SnertSoft: We Serve Your Server

milter-7bit/0.16
«What's A Bit Between Servers»
Content Transfer Encoding Validation


Description & Usage ° Installation & Notes ° License & Support

WARNING

THIS IS MAIL FILTERING SOFTWARE AND WILL BLOCK MAIL THAT FAILS TO PASS A GIVEN SET OF TESTS. SNERTSOFT AND THE AUTHOR DO NOT ACCEPT ANY RESPONSIBLITY FOR MAIL REJECTED OR POSSIBLE LOSS OF BUSINESSS THROUGH THE USE OF THIS SOFTWARE. BY INSTALLING THIS SOFTWARE THE CLIENT UNDERSTANDS AND ACCEPTS THE RISKS INVOLVED.

Description

This Sendmail mail filter will tag or reject email with invalid message content transfer encodings as given by RFC 2045. For example a message that is declared to use MIME 7-bit encoding, but contains an 8-bit octet, NUL byte, and/or unpaired CR or LF characters, would be considered invalid and tagged/rejected. Variations of these checks are performed for MIME parts with 8-bit or binary encodings.

Usage

milter-7bit [options ...][arguments ...]

Options can be expressed in four different ways. Boolean options are expressed as +option or -option to turn the option on or off respectively. Options that required a value are expressed as option=value or option+=value for appending to a value list. Note that the +option and -option syntax are equivalent to option=1 and option=0 respectively. Option names are case insensitive.

Some options, like +help or -help, are treated as immediate actions or commands. Unknown options are ignored. The first command-line argument is that which does not adhere to the above option syntax. The special command-line argument -- can be used to explicitly signal an end to the list of options.

The default options, as shown below, can be altered by specifying them on the command-line or within an option file, which simply contains command-line options one or more per line and/or on multiple lines. Comments are allowed and are denoted by a line starting with a hash (#) character. If the file option is defined and not empty, then it is parsed first followed by the command-line options.

Note that there may be additional options that are listed in the option summary given by +help or -help that are not described here.

Options

access-db=/etc/mail/access.db
The type and location of the read-only access key-value map. It provides a centralised means to black and white list hosts, domains, mail addresses, etc. The following methods are supported:
text!/path/map.txtR/O text file, memory hash
/path/map.dbBerkeley DB hash format
db!/path/map.dbBerkeley DB hash format
db!btree!/path/map.dbBerkeley DB btree format
sql!/path/databaseAn SQLite3 database
socketmap!host:portSendmail style socket-map
socketmap!/path/local/socketSendmail style socket-map
socketmap!123.45.67.89:portSendmail style socket-map
socketmap![2001:0DB8::1234]:portSendmail style socket-map

If :port is omitted, the default is 7953.

The access-db contains key-value pairs. Lookups are performed from most to least specific, stopping on the first entry found. Keys are case-insensitive.

An IPv4 lookup is repeated several times reducing the IP address by one octet from right to left until a match is found.

tag:192.0.2.9
tag:192.0.2
tag:192.0
tag:192

An IPv6 lookup is repeated several times reducing the IP address by one 16-bit word from right to left until a match is found.

tag:2001:0DB8:0:0:0:0:1234:5678
tag:2001:0DB8:0:0:0:0:1234
tag:2001:0DB8:0:0:0:0
tag:2001:0DB8:0:0:0
tag:2001:0DB8:0:0
tag:2001:0DB8:0:0
tag:2001:0DB8:0
tag:2001:0DB8
tag:2001

A domain lookup is repeated several times reducing the domain by one label from left to right until a match is found.

tag:[ipv6:2001:0DB8::1234:5678]
tag:[192.0.2.9]
tag:sub.domain.tld
tag:domain.tld
tag:tld
tag:

An email lookup is similar to a domain lookup, the exact address is first tried, then the address's domain, and finally the local part of the address.

tag:account@sub.domain.tld
tag:sub.domain.tld
tag:domain.tld
tag:tld
tag:account@
tag:

If a key is found and is a milter specific tag (ie. milter-7bit-Connect, milter-7bit-From, milter-7bit-Auth, milter-7bit-To), then the value is processed as a pattern list and the result returned. The Sendmail variants cannot have a pattern list. A pattern list is a whitespace separated list of pattern-action pairs followed by an optional default action. The supported patterns are:

[network/cidr]actionClassless Inter-Domain Routing
!pattern!actionSimple fast text matching.
/regex/actionPOSIX Extended Regular Expressions

The CIDR will only ever match for IP address related lookups.

