Fileserve, a rapid-set up hosting service (or “cyberlocker”), is suffering collateral damage from network level content blocking systems using the IWF block list.

According to ISPreview reports, single Fileserve customer account was added to the IWF block list sometime last week. However, because of an unintended technical interaction between the way some ISPs implement IWF blocking and the way Fileserve limits free use of its service, all Fileserve customer accounts are being blocked for some ISPs. BT and Virgin are both reported to be affected.

Fileserve allows Internet user to download a limited amount of content hosted by their customers free of charge; additional downloading is only permitted having paid a fee. This business model enables the content provider to transfers the cost of hosting to their user. This requires Fileserve to limit the download capacity to each IP address.

This doesn’t interact well with the “Cleanfeed” model of network level blocking system employed by major UK ISPs. Cleanfeed systems attempt to block access to only a particular URL while also allowing the scalability national ISP networks require. This works by diverting all traffic destined for a particular site to a network proxy, which then implements blocking for blocked URLs, and a proxy service for other, unblocked, URLs. Unfortunately, this proxy presents a single IP address to sites like Fileserve, which means all BT/Virgin customers will appear to be using the same IP address to Fileserve – and so be indistinguishable from a single user massively overusing the free download capacity.

As with the time the IWF blocked access to Wikipedia two years ago, the incident demonstrates once again that if you manipulate network traffic in the core of the network in unexpected ways it may have unintended consequences for users at both edges of the network.