The Sender Policy Framework (SPF) is an open standard specifying a technical method to prevent sender address forgery. More precisely, the current version of SPF — called SPFv1 or SPF Classic — protects the envelope sender address, which is used for the delivery of messages. See the box on the right for a quick explanation of the different types of sender addresses in e-mails.
- Basic informations about SPF here
First of all you have to setup your SPF record. You can create it using this great wizard. Basically it will turn to be something like:
yourdomain.tld. IN TXT "v=spf1 mx a ip4:<your-ip>/32 ip4:<your-localnet>/24 include:alloweddomain.tld -all"
If your mx domain also appears in the name server's DMZ (DeMilitarized Zone) you should add such a TXT record in there as well, otherwise you will be exposed to spoofing, as spammers can always send you mail messages with your domain in the envelope.
It is important to have an
SPF record also for the control/me domain, as the system messages like bounces and so on are sent with the null sender, and the remote
MTA will use that domain for the
The SPF behavior of your mail server is controlled by the file /var/qmail/control/spfbehavior. You can specify a value between 0 and 6:
- 0 disabled (Default). Never do SPF lookups, don't create Received-SPF headers
- 1 selects 'annotate-only' mode, where qmail-smtpd will annotate incoming email with Received-SPF fields, but will not reject any messages.
- 2 will produce temporary failures on DNS lookup problems so you can make sure you always have meaningful Received-SPF headers.
- 3 selects 'reject' mode, where incoming mail will be rejected if the SPF record says 'fail'.
- 4 selects a more stricter rejection mode, which is like 'reject' mode, except that incoming mail will also be rejected when the SPF record says 'softfail'.
- 5 will also reject when the SPF record says 'neutral'
- 6 if no SPF records are available at all (or a syntax error was encountered).
You can override the value in /var/qmail/control/spfbehavior by setting the
SPFBEHAVIOR environment variable (typically in /etc/tcprules.d/tcp.smtp or, if you’ve used these notes as your guide, in ~/vpopmail/etc/tcp.smtp).
Values higher than 3 are strongly discouraged. You probably will want to go with 2 or 3.
First of all, check the header of your incoming messages. For email senders who don’t have SPF enabled, you should find a Received-SPF header that looks something like this:
Received-SPF: none (0: domain at xxxxxxxxxx does not designate permitted sender hosts)
For email senders who have SPF enabled, you’ll see a header that looks something like this:
Received-SPF: pass(0: SPF record at xxxxxxxxxx designates x.x.x.x as permitted sender)
To run a rejection test, use the highest value (6 ) and restart qmail. Then, from a remote IP address, try telnetting into your mail server and sending a message using a fake email address:
> telnet qmail.yourserver.net 25 Trying [remote-IP]... Connected to [remote-IP]. Escape character is '^]'. 220 qmail.yourserver.net ESMTP mail from: firstname.lastname@example.org 250 ok rcpt to: email@example.com 550 See http://spf.pobox.com/why.html?sender=test%40nospfdomain.net&ip=[sender-IP]&receiver=0 (#5.7.1) quit 221 qmail.yourserver.net Connection closed by foreign host.
Remember to restore to 2 or 3 your /var/qmail/control/spfbehavior file.