Wikipedia's radical openness sounds too good to be true — how can you let anyone edit a website, without even logging in? Won't it be swamped by obsessive, childish users trying to spray their graffiti all over it in a misguided attempt to feel important? Won't people just set up bots to constantly revert an article to their desired version, 24 hours per day?

In a way, it is too good to be true. Wikipedia relies for its radical openness on IP address-based access control — the ability to block individuals from editing by blocking their IP address, and the difficulty for individuals to obtain more IP addresses.

There are threats to this model. For example, open proxies allow technically capable users to edit from a pool of thousands of IP addresses. Even more capable attackers can break into remote computers and use them as "zombies" to attack Wikipedia. In both cases however, we can block the proxies and the zombies as they appear, with very little inconvenience to other users. Thus the attacker is forced to constantly search for new hosts to compromise, and that is the best way we have to slow down their attack.

A more serious threat is the use of HTTP proxies by Internet service providers. Poorly configured proxies can make it impossible to block one user without blocking thousands of others -- they obscure the identity of the attacker by mixing their traffic in with the traffic of other customers of the same ISP. This is where the XFF project comes in.

The X-Forwarded-For (XFF) header is a de facto standard supported by most HTTP proxy software. It appends the IP address of the client to an HTTP header which is passed on to the server. Thus, the server can determine the client IP. Because this header can be forged, Wikipedia and other Wikimedia websites will only accept XFF headers which come from sources which are known to be trusted. The aims of the XFF project are to:

  • Create a catalogue of ISP proxy addresses and their XFF status
  • Encourage ISPs to configure their proxies to give XFF headers



How to configure your proxy to send XFF headers:

Microsoft ISA Server 2004/2006

  1. Install the Winfrasoft X-Forwarded-For for ISA Server web filter. ISA Server does not support XFF natively.
  2. For forward proxy create a web chaining rule (if one doesn't already exist) to route traffic to an upstream proxy server. There is nothing more to do for a reverse proxy.

Reference: X-Forwarded-For for ISA Server details.



Put this in your squid.conf:

forwarded_for on

This is the default, so really it's only necessary to remove forwarded_for off if it is present.

Reference: Squid Configuration Manual.

Cisco Cache Engine


Switch to configure mode and run:

http append x-forwarded-for-header

Reference: Cisco Cache Engine User Guide, Version 2.1.0, Appendix A

NetApp NetCache


In the Network Settings page of the web-based administration interface, click "off" next to "Privacy". This only affects the X-Forwarded-For header, as far as we know.

Reference: ISP-Planet Cache Review Series - NetApp NetCache C720, Tuning Your Web Cache

Cacheflow Client Accelerator


In configure mode:

reveal http
http add-header x-forwarded-for

Trusted XFF list


If a proxy is on our trusted XFF list, users editing Wikipedia from behind the proxy will appear to be editing from their client IP, not from the proxy IP. Wikipedia administrators will be able to block clients individually; the whole proxy will never be blocked due to the actions of a single vandal.

Proxies will be added to the trusted XFF list when they come to our attention if they:

  • Have a reverse DNS entry that implies their dedicated use as an ISP proxy, and
  • Primarily forward for public IP addresses, not addresses in RFC 1918 private internets. See RFC 1918 for a list of proxies which forward for these addresses.

For listing and delisting inquiries, file a task in Phabricator with the MediaWiki-extensions-TrustedXFF project tag.

As of May 2013, the trusted XFF list is managed under the TrustedXFF MediaWiki extension, in a file called "trusted-hosts.txt". This file is viewable by following this link. Wikimedia wikis take this text file and generate a PHP file containing an array.

Changes to the trusted-hosts.txt file can be proposed through Gerrit.