Mercurial > dnsbl
view xml/dnsbl.in @ 114:f4f5fb263072
cleanup list of tlds, add trailing / on http package directory reference
| author | carl |
|---|---|
| date | Sat, 24 Dec 2005 06:26:48 -0800 |
| parents | d0dad5610980 |
| children | 07e5d4721213 |
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<reference> <title>@PACKAGE@ Sendmail milter - Version @VERSION@</title> <partintro> <title>Packages</title> <para>The various source and binary packages are available at <ulink url="http://www.five-ten-sg.com/@PACKAGE@/packages/">http://www.five-ten-sg.com/@PACKAGE@/packages/</ulink> The most recent documentation is available at <ulink url="http://www.five-ten-sg.com/@PACKAGE@/">http://www.five-ten-sg.com/@PACKAGE@/</ulink> </para> </partintro> <refentry id="@PACKAGE@.1"> <refentryinfo> <date>2005-12-18</date> </refentryinfo> <refmeta> <refentrytitle>@PACKAGE@</refentrytitle> <manvolnum>1</manvolnum> <refmiscinfo>@PACKAGE@ @VERSION@</refmiscinfo> </refmeta> <refnamediv id='name.1'> <refname>@PACKAGE@</refname> <refpurpose>a sendmail milter with per-user dnsbl filtering</refpurpose> </refnamediv> <refsynopsisdiv id='synopsis.1'> <title>Synopsis</title> <cmdsynopsis> <command>@PACKAGE@</command> <arg><option>-c</option></arg> <arg><option>-s</option></arg> <arg><option>-d <replaceable class="parameter">n</replaceable></option></arg> <arg><option>-e <replaceable class="parameter">from|to</replaceable></option></arg> <arg><option>-r <replaceable class="parameter">local-domain-socket</replaceable></option></arg> <arg><option>-p <replaceable class="parameter">sendmail-socket</replaceable></option></arg> <arg><option>-t <replaceable class="parameter">timeout</replaceable></option></arg> </cmdsynopsis> </refsynopsisdiv> <refsect1 id='options.1'> <title>Options</title> <variablelist> <varlistentry> <term>-c</term> <listitem><para> Load the configuration file, print a cannonical form of the configuration on stdout, and exit. </para></listitem> </varlistentry> <varlistentry> <term>-s</term> <listitem><para> Stress test the configuration loading code by repeating the load/free cycle in an infinite loop. </para></listitem> </varlistentry> <varlistentry> <term>-d <replaceable class="parameter">n</replaceable></term> <listitem><para> Set the debug level to <replaceable class="parameter">n</replaceable>. </para></listitem> </varlistentry> <varlistentry> <term>-e <replaceable class="parameter">from|to</replaceable></term> <listitem><para> 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. </para></listitem> </varlistentry> <varlistentry> <term>-r <replaceable class="parameter">local-domain-socket</replaceable></term> <listitem><para> Set the local socket used for the connection to our own dns resolver processes. </para></listitem> </varlistentry> <varlistentry> <term>-p <replaceable class="parameter">sendmail-socket</replaceable></term> <listitem><para> Set the socket used for the milter connection to sendmail. This is either "inet:port@ip-address" or "local:local-domain-socket-file-name". </para></listitem> </varlistentry> <varlistentry> <term>-t <replaceable class="parameter">timeout</replaceable></term> <listitem><para> Set the timeout in seconds used for communication with sendmail. </para></listitem> </varlistentry> </variablelist> </refsect1> <refsect1 id='usage.1'> <title>Usage</title> <para><command>@PACKAGE@</command> -c</para> <para><command>@PACKAGE@</command> -s</para> <para><command>@PACKAGE@</command> -e 'someone@aol.com|localname@mydomain.tld'</para> <para><command>@PACKAGE@</command> -d 10 -r resolver.sock -p local:dnsbl.sock</para> </refsect1> <refsect1 id='installation.1'> <title>Installation</title> <para> 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 <ulink url="http://www.five-ten-sg.com/@PACKAGE@/packages">http://www.five-ten-sg.com/@PACKAGE@/packages</ulink>. </para> <para> 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. </para> <para> Modify your sendmail.mc by removing all the "FEATURE(dnsbl" lines, add the following line in your sendmail.mc and rebuild the .cf file </para> <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> <para> Modify the default <citerefentry> <refentrytitle>@PACKAGE@.conf</refentrytitle> <manvolnum>5</manvolnum> </citerefentry> configuration. </para> </refsect1> <refsect1 id='configuration.1'> <title>Configuration</title> <para> The configuration file is documented in <citerefentry> <refentrytitle>@PACKAGE@.conf</refentrytitle> <manvolnum>5</manvolnum> </citerefentry>. Any change to the config file, or any file included from that config file, will cause it to be reloaded within three minutes. </para> </refsect1> <refsect1 id='introduction.1'> <title>Introduction</title> <para> 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 SPEWS and the SBL, where all other users @example.com use only the SBL. </para> <para> 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). </para> <para> 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. </para> <para> 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@$hostname. You