[ruby-core:114096] [Ruby master Bug#19756] URI::HTTP.build does not accept a host of `_gateway`, but `URI.parse` will.

Issue #19756 has been updated by austin (Austin Ziegler).


This is a better thread overall and there are a *number* of points worth reading in it. It boils down to:

- Underscores are not permitted in hostnames and therefore URLs/URIs.
- Leading underscores are permitted in DNS labels.
- Underscores are not otherwise permitted in DNS labels.

https://stackoverflow.com/questions/2180465/can-domain-name-subdomains-have-an-underscore-in-it

----------------------------------------
Bug #19756: URI::HTTP.build does not accept a host of `_gateway`, but `URI.parse` will.
https://bugs.ruby-lang.org/issues/19756#change-103772

* Author: postmodern (Hal Brodigan)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.2.2 (2023-03-30 revision e51014f9c0) [x86_64-linux]
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN
----------------------------------------
I noticed a difference in behavior between `URI::HTTP.build` and `URI.parse`. `URI::HTTP.build` will not accept `host:` value of `_gateway`, but `URI.parse` will.

## Steps To Reproduce

```ruby
URI::HTTP.build(host: "_gateway")
```
vs.
```ruby
URI.parse("http://_gateway")
```

### Expected Results

Both raise the same exception, or return the same URI object.

### Actual Results

```
URI::HTTP.build(host: "_gateway")
/usr/share/ruby/uri/generic.rb:601:in `check_host': bad component(expected host component): _gateway (URI::InvalidComponentError)
	from /usr/share/ruby/uri/generic.rb:640:in `host='
	from /usr/share/ruby/uri/generic.rb:673:in `hostname='
	from /usr/share/ruby/uri/generic.rb:190:in `initialize'
	from /usr/share/ruby/uri/generic.rb:136:in `new'
	from /usr/share/ruby/uri/generic.rb:136:in `build'
	from /usr/share/ruby/uri/http.rb:61:in `build'
	from (irb):2:in `<main>'
	from /usr/local/share/gems/gems/irb-1.7.0/exe/irb:9:in `<top (required)>'
	from /usr/local/bin/irb:25:in `load'
	from /usr/local/bin/irb:25:in `<main>'
```

```
URI.parse("https://_gateway")
# => #<URI::HTTPS https://_gateway>
```

## Additional Information

```
$ gem list uri
uri (default: 0.12.1)
```



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