How to block mysterious remote requests?

Solution 1:

It is hard to say what exactly is going on here. However you state:

The request looks like my server is spending time serving or getting other pages content.

This together with the "GET http://218.22.14.198/index" sounds like you have misconfigured your system and are accidentally running a open proxy which is getting abused.
Basically other systems are now using your system as a proxy, usually to hide their IP address and not exactly doing things you want to be associated with.
You should as soon as possible investigate if this is the case.
A firewall rule here is just a bandaid and not the real solution.

If this is the case - and with the information provided it is impossible to tell - you need to reconfigure your system to stop being a open proxy. It depends on your specific webserver configuration how to do that.

More information for example for Apache httpd:
https://wiki.apache.org/httpd/ProxyAbuse