Apache HTTP Server, in all releases prior to 2.2.32 and 2.4.25, was liberal
in the whitespace accepted from requests and sent in response lines and
headers. Accepting these different behaviors represented a security concern
when httpd participates in any chain of proxies or interacts with back-end
application servers, either through mod_proxy or using conventional CGI
mechanisms, and may result in request smuggling, response splitting and
cache pollution.

 ratliff> Notes from Debian "The fix is not fully backwards compatible so
 ratliff> upstream have created a new option to control this behaviour.
 ratliff> Affects: 2.2.0 to 2.4.23."
 mdeslaur>
 mdeslaur> This fix no longer allows underscores in host names. Debian
 mdeslaur> added a patch to restore the behaviour:
 mdeslaur> https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=851357
 mdeslaur> http://mail-archives.apache.org/mod_mbox/httpd-dev/201702.mbox/%3C20170202125319.GA15948%40redhat.com%3E
 mdeslaur>
 mdeslaur> The new configuration option doesn't entirely preserve
 mdeslaur> backwards compatibility:
 mdeslaur> https://bz.apache.org/bugzilla/show_bug.cgi?id=60783

mdeslaur