@PACKAGE@ Sendmail milter - Version @VERSION@Packages
The various source and binary packages are available at http://www.five-ten-sg.com/@PACKAGE@/packages/.
The most recent documentation is available at http://www.five-ten-sg.com/@PACKAGE@/.
A Mercurial source
code repository for this project is available at http://hg.five-ten-sg.com/@PACKAGE@/.
Bitcoin donations for this project may be sent to
bitcoin:17n5xJZ9a8csJW2uLeZn7i6jegNKGdLUPJ2016-12-17CarlByington510 Software Group@PACKAGE@1@PACKAGE@ @VERSION@@PACKAGE@a sendmail milter with per-user dnsbl filteringSynopsis@PACKAGE@Options-c
Load the configuration file, print a cannonical form
of the configuration on stdout, and exit.
-s
Stress test the configuration loading code by repeating
the load/free cycle in an infinite loop.
-d n
Set the debug level to n.
-e from|to
Print the results of looking up the from and to addresses in the
current configuration. The | character is used to separate the from and to
addresses in the argument to the -e switch.
-b local-domain-socket-file-name
Set the local socket used for the connection to the dccifd daemon.
This is typically /var/dcc/dccifd.
-r local-domain-socket-file-name
Set the local socket used for the connection to our own dns resolver processes.
-p sendmail-socket
Set the socket used for the milter connection to sendmail. This is either
"inet:port@ip-address" or "local:local-domain-socket-file-name".
-t timeout
Set the timeout in seconds used for communication with sendmail.
Usage@PACKAGE@ -c@PACKAGE@ -s@PACKAGE@ -e 'someone@aol.com|localname@mydomain.tld'@PACKAGE@ -d 10 -r resolver.sock -p local:dnsbl.sockInstallation
This is now a standard GNU autoconf/automake installation, so the normal
"./configure; make; su; make install" works. "make chkconfig" will
setup the init.d runlevel scripts. Alternatively, you can use the
source or binary RPMs at http://www.five-ten-sg.com/@PACKAGE@/packages.
Note that this has ONLY been tested on Linux, specifically RedHat Linux.
In particular, this milter makes no attempt to understand IPv6. Your
mileage will vary. You will need at a minimum a C++ compiler with a
minimally thread safe STL implementation. The distribution includes a
test.cpp program. If it fails this milter won't work. If it passes,
this milter might work.
Modify your sendmail.mc by removing all the "FEATURE(dnsbl" lines, add
the following line in your sendmail.mc and rebuild the .cf file
INPUT_MAIL_FILTER(`dnsbl', `S=local:/var/run/dnsbl/dnsbl.sock, F=T, T=C:30s;S:5m;R:5m;E:5m')
Modify the default @PACKAGE@.conf5 configuration.
Configuration
The configuration file is documented in @PACKAGE@.conf5. Any change to the config file, or any file included
from that config file, will cause it to be reloaded within three
minutes.
Introduction
Consider the case of a mail server that is acting as secondary MX for a
collection of clients, each of which has a collection of mail domains.
Each client may use their own collection of DNSBLs on their primary mail
server. We present here a mechanism whereby the backup mail server can
use the correct set of DNSBLs for each recipient for each message. As a
side-effect, it gives us the ability to customize the set of DNSBLs on a
per-recipient basis, so that fred@example.com could use LOCAL and the
SBL, where all other users @example.com use only the SBL.
This milter can also verify the envelope from/recipient pairs with the
primary MX server. This allows the backup mail servers to properly
reject mail sent to invalid addresses. Otherwise, the backup mail
servers will accept that mail, and then generate a bounce message when
the message is forwarded to the primary server (and rejected there with
no such user). These rejections are the primary cause of such backscatter.
This milter will also decode (uuencode, base64, mime, html entity, url
encodings) and scan for HTTP and HTTPS URLs and bare hostnames in the
body of the mail. If any of those host names have A or NS records on
the SBL (or a single configurable DNSBL), the mail will be rejected
unless previously whitelisted. This milter also counts the number of
invalid HTML tags, and can reject mail if that count exceeds your
specified limit.