probably want to added dnsbl@$hostname to your /etc/mail/virtusertable since otherwise sendmail will reject that message. </para> </refsect1> <refsect1 id='dcc.1'> <title>DCC Issues</title> <para> If you are also using the <ulink url="http://www.rhyolite.com/anti-spam/dcc/">DCC</ulink> 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. </para> <para> 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. </para> <para> 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. </para> <para> 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. </para> </refsect1> <refsect1 id='definitions.1'> <title>Definitions</title> <para> 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. </para> <para> 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. </para> <para> DNSBL-LIST - a named list of DNSBLs that will be used for specific recipients or recipient domains. </para> </refsect1> <refsect1 id='filtering.1'> <title>Filtering Procedure</title> <para> If the client has authenticated with sendmail, the mail is accepted, the filtering contexts are not used, the dns lists are not checked, and the body content is not scanned. Otherwise, we follow these steps for each recipient. </para> <orderedlist> <listitem><para> 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. </para></listitem> <listitem><para> 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. </para></listitem> <listitem><para> 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). </para></listitem> <listitem><para> If the answer is black, mail to this recipient is rejected with "no such user", and the dns lists are not checked. </para></listitem> <listitem><para> If the answer is white, mail to this recipient is accepted and the dns lists are not checked. </para></listitem> <listitem><para> If the answer is unknown, we don't reject yet, but the dns lists will be checked, and the content may be scanned. </para></listitem> <listitem><para> If the answer is inherit, we repeat the envelope from search in the parent context. </para></listitem> <listitem><para> The dns 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). </para></listitem> <listitem><para> 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". </para></listitem> <listitem><para> 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. </para></listitem> </orderedlist> <para> If content filtering is enabled for this body, the mail text is decoded (uuencode, base64, mime, html entity, url encodings), scanned for HTTP and HTTPS URLs, and the first <configurable> host names are checked for their presence on the single <configurable> DNSBL. The only known list that is suitable for this purpose is the SBL. If any of those host names are on that DNSBL (or have nameservers that are on that list), and it is not on the <configurable> ignore list, the mail is rejected. We also scan for excessive bad html tags, and if a <configurable> limit is exceeded, the mail is rejected. </para> </refsect1> <refsect1 id='access.1'> <title>Sendmail access vs. DNSBL</title> <para> 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 <literallayout class="monospaced"><![CDATA[ 192.168.4 OK 192.168.17 OK]]></literallayout> </para> <para> 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. </para> <para> 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). </para> <para> 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 <ulink url="http://www.ists.dartmouth.edu/classroom/sendmail-ssl-how-to.php"> these directions</ulink> helpful for setting up smtp-auth if you are on RH Linux. </para> </refsect1> <refsect1 id='performance.1'> <title>Performance Issues</title> <para> 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. </para> <para> 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. </para> <para> 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. </para> <para> 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. </para> </refsect1> <refsect1 id='rejected.1'> <title>Rejected Ideas</title> <para> The following ideas have been considered and rejected. </para> <para> 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. </para> <para> 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. </para> <para> 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. </para> <para> 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. </para> </refsect1> <refsect1 id='todo.1'> <title>TODO</title> <para> The following ideas are under consideration. </para> <para> Add a per-context option to reject mail if the number of digits in the reverse dns client name exceeds some threshold. </para> </refsect1> <refsect1 id='copyright.1'> <title>Copyright</title> <para> Copyright (C) 2005 by 510 Software Group <carl@five-ten-sg.com> </para> <para> 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 2, or (at your option) any later version. </para> <para> 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. </para> </refsect1> <refsect1 id='version.1'> <title>CVS Version</title> <para> $Id$ </para> </refsect1> </refentry> <refentry id="@PACKAGE@.conf.5"> <refentryinfo> <date>2005-12-18</date> </refentryinfo> <refmeta> <refentrytitle>@PACKAGE@.conf</refentrytitle> <manvolnum>5</manvolnum> <refmiscinfo>@PACKAGE@ @VERSION@</refmiscinfo> </refmeta> <refnamediv id='name.5'> <refname>@PACKAGE@.conf</refname> <refpurpose>configuration file for @PACKAGE@ sendmail milter</refpurpose> </refnamediv> <refsynopsisdiv id='synopsis.5'> <title>Synopsis</title> <cmdsynopsis> <command>@PACKAGE@.conf</command> </cmdsynopsis> </refsynopsisdiv> <refsect1 id='description.5'> <title>Description</title> <para>The <command>@PACKAGE@.conf</command> configuration file is specified by this partial bnf description.