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Update docs for 0.8.10

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Stuart Gathman
2008-08-25 18:14:56 +00:00
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@@ -4,8 +4,7 @@ Title: Python Milter Mail Policy
These are the policies implemented by the <code>bms.py</code> milter
application. The milter and Milter modules do not implement any policies
by themselves. Eventually, I'll get the bms.py milter moved to its
own package.
by themselves.
<h3> Classify connection </h3>
@@ -77,161 +76,119 @@ altered accordingly.
<h2> SPF check </h2>
Finally, the MAIL FROM, connect IP, and HELO name are checked against
any SPF records published via DNS for the alleged sender (MAIL FROM).
If there is no SPF record, we check for a local substitute under the
domain defined in the <code>[spf]delegate</code> configuration.
Further checks depend on the result.
The MAIL FROM, connect IP, and HELO name are checked against
any SPF records published via DNS for the alleged sender (MAIL FROM)
to determine the official SPF policy result.
The offical SPF result is then logged in the Received-SPF header field,
but certain results are subjected to further processing to create
an effective result for policy purposes.
<table border=1>
<tr><th>NONE</th><td>
If there is no SPF record (official or delegated), then we
initiate a "three strikes and your out" regime, which looks for
<b>some</b> form of validated identification.
<ol>
<li>We try a "best guess" SPF record of "v=spf1 a/24 mx/24 ptr". If this
passes, good.
<li> We try to validate the HELO name. First check for an SPF record.
Otherwise, check whether the connect IP matches any A record for
the HELO name, or any A record for any MX name for the HELO name,
or is at least in the same /24 subnet as any of the above.
(In other words, a HELO SPF "best guess" of "v=spf1 a/24 mx/24".)
If so, good. We consider the HELO validated. If the HELO SPF
check fails, we reject the email.
</ol>
If the official result is 'none', we try to turn it into an effective result of
'pass' or 'fail'. First, we check for a local substitute SPF record
under the domain defined in the <code>[spf]delegate</code> configuration.
It is often useful to add local SPF records for correspondents that are
too clueless to add their own. If there is no local substitute, we use a "best
guess" SPF record of "v=spf1 a/24 mx/24 ptr" for MAIL FROM or "v=spf1 a/24
mx/24" for HELO. In addition, a HELO that is a subdomain of MAIL FROM and
resolves to the connect IP results in an effective result of 'pass'.
If there is no local SPF record, and the effective result is still not
'pass', we check for either a valid HELO name or a valid PTR record for
the connect IP. A valid HELO or PTR cannot look like a dynamic name
as determined by the heuristic in <code>Milter.dynip</code>.
If HELO has an SPF record, and the result is anything but pass, we reject
the connection:
<pre>
2005Jul30 19:45:16 [93991] connect from [221.200.41.54] at ('221.200.41.54', 3581) EXTERNAL DYN
2005Jul30 19:45:18 [93991] hello from adelphia.net
2005Jul30 19:45:19 [93991] mail from <wendy.stubbsua@link-it.com> ()
2005Jul30 19:45:19 [93991] REJECT: hello SPF: fail 550 access denied
</pre>
<ol>
<li> If there is a validated PTR name, and it doesn't look
like a dynamic name, good. We consider the connection validated.
</ol>
If any of the above can be validated, we continue on.
If none of the above can be validated, and the <code>[SPF]reject_noptr</code>
option is true, we reject the message immediately with the explanation
that we need some form of valid identification before we accept an email.
If <code>[SPF]reject_noptr</code> is false, we flag the message as
needing Call Back Validation.
The Call Back Valildation sends a DSN to the purported sender informing
them of the lack of identification. If the message is legitimate, the
sender needs to know that their email setup is broken and should be corrected.
If the message is forged, the sender is informed of the forgery,
and their need to publish an SPF record or at least use a valid HELO name.
If the purported sender does not accept the DSN,
then the message is rejected. The CBV status is cached to avoid
annoying the purported sender with too many DSNs. Currently, the DSN
is repeated to the same sender once per month.
<p>
In this example, although 3com.com has no SPF record, we assume that
any legitimate mail from them will at least have a valid HELO or PTR.
