Mercurial > dnsbl
annotate xml/dnsbl.in @ 294:7fb5911fe3a4
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author | Carl Byington <carl@five-ten-sg.com> |
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date | Fri, 18 Jul 2014 15:04:16 -0700 |
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108 | 1 <reference> |
2 <title>@PACKAGE@ Sendmail milter - Version @VERSION@</title> | |
3 <partintro> | |
4 <title>Packages</title> | |
201
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5 |
108 | 6 <para>The various source and binary packages are available at <ulink |
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7 url="http://www.five-ten-sg.com/@PACKAGE@/packages/">http://www.five-ten-sg.com/@PACKAGE@/packages/</ulink>. |
108 | 8 The most recent documentation is available at <ulink |
201
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9 url="http://www.five-ten-sg.com/@PACKAGE@/">http://www.five-ten-sg.com/@PACKAGE@/</ulink>. |
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10 </para> |
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11 |
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12 <para>A <ulink |
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13 url="http://www.selenic.com/mercurial/wiki/">Mercurial</ulink> source |
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14 code repository for this project is available at <ulink |
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15 url="http://hg.five-ten-sg.com/@PACKAGE@/">http://hg.five-ten-sg.com/@PACKAGE@/</ulink>. |
108 | 16 </para> |
94 | 17 |
108 | 18 </partintro> |
94 | 19 |
108 | 20 <refentry id="@PACKAGE@.1"> |
21 <refentryinfo> | |
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22 <date>2013-05-22</date> |
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23 <author> |
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24 <firstname>Carl</firstname> |
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25 <surname>Byington</surname> |
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26 <affiliation><orgname>510 Software Group</orgname></affiliation> |
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27 </author> |
108 | 28 </refentryinfo> |
94 | 29 |
108 | 30 <refmeta> |
31 <refentrytitle>@PACKAGE@</refentrytitle> | |
32 <manvolnum>1</manvolnum> | |
33 <refmiscinfo>@PACKAGE@ @VERSION@</refmiscinfo> | |
34 </refmeta> | |
35 | |
36 <refnamediv id='name.1'> | |
37 <refname>@PACKAGE@</refname> | |
38 <refpurpose>a sendmail milter with per-user dnsbl filtering</refpurpose> | |
39 </refnamediv> | |
94 | 40 |
108 | 41 <refsynopsisdiv id='synopsis.1'> |
42 <title>Synopsis</title> | |
43 <cmdsynopsis> | |
44 <command>@PACKAGE@</command> | |
45 <arg><option>-c</option></arg> | |
46 <arg><option>-s</option></arg> | |
47 <arg><option>-d <replaceable class="parameter">n</replaceable></option></arg> | |
48 <arg><option>-e <replaceable class="parameter">from|to</replaceable></option></arg> | |
179 | 49 <arg><option>-b <replaceable class="parameter">local-domain-socket</replaceable></option></arg> |
108 | 50 <arg><option>-r <replaceable class="parameter">local-domain-socket</replaceable></option></arg> |
51 <arg><option>-p <replaceable class="parameter">sendmail-socket</replaceable></option></arg> | |
52 <arg><option>-t <replaceable class="parameter">timeout</replaceable></option></arg> | |
53 </cmdsynopsis> | |
54 </refsynopsisdiv> | |
94 | 55 |
108 | 56 <refsect1 id='options.1'> |
57 <title>Options</title> | |
58 <variablelist> | |
59 <varlistentry> | |
60 <term>-c</term> | |
111 | 61 <listitem><para> |
62 Load the configuration file, print a cannonical form | |
63 of the configuration on stdout, and exit. | |
64 </para></listitem> | |
108 | 65 </varlistentry> |
66 <varlistentry> | |
67 <term>-s</term> | |
111 | 68 <listitem><para> |
69 Stress test the configuration loading code by repeating | |
70 the load/free cycle in an infinite loop. | |
71 </para></listitem> | |
108 | 72 </varlistentry> |
73 <varlistentry> | |
74 <term>-d <replaceable class="parameter">n</replaceable></term> | |
111 | 75 <listitem><para> |
76 Set the debug level to <replaceable class="parameter">n</replaceable>. | |
77 </para></listitem> | |
108 | 78 </varlistentry> |
79 <varlistentry> | |
80 <term>-e <replaceable class="parameter">from|to</replaceable></term> | |
111 | 81 <listitem><para> |
82 Print the results of looking up the from and to addresses in the | |
83 current configuration. The | character is used to separate the from and to | |
84 addresses in the argument to the -e switch. | |
85 </para></listitem> | |
108 | 86 </varlistentry> |
87 <varlistentry> | |
179 | 88 <term>-b <replaceable class="parameter">local-domain-socket-file-name</replaceable></term> |
89 <listitem><para> | |
90 Set the local socket used for the connection to the dccifd daemon. | |
91 This is typically /var/dcc/dccifd. | |
92 </para></listitem> | |
93 </varlistentry> | |
94 <varlistentry> | |
95 <term>-r <replaceable class="parameter">local-domain-socket-file-name</replaceable></term> | |
111 | 96 <listitem><para> |
97 Set the local socket used for the connection to our own dns resolver processes. | |
98 </para></listitem> | |
108 | 99 </varlistentry> |
100 <varlistentry> | |
101 <term>-p <replaceable class="parameter">sendmail-socket</replaceable></term> | |
111 | 102 <listitem><para> |
103 Set the socket used for the milter connection to sendmail. This is either | |
104 "inet:port@ip-address" or "local:local-domain-socket-file-name". | |
105 </para></listitem> | |
108 | 106 </varlistentry> |
107 <varlistentry> | |
108 <term>-t <replaceable class="parameter">timeout</replaceable></term> | |
111 | 109 <listitem><para> |
110 Set the timeout in seconds used for communication with sendmail. | |
111 </para></listitem> | |
108 | 112 </varlistentry> |
113 </variablelist> | |
114 </refsect1> | |
94 | 115 |
111 | 116 <refsect1 id='usage.1'> |
108 | 117 <title>Usage</title> |
118 <para><command>@PACKAGE@</command> -c</para> | |
119 <para><command>@PACKAGE@</command> -s</para> | |
111 | 120 <para><command>@PACKAGE@</command> -e 'someone@aol.com|localname@mydomain.tld'</para> |
121 <para><command>@PACKAGE@</command> -d 10 -r resolver.sock -p local:dnsbl.sock</para> | |
122 </refsect1> | |
123 | |
124 <refsect1 id='installation.1'> | |
125 <title>Installation</title> | |
126 <para> | |
127 This is now a standard GNU autoconf/automake installation, so the normal | |
128 "./configure; make; su; make install" works. "make chkconfig" will | |
129 setup the init.d runlevel scripts. Alternatively, you can use the | |
