Apache fails to start: undefined symbol: ap_set_content_type_ex

Symptoms

• Apache fails to start with an error like this one:

  Cannot load /usr/lib/apache2/modules/mod_headers.so into server: /usr/lib/apache2/modules/mod_headers.so: undefined symbol: ap_set_content_type_ex

• LiteSpeed is/was installed on the server

Cause

Incorrect package configuration from LiteSpeed.

Resolution

Reinstall Apache

1. Connect to the server over SSH

2. Reinstall the Apache components

   • RHEL-based

     # yum reinstall httpd

   • Debian-based

     # apt reinstall apache2 apache2-bin apache2-data apache2-utils

3. Start Apache

   # systemctl start httpd || systemctl start apache2