System Administration Commands
milter-regex
sendmail milter plugin for regular expression filtering
milter-regex -d -c config -n -p pipe -u user

Description
The milter-regex plugin can be used with the milter API of sendmail 8 sendmail 8 sendmail 1M to filter mails using regular expressions matching SMTP envelope parameters and mail headers and body.

Options
-d 	 Enable verbose debug output, which will be logged via syslog 3 syslog 3 syslog 3C at the debug level for the mail facility. In case you are logging to a file, make sure to have a lot of space at the partition in question!
-c config 	 Use the specified configuration file instead of the default, @CONF@.
-n 	 Usually milter-regex adds a heading to messages that are scanned. The header is of the form "X-Milter: version", this option instructs regex-milter to refrain from adding this heading.
-p pipe 	 Use the specified pipe to interface sendmail 8 sendmail 8 sendmail 1M . Default is @OCONN@.
-u user 	 Run as the specified user instead of the default, smmsp. When milter-regex is started as root, it calls setuid 2 to drop privileges. The non-privileged user should have read access to the configuration file and read-write access to the pipe.

Sendmail Configuration
sendmail 8 sendmail 8 sendmail 1M needs to have milter support. To check, whether your sendmail has milter support, you may run:

  /usr/lib//usr/sbin//usr/sbin/sendmail -d0.1 -bp | grep MILTER

If MILTER appears in the output, sendmail supports Mail Filter.
The milter aka plugin needs to be registered in the sendmail 8 sendmail 8 sendmail 1M configuration, by adding the following lines to your sendmail m4 configuration file

  INPUT_MAIL_FILTER(`milter-regex', `S=@OCONN@, T=S:30s;R:2m')

and rebuilding /etc/mail/sendmail.cf (e.g. cd /etc/mail; ./mknewcf -c server.mc) and restarting sendmail 8 sendmail 8 sendmail 1M .

Plugin Configuration
The configuration file consists of rules that, when matched, cause sendmail 8 sendmail 8 sendmail 1M to reject mails. Emtpy lines and lines starting with # are ignored, as well as leading whitespace (blanks, tabs). Trailing backslashes can be used to wrap long rules into multiple lines. Each rule starts with one of the following commands:
reject "message" 	 Subsequent rules cause the mail to be rejected with a permanent error consisting of the specified text part. The SMTP reply consists of the three-digit code 550 (RFC 2821 "command rejected for policy reasons"), the extended reply code 5.7.1 (RFC 1893 "Permanent Failure", "Security or Policy Status", "Delivery not authorized, message refused") and the text part (which defaults to "Command rejected", if not specified). This is a permanent failure, which causes the sender to remove the message from its queue without trying to retransmit, commonly generating a bounce message to the sender.
tempfail "message" 	 Subsequent matching rules cause the mail to be rejected with a temporary error consisting of the specified text part. The SMTP reply consists of the three-digit code 451 (RFC 2821 "Requested action aborted: local error in processing"), the extended reply code 4.7.1 (RFC 1893 "Persistent Transient Failure", "Security or Policy Status", "Delivery not authorized, message refused") and the text part (which defaults to "Please try again later", if not specified). This is a temporary failure, which causes the sender to keep the message in its queue and try to retransmit it, commonly for several days.
discard 	 Subsequent matching rules cause the mail to be accepted but then discarded silently. Note that connect and helo rules should not use discard.
accept 	 Subsequent matching rules cause the mail to be accepted without further rule evaluation. Can be used for whitelist criteria.
A command is followed by one or more expressions, each causing the previous command to be executed when matched. The following expressions can be used:
connect hostname address 	 Reject the connection if both the sender's hostname and address match the specified regular expressions. The numerical address is either dotted-quad (IPv4) or coloned-hex (IPv6). The hostname is the result of a DNS reverse resolution of the numerical address (which sendmail 8 sendmail 8 sendmail 1M performs independantly of the milter plugin). When resolution fails, the hostname contains the numerical address in square brackets.
helo name 	 Reject the connection if the sender supplied HELO name matches the specified regular expression. Commonly, the sender supplies his fully-qualified hostname as HELO name.
envfrom address 	 Reject the mail if the sender supplied envelope MAIL FROM address matches the specified regular expression. Addresses commonly have the form user@host.doma.in.
envrcpt address 	 Reject the mail if the sender supplied envelope RCPT TO address matches the specified regular expression.
header name value 	 Reject the mail if a header matches the specified name and value. For instance, the header "Subject: Test" matches name Subject and value Test.
body line 	 Reject the mail if a body line matches the specified regular expression.

Regular Expressions
The regular expressions used in the configuration rules are enclosed in arbitrary delimiters, no further escaping is needed.
The first character of an argument is taken as the delimiter, and all subsequent characters up to the next occurance of the same delimiter are taken literally as the regular expression. Since the delimiter itself cannot be part of the regular expression (no escaping is supported), a delimiter must be chosen that doesn't occur in the regular expression itself. Each argument can use a different delimiter, all characters except spaces and tabs are valid.
Two immediately adjacent delimiters form an empty regular expression, which always matches and requires no regexec 3C regexec 3 regexec 3 call. This can be used in rules requiring multiple arguments, to match only some arguments.
See re_format 7 regex 5 regex 7 for a detailed description of basic and extended regular expressions.
Optionally, the following flags can be used after the closing delimiter:
e 	 Extended regular expression. This sets REG_EXTENDED for regcomp 3C regcomp 3 regcomp 3 .
i 	 Ignore upper/lower case. This sets REG_ICASE.
n 	 Not matching. Reverses the matching result, i.e. the mail is rejected if the regular expression does not match.

