Please review /etc/httpd/conf.d/mrtg.conf
mrtg-2.17.4-4.fc18 has been submitted as an update for Fedora 18. https://admin.fedoraproject.org/updates/mrtg-2.17.4-4.fc18
Package mrtg-2.17.4-4.fc18: * should fix your issue, * was pushed to the Fedora 18 testing repository, * should be available at your local mirror within two days. Update it with: # su -c 'yum update --enablerepo=updates-testing mrtg-2.17.4-4.fc18' as soon as you are able to. Please go to the following url: https://admin.fedoraproject.org/updates/FEDORA-2012-17452/mrtg-2.17.4-4.fc18 then log in and leave karma (feedback).
Package has been pushed to stable repositories and is working with F-18/httpd 2.4 :)