130 source or binary RPMs at <ulink | |
131 url="http://www.five-ten-sg.com/@PACKAGE@/packages">http://www.five-ten-sg.com/@PACKAGE@/packages</ulink>. | |
132 </para> | |
133 <para> | |
134 Note that this has ONLY been tested on Linux, specifically RedHat Linux. | |
135 In particular, this milter makes no attempt to understand IPv6. Your | |
136 mileage will vary. You will need at a minimum a C++ compiler with a | |
137 minimally thread safe STL implementation. The distribution includes a | |
138 test.cpp program. If it fails this milter won't work. If it passes, | |
139 this milter might work. | |
140 </para> | |
141 <para> | |
142 Modify your sendmail.mc by removing all the "FEATURE(dnsbl" lines, add | |
143 the following line in your sendmail.mc and rebuild the .cf file | |
144 </para> | |
145 <para><screen>INPUT_MAIL_FILTER(`dnsbl', `S=local:/var/run/dnsbl/dnsbl.sock, F=T, T=C:30s;S:5m;R:5m;E:5m')</screen></para> | |
146 <para> | |
147 Modify the default <citerefentry> | |
148 <refentrytitle>@PACKAGE@.conf</refentrytitle> <manvolnum>5</manvolnum> | |
149 </citerefentry> configuration. | |
150 </para> | |
151 </refsect1> | |
152 | |
153 <refsect1 id='configuration.1'> | |
154 <title>Configuration</title> | |
155 <para> | |
156 The configuration file is documented in <citerefentry> | |
157 <refentrytitle>@PACKAGE@.conf</refentrytitle> <manvolnum>5</manvolnum> | |
158 </citerefentry>. Any change to the config file, or any file included | |
159 from that config file, will cause it to be reloaded within three | |
160 minutes. | |
161 </para> | |
108 | 162 </refsect1> |
94 | 163 |
108 | 164 <refsect1 id='introduction.1'> |
165 <title>Introduction</title> | |
166 <para> | |
167 Consider the case of a mail server that is acting as secondary MX for a | |
168 collection of clients, each of which has a collection of mail domains. | |
169 Each client may use their own collection of DNSBLs on their primary mail | |
170 server. We present here a mechanism whereby the backup mail server can | |
171 use the correct set of DNSBLs for each recipient for each message. As a | |
172 side-effect, it gives us the ability to customize the set of DNSBLs on a | |
183 | 173 per-recipient basis, so that fred@example.com could use LOCAL and the |
108 | 174 SBL, where all other users @example.com use only the SBL. |
175 </para> | |
176 <para> | |
177 This milter can also verify the envelope from/recipient pairs with the | |
178 primary MX server. This allows the backup mail servers to properly | |
179 reject mail sent to invalid addresses. Otherwise, the backup mail | |
180 servers will accept that mail, and then generate a bounce message when | |
181 the message is forwarded to the primary server (and rejected there with | |
127 | 182 no such user). These rejections are the primary cause of such backscatter. |
108 | 183 </para> |
184 <para> | |
185 This milter will also decode (uuencode, base64, mime, html entity, url | |
186 encodings) and scan for HTTP and HTTPS URLs and bare hostnames in the | |
187 body of the mail. If any of those host names have A or NS records on | |
188 the SBL (or a single configurable DNSBL), the mail will be rejected | |
189 unless previously whitelisted. This milter also counts the number of | |
190 invalid HTML tags, and can reject mail if that count exceeds your | |
191 specified limit. | |
192 </para> | |
193 <para> | |
259
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194 This milter can also impose hourly and daily rate |
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195 limits on the number of recipients accepted from SMTP |
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196 AUTH connections, that would otherwise be allowed to |
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197 relay thru this mail server with no spam filtering. If |
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198 the connection does not use SMTP AUTH, the rate limits |
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199 may be specified by the mail from email address or |
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200 domain. |
136 | 201 </para> |
202 <para> | |
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203 This milter can also impose hourly and daily limits on the number of |
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204 different ip addresses used for SMTP AUTH connections. If a single |
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205 user is connecting from too many different ip addresses, we presume that |
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206 their authentication credentials have been discovered, and block their |
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207 outgoing mail. |
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208 </para> |
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209 <para> |
162 | 210 Consider the case of a message from A to B passing thru this milter. If |
211 that message is not blocked, then we might eventually see a reply | |
156 | 212 message from B to A. If the filtering context for A includes an |
162 | 213 autowhite entry, and that context does <emphasis>not</emphasis> cover B |
214 as a recipient, then this milter will add an entry in that file to | |
215 whitelist such replies for a configurable time period. Suppose A and B | |
216 are in the same domain, or at least use the same filtering context. In | |
217 that case we don't want to add a whitelist entry for B, since that would | |
218 then allow spammers to send mail from B (forged) to B. Such autowhite | |
160 | 219 files need to be writeable by the dnsbl user, where all the other dnsbl |
220 configuration files only need to be readable by the dnsbl user. | |
156 | 221 </para> |
222 <para> | |
176
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223 You can manually add such an autowhite entry, by appending a single |
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224 text line to the autowhitelist file, using something like |
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225 <command>echo "$mail 0" >>$autowhitefile</command>. |
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226 You can manually remove such an autowhite entry, by appending a single |
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227 text line to the autowhitelist file, using something like |
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228 <command>echo "$mail 1" >>$autowhitefile</command>. |
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229 </para> |