This milter can also impose hourly and daily rate
limits on the number of recipients accepted from SMTP
AUTH connections, that would otherwise be allowed to
relay thru this mail server with no spam filtering. If
the connection does not use SMTP AUTH, the rate limits
may be specified by the mail from email address or
domain.
This milter can also impose hourly and daily limits on the number of
different ip addresses used for SMTP AUTH connections. If a single
user is connecting from too many different ip addresses, we presume that
their authentication credentials have been discovered, and block their
outgoing mail.
Consider the case of a message from A to B passing thru this milter. If
that message is not blocked, then we might eventually see a reply
message from B to A. If the filtering context for A includes an
autowhite entry, and that context does not cover B
as a recipient, then this milter will add an entry in that file to
whitelist such replies for a configurable time period. Suppose A and B
are in the same domain, or at least use the same filtering context. In
that case we don't want to add a whitelist entry for B, since that would
then allow spammers to send mail from B (forged) to B. Such autowhite
files need to be writeable by the dnsbl user, where all the other dnsbl
configuration files only need to be readable by the dnsbl user.
You can manually add such an autowhite entry, by appending a single
text line to the autowhitelist file, using something like
echo "$mail 0" >>$autowhitefile.
You can manually remove such an autowhite entry, by appending a single
text line to the autowhitelist file, using something like
echo "$mail 1" >>$autowhitefile.
The DNSBL milter reads a text configuration file (dnsbl.conf) on
startup, and whenever the config file (or any of the referenced include
files) is changed. The entire configuration file is case insensitive.
If the configuration cannot be loaded due to a syntax error, the milter
will log the error and quit. If the configuration cannot be reloaded
after being modified, the milter will log the error and send an email to
root from dnsbl@$hostname. You probably want to add dnsbl@$hostname
to your /etc/mail/virtusertable since otherwise sendmail will reject
that message.
DCC Issues
If you are also using the DCC milter, there
are a few considerations. You may need to whitelist senders from the
DCC bulk detector, or from the DNS based lists. Those are two very
different reasons for whitelisting. The former is done thru the DCC
whiteclnt config file, the later is done thru the DNSBL milter config
file.
You may want to blacklist some specific senders or sending domains.
This could be done thru either the DCC (on a global basis, or for a
specific single recipient). We prefer to do such blacklisting via the
DNSBL milter config, since it can be done for a collection of recipient
mail domains. The DCC approach has the feature that you can capture the
entire message in the DCC log files. The DNSBL milter approach has the
feature that the mail is rejected earlier (at RCPT TO time), and the
sending machine just gets a generic "550 5.7.1 no such user" message.
The DCC whiteclnt file can be included in the DNSBL milter config by the
dcc_to and dcc_from statements. This will import the (env_to, env_from,
and substitute mail_host) entries from the DCC config into the DNSBL
config. This allows using the DCC config as the single point for
white/blacklisting.
Consider the case where you have multiple clients, each with their own
mail servers, and each running their own DCC milters. Each client is
using the DCC facilities for envelope from/to white/blacklisting.
Presumably you can use rsync or scp to fetch copies of your clients DCC
whiteclnt files on a regular basis. Your mail server, acting as a
backup MX for your clients, can use the DNSBL milter, and include those
client DCC config files. The envelope from/to white/blacklisting will
be appropriately tagged and used only for the domains controlled by each
of those clients.
You can now use (via dccifd) different dcc filtering parameters on a per
context basis. See the dcc_greylist and dcc_bulk_threshold statements
in the @PACKAGE@.conf5 configuration. Those
statements are only active if you supply the option
on the dnsbl command line. If you use the dcc via the standard dcc
milter (dccm), then connections from clients that use SMTP AUTH are
still subject to greylisting. If you use the dcc via dccifd and this
milter, then connections from clients that use SMTP AUTH are never
subject to greylisting. As part of this per-user greylisting, you need
to move the dnsblnogrey file from the config directory to something
like /var/dcc/userdirs/local/dnsblnogrey/whiteclnt so the dccifd will
properly ignore greylisting for those recipients that don't want it.