A !pattern! uses an astrisk (*) for a wildcard, scanning over zero or more characters; a question-mark (?) matches any single character; a backslash followed by any character treats it as a literal (it loses any special meaning).

!abc!exact match for 'abc'
!abc*!match 'abc' at start of string
!*abc!match 'abc' at the end of string
!abc*def!match 'abc' at the start and match 'def' at the end, maybe with stuff in between.
!*abc*def*!find 'abc', then find 'def'

For black-white lookups, the following actions are recognised: OK or RELAY (white list), REJECT or ERROR (black list), DISCARD (accept & discard), SKIP or DUNNO (stop lookup, no result), and NEXT (opposite of SKIP, resume lookup). Its possible to specify an empty action after a pattern, which is treated like SKIP returning an undefined result. Other options may specify other actions.

Below is a list of supported tags. Other options may specify additional tags.

  
milter-7bit-Connect:client-ip  value   § Can be a pattern list.
Connect:client-ip  value
client-ip  value
 
milter-7bit-Connect:[client-ip]  value   § Can be a pattern list.
milter-7bit-Connect:client-domain  value   § Can be a pattern list.
milter-7bit-Connect:  value   § Can be a pattern list.
Connect:[client-ip] value
Connect:client-domain value
[client-ip] value
client-domain value
 
All mail sent by a connecting client-ip, unresolved client-ip address or IP addresses that resolve to a client-domain are black or white-listed. These allows you to white-list your network for mail sent internally and off-site, or connections from outside networks. Note that Sendmail also has special semantics for Connect: and untagged forms.
 
milter-7bit-From:sender-address  value   § Can be a pattern list.
milter-7bit-From:sender-domain  value   § Can be a pattern list.
milter-7bit-From:sender@  value   § Can be a pattern list.
milter-7bit-From:  value   § Can be a pattern list.
From:sender-address value
From:sender-domain value
From:sender@ value
sender-address value
sender-domain value
sender@ value
 
All mail from the sender-address, sender-domain, or that begins with sender is black or white-listed. In the case of a +detailed email address, the left hand side of the +detail is used for the sender@ lookup. Note that Sendmail also has special semantics for From: and untagged forms.
 
milter-7bit-Auth:auth_authenvalue   § Can be a pattern list.
milter-7bit-Auth:value   § Can be a pattern list.
 
All mail from the authenticated sender, as given by sendmail's {auth_authen} macro, is black or white-listed. The string searched by the pattern list will be the sender-address. The empty form of milter-7bit-Auth: allows for a milter specific default only when {auth_authen} is defined.
 
milter-7bit-To:recipient-address  value   § Can be a pattern list.
milter-7bit-To:recipient-domain  value   § Can be a pattern list.
milter-7bit-To:recipient@  value   § Can be a pattern list.
milter-7bit-To:  value   § Can be a pattern list.
Spam:recipient-address value   * (FRIEND or HATER are recognised)
Spam:recipient-domain value   * (FRIEND or HATER are recognised)
Spam:recipient@ value   * (FRIEND or HATER are recognised)
To:recipient-address value
To:recipient-domain value
To:recipient@ value
recipient-address value
recipient-domain value
recipient@ value
 
All mail to the recipient-address, recipient-domain, or that begins with recipient is black or white-listed. In the case of a +detailed email address, the left hand side of the +detail is used for the recipient@ lookup. Note that Sendmail also has special semantics for Spam:, To:, and untagged forms.
 

