SPF

Phil Pennock edited this page Nov 21, 2014 · 2 revisions
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Sender Policy Framework (SPF) support

SPF support is Experimental

To learn more about SPF, visit http://www.openspf.org. This document does not explain the SPF fundamentals, you should read and understand the implications of deploying SPF on your system before doing so.

SPF support is added via the libspf2 library. Visit

http://www.libspf2.org/

to obtain a copy, then compile and install it. By default, this will put headers in /usr/local/include and the static library in /usr/local/lib.

To compile Exim with SPF support, set these additional flags in Local/Makefile:

EXPERIMENTAL_SPF=yes
CFLAGS=-DSPF -I/usr/local/include
EXTRALIBS_EXIM +=-L/usr/local/lib -lspf2

This assumes that the libspf2 files are installed in their default locations.

NOTE: on Fedora/RedHat/Centos 64 bit systems you can compile the SPF libs running configure as follows:

configure --prefix=/usr --libdir=/usr/lib64

Then the Local/Makefile would be:

EXPERIMENTAL_SPF=yes
CFLAGS=-DSPF -I/usr/include/spf2
EXTRALIBS_EXIM +=-L/usr/lib64 -lspf2

You can now run SPF checks in incoming SMTP by using the "spf" ACL condition in either the MAIL, RCPT or DATA ACLs. When using it in the RCPT ACL, you can make the checks dependend on the RCPT address (or domain), so you can check SPF records only for certain target domains. This gives you the possibility to opt-out certain customers that do not want their mail to be subject to SPF checking.

The spf condition takes a list of strings on its right-hand side. These strings describe the outcome of the SPF check for which the spf condition should succeed. Valid strings are:

string meaning
pass The SPF check passed, the sending host is positively verified by SPF.
fail The SPF check failed, the sending host is NOT allowed to send mail for the domain in the envelope-from address.
softfail The SPF check failed, but the queried domain can't absolutely confirm that this is a forgery.
none The queried domain does not publish SPF records.
neutral The SPF check returned a "neutral" state. This means the queried domain has published a SPF record, but wants to allow outside servers to send mail under its domain as well.
err_perm This indicates a syntax error in the SPF record of the queried domain. This should be treated like "none".
err_temp This indicates a temporary error during all processing, including Exim's SPF processing. You may defer messages when this occurs.

You can prefix each string with an exclamation mark to invert is meaning, for example "!fail" will match all results but "fail". The string list is evaluated left-to-right, in a short-circuit fashion. When a string matches the outcome of the SPF check, the condition succeeds. If none of the listed strings matches the outcome of the SPF check, the condition fails.

Here is a simple example to fail forgery attempts from domains that publish SPF records:

deny message = $sender_host_address is not allowed to send mail from $sender_address_domain
     spf = fail

You can also give special treatment to specific domains:

deny message = AOL sender, but not from AOL-approved relay.
     sender_domains = aol.com
     spf = fail:neutral

Explanation: AOL publishes SPF records, but is liberal and still allows non-approved relays to send mail from aol.com. This will result in a "neutral" state, while mail from genuine AOL servers will result in "pass". The example above takes this into account and treats "neutral" like "fail", but only for aol.com. Please note that this violates the SPF draft.

When the spf condition has run, it sets up several expansion variables.

Variable Description
$spf_header_comment This contains a human-readable string describing the outcome of the SPF check. You can add it to a custom header or use it for logging purposes.
$spf_received This contains a complete SPF-Received: header that can be added to the message. Please note that according to the SPF draft, this header must be added at the top of the header list. Please see section 10 on how you can do this.
$spf_result This contains the outcome of the SPF check in string form, one of pass, fail, softfail, none, neutral, err_perm or err_temp.
$spf_smtp_comment This contains a string that can be used in a SMTP response to the calling party. Useful for "fail".

Making SPF Useful

Exim processes SPF based on raw logic. Thus if a domain uses +all, which means every host will pass, this doesn't provide any useful information. Here's an ACL that filters out the +all and returns more useful information.

spf_test:
warn    set acl_m_spf_record = ${lookup dnsdb{txt=$sender_address_domain}{$value}}

# No SPF record
accept  !condition = ${if def:acl_m_spf_record}

# SPF +all is meaningless
accept  condition = ${if match {$acl_m_spf_record}{\\+all}}

accept  spf = pass
        set acl_m_spf_pass = $acl_m_spf_record

accept  spf = fail
        set acl_m_spf_fail = $acl_m_spf_record

accept

Here's an example of how to use the ACL. In this case we are combining good SPF with a good white list lookup.

accept  acl = spf_test
        condition = ${if def:acl_m_spf_pass}
        dnslists = hostkarma.junkemailfilter.com=127.0.0.1/$sender_address_domain