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230 <para> |
108 | 231 The DNSBL milter reads a text configuration file (dnsbl.conf) on |
232 startup, and whenever the config file (or any of the referenced include | |
233 files) is changed. The entire configuration file is case insensitive. | |
234 If the configuration cannot be loaded due to a syntax error, the milter | |
235 will log the error and quit. If the configuration cannot be reloaded | |
236 after being modified, the milter will log the error and send an email to | |
152 | 237 root from dnsbl@$hostname. You probably want to add dnsbl@$hostname |
108 | 238 to your /etc/mail/virtusertable since otherwise sendmail will reject |
239 that message. | |
240 </para> | |
241 </refsect1> | |
94 | 242 |
111 | 243 <refsect1 id='dcc.1'> |
108 | 244 <title>DCC Issues</title> |
245 <para> | |
246 If you are also using the <ulink | |
247 url="http://www.rhyolite.com/anti-spam/dcc/">DCC</ulink> milter, there | |
248 are a few considerations. You may need to whitelist senders from the | |
249 DCC bulk detector, or from the DNS based lists. Those are two very | |
250 different reasons for whitelisting. The former is done thru the DCC | |
251 whiteclnt config file, the later is done thru the DNSBL milter config | |
252 file. | |
253 </para> | |
254 <para> | |
255 You may want to blacklist some specific senders or sending domains. | |
256 This could be done thru either the DCC (on a global basis, or for a | |
257 specific single recipient). We prefer to do such blacklisting via the | |
258 DNSBL milter config, since it can be done for a collection of recipient | |
259 mail domains. The DCC approach has the feature that you can capture the | |
260 entire message in the DCC log files. The DNSBL milter approach has the | |
261 feature that the mail is rejected earlier (at RCPT TO time), and the | |
262 sending machine just gets a generic "550 5.7.1 no such user" message. | |
263 </para> | |
264 <para> | |
265 The DCC whiteclnt file can be included in the DNSBL milter config by the | |
266 dcc_to and dcc_from statements. This will import the (env_to, env_from, | |
267 and substitute mail_host) entries from the DCC config into the DNSBL | |
268 config. This allows using the DCC config as the single point for | |
269 white/blacklisting. | |
270 </para> | |
271 <para> | |
272 Consider the case where you have multiple clients, each with their own | |
273 mail servers, and each running their own DCC milters. Each client is | |
274 using the DCC facilities for envelope from/to white/blacklisting. | |
275 Presumably you can use rsync or scp to fetch copies of your clients DCC | |
276 whiteclnt files on a regular basis. Your mail server, acting as a | |
277 backup MX for your clients, can use the DNSBL milter, and include those | |
278 client DCC config files. The envelope from/to white/blacklisting will | |
279 be appropriately tagged and used only for the domains controlled by each | |
280 of those clients. | |
281 </para> | |
179 | 282 <para> |
283 You can now use (via dccifd) different dcc filtering parameters on a per | |
284 context basis. See the dcc_greylist and dcc_bulk_threshold statements | |
285 in the <citerefentry> <refentrytitle>@PACKAGE@.conf</refentrytitle> | |
286 <manvolnum>5</manvolnum> </citerefentry> configuration. Those | |
287 statements are only active if you supply the <option>-b</option> option | |
288 on the dnsbl command line. If you use the dcc via the standard dcc | |
289 milter (dccm), then connections from clients that use SMTP AUTH are | |
290 still subject to greylisting. If you use the dcc via dccifd and this | |
291 milter, then connections from clients that use SMTP AUTH are never | |
180 | 292 subject to greylisting. As part of this per-user greylisting, you need |
293 to move the dnsblnogrey file from the config directory to something | |
294 like /var/dcc/userdirs/local/dnsblnogrey/whiteclnt so the dccifd will | |
295 properly ignore greylisting for those recipients that don't want it. | |
179 | 296 </para> |
108 | 297 </refsect1> |
94 | 298 |
111 | 299 <refsect1 id='definitions.1'> |
108 | 300 <title>Definitions</title> |
301 <para> | |
302 CONTEXT - a collection of parameters that defines the filtering context | |
303 to be used for a collection of envelope recipient addresses. The | |
304 context includes such things as the list of DNSBLs to be used, and the | |
305 various content filtering parameters. | |
306 </para> | |
307 <para> | |
308 DNSBL - a named DNS based blocking list is defined by a dns suffix (e.g. | |
309 sbl-xbl.spamhaus.org) and a message string that is used to generate the | |
310 "550 5.7.1" smtp error return code. The names of these DNSBLs will be | |
311 used to define the DNSBL-LISTs. | |
312 </para> | |
313 <para> | |
314 DNSBL-LIST - a named list of DNSBLs that will be used for specific | |
315 recipients or recipient domains. | |
316 </para> | |
249 | 317 <para> |
318 DNSWL - a named DNS based white list is defined by a dns suffix (e.g. | |
319 list.dnswl.org) and an integer level. If the level is greater than or | |
320 equal to x in the 127.0.z.x return code from the white list, then the | |
321 ip address is considered to match, and the message will be whitelisted. | |
322 The names of these DNSWLs will be used to define the DNSWL-LISTs. | |
323 </para> | |
324 <para> | |
325 DNSWL-LIST - a named list of DNSWLs that will be used for specific | |
326 recipients or recipient domains. | |
327 </para> | |
108 | 328 </refsect1> |
94 | 329 |
111 | 330 <refsect1 id='filtering.1'> |
108 | 331 <title>Filtering Procedure</title> |
332 <para> | |
152 | 333 The SMTP envelope 'from' and 'to' values are used in various checks. |
334 The first check is to see if a reply message (swapping the env_from and | |
160 | 335 env_to values) would be unconditionally blocked (just based on the |
336 envelope from address). That check is similar to the main check | |
337 described below, but there is no body content to be scanned, and there | |
338 is no client connection ip address to be checked against DNSBLs. If | |
339 such a reply message would be blocked, we also block the original | |
340 outgoing message. This prevents folks from sending mail to recipients | |
341 that are unable to reply. | |
152 | 342 </para> |
343 <para> | |
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344 If the client has authenticated with sendmail, the recipient rate limits |
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345 and connection ip address limits are |
