why strict_hostname_check treats '+' invalid character

[建民郭 <mrblack.tw@xxxxxxxxx>]

In apache 2.4.25, there has a new function in vhost.c, strict_hostname_check

https://github.com/apache/httpd/blob/639df4136b46642cdfdc390b34e446501597abba/server/vhost.c#L754

I current get 400 response, if send a request likes the following:

http://abc+def:8088/test/auth.cgi

Base on log, '+' is not a valid character.

I check history and find it comes from the following commit:

https://github.com/apache/httpd/commit/741c7f335bdfb133404baf614ffa03db262b1e48

Is there any one know why strict_hostname_check implements the following rule?

* for the host name in the URL or Host header:

   - if an IPv4 dotted decimal address: Reject octal or hex values, require

     exactly four parts

   - if a DNS host name: Reject non-alphanumeric characters besides '.' and

     '-'. As a side effect, this rejects multiple Host headers.

Is it based on RFC or any other document?

I will appreciate it if any one can share with me.

Thank you