Definitions
CONTEXT - a collection of parameters that defines the filtering context
to be used for a collection of envelope recipient addresses. The
context includes such things as the list of DNSBLs to be used, and the
various content filtering parameters.
DNSBL - a named DNS based blocking list is defined by a dns suffix (e.g.
sbl-xbl.spamhaus.org) and a message string that is used to generate the
"550 5.7.1" smtp error return code. The names of these DNSBLs will be
used to define the DNSBL-LISTs.
DNSBL-LIST - a named list of DNSBLs that will be used for specific
recipients or recipient domains.
DNSWL - a named DNS based white list is defined by a dns suffix (e.g.
list.dnswl.org) and an integer level. If the level is greater than or
equal to x in the 127.0.z.x return code from the white list, then the
ip address is considered to match, and the message will be whitelisted.
The names of these DNSWLs will be used to define the DNSWL-LISTs.
DNSWL-LIST - a named list of DNSWLs that will be used for specific
recipients or recipient domains.
Filtering Procedure
The SMTP envelope 'from' and 'to' values are used in various checks.
The first check is to see if a reply message (swapping the env_from and
env_to values) would be unconditionally blocked (just based on the
envelope from address). That check is similar to the main check
described below, but there is no body content to be scanned, and there
is no client connection ip address to be checked against DNSBLs. If
such a reply message would be blocked, we also block the original
outgoing message. This prevents folks from sending mail to recipients
that are unable to reply.
If the client has authenticated with sendmail, the recipient rate limits
and connection ip address limits are
checked. If the authenticated user has not exceeded the hourly or daily rate
limits, then the mail is accepted, the filtering contexts are not used,
the dns lists are not checked, and the body content is not scanned. These
rate limits can also be applied to unauthenticated connections, in which case
the envelope from value is used as the authentication id for lookup purposes.
If
the client has not authenticated with sendmail, we follow these steps
for each recipient.
The envelope to email address is used to find an initial filtering
context. We first look for a context that specified the full email
address in the env_to statement. If that is not found, we look for a
context that specified the entire domain name of the envelope recipient
in the env_to statement. If that is not found, we look for a context
that specified the user@ part of the envelope recipient in the env_to
statement. If that is not found, we use the first top level context
defined in the config file.
The initial filtering context may redirect to a child context based on
the values in the initial context's env_from statement. We look for [1)
the full envelope from email address, 2) the domain name part of the
envelope from address, 3) the user@ part of the envelope from address]
in that context's env_from statement, with values that point to a child
context. If such an entry is found, we switch to that child filtering
context.
We lookup [1) the full envelope from email address, 2) the domain name
part of the envelope from address, 3) the user@ part of the envelope
from address] in the filtering context env_from statement. That results
in one of (white, black, unknown, inherit).
If the answer is black, mail to this recipient is rejected with "no such
user", and the dns lists are not checked.
If the answer is white, mail to this recipient is accepted and the dns
lists are not checked.
If the answer is unknown, we don't reject yet, but the dns lists will be
checked, and the content may be scanned.
If the answer is inherit, we repeat the envelope from search in the
parent context.
If the mail has not been accepted or rejected yet, and the filtering
context (or any ancestor context) specifies a non-empty whitelist regular
expression, then we check the envelope from value against that regex.
The mail is accepted if the envelope from value matches the specified regular
expression.
If the mail has not been accepted or rejected yet, the dns white lists
specified in the filtering context are checked and the mail is accepted
if any list has an A record for the standard dns based lookup scheme
(reversed octets of the client followed by the dns suffix) with a final
octet greater than or equal to the level specified for that dnswl.
If the mail has not been accepted or rejected yet, the dns black lists
specified in the filtering context are checked and the mail is rejected
if any list has an A record for the standard dns based lookup scheme
(reversed octets of the client followed by the dns suffix).
