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First up, check out this question:

Why are external domains appearing in my apache logs?

We're seeing the exact same thing on one of our Debian VPS servers - with one big difference, the regularity of said requests. We're seeing 5000+ requests per minute - sometimes as many as 100 requests per second - all for domains that we don't host - this is a big problem as apache is chomping up gigs of ram then crashing the server.

Ive taken a tail snapshot of the other_vhosts_access.log from apache and uploaded here:

http://www.heavencore.co.uk/filehub/20140411101731_apache_log_tail.txt

Two examples of said log:

vps.XXXXXX.co.uk:80 216.244.68.216 - - [10/Apr/2014:14:57:15 +0000] "GET http://advs.adgorithms.com/ttj?id=2440545 HTTP/1.0" 404 527 "http://www.cashwargames.com/girls-games.htm" "Mozilla/4.0 (compatible; MSIE 5.0; Mac_PowerPC; Alexa Toolbar)"
vps.XXXXXX.co.uk:80 192.157.246.54 - - [10/Apr/2014:14:57:15 +0000] "GET http://ads.yahoo.com/st?ad_type=ad&ad_size=300x250&section=5452138&pub_url=${PUB_URL} HTTP/1.0" 404 520 "http://www.verlyer.com/?p=866" "Mozilla/5.0 (Windows; U; Windows NT 5.1; ru-RU) AppleWebKit/533.18.1 (KHTML, like Gecko) Version/5.0.2 Safari/533.18.5"

Even if we delete all virtual hosts - apache still goes mental replying with 404's to all these requests, and if we configure apache to deny all - it just starts replying with 403's

as a stop gap measure, we've used IPTABLES to block ALL traffic to the server except a few IP addresses of our primary users.

As you can see from the tail log though, these requests are coming from thousands of unique addresses - presumably some botnet has latched on to our server??

How can we stop this attack & re-instate public http access to the server??? (its been going on for over 48 hours now)

The VPS is hosted by a 3rd party and we have no control over the perimeter firewall / hypervisor.

The only thing the 3rd parties support people could recommend was using IPT to block all the IP's - we don't see how that's possible given the vast number of them?

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I'd try to use a reverse proxy with some vary fast webserver (apache 2.4 with worker or event MPM, or ngix) which would only allow valid requests to real webserver (probably apache with prefork MPM for PHP compatibility).

Both can run on the same VPS. Real webserver can for example listen on 127.0.0.2 on non-standard port.

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You could configure a default virtual host to handle all domains except your server. Configure it to return a 403 request to all queries. This will have minimal content. Also configure it not to allow keep alive to free resources quickly.

Using a separate log for the default virtual host with fail2ban configured to drop any connecting hosts will quickly block the hosts. Fail2ban will clear the blocks after a configurable time.

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