NAME

Mail::SPF::Query - query Sender Permitted From for an IP,email,helo

SYNOPSIS

  my $query = new Mail::SPF::Query (ip => "127.0.0.1", sender=>'foo@example.com', helo=>"somehost.example.com", trusted=>1, guess=>1);
  my ($result,           # pass | fail | error | unknown [mechanism]
      $smtp_comment,     # "please see http://spf.pobox.com/why.html?..."  when rejecting, return this string to the SMTP client
      $header_comment,   # prepend_header("Received-SPF" => "$result ($header_comment)")
      $spf_record,       # "v=spf1 ..." original SPF record for the domain
     ) = $query->result();

    if    ($result eq "pass") { "domain is (probably) not forged.  apply RHSBL and content filters" }
    elsif ($result eq "fail") { "domain is forged.  reject or save to spambox" }
    else                      { "domain has no SPF, or broken SPF, may be forged.  apply content filters" }

  The default mechanism for trusted=>1 is "include:spf.trusted-forwarder.org".
  The default mechanisms for guess=>1 are "a/24 mx/24 ptr".

ABSTRACT

The SPF protocol relies on sender domains to publish a DNS whitelist of their designated outbound mailers. Given an envelope sender, Mail::SPF::Query determines the legitimacy of an SMTP client IP.

DESCRIPTION

There are two ways to use Mail::SPF::Query. Which you choose depends on whether the domains your server is an MX for have secondary MXes which your server doesn't know about.

The first style, calling ->result(), is suitable when all mail is received directly from the originator's MTA. If the domains you receive do not have secondary MX entries, this is appropriate. This style of use is outlined in the SYNOPSIS above. This is the common case.

The second style is more complex, but works when your server receives mail from secondary MXes. This performs checks as each recipient is handled. If the message is coming from a valid MX secondary for a recipient, then the SPF check is not performed, and a "pass" response is returned right away. To do this, call result2() and message_result2() instead of result().

If you do not know what a secondary MX is, you probably don't have one. Use the first style.

METHODS

Mail::SPF::Query->new()

  my $query = eval { new Mail::SPF::Query (ip    =>"127.0.0.1",
                                           sender=>'foo@example.com',
                                           helo  =>"host.example.com") };

  optional parameters:
     debug => 1, debuglog => sub { print STDERR "@_\n" },
     local => 'extra mechanisms',
     trusted => 1,                    # do trusted forwarder processing
     guess => 1,                      # do best_guess if no SPF record
     default_explanation => 'Please see http://spf.my.isp/spferror.html for details',
     max_lookup_count => 20,          # total number of SPF include/redirect queries
     sanitize => 1,                   # sanitize all returned strings
     myhostname => "foo.example.com", # prepended to header_comment

  if ($@) { warn "bad input to Mail::SPF::Query: $@" }

Set trusted=>1 to turned on automatic trusted_forwarder processing. The mechanism include:spf.trusted-forwarder.org is used just before a -all or ?all. The precise circumstances are somewhat more complicated, but it does get the case of v=spf1 -all right -- i.e. spf.trusted-forwarder.org is not checked.

Set guess=>1 to turned on automatic best_guess processing. This will use the best_guess SPF record when one cannot be found in the DNS. Note that this can only return pass or unknown. The trusted and local flags also operate when the best_guess is being used.

Set local=>'include:local.domain' to include some extra processing just before a -all or ?all. The local processing happens just before the trusted processing.

Set default_explanation to a string to be used if the SPF record does not provide a specific explanation. The default value will direct the user to a page at spf.pobox.com with "Please see http://spf.pobox.com/why.html?sender=%{S}&ip=%{I}&receiver=%{xR}". Note that the string has macro substitution performed.

Set sanitize to 1 to get all the returned strings sanitized. Alternatively, pass a function reference and this function will be used to sanitize the returned values. The function must take a single string argument and return a single string which contains the sanitized result.

Set debug=>1 to watch the queries happen.

$query->result()

  my ($result, $smtp_comment, $header_comment, $spf_record) = $query->result();

$result will be one of pass, fail, unknown [...], or error.

pass means the client IP is a designated mailer for the sender. The mail should be accepted subject to local policy regarding the sender.

fail means the client IP is not a designated mailer, and the sender wants you to reject the transaction for fear of forgery.

unknown [...] means the domain either does not publish SPF data or has a configuration error in the published data. If the data contained an unrecognized mechanism, it will be presented following "unknown". You should test for unknown using a regexp /^unknown/i rather than eq "unknown".

error means the DNS lookup encountered a temporary error during processing.

Results are cached internally for a default of 120 seconds. You can call ->result() repeatedly; subsequent lookups won't hit your DNS.

The smtp_comment should be displayed to the SMTP client.

