X-Forwarded-For DDoS

Posted at 10:55 pm in Uncategorized

A discussion forum of one of Redpill Linpro‘s customers has been under attack lately, through a number of DoS and DDoS variants. Today’s attack strain was of the rather interesting kind, as one of its very distinctive identifiers was a suspicious, not to say ridiculous, amount of IP addresses in the incoming X-Forwarded-For HTTP header. The X-Forwarded-For IP addresses included both IPv4 and IPv6 addresses.

The longest X-F-F header observed contained no less than 20 IP addresses that the HTTP request had allegedly been forwarded through on its way to the forum. If we are to believe the headers, this particular request has been following this route: United States → United States → South Africa → United States → United States → Mexico → Uruguay → China → Germany → United States → United States → South Africa → United States → United States → Mexico → Uruguay → China → Germany → Costa Rica → Norway.

This short animation (click to play) illustrates a few of the the alleged routes:

Whether the HTTP requests have indeed been proxied through all these relays is difficult to confirm. By their reverse DNS lookup, quite a few of the IP addresses identify themselves as proxy servers. Checking a sample of the listed IP addresses did not reveal any open proxies or other kinds of relays, neither were they listed on random open relay blacklists. The HTTP headers included the “Via:” header as well, indicating that the request did pass through some HTTP proxies. But as we know, incoming headers can’t be trusted and should never be treated as if they could.

For the purpose of blocking the DDoS attack, it’s not really interesting whether the intermediate IP addresses are real or just faked. We simply reconfigured Varnish to check each incoming HTTP request for two things:

  • Does the X-Forwarded-For header have more than five IP addresses?
  • Is the request destined for the forum currently under siege?

All requests matching the above criteria were then efficiently rejected with the well-known, all-purpose 418 I’m a teapot HTTP response. After a minute or two of serving 418 responses, the attack stopped abruptly.

Written by bjorn on March 19th, 2018

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