Boolean Expressions
A rule can consist of either a simple term or more complex expressions. A term has the form

  header /From/ /domain/i

and expressions can be built combining terms with operators and, or, not and ( ), as in

  header /From/ /domain/i and body /money/ \
      ( not header /From/ /domain/ ) and ( body /sex/ or body /fast/ )

Operator precedence should not be relied on, instead parentheses should be used to resolve any ambiguities (they usually produce syntax errors from the parser).

Macros
Macros allow to store terms or expressions as a name, and $name can be used as term within other rules, expressions or macro definitions. Example:

  friends         = header /^Received$/ /^from [^ ]*(ork.net|home.com)/e 
  attachments     = header ,^Content-Type$, ,multipart/mixed, and \ 
                    body ,^Content-Type: application/,
  executables     = $attachments and body ,name=".*.(pif|exe|scr)"$,e
  
  reject "executable attachment from non-friends"
  $executables and not $friends

Macro names must begin with a letter and may contain alphanumeric characters and punctuation characters. Reserved keywords (like reject or header) cannot be used as macro names. Macros must be defined before use, the definition must precede the use in the configuration file, read from top to bottom.

Evaluation
Rules are evaluated in the order specified in the configuration file, from top to bottom. When a rule matches, the correpsonding action is taken, that is the last action specified before the matching rule.
The plugin evaluates the rules every time a line of mail (or envelope) is received. As soon as a rule matches, the action is taken immediately, possibly before the entire mail is received, even if further lines might possibly make other rules match, too. This means the first rule matching chronologically has precendence.
If evaluation for a line of mail makes two (or more) rules match, the rule that comes first in the configuration file has precendence.
Boolean expressions are short-circuit evaluated, that means "a or b" becomes true as soon as one of the terms is true and "a and b" becomes false as soon as one of the terms is false, even if the other term is not known, possibly because the relevant mail line has not been received yet.

Examples

   # /etc/mail/milter-regex.conf example

   tempfail "Sender IP address not resolving"
   connect /\[.*\]/ //

   reject "Malformed HELO (not a domain, no dot)"
   helo /\./n

   reject "Malformed RCPT TO (not an email address, not <.*@.*>)"
   envrcpt /<(.*@.*|Postmaster)>/ein

   reject "HTML mail not accepted"
   # use comma as delimiter here, as / occurs within RE
   header /^Content-type$/i ,^text/html,i
   body ,^Content-type: text/html,i

   # Swen worm
   discard
   header /^(TO|FROM|SUBJECT)$/e //
   header /^Content-type$/i /boundary="Boundary_(ID_/i
   header /^Content-type$/i /boundary="[a-z]*"/
   body ,^Content-type: audio/x-wav; name="[a-z]*\.[a-z]*",i

   # Some nasty spammer
   reject "Business Corp spam, get lost"
   body /^Business Corp. for W.& L. AG/i and \
       ( body /043.*317.*0285/ or body /0041.43.317.02.85/ )

Logging
milter-regex sends log messages to syslogd 1M syslogd 8 syslogd 8 using facility mail and, with increasing verbosity, level err, notice, info and debug. The following syslog.conf 4 syslog.conf 5 syslog.conf 5 section can be used to log messages to a dedicated file:

  mail.err;mail.notice    /var/log/milter-regex.log
  mail.debug              /var/aLotOfSpaceAvailable/mail-debug.log

Grammar
Syntax for milter-regex in BNF:

   file            = ( rule | macro ) file
   rule            = action expr-list
   action          = "reject" msg | "tempfail" msg | "discard" | "accept"
   msg             = ( '"' | "'" ) string ( '"' | "'" )
   expr-list       = expr [ expr-list ]
   expr            = term | term "and" expr | term "or" expr | "not" term
   term            = '(' expr ')' | "connect" arg arg | "helo" arg |
                     "envfrom" arg | "envrcpt" arg | "header" arg arg |
                     "body" arg | '$' name
   arg             = del regex del flags
   del             = '/' | ',' | '-' | ...
   flags           = [ 'e' ] [ 'i' ] [ 'n' ]
   macro           = name '=' expr

Files
@CONF@ 	default configuration file

See Also
regcomp 3C regcomp 3 regcomp 3 , syslog.conf 4 syslog.conf 5 syslog.conf 5 , re_format 7 regex 5 regex 7 , sendmail 8 sendmail 8 sendmail 1M , syslogd 1M syslogd 8 syslogd 8
The Internet Society (2001), RFC 2821 - Simple Mail Transfer Protocol, AT&T Laboratories, April 2001
G. Vaudreuil, RFC 1893 - Enhanced Mail System Status Codes, Octel Network Services, January 1996

History
The first version of milter-regex was written in 2003. Boolean expression evaluation was added in 2004.

Authors
Daniel Hartmeier daniel@benzedrine.cx