:

```ruby
require "socket"
p TCPSocket.new("bbc.co.uk", 80, fast_fallback: true)
```

- Could you verify via packet capture whether the source IP of the RST is the client host itself? You can see which IP address sends an RST by using tcpdump, for example, by running `sudo tcpdump -i <interface name> host bbc.co.uk`, and then executing `Net::HTTP.get(URI("https://bbc.co.uk"))`.

I'd appreciate your help with further debugging.

----------------------------------------
Bug #21104: Net::HTTP connections failing in Ruby >= 3.4.0 on macOS with Happy Eyeballs enabled
https://bugs.ruby-lang.org/issues/21104#change-111751

* Author: mjt58 (Mike Thompson)
* Status: Open
* ruby -v: ruby 3.4.1 (2024-12-25 revision 48d4efcb85) +PRISM [arm64-darwin24]
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
A project I work on recently upgraded Ruby to 3.4.1 from 3.3.5. Following the upgrade, and when running locally on my Mac, all attempts to connect to an external service within the project over http(s) fail.

We use `mise` for managing development tool dependencies, including Ruby, and I am using macOS 15.3.

For example running something as simple as:
```
require 'net/http'
puts Net::HTTP.get(URI('https://bbc.co.uk'))
```

Will fail with the following stack trace:
```
/path/to/.local/share/mise/installs/ruby/3.4.1/lib/ruby/3.4.0/net/protocol.rb:46:in 'OpenSSL::SSL::SSLSocket#connect_nonblock': Connection reset by peer - SSL_connect (Errno::ECONNRESET)
	from /path/to/.local/share/mise/installs/ruby/3.4.1/lib/ruby/3.4.0/net/protocol.rb:46:in 'Net::Protocol#ssl_socket_connect'
	from /path/to/.local/share/mise/installs/ruby/3.4.1/lib/ruby/3.4.0/net/http.rb:1736:in 'Net::HTTP#connect'
	from /path/to/.local/share/mise/installs/ruby/3.4.1/lib/ruby/3.4.0/net/http.rb:1636:in 'Net::HTTP#do_start'
	from /path/to/.local/share/mise/installs/ruby/3.4.1/lib/ruby/3.4.0/net/http.rb:1625:in 'Net::HTTP#start'
	from /path/to/.local/share/mise/installs/ruby/3.4.1/lib/ruby/3.4.0/net/http.rb:1064:in 'Net::HTTP.start'
	from /path/to/.local/share/mise/installs/ruby/3.4.1/lib/ruby/3.4.0/net/http.rb:824:in 'Net::HTTP.get_response'
	from /path/to/.local/share/mise/installs/ruby/3.4.1/lib/ruby/3.4.0/net/http.rb:805:in 'Net::HTTP.get'
	from request.rb:2:in '<main>'
```

I tried different versions of Ruby and confirmed that the issue appears with 3.4.0. After reading the release notes for this version, I tried setting `RUBY_TCP_NO_FAST_FALLBACK=1` and this worked, allowing me to work around the problem.

This issue has also been encountered by others, please see:
https://github.com/rubygems/rubygems/issues/8390



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