<pre>
2005Jul30 23:52:03 [96777] connect from [222.252.233.200] at ('222.252.233.200', 29934) EXTERNAL DYN
2005Jul30 23:52:03 [96777] hello from 3mail.3com.com
2005Jul30 23:52:04 [96777] mail from <etec_nic_family@3mail.3com.com> ()
2005Jul30 23:52:04 [96777] REJECT: no PTR, HELO or SPF
</pre>
</td></tr>
Note that HELO does not have any forwarding issues like MAIL FROM, and so
any result other than 'pass' or 'none' should be treated like 'fail'.
<tr><th>PASS</th><td>
A pass result normally lets the email continue on, but the domain is
tracked for reputation (and may be blocked), and may skip content scanning if
it matches a whitelist.
<pre>
2005Jul24 17:44:26 [2104] mail from <gnucash-devel-bounces@gnucash.org> ('SIZE=4410',)
2005Jul24 17:44:26 [2104] Received-SPF: pass (mail.bmsi.com: domain of gnucash.org
designates 204.107.200.65 as permitted sender)
client-ip=204.107.200.65; envelope-from=gnucash-devel-bounces@gnucash.org; helo=cvs.gnucash.org;
</pre>
</td></tr>
Only if nothing about the SMTP envelope can be validated does the effective
result remain 'none. I call this the "3 strikes" rule.
<tr><th>NEUTRAL</th><td>
A neutral result normally lets the email continue on, but the domain is not
tracked for reputation or matched against any whitelists.
Highly forged domains listed in <code>[SPF]reject_neutral</code> are
rejected.
<pre>
2005Jul24 17:41:37 [2070] connect from cp500627-a.dbsch1.nb.home.nl at ('84.27.225.3', 3465) EXTERNAL
2005Jul24 17:41:37 [2070] hello from cp500627-a.dbsch1.nb.home.nl
2005Jul24 17:41:38 [2070] mail from <nwarjejkw@yahoo.com> ()
2005Jul24 17:41:38 [2070] REJECT: SPF neutral for nwarjejkw@yahoo.com
</pre>
</td></tr>
If the official result is 'permerror' (a syntax error in the sender's
policy), we use the 'lax' option in pyspf to try various heuristics to guess
what they really meant. For instance, the invalid mechanism "ip:1.2.3.4" is
treated as "ip4:1.2.3.4". The result of lax processing is then used
as the effective result for policy purposes.
<tr><th>SOFTFAIL</th><td>
A softfail result normally lets the email continue on, but the domain is not
tracked for reputation or matched against any whitelists. Furthermore,
the message is flagged as needing Call Back Validation,
and the highly forged domains listed in <code>[SPF]reject_neutral</code> are
rejected as well.
<p>
At present, we also require a valid HELO or PTR to avoid rejecting
a softfail. But this should probably change to only require a
successful CBV.
<p>
The Call Back Valildation sends a DSN to the purported sender informing
them of the softfail. If the message is legitimate, the sender needs
to know about the softfail so that their email setup can be corrected.
If the message is forged, the sender is informed of the forgery, confirming
that SPF is protecting their reputation and encouraging a rapid transition
to a strict policy. If the purported sender does not accept the DSN,
then the message is rejected. The CBV status is cached to avoid
annoying the purported sender with too many DSNs. Currently, the DSN
is repeated to the same sender once per month.
<pre>
2005Jul24 15:41:33 [801] mail from <Aitp@horafeliz.com> ()
2005Jul24 15:41:33 [801] Received-SPF: softfail (mail.bmsi.com: transitioning domain of horafeliz.com
does not designate 221.184.83.185 as permitted sender)
client-ip=221.184.83.185; envelope-from=Aitp@horafeliz.com;
helo=p8185-ipad30funabasi.chiba.ocn.ne.jp;
2005Jul24 15:41:33 [801] rcpt to <david@example.com> ()
2005Jul24 15:41:35 [801] Subject: Microsoft, Adobe, Macromedia, Corel software. Up to 80% discount.
2005Jul24 15:41:35 [801] X-Mailer: Microsoft Outlook, Build 10.0.2605
2005Jul24 15:41:35 [801] CBV: Aitp@horafeliz.com
2005Jul24 15:41:38 [801] REJECT: CBV: 550 <Aitp@horafeliz.com>: User unknown
</pre>
</td></tr>
With an effective SPF result in hand, we consult the sendmail access
database to find our receiver policy for the sender.
<tr><th>FAIL</th><td>
The message is rejected with a reference the SPF why page.