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346 checked. If the authenticated user has not exceeded the hourly or daily rate |
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347 limits, then the mail is accepted, the filtering contexts are not used, |
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348 the dns lists are not checked, and the body content is not scanned. These |
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349 rate limits can also be applied to unauthenticated connections, in which case |
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350 the envelope from value is used as the authentication id for lookup purposes. |
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351 If |
136 | 352 the client has not authenticated with sendmail, we follow these steps |
353 for each recipient. | |
108 | 354 </para> |
355 <orderedlist> | |
111 | 356 <listitem><para> |
108 | 357 The envelope to email address is used to find an initial filtering |
358 context. We first look for a context that specified the full email | |
359 address in the env_to statement. If that is not found, we look for a | |
360 context that specified the entire domain name of the envelope recipient | |
361 in the env_to statement. If that is not found, we look for a context | |
362 that specified the user@ part of the envelope recipient in the env_to | |
363 statement. If that is not found, we use the first top level context | |
364 defined in the config file. | |
111 | 365 </para></listitem> |
366 <listitem><para> | |
108 | 367 The initial filtering context may redirect to a child context based on |
368 the values in the initial context's env_from statement. We look for [1) | |
369 the full envelope from email address, 2) the domain name part of the | |
370 envelope from address, 3) the user@ part of the envelope from address] | |
371 in that context's env_from statement, with values that point to a child | |
372 context. If such an entry is found, we switch to that child filtering | |
373 context. | |
111 | 374 </para></listitem> |
375 <listitem><para> | |
108 | 376 We lookup [1) the full envelope from email address, 2) the domain name |
377 part of the envelope from address, 3) the user@ part of the envelope | |
378 from address] in the filtering context env_from statement. That results | |
379 in one of (white, black, unknown, inherit). | |
111 | 380 </para></listitem> |
381 <listitem><para> | |
108 | 382 If the answer is black, mail to this recipient is rejected with "no such |
383 user", and the dns lists are not checked. | |
111 | 384 </para></listitem> |
385 <listitem><para> | |
108 | 386 If the answer is white, mail to this recipient is accepted and the dns |
387 lists are not checked. | |
111 | 388 </para></listitem> |
389 <listitem><para> | |
108 | 390 If the answer is unknown, we don't reject yet, but the dns lists will be |
391 checked, and the content may be scanned. | |
111 | 392 </para></listitem> |
393 <listitem><para> | |
108 | 394 If the answer is inherit, we repeat the envelope from search in the |
395 parent context. | |
111 | 396 </para></listitem> |
397 <listitem><para> | |
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398 If the mail has not been accepted or rejected yet, and the filtering |
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399 context (or any ancestor context) specifies a non-empty whitelist regular |
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400 expression, then we check the envelope from value against that regex. |
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401 The mail is accepted if the envelope from value matches the specified regular |
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402 expression. |
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403 </para></listitem> |
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404 <listitem><para> |
249 | 405 If the mail has not been accepted or rejected yet, the dns white lists |
406 specified in the filtering context are checked and the mail is accepted | |
407 if any list has an A record for the standard dns based lookup scheme | |
408 (reversed octets of the client followed by the dns suffix) with a final | |
409 octet greater than or equal to the level specified for that dnswl. | |
410 </para></listitem> | |
411 <listitem><para> | |
412 If the mail has not been accepted or rejected yet, the dns black lists | |
168 | 413 specified in the filtering context are checked and the mail is rejected |
414 if any list has an A record for the standard dns based lookup scheme | |
415 (reversed octets of the client followed by the dns suffix). | |
416 </para></listitem> | |
417 <listitem><para> | |
418 If the mail has not been accepted or rejected yet, and the filtering | |
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419 context (or any ancestor context) requires matching reverse dns client |
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420 name, the mail is rejected if the client name is empty or forged. |
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421 </para></listitem> |
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422 <listitem><para> |
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423 If the mail has not been accepted or rejected yet, and the filtering |
170 | 424 context (or any ancestor context) specifies a non-empty generic regular |
425 expression, then we check the fully qualified client name (obtained via | |
426 the sendmail macro "_"). The mail is rejected if the client name | |
427 matches the specified regular expression. | |
111 | 428 </para></listitem> |
429 <listitem><para> | |
108 | 430 If the mail has not been accepted or rejected yet, we look for a |
431 verification context, which is the closest ancestor of the filtering | |
432 context that both specifies a verification host, and which covers the | |
433 envelope to address. If we find such a verification context, and the | |
434 verification host is not our own hostname, we open an smtp conversation | |
435 with that verification host. The current envelope from and recipient to | |
436 values are passed to that verification host. If we receive a 5xy | |
437 response those commands, we reject the current recipient with "no such | |
438 user". | |
111 | 439 </para></listitem> |
440 <listitem><para> | |
108 | 441 If the mail has not been accepted or rejected yet, and the filtering |
442 context enables content filtering, and this is the first such recipient | |
443 in this smtp transaction, we set the content filtering parameters from | |
