Nolisting - poor man's greylisting

Tuesday, 23 January 2007, 11:52 AM EST

Nolisting fights spam by specifying a primary MX that is always unavailable.

How does Nolisting work?

It has been observed that when a domain has both a primary (high priority, low number) and a secondary (low priority, high number) MX record configured in DNS, overall SMTP connections will decrease when the primary MX is unavailable. This decrease is unexpected because RFC 2821 (Simple Mail Transfer Protocol) specifies that a client MUST try and retry each MX address in order, and SHOULD try at least two addresses. It turns out that nearly all violators of this specification exist for the purpose of sending spam or viruses. Nolisting takes advantage of this behaviour by configuring a domain's primary MX record to use an IP address that does not have an active service listening on SMTP port 25. RFC-compliant clients will retry delivery to the secondary MX, which is configured to serve the role normally performed by the primary MX (final delivery, transport rerouting, etc.).

[ Read more ]




Spotlight

USBdriveby: Compromising computers with a $20 microcontroller

Posted on 19 December 2014.  |  Security researcher Samy Kamkar has devised a fast and easy way to compromise an unlocked computer and open a backdoor on it: a simple and cheap ($20) pre-programmed Teensy microcontroller.


Weekly newsletter

Reading our newsletter every Monday will keep you up-to-date with security news.
  



Daily digest

Receive a daily digest of the latest security news.
  
DON'T
MISS

Fri, Dec 19th
    COPYRIGHT 1998-2014 BY HELP NET SECURITY.   // READ OUR PRIVACY POLICY // ABOUT US // ADVERTISE //