The milter-7bit-Connect:, milter-7bit-From:, and milter-7bit-To: tags provide a milter specific means to override the Sendmail variants. For example, you normally white list your local network through any and all milters, but on the odd occasion you might want to actually scan mail from inside going out, without removing the Connect: tag that allows Sendmail to relay for your network or white listing for other milters. So for example if you have Sendmail tags like:

To:mx.example.comRELAY

You might have to add milter specific overrides in order to make sure the mail still gets filtered:

To:mx.example.comRELAY
milter-7bit-To:mx.example.comSKIP

Some additional examples:

milter-7bit-Connect:80.94 [80.94.96.0/20]OK  REJECT
 
Accept connections from the netblock 80.94.96.0/20 (80.94.96.0 through to 80.94.111.255) and rejecting anything else in 80.94.0.0/16.
 
milter-7bit-Connect:192.0.2 /^192\.0\.2\.8[0-9]/OK  REJECT
 
Accept connections from 192.0.2.80 through to 192.0.2.89, reject everything else in 192.0.2.0/24.
 
milter-7bit-From:example.com /^john@.+/OK  /^fred\+.*@.*/OK  REJECT
 
Accept mail from <john@example.com> and <fred@example.com> when fred's address contains a plus-detail in the address. Reject everything else from example.com.
 
milter-7bit-To:example.net !*+*@*!REJECT  !*.smith@*!REJECT  /^[0-9].*/REJECT
 
Reject mail to example.net using a plus-detail address or to any user who's last name is "smith" or addresses starting with a digit. No default given, so B/W processing would continue.
 

Normally when the access.db lookup matches a milter tag, then the value pattern list is processed and there are no further access.db lookups. The NEXT action allows the access.db lookups to resume and is effectively the opposite of SKIP. Consider the following examples:

milter-7bit-From:com
From:com
/@com/REJECT  NEXT
OK
 
Reject mail from places like compaq.com or com.com if the pattern matches, but resume the access.db lookups otherwise.
 
milter-7bit-From:aol.com  
From:fred@aol.com  
/^[a-zA-Z0-9!#$&'*+=?^_`{|}~.-]{3,16}@aol.com$/NEXT  REJECT
OK
 
AOL local parts are between 3 and 16 characters long and can contain dots and RFC 2822 atext characters except % and /. The NEXT used above allows one simple regex to validate the format of the address and proceed to lookup white listed and/or black listed addresses.
 
+daemon
Start as a background daemon or foreground application.
file=/etc/mail/milter-7bit.cf
Read the option file before command line options. This option is set by default. To disable the use of an option file, simply say file=''
-help or +help
Write the option summary to standard output and exit. The output is suitable for use as an option file.
milter-socket=unix:/var/run/milter/milter-7bit.socket
A socket specifier used to communicate between Sendmail and milter-7bit. Typically a unix named socket or a host:port. This value must match the value specified for the INPUT_MAIL_FILTER() macro in the sendmail.mc file. The accepted syntax is:
{unix|local}:/path/to/file
A named pipe. (default)
inet:port@{hostname|ip-address}
An IPV4 socket.
inet6:port@{hostname|ip-address}
An IPV6 socket.
milter-timeout=7210
The sendmail/milter I/O timeout in seconds.
pid-file=/var/run/milter/milter-7bit.pid
The file path of where to save the process-id.
policy=reject
If a URL is invalid in some way, then apply one of the following policies:
 none Do nothing.
 tag Tag the message subject. See subject-tag.
 quarantine Quarantine the message. See sendmail's queue management options -qQ and -Q.
 reject Reject the message. The Sendmail option EightBitMode=strict (see The Bat Book 3ed § 24.9.42) appears to be the equivalent of this milter option.
 discard Discard the message.
-printable-only
Restrict MIME 7bit to ASCII printable and whitespace characters and exclude ASCII control characters. This does NOT conform to RFC 2045, though it might prove useful for some.
-quit or +quit
Quit an already running instance of the milter and exit. This is equivalent to: kill -QUIT `cat /var/run/milter/milter-7bit.pid`
-restart or +restart
Terminate an already running instance of the milter before starting.
run-group=milter
The process runtime group name to be used when started by root.
run-user=milter
The process runtime user name to be used when started by root.
subject-tag=[SPAM]
Subject tag prefix for invalid messages. To disable the subject tag specify: subject-tag='' or see policy=none
verbose=info
A comma separated list of how much detail to write to the mail log. Those mark with § have meaning for this milter.
§ all All messages
§ 0 Log nothing.
§ info General info messages. (default)
§ trace Trace progress through the milter.
§ parse Details from parsing addresses or special strings.
§ debug Lots of debug messages.
  dialog I/O from Communications dialog
§ state State transitions of message body scanner.
  dns Trace & debug of DNS operations
  cache Cache get/put/gc operations.
§ database Sendmail database lookups.
  socket-fd Socket open & close calls
  socket-all All socket operations & I/O
§ libmilter libmilter engine diagnostics
work-dir=/var/tmp
The working directory of the process. Normally serves no purpose unless the kernel option that permits daemon process core dumps is set.