444 this context, and enable content filtering for the body of this message. | |
111 | 445 </para></listitem> |
108 | 446 </orderedlist> |
447 <para> | |
160 | 448 For each recipient that was accepted, we search for an autowhite entry |
449 starting in the reply filtering context. If an autowhite entry is found, | |
450 we add the recipient to that auto whitelist file. This will prevent reply | |
451 messages from being blocked by the dnsbl or content filtering. | |
452 </para> | |
453 <para> | |
108 | 454 If content filtering is enabled for this body, the mail text is decoded |
119 | 455 (uuencode, base64, mime, html entity, url encodings), and scanned for HTTP |
456 and HTTPS URLs or bare host names. Hostnames must be either ip address | |
457 literals, or must end in a string defined by the TLD list. The first | |
458 <configurable> host names are checked as follows. | |
459 </para> | |
460 <para> | |
461 The only known list that is suitable for the content filter DNSBL is the | |
462 SBL. If the content filter DNSBL is defined, and any of those host | |
463 names resolve to ip addresses that are on that DNSBL (or have | |
464 nameservers that are on that list), and the host name is not on the | |
465 <configurable> ignore list, the mail is rejected. | |
466 </para> | |
467 <para> | |
468 If the content uribl DNSBL is defined, and any of those host names are | |
469 on that DNSBL, and the host name is not on the <configurable> | |
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470 ignore list, the mail is rejected. There are three lists that are suitable |
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471 here, URIBL, SURBL, and DBL. |
119 | 472 </para> |
473 <para> | |
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474 If any non-whitelisted recipient has a filtering context with a non-zero |
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475 spamassassin limit, then the message is passed thru spamassassin (via |
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476 spamc), and the message is rejected for those recipients with spamassassin |
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477 limits less than the resulting spamassassin score. For example, a |
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478 spamassassin limit of three will reject messages with spamassassin scores |
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479 of four or greater. If the filtering context has a spamassassin limit of |
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480 zero, then spamassassin is not called (or if called the results are not used) |
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481 for this recipient. |
203
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482 </para> |
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483 <para> |
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484 If any non-whitelisted recipient has a filtering context that specifies |
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485 DCC greylisting, then the message is passed thru the DCC bulk detector, |
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486 and the message is greylisted (for all recipients) if the DCC says this |
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487 message should be delayed. |
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488 </para> |
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489 <para> |
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490 If any non-whitelisted recipient has a filtering context with a non-zero |
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491 DCC bulk threshold, then the message is passed thru the DCC bulk detector, |
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492 and the message is rejected for those recipients with DCC thresholds less |
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493 than or equal to the DCC bulk score. |
163 | 494 </para> |
495 <para> | |
119 | 496 We also scan for excessive bad html tags, and if a <configurable> |
497 limit is exceeded, the mail is rejected. | |
108 | 498 </para> |
499 </refsect1> | |
94 | 500 |
111 | 501 <refsect1 id='access.1'> |
108 | 502 <title>Sendmail access vs. DNSBL</title> |
503 <para> | |
504 With the standard sendmail.mc dnsbl FEATURE, the dnsbl checks may be | |
505 suppressed by entries in the /etc/mail/access database. For example, | |
506 suppose you control a /18 of address space, and have allocated some /24s | |
507 to some clients. You have access entries like | |
111 | 508 <literallayout class="monospaced"><![CDATA[ |
509 192.168.4 OK | |
510 192.168.17 OK]]></literallayout> | |
108 | 511 </para> |
512 <para> | |
513 to allow those clients to smarthost thru your mail server. Now if one | |
514 of those clients happens get infected with a virus that turns a machine | |
515 into an open proxy, and their 192.168.4.45 lands on the SBL-XBL, you | |
516 will still wind up allowing that infected machine to smarthost thru your | |
517 mail servers. | |
518 </para> | |
519 <para> | |
520 With this DNSBL milter, the sendmail access database cannot override the | |
521 dnsbl checks, so that machine won't be able to send mail to or thru your | |
522 smarthost mail server (unless the virus/proxy can use smtp-auth). | |
523 </para> | |
524 <para> | |
525 Using the standard sendmail features, you would add access entries to | |
526 allow hosts on your local network to relay thru your mail server. Those | |
527 OK entries in the sendmail access database will override all the dnsbl | |
528 checks. With this DNSBL milter, you will need to have the local users | |
529 authenticate with smtp-auth to get the same effect. You might find | |
530 <ulink | |
531 url="http://www.ists.dartmouth.edu/classroom/sendmail-ssl-how-to.php"> | |
532 these directions</ulink> helpful for setting up smtp-auth if you are on | |
533 RH Linux. | |
534 </para> | |
535 </refsect1> | |
94 | 536 |
111 | 537 <refsect1 id='performance.1'> |
108 | 538 <title>Performance Issues</title> |
539 <para> | |
540 Consider a high volume high performance machine running sendmail. Each | |
541 sendmail process can do its own dns resolution. Typically, such dns | |
542 resolver libraries are not thread safe, and so must be protected by some | |
543 sort of mutex in a threaded environment. When we add a milter to | |
544 sendmail, we now have a collection of sendmail processes, and a | |
545 collection of milter threads. | |
546 </para> | |
547 <para> | |
548 We will be doing a lot of dns lookups per mail message, and at least | |