If the mail has not been accepted or rejected yet, and the filtering
context (or any ancestor context) requires matching reverse dns client
name, the mail is rejected if the client name is empty or forged.
If the mail has not been accepted or rejected yet, and the filtering
context (or any ancestor context) specifies a non-empty generic regular
expression, then we check the fully qualified client name (obtained via
the sendmail macro "_"). The mail is rejected if the client name
matches the specified regular expression.
If the mail has not been accepted or rejected yet, we look for a
verification context, which is the closest ancestor of the filtering
context that both specifies a verification host, and which covers the
envelope to address. If we find such a verification context, and the
verification host is not our own hostname, we open an smtp conversation
with that verification host. The current envelope from and recipient to
values are passed to that verification host. If we receive a 5xy
response those commands, we reject the current recipient with "no such
user".
If the mail has not been accepted or rejected yet, and the filtering
context enables content filtering, and this is the first such recipient
in this smtp transaction, we set the content filtering parameters from
this context, and enable content filtering for the body of this message.
For each recipient that was accepted, we search for an autowhite entry
starting in the reply filtering context. If an autowhite entry is found,
we add the recipient to that auto whitelist file. This will prevent reply
messages from being blocked by the dnsbl or content filtering.
If content filtering is enabled for this body, the mail text is decoded
(uuencode, base64, mime, html entity, url encodings), and scanned for HTTP
and HTTPS URLs or bare host names. Hostnames must be either ip address
literals, or must end in a string defined by the TLD list. The first
<configurable> host names are checked as follows.
The only known list that is suitable for the content filter DNSBL is the
SBL. If the content filter DNSBL is defined, and any of those host
names resolve to ip addresses that are on that DNSBL (or have
nameservers that are on that list), and the host name is not on the
<configurable> ignore list, the mail is rejected.
If the content uribl DNSBL is defined, and any of those host names are
on that DNSBL, and the host name is not on the <configurable>
ignore list, the mail is rejected. There are three lists that are suitable
here, URIBL, SURBL, and DBL.
If any non-whitelisted recipient has a filtering context with a non-zero
spamassassin limit, then the message is passed thru spamassassin (via
spamc), and the message is rejected for those recipients with spamassassin
limits less than the resulting spamassassin score. For example, a
spamassassin limit of three will reject messages with spamassassin scores
of four or greater. If the filtering context has a spamassassin limit of
zero, then spamassassin is not called (or if called the results are not used)
for this recipient.
If any non-whitelisted recipient has a filtering context that specifies
DCC greylisting, then the message is passed thru the DCC bulk detector,
and the message is greylisted (for all recipients) if the DCC says this
message should be delayed.
If any non-whitelisted recipient has a filtering context with a non-zero
DCC bulk threshold, then the message is passed thru the DCC bulk detector,
and the message is rejected for those recipients with DCC thresholds less
than or equal to the DCC bulk score.
We also scan for excessive bad html tags, and if a <configurable>
limit is exceeded, the mail is rejected.
Sendmail access vs. DNSBL
With the standard sendmail.mc dnsbl FEATURE, the dnsbl checks may be
suppressed by entries in the /etc/mail/access database. For example,
suppose you control a /18 of address space, and have allocated some /24s
to some clients. You have access entries like
to allow those clients to smarthost thru your mail server. Now if one
of those clients happens get infected with a virus that turns a machine
into an open proxy, and their 192.168.4.45 lands on the SBL-XBL, you
will still wind up allowing that infected machine to smarthost thru your
mail servers.
With this DNSBL milter, the sendmail access database cannot override the
dnsbl checks, so that machine won't be able to send mail to or thru your
smarthost mail server (unless the virus/proxy can use smtp-auth).
Using the standard sendmail features, you would add access entries to
allow hosts on your local network to relay thru your mail server. Those
OK entries in the sendmail access database will override all the dnsbl
checks. With this DNSBL milter, you will need to have the local users
authenticate with smtp-auth to get the same effect. You might find
these directions helpful for setting up smtp-auth if you are on
RH Linux.
