Distributed Tarpitting: Impeding Spam Across Multiple Servers

This paper describes an Irish ISP's attempts to combat the abuse of resources caused by unsolicited commercial email. We describe the extension of a multicast system, used to implement POP-before-SMTP relaying, to share information about remote mail servers between multiple mail systems. The information may then be used to tarpit abusive servers - placing delays between SMTP protocol answers thus mitigating their impact on our systems. We then examine how effective this has been, and come up with some ideas for future development.

We also discuss building a policy around this and other measures we use to combat spam. An ISP is in the business of sending and receiving mail - this makes slowing or blocking mail a delicate subject.