549 some of those will take many tens of seconds. If all this dns work is | |
550 serialized inside the milter, we have an upper limit of about 25K mail | |
551 messages per day. That is clearly not sufficient for many sites. | |
552 </para> | |
553 <para> | |
554 Since we want to do parallel dns resolution across those milter threads, | |
555 we add another collection of dns resolver processes. Each sendmail | |
556 process is talking to a milter thread over a socket, and each milter | |
557 thread is talking to a dns resolver process over another socket. | |
558 </para> | |
559 <para> | |
560 Suppose we are processing 20 messages per second, and each message | |
561 requires 20 seconds of dns work. Then we will have 400 sendmail | |
562 processes, 400 milter threads, and 400 dns resolver processes. Of | |
563 course that steady state is very unlikely to happen. | |
564 </para> | |
565 </refsect1> | |
94 | 566 |
567 | |
111 | 568 <refsect1 id='rejected.1'> |
108 | 569 <title>Rejected Ideas</title> |
570 <para> | |
571 The following ideas have been considered and rejected. | |
572 </para> | |
573 <para> | |
111 | 574 Add max_recipients setting to the context configuration. Recipients in |
575 excess of that limit will be rejected, and all the non-whitelisted | |
576 recipients will be removed. Current spammers *very* rarely send more | |
577 than ten recipients in a single smtp transaction, so this won't stop any | |
108 | 578 significant amount of spam. |
579 </para> | |
580 <para> | |
581 Add poison addresses to the configuration. If any recipient is | |
582 poison, all recipients are rejected even if they would be whitelisted, | |
583 and the data is rejected if sent. I have a collection of spam trap | |
584 addresses that would be suitable for such use. Based on my log files, | |
585 any mail to those spam trap addresses is rejected based on either dnsbl | |
586 lookups or the DCC. So this won't result in blocking any additional | |
587 spam. | |
588 </para> | |
589 <para> | |
590 Add an option to only allow one recipient if the return path is | |
591 empty. Based on my log files, there is no mail that violates this | |
592 check. | |
593 </para> | |
594 <para> | |
595 Reject the mail if the envelope from domain name contains any MX | |
596 records pointing to 127.0.0.0/8. I don't see any significant amount of | |
597 spam sent with such domain names. | |
598 </para> | |
599 </refsect1> | |
94 | 600 |
108 | 601 <refsect1 id='todo.1'> |
602 <title>TODO</title> | |
603 <para> | |
604 The following ideas are under consideration. | |
605 </para> | |
606 <para> | |
276 | 607 Add config switch to require the HELO argument to resolve to an ip address. |
608 </para> | |
609 <para> | |
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610 Add white/unknown to config for smtp authenticated connections. Currently |
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611 any authenticated connection is fully whitelisted. The only spam control |
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612 on those connections is rate limiting. This feature would allow content based |
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613 spam controls to be applied even to authenticated connections. Add |
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614 context/authenticated_dnsbl_list and context/content/authenticated. |
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615 </para> |
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616 <para> |
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617 Add an optional list of domains to be enforced on the env_from value for |
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618 authenticated connections. User abc could be restricted to envelope from |
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619 values of a.com and b.com, user def could be restricted to envelope from |
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620 values of dd.com and ee.com. |
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621 </para> |
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622 <para> |
115 | 623 Look for href="hostname/path" strings that are missing the required |
624 http:// protocol header. Such references are still clickable in common | |
625 mail software. | |
626 </para> | |
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627 <para> |
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628 Add spf to the white/black/unknown values in env_from blocks. This |
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629 results in whitelisting that envelope from value as long as the connection |
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630 is made from an ip address listed in the domain spf txt record. |
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631 </para> |
108 | 632 </refsect1> |
94 | 633 |
111 | 634 <refsect1 id='copyright.1'> |
108 | 635 <title>Copyright</title> |
636 <para> | |
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637 Copyright (C) 2012 by 510 Software Group <carl@five-ten-sg.com> |
108 | 638 </para> |
639 <para> | |
640 This program is free software; you can redistribute it and/or modify it | |
641 under the terms of the GNU General Public License as published by the | |
160 | 642 Free Software Foundation; either version 3, or (at your option) any |
108 | 643 later version. |
644 </para> | |
645 <para> | |
646 You should have received a copy of the GNU General Public License along | |
647 with this program; see the file COPYING. If not, please write to the | |
648 Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA. | |
649 </para> | |
650 </refsect1> | |
94 | 651 |
111 | 652 <refsect1 id='version.1'> |
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653 <title>Version</title> |
108 | 654 <para> |
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655 @VERSION@ |
108 | 656 </para> |
657 </refsect1> | |
658 </refentry> | |
659 | |
660 | |
661 <refentry id="@PACKAGE@.conf.5"> | |
662 <refentryinfo> | |
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663 <date>2013-05-22</date> |
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664 <author> |
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665 <firstname>Carl</firstname> |
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666 <surname>Byington</surname> |
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667 <affiliation><orgname>510 Software Group</orgname></affiliation> |
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668 </author> |
108 | 669 </refentryinfo> |
94 | 670 |