The header_comment goes into a Received-SPF header, like so: Received-SPF: $result ($header_comment)

The spf_record shows the original SPF record fetched for the query. If there is no SPF record, it is blank. Otherwise, it will start with "v=spf1" and contain the SPF mechanisms and such that describe the domain.

Note that the strings returned by this method (and most of the other methods) are (at least partially) under the control of the sender's domain. This means that, if the sender is an attacker, the contents can be assumed to be hostile. Thus they should be sanitized before being used for sensitive purposes. In particular, assume that the smtp_comment might contain a newline character.

$query->result2()

  my ($result, $smtp_comment, $header_comment, $spf_record) = $query->result2('recipient@domain', 'recipient2@domain');

result2 does everything that result does, but it first checks to see if the sending system is a recognized MX secondary for the recipient(s). If so, then it returns pass and does not perform the SPF query. Note that the sending system may be a MX secondary for some (but not all) of the recipients for a multi-recipient message, which is why result2 takes an argument list. See also message_result2().

$result will be one of pass, fail, unknown [...], or error. See result() above for meanings.

If you have MX secondaries and if you are unable to explicitly whitelist those secondaries before SPF tests occur, you can use this method in place of result(), calling it as many times as there are recipients, or just providing all the recipients at one time.

The smtp_comment can be displayed to the SMTP client.

For example:

  my $query = new Mail::SPF::Query (ip => "127.0.0.1",
                                    sender=>'foo@example.com',
                                    helo=>"somehost.example.com");

  ...

  my ($result, $smtp_comment, $header_comment);

  ($result, $smtp_comment, $header_comment) = $query->result2('recip1@mydomain.com');
  # return suitable error code based on $result eq 'fail' or not

  ($result, $smtp_comment, $header_comment) = $query->result2('recip2@mydom.org');
  # return suitable error code based on $result eq 'fail' or not

  ($result, $smtp_comment, $header_comment) = $query->message_result2();
  # return suitable error if $result eq 'fail'
  # prefix message with "Received-SPF: $result ($header_comment)"

This feature is relatively new to the module. You can get support on the mailing list spf-devel@listbox.com.

The methods result2() and message_result2() use "2" because they work for secondary MXes. result2() takes care to minimize the number of DNS operations so that there is little performance penalty from using it in place of result(). In particular, if no arguments are supplied, then it just calls result() and returns the method response.

$query->message_result2()

  my ($result, $smtp_comment, $header_comment, $spf_record) = $query->message_result2();

message_result2() returns an overall status for the message after zero or more calls to result2(). It will always be the last status returned by result2(), or the status returned by result() if result2() was never called.

$result will be one of pass, fail, unknown [...], or error. See result() above for meanings.

$query->best_guess()

      my ($result, $smtp_comment, $header_comment) = $query->best_guess();

When a domain does not publish SPF records, this library can produce an educated guess anyway.

It pretends the domain defined A, MX, and PTR mechanisms, plus a few others. The default set of directives is

  "a/24 mx/24 ptr"

That default set will return either "pass" or "unknown".

If you want to experiment with a different default, you can pass it as an argument: $query->best_guess("a mx ptr")

$query->trusted_forwarder()

      my ($result, $smtp_comment, $header_comment) = $query->best_guess();

It is possible that the message is coming through a known-good relay like acm.org or pobox.com. During the transitional period, many legitimate services may appear to forge a sender address: for example, a news website may have a "send me this article in email" link.

The trusted-forwarder.org domain is a whitelist of known-good hosts that either forward mail or perform legitimate envelope sender forgery.

  "include:spf.trusted-forwarder.org"

This will return either "pass" or "unknown".

$query->sanitize('string')

This applies the sanitization rules for the particular query object. These rules are controlled by the sanitize parameter to the Mail::SPF::Query new method.

strict_sanitize('string')

This ensures that all the characters in the returned string are printable. All whitespace is converted into spaces, and all other non-printable characters are converted into question marks. This is probably over aggressive for many applications.

This function is used by default when the sanitize option is passed to the new method of Mail::SPF::Query.

Note that this function is not a class method.

$query->debuglog()

Subclasses may override this with their own debug logger. I recommend Log::Dispatch.

Alternatively, pass the new() constructor a debuglog = sub { ... }> callback, and we'll pass debugging lines to that.

EXPORT

None by default.

WARNINGS

Mail::Query::SPF should only be used at the point where messages are received from the Internet. The underlying assumption is that the sender of the email is sending the message directly to you or one of your secondaries. If your MTA does not have an exhaustive list of secondaries, then the result2() and message_result2() methods should be used. These methods take care to permit mail from secondaries.

AUTHORS

Meng Weng Wong, <mengwong+spf@pobox.com>

Philip Gladstone

SEE ALSO

http://spf.pobox.com/

cpanm Mail::SPF::Query

perl -MCPAN -e shell
install Mail::SPF::Query