Description of problem:
The mod_rewrite (and possibly other modules) can't send a http 451 error code unless it is supported by the server, the current httpd version (httpd-2.4.6-40) in RHEL 7.2 doesn't support it. There is upstream bugzilla https://bz.apache.org/bugzilla/show_bug.cgi?id=58985 which has been resloved and patch is available which added this support.

Version-Release number of selected component (if applicable):
httpd-2.4.6-40

How reproducible:
Always.

Steps to Reproduce:
1. Add following configuration to "/etc/httpd/conf/httpd.conf".
RewriteEngine On
RewriteRule ^ - [R=451,L]
2.Now start the httpd service
# systemctl start httpd
3. Now check httpd status.
# systemctl -l status  httpd

Actual results:

The httpd fails to start with below error message.
[root@rhle7]# systemctl -l status  httpd
httpd.service - The Apache HTTP Server
   Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled)
   Active: failed (Result: exit-code) since Tue 2016-06-07 18:50:12 IST; 9s ago
     Docs: man:httpd(8)
           man:apachectl(8)
  Process: 26984 ExecStop=/bin/kill -WINCH ${MAINPID} (code=exited, status=1/FAILURE)
  Process: 26982 ExecStart=/usr/sbin/httpd $OPTIONS -DFOREGROUND (code=exited, status=1/FAILURE)
 Main PID: 26982 (code=exited, status=1/FAILURE)

Jun 07 18:50:12 localhost httpd[26982]: AH00526: Syntax error on line 355 of /etc/httpd/conf/httpd.conf:
Jun 07 18:50:12 localhost httpd[26982]: RewriteRule: invalid HTTP response code '451' for flag 'R'
Jun 07 18:50:12 localhost systemd[1]: httpd.service: main process exited, code=exited, status=1/FAILURE
Jun 07 18:50:12 localhost kill[26984]: kill: cannot find process ""
Jun 07 18:50:12 localhost systemd[1]: httpd.service: control process exited, code=exited status=1
Jun 07 18:50:12 localhost systemd[1]: Failed to start The Apache HTTP Server.
Jun 07 18:50:12 localhost systemd[1]: Unit httpd.service entered failed state.

Expected results:

The httpd should start and allow mod_rewrite to send a http 451 error code.

Additional info:
Refer https://bz.apache.org/bugzilla/show_bug.cgi?id=58985