Performance Issues
Consider a high volume high performance machine running sendmail. Each
sendmail process can do its own dns resolution. Typically, such dns
resolver libraries are not thread safe, and so must be protected by some
sort of mutex in a threaded environment. When we add a milter to
sendmail, we now have a collection of sendmail processes, and a
collection of milter threads.
We will be doing a lot of dns lookups per mail message, and at least
some of those will take many tens of seconds. If all this dns work is
serialized inside the milter, we have an upper limit of about 25K mail
messages per day. That is clearly not sufficient for many sites.
Since we want to do parallel dns resolution across those milter threads,
we add another collection of dns resolver processes. Each sendmail
process is talking to a milter thread over a socket, and each milter
thread is talking to a dns resolver process over another socket.
Suppose we are processing 20 messages per second, and each message
requires 20 seconds of dns work. Then we will have 400 sendmail
processes, 400 milter threads, and 400 dns resolver processes. Of
course that steady state is very unlikely to happen.
Rejected Ideas
The following ideas have been considered and rejected.
Add max_recipients setting to the context configuration. Recipients in
excess of that limit will be rejected, and all the non-whitelisted
recipients will be removed. Current spammers *very* rarely send more
than ten recipients in a single smtp transaction, so this won't stop any
significant amount of spam.
Add poison addresses to the configuration. If any recipient is
poison, all recipients are rejected even if they would be whitelisted,
and the data is rejected if sent. I have a collection of spam trap
addresses that would be suitable for such use. Based on my log files,
any mail to those spam trap addresses is rejected based on either dnsbl
lookups or the DCC. So this won't result in blocking any additional
spam.
Add an option to only allow one recipient if the return path is
empty. Based on my log files, there is no mail that violates this
check.
Reject the mail if the envelope from domain name contains any MX
records pointing to 127.0.0.0/8. I don't see any significant amount of
spam sent with such domain names.
TODO
The following ideas are under consideration.
Add config switch to require the HELO argument to resolve to an ip address.
Add white/unknown to config for smtp authenticated connections. Currently
any authenticated connection is fully whitelisted. The only spam control
on those connections is rate limiting. This feature would allow content based
spam controls to be applied even to authenticated connections. Add
context/authenticated_dnsbl_list and context/content/authenticated.
Add an optional list of domains to be enforced on the env_from value for
authenticated connections. User abc could be restricted to envelope from
values of a.com and b.com, user def could be restricted to envelope from
values of dd.com and ee.com.
Look for href="hostname/path" strings that are missing the required
http:// protocol header. Such references are still clickable in common
mail software.
Add spf to the white/black/unknown values in env_from blocks. This
results in whitelisting that envelope from value as long as the connection
is made from an ip address listed in the domain spf txt record.
Copyright
Copyright (C) 2012 by 510 Software Group <carl@five-ten-sg.com>
This program is free software; you can redistribute it and/or modify it
under the terms of the GNU General Public License as published by the
Free Software Foundation; either version 3, or (at your option) any
later version.