108 | 671 <refmeta> |
672 <refentrytitle>@PACKAGE@.conf</refentrytitle> | |
673 <manvolnum>5</manvolnum> | |
674 <refmiscinfo>@PACKAGE@ @VERSION@</refmiscinfo> | |
675 </refmeta> | |
94 | 676 |
108 | 677 <refnamediv id='name.5'> |
678 <refname>@PACKAGE@.conf</refname> | |
111 | 679 <refpurpose>configuration file for @PACKAGE@ sendmail milter</refpurpose> |
108 | 680 </refnamediv> |
681 | |
682 <refsynopsisdiv id='synopsis.5'> | |
683 <title>Synopsis</title> | |
684 <cmdsynopsis> | |
685 <command>@PACKAGE@.conf</command> | |
686 </cmdsynopsis> | |
687 </refsynopsisdiv> | |
94 | 688 |
108 | 689 <refsect1 id='description.5'> |
690 <title>Description</title> | |
691 <para>The <command>@PACKAGE@.conf</command> configuration file is | |
148
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692 specified by this partial bnf description. Comments start with // |
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693 or # and extend to the end of the line. To include the contents |
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694 of some file verbatim in the dnsbl.conf file, use |
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695 <literallayout class="monospaced"><![CDATA[include "<file>";]]></literallayout> |
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696 </para> |
108 | 697 |
698 <literallayout class="monospaced"><![CDATA[ | |
699 CONFIG = {CONTEXT ";"}+ | |
700 CONTEXT = "context" NAME "{" {STATEMENT}+ "}" | |
249 | 701 STATEMENT = (DNSBL | DNSBLLIST | DNSWL | DNSWLLIST | CONTENT | ENV-TO |
702 | VERIFY | GENERIC | W_REGEX | AUTOWHITE | CONTEXT | ENV-FROM | |
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703 | RATE-LIMIT | REQUIRERDNS) ";" |
108 | 704 |
124 | 705 DNSBL = "dnsbl" NAME DNSPREFIX ERROR-MSG1 |
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706 DNSBLLIST = "dnsbl_list" {NAME}* |
108 | 707 |
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708 DNSWL = "dnswl" NAME DNSPREFIX LEVEL |
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709 DNSWLLIST = "dnswl_list" {NAME}* |
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710 LEVEL = INTEGER |
94 | 711 |
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712 REQUIRERDNS = "require_rdns" ("yes" | "no") |
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713 |
108 | 714 CONTENT = "content" ("on" | "off") "{" {CONTENT-ST}+ "}" |
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715 CONTENT-ST = (FILTER | URIBL | IGNORE | TLD | HTML-TAGS | HTML-LIMIT | |
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716 HOST-LIMIT | SPAMASS | REQUIRE | DCCGREY | DCCBULK) ";" |
124 | 717 FILTER = "filter" DNSPREFIX ERROR-MSG2 |
718 URIBL = "uribl" DNSPREFIX ERROR-MSG3 | |
108 | 719 IGNORE = "ignore" "{" {HOSTNAME [";"]}+ "}" |
720 TLD = "tld" "{" {TLD [";"]}+ "}" | |
721 HTML-TAGS = "html_tags" "{" {HTMLTAG [";"]}+ "}" | |
124 | 722 ERROR-MSG1 = string containing exactly two %s replacement tokens |
723 both are replaced with the client ip address | |
724 ERROR-MSG2 = string containing exactly two %s replacement tokens | |
725 the first is replaced with the hostname, and the second | |
726 is replaced with the ip address | |
727 ERROR-MSG3 = string containing exactly two %s replacement tokens | |
728 both are replaced with the hostname | |
108 | 729 |
730 HTML-LIMIT = "html_limit" ("on" INTEGER ERROR-MSG | "off") | |
731 | |
111 | 732 HOST-LIMIT = "host_limit" ("on" INTEGER ERROR-MSG | "off" | |
733 "soft" INTEGER) | |
178 | 734 SPAMASS = "spamassassin" INTEGER |
735 REQUIRE = "require_match" ("yes" | "no") | |
736 DCCGREY = "dcc_greylist" ("yes" | "no") | |
737 DCCBULK = "dcc_bulk_threshold" (INTEGER | "many" | "off") | |
94 | 738 |
108 | 739 ENV-TO = "env_to" "{" {(TO-ADDR | DCC-TO)}+ "}" |
740 TO-ADDR = ADDRESS [";"] | |
741 DCC-TO = "dcc_to" ("ok" | "many") "{" DCCINCLUDEFILE "}" ";" | |
742 | |
743 VERIFY = "verify" HOSTNAME ";" | |
168 | 744 GENERIC = "generic" REGULAREXPRESSION ERROR-MSG4 ";" |
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745 W-REGEX = "white_regex" REGULAREXPRESSION ";" |
168 | 746 ERROR-MSG4 = string containing exactly one %s replacement token |
747 which is replaced with the client name | |
153 | 748 AUTOWHITE = "autowhite" DAYS FILENAME ";" |
108 | 749 |
750 ENV_FROM = "env_from" [DEFAULT] "{" {(FROM-ADDR | DCC-FROM)}+ "}" | |
751 FROM-ADDR = ADDRESS VALUE [";"] | |
752 DCC-FROM = "dcc_from" "{" DCCINCLUDEFILE "}" ";" | |
136 | 753 |
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754 RATE-LIMIT = "rate_limit" DEFAULT_RCPT_LIMIT DAILY_MULTIPLE_RCPT |
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755 DEFAULT_IP_LIMIT DAILY_MULTIPLE_IP "{" (RATE)+ "}" |
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756 RATE = USER RCPTLIMIT IPLIMIT ";" |
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757 RCPTLIMIT = INTEGER |
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758 DEFAULT_RCPT_LIMIT = INTEGER |
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759 DAILY_MULTIPLE_RCPT = INTEGER |
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760 DEFAULT_IP_LIMIT = INTEGER |
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761 DAILY_MULTIPLE_IP = INTEGER |
136 | 762 |
108 | 763 DEFAULT = ("white" | "black" | "unknown" | "inherit" | "") |
764 ADDRESS = (USER@ | DOMAIN | USER@DOMAIN) | |
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765 VALUE = ("white" | "black" | "unknown" | "inherit" | CHILD-CONTEXT-NAME)]]></literallayout> |
108 | 766 </refsect1> |
94 | 767 |
108 | 768 <refsect1 id='sample.5'> |
769 <title>Sample</title> | |
770 <literallayout class="monospaced"><![CDATA[ | |
127 | 771 context main-default { |
772 // outbound dnsbl filtering to catch our own customers that end up on the sbl | |
773 dnsbl sbl sbl-xbl.spamhaus.org "Mail from %s rejected - sbl; see http://www.spamhaus.org/query/bl?ip=%s"; | |
174 | 774 dnsbl_list sbl; |
127 | 775 |
776 // outbound content filtering to prevent our own customers from sending spam | |
777 content on { | |
778 filter sbl-xbl.spamhaus.org "Mail containing %s rejected - sbl; see http://www.spamhaus.org/query/bl?ip=%s"; | |
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779 uribl multi.surbl.org "Mail containing %s rejected - surbl; see http://www.surbl.org/surbl-analysis?d=%s"; |
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780 #uribl multi.uribl.com "Mail containing %s rejected - uribl; see http://l.uribl.com/?d=%s"; |
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781 #uribl dbl.spamhaus.org "Mail containing %s rejected - dbl; see http://www.spamhaus.org/query/domain?domain=%s"; |
