SPF Overview

You can help eliminate the spam problem by making it easy to detect forgeries. Protect your e-mail address reputation with a simple DNS technique.
Traveling Mailman and the Forwarding Problem

SPF was designed to give the greatest benefit for the least cost. It tightens the rules in a way that makes it hard for bad people to do bad things, while not bothering the good people who do good things. Even so, some power users who have taken advantage of SMTP's lax rules may be inconvenienced by SPF. This section describes the two problems SPF causes power users and offers ways to work around them.

Most end users relay their outbound mail through their ISPs' SMTP servers. Most modern clients also support SASL authentication or POP-before-SMTP for users who need to phone home from outside the ISPs' networks. Users who always send mail through their ISPs' SMTP servers are automatically SPF-compliant and don't need to do a thing.

But some power users with an MTA on their laptop are used to originating mail from random IP addresses, bypassing their ISPs' SMTP servers entirely. SPF accommodates these users: the advanced mechanism (see the “Advanced SPF” sidebar) is a way to exempt certain users from being required to use their ISPs' SMTP servers. They can keep doing what they want.

Some power users have a dozen or more addresses that forward all over the place by using entries in /etc/aliases or .forward files. In classical forwarding, the envelope sender remains unchanged while the recipient address is rewritten. This becomes a problem, though, when the message arrives at the destination—it still has the original sender address, and SPF tests fail.

The workaround is easy, however; you simply need to switch to remailing, where the sender address changes as well. There are many ways to accomplish this. Read the SPF FAQ (spf.pobox.com/faq.html#forwarding) to pick up the one that's right for you. Most end users have nothing to do with forwarding; only power users need to implement this workaround. If you have third-party service through an alumni, vanity domain or other commercial forwarding provider (such as pobox.com), you should expect them to implement remailing for you.

Stopping Spam: It's Part of the Solution

The primary goal of SPF is to stop forgery. I don't want to get any more spam from myself, and I certainly don't want you to receive any spam that claims to be from me. Worms and viruses tend to forge the envelope sender, too, and we can block them with SPF. And, stopping forgery carries a bonus. When spammers are forced to use their true names, we can figure out which domains are legitimate and which are spammers. People already are doing this: a right-hand side block list (RHSBL) is the domain name version of a DNS block list (DNSBL). Spammers who aren't afraid of using their own domains end up on RHSBLs quickly, and they can be blocked that way. In an SPF world, RHSBLs will become more important and effective.



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Re: SPF Overview

Anonymous's picture

I read this article after reading the May 2004 article because my subscription just started up. So, interestingly, everything seemed to flow. Thank goodness Linux Journal posts their articles!

I found this one here because I read the other article and thought "Hey, I need to do/learn this! Where's that previous article." A few clicks later, there it was. :)

I can understand the first person's frustrational comment about the definition of SPF. I explained a little of what I was doing to my signifigant other, and the initial response, after defining SPF, was "Oh, I thought it was like sunblock. You know, like SPF-15, SPF-45.."

In a way, it is like sunblock. It prevents your systems from burning up from processing all of that spam!

Re: SPF Overview

Anonymous's picture

Just for the record, the other half of this article, which is obviously more detailed due to the complexity (setting up the email side of thigns), is in the April issue:



Anonymous's picture

I don't believe this article even once tells us what SPF stands for.* Perhaps it means nothing, the latest fad in (non-) acronyms.

How did this piece make it past an editor?

*For the frustrated reader: it stands for Sender Policy Framework.

Re: SPF?

Anonymous's picture

You seem to be in the possession of Internet access... A quick trip over to spf.pobox.com would have answered your question rather quickly ;)

Re: SPF?

Anonymous's picture

I think you are missing out on some experience when it comes to technical writing. Technical lingo should be explained, but not to the depth of the PDR (Physician's Desk Refeence). Because the industry is so acronym-laden, many have found a common style to use the acronum once and place an explanation in parantheses afterwards, describing the acronym or term, but after that, to use the acronym (only) as the meaning has been explained. The initial reference can be seen in my reference to the PDR above.
And finally, but most importantly, material should be written|developed|edited so a reader doesn't have to read a sentence more than once for comprehension. Have you ever found yourself halfway into a sentence (technical, romance, etc. and say, "Huh?" then go back to the beginning of the sentence and start reading it again? That's an example of a bad book. Bad book! Bad bad book.

Re: SPF?

Anonymous's picture

I agree. Thanks for suppling the definition. I though the article was good.