Mercurial > dnsbl
view xml/dnsbl.in @ 423:c9b7b6dd1206 stable-6-0-59
use both envelope from and header from for spf checks when envelope from is a subdomain of the header from domain
| author | Carl Byington <carl@five-ten-sg.com> |
|---|---|
| date | Wed, 26 Jul 2017 08:52:31 -0700 |
| parents | d5a1ed33d3ae |
| children | beda588f2881 |
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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> <para> A <ulink url="http://www.selenic.com/mercurial/wiki/">Mercurial</ulink> source code repository for this project is available at <ulink url="http://hg.five-ten-sg.com/@PACKAGE@/">http://hg.five-ten-sg.com/@PACKAGE@/</ulink>. </para> <para> Bitcoin donations for this project may be sent to <ulink url="bitcoin:17n5xJZ9a8csJW2uLeZn7i6jegNKGdLUPJ">bitcoin:17n5xJZ9a8csJW2uLeZn7i6jegNKGdLUPJ</ulink> </para> </partintro> <refentry id="@PACKAGE@.1"> <refentryinfo> <date>2017-03-30</date> <author> <firstname>Carl</firstname> <surname>Byington</surname> <affiliation><orgname>510 Software Group</orgname></affiliation> <personblurb><para></para></personblurb> </author> </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>-b <replaceable class="parameter">local-domain-socket</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>-b <replaceable class="parameter">local-domain-socket-file-name</replaceable></term> <listitem><para> Set the local socket used for the connection to the dccifd daemon. This is typically /var/dcc/dccifd. </para></listitem> </varlistentry> <varlistentry> <term>-r <replaceable class="parameter">local-domain-socket-file-name</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 LOCAL 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). These rejections are the primary cause of such backscatter. </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> 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. </para> <para> 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. </para> <para> 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 <emphasis>not</emphasis> 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. </para> <para> You can manually add such an autowhite entry, by appending a single text line to the autowhitelist file, using something like <command>echo "$mail 0" >>$autowhitefile</command>. You can manually remove such an autowhite entry, by appending a single text line to the autowhitelist file, using something like <command>echo "$mail 1" >>$autowhitefile</command>. </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@$hostname. You probably want to add dnsbl@$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> <para> 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 <citerefentry> <refentrytitle>@PACKAGE@.conf</refentrytitle> <manvolnum>5</manvolnum> </citerefentry> configuration. Those statements are only active if you supply the <option>-b</option> 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/dnsblnogrey/whiteclnt so the dccifd will properly ignore greylisting for those recipients that don't want it. </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> <para> 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. </para> <para> DNSWL-LIST - a named list of DNSWLs that will be used for specific recipients or recipient domains. </para> </refsect1> <refsect1 id='filtering.1'> <title>Filtering Procedure</title> <para> 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. </para> <para> 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. </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, and the envelope from domain name is listed in the current (or parents) filtering contexts dkim_from with "required_signed", we downgrade this white answer to unknown. If the answer is still 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> 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. </para></listitem> <listitem><para> 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. </para></listitem> <listitem><para> 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). </para></listitem> <listitem><para> 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. </para></listitem> <listitem><para> 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. </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> 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. </para> <para> If content filtering is enabled for this body, we look for dkim_signer and dkim_from sections in the current context and parents. We collect the signers of this message from the header added by the dkim-milter. If any of the message signers are whitelisted, the message is accepted. </para> <para> If the header from domain maps to required_signed then: If any of the message signers are in that list, the message is accepted. If the source ip address passes a strong spf check for the header from domain, the message is accepted. Otherwise, the message is rejected. </para> <para> If the header from domain maps to signed_white then: If any of the message signers are in that list, the message is accepted. If the source ip address passes a strong spf check for the header from domain, the