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 teenagers, 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
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.
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.
xff AT wikimedia DOT org for listing and delisting inquiries.
Listed ISPs 
- America Online (not sending XFF headers consistently)
- Cable & Wireless (Energis proxies *.cache.pol.co.uk)
- Demon.net (IWF proxies)
- Deutsche Telekom
- Easynet (UK Online IWF proxies)
- Gallaudet University
- KCOM (IWF proxies)
- LTT Broadband
- MaxOnline, StarHub Cable Vision Ltd
- Opera Mini
- Oulu Telephone Company
- PlusNet (IWF proxies)
- Qatar Telecom
- Saudi Arabia national proxy service
- Shaw cable
- Telefónica de España
- TeliaSonera Finland
- TPG Internet
- True Internet
- TSN Communications
- United Online
- University of Cape Town
- University of Cambridge
- University College Cork
- University of Maine (MSLN)
- Virgin Media (both the NTL and Telewest networks)
- Daisy Group Plc