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milter-length/0.10
«When size matters.»


Description & Usage ° Installation & Notes ° License & Support

Description

This is a Sendmail utility milter that imposes message size limits by IP address, domain name, or sender address on a message body length, excluding the message headers. Sendmail's MaxMessageSize option only allows for a single global server wide message size limit, which is insufficient for some sites that would prefer finer granularity in the application of message size limits. This is particularly useful for mail hosts that manage several domains and/or a large number of users, such as an ISP.

Note that if the Sendmail option MaxMessageSize is specified in the sendmail.cf configuration, then that will impose an upper limit to any milter-length message size limits.

Usage

milter-length [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.

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-length-Connect, milter-length-From, milter-length-Auth, milter-length-To), then the value is processed as a pattern list and the result returned. A pattern list is a whitespace separated list of pattern-size pairs followed by an optional default size. The supported patterns are:

[network/cidr]sizeClassless Inter-Domain Routing
!pattern!sizeSimple fast text matching.
/regex/sizePOSIX 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-length-Connect:client-ip  value   § Can be a pattern list.
milter-length-Connect:[client-ip]  value   § Can be a pattern list.
milter-length-Connect:client-domain  value   § Can be a pattern list.
milter-length-Connect:  value   § Can be a pattern list.
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.
 
milter-length-From:sender-address  value   § Can be a pattern list.
milter-length-From:sender-domain  value   § Can be a pattern list.
milter-length-From:sender@  value   § Can be a pattern list.
milter-length-From:  value   § Can be a pattern list.
 
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.
 
milter-length-Auth:auth_authenvalue   § Can be a pattern list.
milter-length-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-length-Auth: allows for a milter specific default only when {auth_authen} is defined.
 
milter-length-To:recipient-address  value   § Can be a pattern list.
milter-length-To:recipient-domain  value   § Can be a pattern list.
milter-length-To:recipient@  value   § Can be a pattern list.
milter-length-To:  value   § Can be a pattern list.
 
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.
 

The size is the maximum number of octets permitted in a message and is expressed as a number with an optional scale suffix: K (kilo), M (mega), or G (giga). If no size is specified (or -1), then the message can be any length (ULONG_MAX).

When there are multiple message size limits assigned, then the limit applied, in order of precedence, will be: the maximum value of all relevant milter-length-to:, milter-length-auth:, milter-length-from:, milter-length-connection:, or an unlimited default. Essentially each matching milter tag replaces the previous value. When there are multiple recipients with one or more possible milter-length-to: entries, then maximum length limit found is applied.

Below are only some examples of what is possible:

milter-length-Connect:80.94 [80.94.96.0/20]2M  500K
 
All the mail from netblock 80.94.96.0/20 (80.94.96.0 through to 80.94.111.255) is limited to 2 megabytes, while anything else in 80.94.0.0/16 is limited to 500 kilobytes.
 
milter-length-Connect:192.0.2 10M
 
All the mail from the network in 192.0.2.0/24 is limited to 10 megabytes.
 
milter-length-Connect:192.0.2 /^192\.0\.2\.8[0-9]/1M  10M

Mail sent from IP addresses 192.0.2.80 through to 192.0.2.89 is limited to 1 megabyte, while the rest of the network in 192.0.2.0/24 is limited to 10 megabytes.

This example is rather contrived to show some of what is possible with patterns and is the equivalent of ten simple milter-length-connect: entries. When using patterns, weigh the need vs. simplicity, readability, and administration.
 
milter-length-From:example.com /^[^+]+@/500K
 
Mail from within example.com, where the sender's address does not contain a plus-detail, is limited to 500 kilobytes, otherwise no limit is imposed.
 
milter-length-To:example.net !*+*@*!  /^[0-9].*/20K  !*smith*@*!1M 4M
 
Mail to addresses within example.net containing a plus-detail have no limit; those starting with a digit are limited to 20 kilobytes; mail to anyone with "smith" as part of their address is limited to 1 megabyte, while the rest of the domain is limited to 4 megabytes.
 
+daemon
Start as a background daemon or foreground application.
file=/etc/mail/milter-length.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-length.socket
A socket specifier used to communicate between Sendmail and milter-length. 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-length.pid
The file path of where to save the process-id.
-quit or +quit
Quit an already running instance of the milter and exit. This is equivalent to: kill -QUIT `cat /var/run/milter/milter-length.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.
-smtp-auth-ok
Allow SMTP authenticated senders to send unscanned mail. See also the milter-length-auth: tag (access-db=) for finer granularity of control.
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 generated by milter-length.

550 5.7.1 max. message size \([0-9]+\) exceeded
The message body, not counting the headers, is too big.
553 5.1.0 imbalanced angle brackets in path
The path given for a MAIL or RCPT command is missing a closing angle bracket
553 5.1.0 address does not conform to RFC 2821 syntax
The address is missing the angle brackets, < and >, as required by the RFC grammar.
553 5.1.0 local-part too long
The stuff before the @ is too long.
553 5.1.[37] invalid local part
The stuff before the @ sign contains unacceptable characters.
553 5.1.0 domain name too long
The stuff after the @ is too long.
553 5.1.7 address incomplete
Expecting a domain.tld after the @ sign and found none.
553 5.1.[37] invalid domain name
The domain after the @ sign contains unacceptable characters.

Installation

  1. Download:

    milter-length/0.10 md5sum Change Log
    LibSnert md5sum Change Log
    Sendmail 8.13   http://www.sendmail.org/
    Berkeley DB   http://www.sleepycat.com/

  2. In order to support the access database milter-length requires Berkeley DB. You should build and install Berkeley DB library first, if you do not already have it. Please read the Berkeley DB documentation on how to build the library. Briefly, it should be something like this: 

    cd (path to)/db-4.2.52/build_unix
../dist/configure
make
make install

    If your system is Linux and you install Berkeley DB in the default, non- standard, location then you must remember to update /etc/ld.so.conf and run ldconfig. You can change the default install location by specifying the ../dist/configure option --prefix=/usr/local for example.

  3. 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 prior to 8.13.0. Please read the libmilter documentation. Briefly, it should be something like this: 

    cd (path to)/sendmail-8.12.11/libmilter
sh Build -c
sh Build install

  4. The build process for libsnert and milter-length is pretty straight forward once you have libmilter installed: 

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

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

    ./configure --help

  5. An example /usr/local/share/examples/milter-length/milter-length.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.

  6. Once installed and configured, start milter-length and then restart Sendmail. An example startup script is provided in /usr/local/share/examples/milter-length/milter-length.sh. The default options can be altered by specifying them on the command-line or within a /etc/mail/milter-length.cf. The milter-length.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-length.cfroot:root0644 -rw-r--r--
    /var/run/milter/milter-length.pidmilter:milter0644 -rw-r--r--
    /var/run/milter/milter-length.socketmilter:milter0644 srw-r--r--
    /var/db/milter-lengthmilter:milter0644 -rw-r--r-- (*BSD)
    /var/cache/milter-lengthmilter:milter0644 -rw-r--r-- (linux)
    /usr/local/libexec/milter-lengthroot:milter0550 -r-xr-x---

  • I would like to express my thanks to Derek Balling for his support at http://www.milter.org/.

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parsecs since 23 October 2005