message is accepted. Otherwise, processing continues. </para> <para> If the header from domain maps to signed_black then: If any of the message signers are in that list, the message is rejected. Otherwise, processing continues. </para> <para> If any of the message signers are blacklisted, the message is rejected. </para> <para> 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. </para> <para> 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. </para> <para> 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. </para> <para> 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. </para> <para> 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. </para> <para> 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. </para> <para> We also scan for excessive bad html tags, and if a <configurable> limit is exceeded, the mail is rejected. </para> </refsect1> <refsect1 id='dmarc.1'> <title>DMARC vs dkim_from require_signed</title> <para> Note that DNSBL does not implement rfc7489 DMARC. We do not look for _dmarc.$DOMAIN txt records. </para> <para> The restrictions imposed by require_signed are similar but not identical to a DMARC reject policy with strict identifier alignment. When doing SPF fallback, DMARC checks SPF based on the rfc5321 envelope from domain. DNSBL checks SPF based on the rfc5322 header from domain, but we ignore mx: and ptr: elements in the txt record. DMARC does not allow mail from good.example.com to be signed by trusted.example.net - which is a common case. Both Microsoft Office365 and Google run mail for customer domains, but use DKIM signing domains in onmicrosoft.com and gappssmtp.com, which are unrelated to the customer domain. DMARC in the default relaxed alignment mode allows evil.example.com to sign mail from good.example.com. DNSBL specifies the exact list of acceptable signing domains, rather than inferring it from child/parent relationships, or using public suffix lists to find the organizational domain. We can block mail from marketing.example.com while accepting mail from billing.example.com, even if both are DKIM signed by example.com. </para> <para> Suppose we have: <literallayout class="monospaced"><![CDATA[ rfc5321 envelope from = one@evil.example.com rfc5322 header from = two@good.example.com authentication results = dkim pass header.d=other.example.com _dmarc.good.example.com txt = "v=DMARC1; p=reject; adkim:s aspf:s" dkim_from {good.example.com require_signed other.example.com;} ]]></literallayout> DMARC would fail the strict identifier alignment. DNSBL allows us to require DKIM signatures that are unrelated to the rfc5322 header from, so we accept this message. </para> <para> Suppose we have: <literallayout class="monospaced"><![CDATA[ rfc5321 envelope from = one@evil.example.com rfc5322 header from = two@good.example.com authentication results = dkim pass header.d=other.example.net _dmarc.good.example.com txt = "v=DMARC1; p=reject; adkim:r aspf:r" dkim_from {good.example.com require_signed other.example.net;} ]]></literallayout> DMARC would pass the relaxed spf identifier alignments, and would check the evil.example.com spf record. If that allowed the source ip, DMARC would accept the message. DMARC would not check DKIM since example.com and example.net do not pass even the relaxed identifer alignment requirement. DNSBL allows us to require DKIM signatures that are not related to the rfc5322 header from domain, so we accept the message based on the DKIM signature and don't need to fall back to SPF. </para> <para> Suppose we have: <literallayout class="monospaced"><![CDATA[ rfc5321 envelope from = one@evil.example.com rfc5322 header from = two@good.example.com authentication results = dkim fail header.d=other.example.net _dmarc.good.example.com txt = "v=DMARC1; p=reject; adkim:r aspf:r" evil.example.com txt = "v=spf1 ... including the source ip good.example.com txt = "v=spf1 ... not including the source ip dkim_from {good.example.com require_signed other.example.net;} ]]></literallayout> DNSBL allows us to require DKIM signatures that are not related to the rfc5322 header from domain. In this case the signature fails, so we fall back to an SPF check. We check SPF based on the rfc5322 header from, and good.example.com does not allow the source ip, so we reject this message. DMARC would accept that message based on the SPF check for evil.example.com </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> More complete SPF check. </para> <para> Add config switch to require the HELO argument to resolve to an ip address. </para> <para> 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. </para> <para> 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. </para> <para> Look for href="hostname/path" strings that are missing the required http:// protocol header. Such references are still clickable in common mail software. </para> </refsect1> <refsect1 id='copyright.1'> <title>Copyright</title> <para> Copyright (C) 2012 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 3, 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>Version</title> <para> @VERSION@ </para> </refsect1> </refentry> <refentry id="@PACKAGE@.conf.5"> <refentryinfo> <date>2017-03-30</date> <author> <firstname>Carl</firstname> <surname>Byington</surname> <affiliation><orgname>510 Software Group</orgname></affiliation> <personblurb><para></para></personblurb> </author> </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. 