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4 <meta http-equiv="Content-Type" content="text/html; charset=windows-1252">
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5 <title>DNSBL Sendmail milter</title>
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6 </head>
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7
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8 <p>This milter is released under the GPL license version 2 included in
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9 the LICENSE file in the distribution, and also available at
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10 <a href="http://www.gnu.org/licenses/gpl.html">http://www.gnu.org/licenses/gpl.html</a>
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11
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12 <p>Consider the case of a mail server that is acting as secondary MX
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13 for a collection of clients, each of which has a collection of mail
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14 domains. Each client may use their own collection of DNSBLs on their
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15 primary mail server. We present here a mechanism whereby the backup
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16 mail server can use the correct set of DNSBLs for each message. As a
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17 side-effect, it gives us the ability to customize the set of DNSBLs on a
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18 per-recipient basis, so that fred@example.com could use SPEWS and the
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19 SBL, where all other users @example.com use only the SBL.
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20
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21 <p>The DNSBL milter reads a text configuration file on startup, and
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22 whenever the config file (or any of the referenced include files) is
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23 changed. The entire configuration file is case insensitive.
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24
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25 <p>If you are also using the <a
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26 href="http://www.rhyolite.com/anti-spam/dcc/">DCC</a> milter, there are
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27 a few considerations. You may need to whitelist senders from the DCC
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28 bulk detector, or from the DNS based lists. Those are two very
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29 different reasons for whitelisting. The former is done thru the DCC
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30 whiteclnt config file, the later is done thru the DNSBL milter config
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31 file.
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32
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33 <p>You may want to blacklist some specific senders or sending domains.
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34 This could be done thru either the DCC (on a global basis, or for a
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35 specific single recipient). We prefer to do such blacklisting via the
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36 DNSBL milter config, since it can be done for an entire recipient mail
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37 domain. The DCC approach has the feature that you can capture the
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38 entire message in the DCC log files. The DNSBL milter approach has the
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39 feature that the mail is rejected earlier (at RCPT TO time), and the
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40 sending machine just gets a generic "550 5.7.1 no such user" message.
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41
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42 <p>Definitions:
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43
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44 <p>DNSBL - a named DNS based blocking list is defined by a dns suffix
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45 (e.g. sbl-xbl.spamhaus.org) and a message string that is used to
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46 generate the "550 5.7.1" smtp error return code. The names of these
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47 DNSBLs will be used to define the DNSBL-LISTs.
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48
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49 <p>DNSBL-LIST - a named list of DNSBLs that will be used for specific
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50 recipients or recipient domains.
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51
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52 <p>ENVELOPE-FROM-MAP - a named collection of mappings (key->value pairs)
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53 from envelope-from values to the WHITE, BLACK, or DEFAULT keywords. The
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54 names of these maps will be used for specific recipients or recipient
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55 domains.
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56
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57 <p>The configuration file maps each recipient (or recipient domain) to
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58 two names (a named DNSBL-LIST, and a named ENVELOPE-FROM-MAP). If the
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59 recipient is not found in the configuration, the named DEFAULT
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60 dnsbl-list and DEFAULT envelope-from-map will be used. When mail is
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61 received for that recipient,
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62
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63 <ol>
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64
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65 <li>If the client has authenticated with sendmail, the mail is accepted
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66 and the dns lists are not checked.
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67
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68 <li>If either one is BLACK, mail to this recipient is rejected with "no
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69 such user", and the dns lists are not checked.
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70
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71 <li>If the envelope-from-map name is WHITE, mail to this recipient is
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72 accepted and the dns lists are not checked.
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73
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74 <li>If the envelope-from-map exists, the map is checked for the presence
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75 of the sender. A WHITE or BLACK answer is definitive and the dns lists
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76 are not checked.
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77
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78 <li>If the dnsbl-list name is WHITE, the dns lists are not checked and
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79 the mail is accepted. Otherwise, the dns lists are checked and the mail
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80 is rejected if any list has an A record for the standard dns based
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81 lookup scheme (reversed octets of the client followed by the dns
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82 suffix).
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83
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84 </ol>
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85
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86
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87 <p>Usage: Note that this has ONLY been tested on Linux, specifically
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88 RedHat Linux. Your mileage will vary. In particular, this milter makes no
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89 attempt to understand IPv6.
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90
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91 Fetch <a href="http://www.five-ten-sg.com/util/dnsbl.tar.gz">dnsbl.tar.gz</a>
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92 and
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93
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94 <pre>
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95 tar xfvz dnsbl.tar.gz
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96 bash install.bash
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97 </pre>
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98
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99 Read and understand the contents of that install.bash script before you
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100 run it. It may not be suitable for your system. Modify your
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101 sendmail.mc by removing all the "FEATURE(dnsbl" lines, add the following
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102 line in your sendmail.mc and rebuild the .cf file
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103
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104 <pre>
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105 INPUT_MAIL_FILTER(`dnsbl', `S=local:/var/run/dnsbl/dnsbl.sock, F=T, T=S:30s;R:30s;E:30s')
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106 </pre>
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107
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108 Read the sample <a
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109 href="http://www.five-ten-sg.com/dnsbl.conf">var/dnsbl/dnsbl.conf</a>
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110 file and modify it to fit your configuration.
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111
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112
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113 </body>
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114 </html>
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