</para> <literallayout class="monospaced"><![CDATA[ CONFIG = {CONTEXT ";"}+ CONTEXT = "context" NAME "{" {STATEMENT}+ "}" STATEMENT = (DNSBL | DNSBLLIST | CONTENT | ENV-TO | VERIFY | CONTEXT | ENV-FROM) ";" DNSBL = "dnsbl" NAME DNSPREFIX ERROR-MSG DNSBLLIST = "dnsbl_list" {NAME}+ CONTENT = "content" ("on" | "off") "{" {CONTENT-ST}+ "}" CONTENT-ST = (FILTER | IGNORE | TLD | HTML-TAGS | HTML-LIMIT | HOST-LIMIT) ";" FILTER = "filter" DNSPREFIX ERROR-MSG IGNORE = "ignore" "{" {HOSTNAME [";"]}+ "}" TLD = "tld" "{" {TLD [";"]}+ "}" HTML-TAGS = "html_tags" "{" {HTMLTAG [";"]}+ "}" ERROR-MSG = string containing exactly two %s replacement tokens for the client ip address HTML-LIMIT = "html_limit" ("on" INTEGER ERROR-MSG | "off") HOST-LIMIT = "host_limit" ("on" INTEGER ERROR-MSG | "off" | "soft" INTEGER) ENV-TO = "env_to" "{" {(TO-ADDR | DCC-TO)}+ "}" TO-ADDR = ADDRESS [";"] DCC-TO = "dcc_to" ("ok" | "many") "{" DCCINCLUDEFILE "}" ";" VERIFY = "verify" HOSTNAME ";" ENV_FROM = "env_from" [DEFAULT] "{" {(FROM-ADDR | DCC-FROM)}+ "}" FROM-ADDR = ADDRESS VALUE [";"] DCC-FROM = "dcc_from" "{" DCCINCLUDEFILE "}" ";" DEFAULT = ("white" | "black" | "unknown" | "inherit" | "") ADDRESS = (USER@ | DOMAIN | USER@DOMAIN) VALUE = ("white" | "black" | "unknown" | CHILD-CONTEXT-NAME)]]></literallayout> </refsect1> <refsect1 id='sample.5'> <title>Sample</title> <literallayout class="monospaced"><![CDATA[ context sample { dnsbl local blackholes.five-ten-sg.com "Mail from %s rejected - local; see http://www.five-ten-sg.com/blackhole.php?%s"; dnsbl sbl sbl-xbl.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"; dnsbl dul dul.dnsbl.sorbs.net "Mail from %s rejected - dul; see http://www.sorbs.net/lookup.shtml?%s"; dnsbl_list local sbl dul; content on { filter sbl-xbl.spamhaus.org "Mail containing %s rejected - sbl; see http://www.spamhaus.org/query/bl?ip=%s"; ignore { include "hosts-ignore.conf"; }; tld { include "tld.conf"; }; html_tags { include "html-tags.conf"; }; html_limit on 20 "Mail containing excessive bad html tags rejected"; html_limit off; host_limit on 20 "Mail containing excessive host names rejected"; host_limit soft 20; }; env_to { # child contexts are not allowed to specify recipient addresses outside these domains # leave this outer global context env_to empty to allow arbitrary recipients in child contexts mydomain.com; customer1.com; customer1a.com; customer1b.com; customer2.com; customer2a.com; customer2b.com; }; context whitelist { content off {}; env_to { # dcc_to ok { include "/var/dcc/whitecommon"; }; # copy the dcc OK values (env_to) into this context }; 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 {}; env_to { abuse@; # no content filtering on abuse reports postmaster@; # "" }; env_from unknown {}; # ignore all parent white/black listing }; context minimal { dnsbl_list sbl dul; content on {}; env_to { sales@mydomain.com; }; }; context blacklist { env_to { dcc_to many { include "/var/dcc/whitecommon"; }; # copy the dcc MANY values (env_to) into this context old-employee@mydomain.com; }; 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 }; context vp { # special context for the vp env_to { vp@mydomain.com; }; env_from inherit { nai.com black; # the vp does not like nai yahoo.com unknown; # override parent context blacklisting mother@spammyisp.com white; # suppress dnsbl checking }; }; context customer1 { dnsbl_list sbl dul; env_to { customer1.com; customer1a.com; customer1b.com; }; verify mail.customer1.com; context customer1a { env_to { customer1a.com; } env_from black { # blacklist everything first@acceptable.com unknown; # except these specific envelope senders second@another.com unknown; yahoo.com inherit; # delegate to the parent }; }; env_from { # default value of the default is inherit yahoo.com black; # no mail from yahoo first@yahoo.com unknown; # except this one }; }; context customer2 { dnsbl_list sbl; env_to { customer2.com; customer2a.com; customer2b.com; }; }; env_from unknown { dcc_from { include "/var/dcc/whitecommon"; }; # copy the dcc OK/MANY values (env_from, substitute mail_host) into this context abuse@ abuse; # replies to abuse reports use the abuse context yahoo.com black; # don't take mail from yahoo spammer@example.com black; }; };]]></literallayout> </refsect1> <refsect1 id='version.5'> <title>CVS Version</title> <para> $Id$ </para> </refsect1> </refentry> </reference>