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 <literallayout class="monospaced"><![CDATA[include "<file>";]]></literallayout> </para> <literallayout class="monospaced"><![CDATA[ CONFIG = {CONTEXT ";"}+ CONTEXT = "context" NAME "{" {STATEMENT}+ "}" STATEMENT = ( DNSBL | DNSBLLIST | DNSWL | DNSWLLIST | CONTENT | ENV-TO | VERIFY | GENERIC | W_REGEX | AUTOWHITE | CONTEXT | ENV-FROM | RATE-LIMIT | REQUIRERDNS) ";" DNSBL = "dnsbl" NAME DNSPREFIX ERROR-MSG1 DNSBLLIST = "dnsbl_list" {NAME}* DNSWL = "dnswl" NAME DNSPREFIX LEVEL DNSWLLIST = "dnswl_list" {NAME}* LEVEL = INTEGER REQUIRERDNS = "require_rdns" ("yes" | "no") CONTENT = "content" ("on" | "off") "{" {CONTENT-ST}+ "}" CONTENT-ST = (FILTER | URIBL | IGNORE | TLD | HTML-TAGS | HTML-LIMIT | HOST-LIMIT | SPAMASS | REQUIRE | DCCGREY | DCCBULK | DKIM_SIGNER | DKIM_FROM) ";" FILTER = "filter" DNSPREFIX ERROR-MSG2 URIBL = "uribl" DNSPREFIX ERROR-MSG3 IGNORE = "ignore" "{" {HOSTNAME [";"]}+ "}" TLD = "tld" "{" {TLD [";"]}+ "}" HTML-TAGS = "html_tags" "{" {HTMLTAG [";"]}+ "}" ERROR-MSG1 = string containing exactly two %s replacement tokens both are replaced with the client ip address ERROR-MSG2 = string containing exactly two %s replacement tokens the first is replaced with the hostname, and the second is replaced with the ip address ERROR-MSG3 = string containing exactly two %s replacement tokens both are replaced with the hostname HTML-LIMIT = "html_limit" ("on" INTEGER ERROR-MSG | "off") HOST-LIMIT = "host_limit" ("on" INTEGER ERROR-MSG | "off" | "soft" INTEGER) SPAMASS = "spamassassin" INTEGER REQUIRE = "require_match" ("yes" | "no") DCCGREY = "dcc_greylist" ("yes" | "no") DCCBULK = "dcc_bulk_threshold" (INTEGER | "many" | "off") DKIMSIGNER = "dkim_signer" "{" {SIGNING_DOMAIN DEF [";"]}+ "}" DKIMFROM = "dkim_from" "{" {HEADER_FROM_DOMAIN DKIMVALUE SIGNERS [";"]}+ "}" DKIMVALUE = "signed_white" | "signed_black" | "require_signed" SIGNERS = quoted comma separated SIGNING_DOMAINs no whitespace ENV-TO = "env_to" "{" {(TO-ADDR | DCC-TO)}+ "}" TO-ADDR = ADDRESS [";"] DCC-TO = "dcc_to" ("ok" | "many") "{" DCCINCLUDEFILE "}" ";" VERIFY = "verify" HOSTNAME ";" GENERIC = "generic" REGULAREXPRESSION ERROR-MSG4 ";" W-REGEX = "white_regex" REGULAREXPRESSION ";" ERROR-MSG4 = string containing exactly one %s replacement token which is replaced with the client name AUTOWHITE = "autowhite" DAYS FILENAME ";" ENV_FROM = "env_from" [DEFAULT] "{" {(FROM-ADDR | DCC-FROM)}+ "}" FROM-ADDR = ADDRESS VALUE [";"] DCC-FROM = "dcc_from" "{" DCCINCLUDEFILE "}" ";" RATE-LIMIT = "rate_limit" DEFAULT_RCPT_LIMIT DAILY_MULTIPLE_RCPT DEFAULT_IP_LIMIT DAILY_MULTIPLE_IP "{" (RATE)+ "}" RATE = USER RCPTLIMIT IPLIMIT ";" RCPTLIMIT = INTEGER DEFAULT_RCPT_LIMIT = INTEGER DAILY_MULTIPLE_RCPT = INTEGER DEFAULT_IP_LIMIT = INTEGER DAILY_MULTIPLE_IP = INTEGER DEF = ("white" | "black" | "unknown") DEFAULT = (DEF | "inherit" | "") ADDRESS = (USER@ | DOMAIN | USER@DOMAIN) VALUE = (DEF | "inherit" | CHILD-CONTEXT-NAME)]]></literallayout> </refsect1> <refsect1 id='sample.5'> <title>Sample</title> <literallayout class="monospaced"><![CDATA[ context main-default { // outbound dnsbl filtering to catch our own customers that end up on the sbl dnsbl sbl sbl-xbl.spamhaus.org "Mail from %s rejected - sbl; see http://www.spamhaus.org/query/bl?ip=%s"; dnsbl_list sbl; // outbound content filtering to prevent our own customers from sending spam content on { 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 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; spamassassin 4; require_match yes; dcc_greylist yes; dcc_bulk_threshold 50; }; // backscatter prevention - do not send bounces for mail that we accepted but could not forward // we only send bounces to our own customers env_from unknown { "<>" 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 { # # anything signed by this is accepted. accounts.google.com white; }; dkim_from { # # white/blacklisting based on presence of valid signatures 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"; # # blacklisting based on header from value - requiring signatures # from an impossible signer. spammer.domain require_signed .; # # whitelisting based on strong spf pass - whitelisted if signed by # an impossible signer (which will never happen) or strong spf pass. some.domain signed_white .; # # whitelisting based on valid signature or strong spf pass. # some paychex mail is signed, some is unsigned but passes strong spf. paychex.com require_signed paychex.com; # # whitelisting from mailchimp which needs wildcards princetheater.org require_signed "mandrillapp.com,*.mcsignup.com,*.mcsv.net,*.rsgsv.net,*.mcdlv.net"; # }; 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 };]]></literallayout> </refsect1> <refsect1 id='version.5'> <title>Version</title> <para> @VERSION@ </para> </refsect1> </refentry> </reference>