SMTP Responses

This is the list of possible SMTP responses.

550 5.7.1 invalid MIME header syntax; see grammar RFC 2045 et al.
The message was rejected (see policy option) because of a syntax error in the MIME headers. See RFC 2045 et al.
550 5.7.1 invalid Content-Transfer-Encoding, octet [0-9A-F]+ at [0-9]+ not [^;]+; see RFC 2045 et al.
The message was rejected (see policy option) for not adhering to the content transfer encoding rules as given by RFC 2045 et al.

Installation

  1. Download:

    milter-7bit/0.16 md5sum Change Log
    LibSnert md5sum Change Log
    Sendmail 8.14   http://www.sendmail.org/
    Berkeley DB   http://www.sleepycat.com/
  2. If you have never built a milter for Sendmail, then please make sure that you build and install libmilter, which is not built by default when you build Sendmail. Please read the libmilter documentation. Briefly, it should be something like this:

    cd (path to)/sendmail-8.14.0/libmilter
    sh Build -c install
    
  3. The build process for libsnert and milter-7bit is pretty straight forward once you have libmilter installed:

    cd (path to)/com/snert/src/lib
    ./configure
    make build
    cd ../milter-7bit
    ./configure
    make build
    make install
    

    Both configuration scripts have some options that allow you to override defaults. Those options are listed with:

    ./configure --help
    
  4. An example ${prefix}/share/examples/milter-7bit/milter-7bit.mc is supplied. This file should be reviewed and the necessary elements inserted into your Sendmail .mc file and sendmail.cf rebuilt. Please note the comments on the general milter flags.

    
    
  5. Once installed and configured, start milter-7bit and then restart Sendmail. An example startup script is provided in ${prefix}/share/examples/milter-7bit/milter-7bit.sh. The default options can be altered by specifying them on the command-line or within a /etc/mail/milter-7bit.cf. The milter-7bit.cf is parsed first followed by the command-line options.

Notes

  • Currently tested platforms:

    Cobalt Qube 1 with Linux RH 5.1 (mips 2.0.34 kernel); Linux RH 5.1 (Intel x386 2.2.25 kernel); FreeBSD 4.8, 4.9 (Intel x386); OpenBSD 3.6 (Intel x386)
  • The minimum desired file ownership and permissions are as follows for a typical Linux system. For FreeBSD, NetBSD, and OpenBSD the binary and cache locations may differ, but have the same permissions.

    Process user ``milter'' is primary member of group ``milter'' and secondary member of group ``smmsp''. Note that the milter should be started as root, so that it can create a .pid file and .socket file in /var/run; after which it will switch process ownership to milter:milter before starting the accept socket thread.

    /etc/mail/root:smmsp0750 drwxr-x---
    /etc/mail/access.dbroot:smmsp0640 -rw-r-----
    /etc/mail/sendmail.cfroot:smmsp0640 -rw-r-----
    /etc/mail/milter-7bit.cfroot:root0644 -rw-r--r--
    /var/run/milter/milter-7bit.pidmilter:milter0644 -rw-r--r--
    /var/run/milter/milter-7bit.socketmilter:milter0644 srw-r--r--
    /var/db/milter-7bitmilter:milter0644 -rw-r--r-- (*BSD)
    /var/cache/milter-7bitmilter:milter0644 -rw-r--r-- (linux)
    /usr/local/libexec/milter-7bitroot:milter0550 -r-xr-x---
  • I would like to express my thanks to Derek Balling for his support at http://www.milter.org/.

User Feedback

Anthony Howe
"If you're going to write a virus, damn it, make sure it conforms to standards."
Chris from Xymox1
Fun tools. You have some of the most interesting SPAM tools on the Internet! RFC Compliance or die I say! Its great !...

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Gifts

Gifts from the author's Amazon US or Amazon UK wishlist (search by mail address <achowe at snert dot com>) are welcomed for the continued encouragement, moral support, and ego pumping needed to work in foreign non-english speaking lands.

Eat-More bars since 24 January 2004