Reverse MX

Reverse MX (RMX) is an email authentication method developed by Hadmut Danisch. It became a basis for the two most commonly used methods, Sender Policy Framework and Sender ID.

Development of RMX

Background and motivation

Between 1990 and 1998, the author of RMX, Hadmut Danisch, was working as a security researcher and system administrator at the European Institute for System Security (E.I.S.S.) at the University of Karlsruhe, Germany. Subject of research at the E.I.S.S. were both cryptographical (e.g. RFC 1824) and non-cryptographical methods (such as early firewall technology), with special focus on authentication and communication security. As a demonstration of a so called 'organizational security measure', Danisch had developed a scheme to prevent forgery of SMTP e-mail sender addresses, implemented as a complex and recursive rule set for sendmail. The basic idea was to perform a recursive sequence of database lookups with both the sender's IP address and given e-mail address after the 'MAIL FROM' command in the SMTP protocol. The first matching database lookup would tell whether to accept or to decline the message for delivery. At that time, the system was working well under lab conditions and in experimental implementations, but did not yet have a particular purpose, except for the demonstration of security technologies.

Around 1996/1997 the system became practically useful when suddenly there was an increasing number of spam messages began to fill and jam the mail system and mailboxes. While spam messages had formerly been seen merely on Usenet, spammers now began to systematically collect e-mail addresses and send spam by e-mail. At first, spammers used their real sender addresses, which were blacklisted soon. Later they used random sender addresses, but again this could easily be filtered by querying whether the sender's domain has a valid MX record. Then spammers started to use real domains to forge sender addresses, thus bringing the first spam storms that could not be systematically detected and blocked by SMTP-based mailsystems. Mailboxes started to contain more spam than regular mail. Since the number of registered domains and personal e-mail contacts was rather small and surveyable at that time, the sendmail ruleset drastically reduced the amount of spam coming into the institute's mailboxes, once the most important sender domains and their legitimate sender machines had been put in the local database (and optionally been whitelisted).

In 1998, Danisch left the E.I.S.S. and became a security consultant at the first german internet provider, soon beeing confronted with the increasing number of harsh complaints of commercial internet customers, who's leased lines had been jammed by spammers, and who had, on top of that, been billed for the spam traffic. At that time, internet traffic was expensive and billed on a volume base, and spam could easily increase the costs by ten times or even more, so a technical solution was urgently needed. Although - and because - the provider had bursted with the dot-com bubble in spring 2002, Danisch was still busy with finding a technical solution.

Beyond that, there was an ongoing intense dispute with the University of Karlsruhe about the nature, the basics, and the principles of IT security in common, and the asserted necessity of cryptography in particular. Therefore, a hard and large scale technical proof of concept in the real world outside the university labs was needed to prove that real, robust, and easy to use security can be achieved by organizational methods without cryptography.