127 | 782 ignore { include "hosts-ignore.conf"; }; |
783 tld { include "tld.conf"; }; | |
784 html_tags { include "html-tags.conf"; }; | |
785 html_limit on 20 "Mail containing excessive bad html tags rejected"; | |
786 html_limit off; | |
787 host_limit on 20 "Mail containing excessive host names rejected"; | |
788 host_limit soft 20; | |
178 | 789 spamassassin 4; |
790 require_match yes; | |
791 dcc_greylist yes; | |
792 dcc_bulk_threshold 50; | |
127 | 793 }; |
794 | |
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795 // backscatter prevention - do not send bounces for mail that we accepted but could not forward |
127 | 796 // we only send bounces to our own customers |
797 env_from unknown { | |
798 "<>" black; | |
799 }; | |
136 | 800 |
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801 // hourly recipient rate limit by smtp auth client id, or unauthenticated mail from address |
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802 // hourly unique ip addresses by smtp auth client id, or unauthenticated mail from address |
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803 // default hourly recipient rate limit is 30 |
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804 // daily recipient rate limits are 4 times the hourly limit |
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805 // default hourly unique ip addresses is 5 |
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806 // daily unique ip addresses are 4 times the hourly limit |
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807 rate_limit 30 4 5 4 { // default |
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808 fred 100 10; // override default limits |
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809 joe 10 2; // "" |
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810 "sam@somedomain.tld" 500 2; |
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811 "@otherdomain.tld" 100 2; |
136 | 812 }; |
127 | 813 }; |
814 | |
171 | 815 context main { |
816 dnsbl localp partial.blackholes.five-ten-sg.com "Mail from %s rejected - local; see http://www.five-ten-sg.com/blackhole.php?%s"; | |
108 | 817 dnsbl local blackholes.five-ten-sg.com "Mail from %s rejected - local; see http://www.five-ten-sg.com/blackhole.php?%s"; |
174 | 818 dnsbl sbl zen.spamhaus.org "Mail from %s rejected - sbl; see http://www.spamhaus.org/query/bl?ip=%s"; |
108 | 819 dnsbl xbl xbl.spamhaus.org "Mail from %s rejected - xbl; see http://www.spamhaus.org/query/bl?ip=%s"; |
249 | 820 dnswl dnswl.org list.dnswl.org 2; |
171 | 821 dnsbl_list local sbl; |
249 | 822 dnswl_list dnswl.org; |
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823 require_rdns yes; |
94 | 824 |
108 | 825 content on { |
826 filter sbl-xbl.spamhaus.org "Mail containing %s rejected - sbl; see http://www.spamhaus.org/query/bl?ip=%s"; | |
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827 uribl multi.surbl.org "Mail containing %s rejected - surbl; see http://www.surbl.org/surbl-analysis?d=%s"; |
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828 #uribl multi.uribl.com "Mail containing %s rejected - uribl; see http://l.uribl.com/?d=%s"; |
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829 #uribl dbl.spamhaus.org "Mail containing %s rejected - dbl; see http://www.spamhaus.org/query/domain?domain=%s"; |
108 | 830 ignore { include "hosts-ignore.conf"; }; |
831 tld { include "tld.conf"; }; | |
832 html_tags { include "html-tags.conf"; }; | |
833 html_limit off; | |
834 host_limit soft 20; | |
178 | 835 spamassassin 5; |
836 require_match yes; | |
837 dcc_greylist yes; | |
838 dcc_bulk_threshold 20; | |
108 | 839 }; |
94 | 840 |
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841 generic "^dsl.static.*ttnet.net.tr$|(^|[x.-])(ppp|h|host)?([0-9]{1,3}[x.-](Red-|dynamic[x.-])?){4}" |
171 | 842 "your mail server %s seems to have a generic name"; |
843 | |
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844 white_regex "=example.com=user@yourhostingaccount.com$"; |
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845 |
108 | 846 env_to { |
171 | 847 # !! replace this with your domain names |
108 | 848 # child contexts are not allowed to specify recipient addresses outside these domains |
179 | 849 # if this is a backup-mx, you need to include here domains for which you relay to the primary mx |
174 | 850 include "/etc/mail/local-host-names"; |
108 | 851 }; |
94 | 852 |
108 | 853 context whitelist { |
854 content off {}; | |
855 env_to { | |
171 | 856 # dcc_to ok { include "/var/dcc/whitecommon"; }; |
108 | 857 }; |
858 env_from white {}; # white forces all unmatched from addresses (everyone in this case) to be whitelisted | |
859 # so all mail TO these env_to addresses is accepted | |
860 }; | |
94 | 861 |
171 | 862 context abuse { |
863 dnsbl_list xbl; | |
864 content off {}; | |
174 | 865 generic "^$ " " "; # regex cannot match, to disable generic rdns rejects |
171 | 866 env_to { |
867 abuse@ # no content filtering on abuse reports | |
868 postmaster@ # "" | |
869 }; | |
870 env_from unknown {}; # ignore all parent white/black listing | |
871 }; | |
872 | |
108 | 873 context minimal { |
171 | 874 dnsbl_list sbl; |
178 | 875 content on { |
876 spamassassin 10; | |
877 dcc_bulk_threshold many; | |
878 }; | |
171 | 879 generic "^$ " " "; # regex cannot match, to disable generic rdns rejects |
108 | 880 env_to { |
881 }; | |
882 }; | |
94 | 883 |
108 | 884 context blacklist { |
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885 dnsbl_list ; |
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886 dnswl_list ; |
108 | 887 env_to { |
171 | 888 # dcc_to many { include "/var/dcc/whitecommon"; }; |
108 | 889 }; |
890 env_from black {}; # black forces all unmatched from addresses (everyone in this case) to be blacklisted | |
891 # so all mail TO these env_to addresses is rejected | |
892 }; | |
94 | 893 |
171 | 894 env_from unknown { |
895 abuse@ abuse; # replies to abuse reports use the abuse context | |
896 # dcc_from { include "/var/dcc/whitecommon"; }; | |
108 | 897 }; |
898 | |
171 | 899 autowhite 90 "autowhite/my-auto-whitelist"; |
900 # install should create /etc/dnsbl/autowhite writable by userid dnsbl | |
108 | 901 };]]></literallayout> |
902 </refsect1> | |
94 | 903 |
111 | 904 <refsect1 id='version.5'> |
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905 <title>Version</title> |
108 | 906 <para> |
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907 @VERSION@ |
108 | 908 </para> |
909 </refsect1> | |
910 | |
911 </refentry> | |
912 </reference> |