Tag Archives: SPF

Detecting Email Server Forgery

Most of the spam I see has been sent by servers forging or otherwise obscuring their server identity.  RFC2505 states that the server identity and sender address are easily forged.  Of these, it is easiest to identify server forgery.  Very little, if any, of the personal email has a forged server identity.  Unfortunately, legitimate bulk and automated email often shows signs of server identity.   If you deliver either of these types of email this article will provide information of fixing the situation.

The rules here apply email originating from the Internet only.    Mail User Agents submitting email are expected to violate these rules.  MUAs should use an authenticated encrypted connection to the Submission port (576).  Relay servers should not apply these rules to connections originating form the local network. Continue reading

Securing your Email Reputation with SPF

SPF (Server Policy Framework) is a simple means to limit the ability of  others to forge your identity in email.  I first implemented it after a forged identity under my domain was used to send Spam.  Once SPF was configured,  the bounce messages quickly dropped off.

Although not as frequently implemented as sender address checks, SPF can  be used to prevent forgery of the HELO identity.  My mail server uses SPF to check the Identity of the server.  This is easier to configure and more reliable than checking the domain in the Mail from address.  Even though I treat Neutral and Softfail policies as a Fail policy, I have not detected any false negatives.   I verify both the address returned by the PTR record for the host and the address in the HELO command.  This is primarily because the PTR record is more likely to have a valid domain. Continue reading

Implementing IPv6 Part 2

We are quickly running out of IPv4 addresses. Are you ready for World IPv6 Day on June 8th, 2011? I have prepared my configuration on OpenWRT and Ubuntu. This includes configuring DNS using bind, email using Exim, and a Squid web proxy.

Having verified that I could establish IPv6 connectivity, I chose to improve my connectivity. This started with getting a tunnel from Hurricane Electric and updating my configuration. I then updated my bind server and Exim mail server support IPv6 addresses. This posting updates and continues from my post on Implementing IPv6 6to4 on OpenWRT.   Review it for information on creating a tunnel and running radvd on OpenWRT. Continue reading

Signing Return Path Addresses with Exim

I have been receiving a fair amount of Spam from an e-mail forwarder.  They are unwilling to correct their problems.  Much of the Spam they forward is the form of bounce notifications.  Attempting to reject other Spam resulted in more notifications.  To control this Spam I implemented signed return path addresses.  As a side benefit, I am also rejecting bogus notifications sent directly to me.

Signing my return path allows me to reject faked notification e-mail.  The SMTP standard requires that no email sent with a null return path “<>” (aka Envelope Sender) be returned.  Its purpose is for allow for notifications about existing messages.  These includes notifications such as address unknown, message delivered, and message read.  E-mail notification which are not about a previously sent message can be refused . Signing the return path allowed me to reject such invalid notifications. Continue reading