<pre>
2005Jul30 19:53:27 [94070] connect from [212.70.52.16] at ('212.70.52.16', 3192) EXTERNAL DYN
2005Jul30 19:53:27 [94070] hello from winzip.com
2005Jul30 19:53:27 [94070] mail from <dan@winzip.com> ()
2005Jul30 19:53:27 [94070] REJECT: SPF fail 550 SPF fail:
see http://openspf.com/why.html?sender=dan@winzip.com&ip=212.70.52.16
</pre>
<table border=1>
<tr><th>REJECT</th><td>
Reject the sender with a 550 5.7.1 SMTP code. The SMTP rejection
includes a detailed description of the problem.
</td></tr>
<tr><th>PERMERROR</th><td>
Permanent errors were called "unknown", and are still show that way
in the log. The message is rejected. Previously, we enabled "lax" parsing
of the SPF record, but rejecting is better because it informs the
sender about their problem. The next milter version will
look for a local substitute SPF record (as for a missing SPF record)
before rejecting. This will inform the sender of their problem, but
also let the receiver install a temporary workaround.
<pre>
2005Jul24 18:05:37 [2312] mail from <b-mihdbcgaacaa-becibijh-000-@msg.euxiphipops.com> ()
2005Jul24 18:05:37 [2312] REJECT: SPF unknown 550 SPF Permanent Error:
include mechanism missing domain: include
</pre>
The SPF record for msg.euxiphipops.com looked like this at the time of the
above error:
<pre>
msg.euxiphipops.com TXT "v=spf1 mx ptr a include"
</pre>
<tr><th>CBV</th><td>
Do a Call Back Validation by connecting to an MX of the sender
and checking that using the sender as the RCPT TO is not rejected.
We quit the CBV connection before actualling sending a message.
If the CBV is rejected, our SMTP connection is rejected with the
same error code and message. CBV results are cached.
</td></tr>
<tr><th>TEMPERROR</th><td>
Temporary errors result in a 451 "Try again later" response. The sender
should retry the message at a later time.
<pre>
2005Jul24 07:33:13 [29846] mail from <quickenloans@rate.quicken.com> ('SIZE=73775', 'BODY=8BITMIME')
2005Jul24 07:33:43 [29846] TEMPFAIL: SPF error 450 SPF Temporary Error: DNS Timeout
</pre>
<tr><th>DSN</th><td>
Do a Call Back Validation by connecting to an MX of the sender
and checking that using the sender as the RCPT TO is not rejected.
Unlike a CBV, we continue on to data and send a detailed message
explaining the problem. This can be useful for reporting PermError
or SoftFail to the sender. Keep in mind that for any result other
than 'pass', the sender could be forged, and your DSN could annoy the
wrong person. However, a SoftFail result is requesting such feedback
for debugging and a PermError result needs to be fixed by the sender ASAP
whether forged or not. DSN results are cached so that senders are
annoyed only weekly.
</td></tr>
<tr><th>OK</th><td>
Accept the sender. The message may still be rejected via reputation
or content filtering.
</td></tr>
</table>
<h3> SPF policy syntax </h3>
First, the full sender is checked:
<pre>
SPF-Fail:abeb@adelphia.net DSN
</pre>
This says to accept mail from that adelphia.net user despite the
SPF fail, but only after annoying them with a DSN about their ISP's broken
policy.
If there is no match on the full sender, the domain is checked:
<pre>
SPF-Neutral:aol.com REJECT
</pre>
This says to reject mail from AOL with an SPF result of neutral.
This means AOL users can't use their AOL address with another mail service
to send us mail. This is good because the other mail service is
likely a badly configured greeting card site or a virus.
Finally, a default policy for the result is checked. While there are program
defaults, you should have defaults in the access database for SPF results:
<pre>
SPF-Neutral: CBV
SPF-Softfail: DSN
SPF-PermError: DSN
SPF-TempError: REJECT
SPF-None: REJECT
SPF-Fail: REJECT
SPF-Pass: OK
</pre>
<h2> Reputation </h2>
If the sender has not been rejected by this point, and if a GOSSiP server is
configured, we consult GOSSiP for the reputation score of the sender and
SPF result. The score is a number from -100 to 100 with a confidence
percentage from 0 to 100. A really bad reputation (less than -50 with
confidence greater than 3) is rejected. Note that the reputation is tracked
independently for each SPF result and sender combination. So aol.com:neutral
might have a really bad reputation, while aol.com:pass would be ok.
Furthermore, when a sender finally publishes an SPF policy and starts
getting SPF pass, their reputation is effectively reset.