You should have received a copy of the GNU General Public License along
with this program; see the file COPYING. If not, please write to the
Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
Version
@VERSION@
2016-12-17CarlByington510 Software Group@PACKAGE@.conf5@PACKAGE@ @VERSION@@PACKAGE@.confconfiguration file for @PACKAGE@ sendmail milterSynopsis@PACKAGE@.confDescriptionThe @PACKAGE@.conf configuration file is
specified by this partial bnf description. Comments start with //
or # and extend to the end of the line. To include the contents
of some file verbatim in the dnsbl.conf file, use
";]]>Sample" black;
};
// hourly recipient rate limit by smtp auth client id, or unauthenticated mail from address
// hourly unique ip addresses by smtp auth client id, or unauthenticated mail from address
// default hourly recipient rate limit is 30
// daily recipient rate limits are 4 times the hourly limit
// default hourly unique ip addresses is 5
// daily unique ip addresses are 4 times the hourly limit
rate_limit 30 4 5 4 { // default
fred 100 10; // override default limits
joe 10 2; // ""
"sam@somedomain.tld" 500 2;
"@otherdomain.tld" 100 2;
};
};
context main {
dnsbl localp partial.blackholes.five-ten-sg.com "Mail from %s rejected - local; see http://www.five-ten-sg.com/blackhole.php?%s";
dnsbl local blackholes.five-ten-sg.com "Mail from %s rejected - local; see http://www.five-ten-sg.com/blackhole.php?%s";
dnsbl sbl zen.spamhaus.org "Mail from %s rejected - sbl; see http://www.spamhaus.org/query/bl?ip=%s";
dnsbl xbl xbl.spamhaus.org "Mail from %s rejected - xbl; see http://www.spamhaus.org/query/bl?ip=%s";
dnswl dnswl.org list.dnswl.org 2;
dnsbl_list local sbl;
dnswl_list dnswl.org;
require_rdns yes;
content on {
dkim_signer {
credit.paypal.com require_signed credit.paypal.com;
paypal.com require_signed paypal.com;
dhl.com require_signed dhl.com;
adp.com require_signed "adp.com,bmi.adp.com";
};
dkim_from {
accounts.google.com white;
};
filter sbl-xbl.spamhaus.org "Mail containing %s rejected - sbl; see http://www.spamhaus.org/query/bl?ip=%s";
uribl multi.surbl.org "Mail containing %s rejected - surbl; see http://www.surbl.org/surbl-analysis?d=%s";
#uribl multi.uribl.com "Mail containing %s rejected - uribl; see http://l.uribl.com/?d=%s";
#uribl dbl.spamhaus.org "Mail containing %s rejected - dbl; see http://www.spamhaus.org/query/domain?domain=%s";
ignore { include "hosts-ignore.conf"; };
tld { include "tld.conf"; };
html_tags { include "html-tags.conf"; };
html_limit off;
host_limit soft 20;
spamassassin 5;
require_match yes;
dcc_greylist yes;
dcc_bulk_threshold 20;
};
generic "^dsl.static.*ttnet.net.tr$|(^|[x.-])(ppp|h|host)?([0-9]{1,3}[x.-](Red-|dynamic[x.-])?){4}"
"your mail server %s seems to have a generic name";
white_regex "=example.com=user@yourhostingaccount.com$";
env_to {
# !! replace this with your domain names
# child contexts are not allowed to specify recipient addresses outside these domains
# if this is a backup-mx, you need to include here domains for which you relay to the primary mx
include "/etc/mail/local-host-names";
};
context whitelist {
content off {};
env_to {
# dcc_to ok { include "/var/dcc/whitecommon"; };
};
env_from white {}; # white forces all unmatched from addresses (everyone in this case) to be whitelisted
# so all mail TO these env_to addresses is accepted
};
context abuse {
dnsbl_list xbl;
content off {};
generic "^$ " " "; # regex cannot match, to disable generic rdns rejects
env_to {
abuse@ # no content filtering on abuse reports
postmaster@ # ""
};
env_from unknown {}; # ignore all parent white/black listing
};
context minimal {
dnsbl_list sbl;
content on {
spamassassin 10;
dcc_bulk_threshold many;
};
generic "^$ " " "; # regex cannot match, to disable generic rdns rejects
env_to {
};
};
context blacklist {
dnsbl_list ;
dnswl_list ;
env_to {
# dcc_to many { include "/var/dcc/whitecommon"; };
};
env_from black {}; # black forces all unmatched from addresses (everyone in this case) to be blacklisted
# so all mail TO these env_to addresses is rejected
};
env_from unknown {
abuse@ abuse; # replies to abuse reports use the abuse context
# dcc_from { include "/var/dcc/whitecommon"; };
};
autowhite 90 "autowhite/my-auto-whitelist";
# install should create /etc/dnsbl/autowhite writable by userid dnsbl
};]]>Version
@VERSION@