milter-link/2.0
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| text!/path/map.txt | R/O text file, memory hash |
| /path/map.db | Berkeley DB hash format |
| db!/path/map.db | Berkeley DB hash format |
| db!btree!/path/map.db | Berkeley DB btree format |
| sql!/path/database | An SQLite3 database |
| socketmap!host:port | Sendmail style socket-map |
| socketmap!/path/local/socket | Sendmail style socket-map |
| socketmap!123.45.67.89:port | Sendmail style socket-map |
| socketmap![2001:0DB8::1234]:port | Sendmail 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.
There are essentially three types of keys used in the access-db. Many of the tags available will use one or more of these lookup sequences.
- IP Address Lookups
-
An IP address lookup is typically applied to the connected SMTP client.
It will start with a complete IPv4 or IPv6 address and break it down
on delimiter boundaries from right to left.
IPv6 Lookup IPv4 Lookup tag:2001:0DB8:0:0:0:0:1234:5678 tag:192.0.2.9 tag:2001:0DB8:0:0:0:0:1234 tag:192.0.2 tag:2001:0DB8:0:0:0:0 tag:192.0 tag:2001:0DB8:0:0:0 tag:192 tag:2001:0DB8:0:0 tag:2001:0DB8:0 tag:2001:0DB8 tag:2001 Note that the compact form of an IPv6 address, "
2001:0DB8::1234:5678", cannot be used. Only the full IPv6 address format, with all intervening zeros, is currently supported. - Domain Name Lookups
-
A domain lookup may be applied to either the connected SMTP client,
where the client's host name found through a DNS PTR record is searched for, or
using the domain portion of an mail address (see below). A domain lookup will try the IP-domain literal
if applicable, then continue with the FQDN, breaking it down one label
at a time from left to right.
tag:[ipv6:2001:0DB8::1234:5678] tag:[192.0.2.9] tag:sub.domain.tld tag:domain.tld tag:tld tag: Note that the bare tag is often used to specify system wide defaults.
- Mail Address Lookups
-
A mail address lookup is similar to a domain lookup, but the search
first starts with a complete mail address, before trying the address's
domain, and finally only the local part of the address.
tag:account@sub.domain.tld tag:sub.domain.tld tag:domain.tld tag:tld tag:account@ tag: Note that the bare tag is often used to specify system wide defaults.
Tags
The following list outlines the available tags and their supported key lookups:
- milter-link-Auth:auth_authen
- milter-link-Auth:
-
Used to black or white list by authenticated user (SMTP AUTH) as given
by the macro
{auth_authen}. The string searched by the pattern list will be the sender's mail address. The empty form of milter-link-Auth: allows for a milter specific default only when{auth_authen}is defined. - milter-link-Body:ip:From:mail
- milter-link-Body:domain:From:mail
- milter-link-Body:mail:From:mail
- milter-link-Body:ip:To:mail
- milter-link-Body:domain:To:mail
- milter-link-Body:mail:To:mail
- The above are only applied when access-check-body is enabled, followed by the simple milter-link-Body: tags. Used to black (REJECT) or ignore (OK) domains that make up mail addresses or URIs found within the header or body content of a message in combination with a sender or recipient.
- milter-link-Body:ip
- milter-link-Body:domain
- milter-link-Body:mail
-
Used to black (REJECT) or ignore (OK) domains that make up
mail addresses or URIs found within the header or body content
of a message.
This can be used to white list domains that are know to be good and so avoid redundant lookups. Consider listing your own web site that might appear in your signatures, common and popular sites like hotmail.com or yahoo.com, etc. If black listed, the message will be rejected when found in the message body. See domain-bl, uri-bl, uri-a-bl, uri-ns-bl, and uri-ns-a-bl options.
- milter-link-Connect:ip:From:mail
- milter-link-Connect:domain:From:mail
- This set of combination tags are used to black or white list sender addresses when sent from a given SMTP client. The sender address can be easily forged and using the From: tag by itself could allow spam with a forged address. By adding the sender's SMTP client as an extra constraint, it is possible to limit such abuse. Note that the lookup variants with blank IP, domain, or mail are not supported.
- milter-link-Connect:ip
- milter-link-Connect:domain
- Connect:ip
- Connect:domain
- Used to black or white list an SMTP client. Note that Sendmail also has special semantics for Connect:.
- milter-link-Connect:ip:To:mail
- milter-link-Connect:domain:To:mail
- This set of combination tags are used to black or white list recipient addresses that a given SMTP client may contact. This allows for finer granularity of control in place of the To: tag. Note that the lookup variants with blank IP, domain, or mail are not supported.
- milter-link-From:mail:To:mail
- This set of combination tags are used to black or white list a pair of sender and recipient addresses. This allows for finer granularity of control in place of the To: tag. Note that the lookup variants with blank mail elements are not supported.
- milter-link-From:mail
- From:mail
- Used to black or white list a sender's mail address. Black listing using this tag is fine, but white listing is not recommended as it is too easy for someone to fake the sender address or domain. In the case of a +detailed mail address, the left hand side of the +detail is used for the sender@ lookup. Note that Sendmail also has special semantics for From:.
- milter-link-Helo:domain
- Used to black or white list an SMTP client based on the HELO / EHLO argument. This tag is not recommended for white listing as the HELO argument can be too easily falsified.
- milter-link-To:mail
- Spam:mail
- To:mail
- Used to black or white list a recipient's mail address. In the case of a +detailed mail address, the left hand side of the +detail is used for the sender@ lookup. Note that Sendmail also has special semantics for To: and Spam:.
Action
When a key lookup matches, then the value returned is a pattern list, which in
its simplest and most common form is either an action word like
OK, REJECT, etc.;
a numerical value; or some arbitrary string depending on the tag involved. For example:
Connect:192.168.0 OK Rate-Connect:fsl.com 17 Msg-Limit-From:hotmail.com 150/30m
The action words supported by milter-link are:
OK White list, by-pass one or more tests. REJECT Black list, reject connection, sender, recipient, etc. SKIP Stop lookup and return no result ie. continue testing. DUNNO Same as SKIP, commonly used by postfix.
Pattern Lists
In most instances, the above forms of key lookup and values are sufficient. However, there may be times when finer granularity of control is required, in which case pattern lists can be used. A pattern list is a white space separated list of pattern-action pairs followed by an optional default action. The supported types are:
[network/cidr]action Classless Inter-Domain Routing
(only with IP address lookups)!pattern!action Simple fast text matching. /regex/action POSIX Extended Regular Expressions.
The simple pattern matching, !pattern!, uses an asterisk (*) for a wildcard, scanning over zero or more characters; a question-mark (?) matches any single character; a backslash followed by any character treats that character as a literal. This method always tries to match the beginning and end of string. For example:
!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'
The following is an example using a simple pattern to reject client connections that originate from a range of IP addresses of an ISP assigned to ADSL customers. Using a pattern like this allows you to drop connects from the ISP's ADSL, while still accepting connections from mail and web servers.
Connect:hananet.net !adsl-*-*.usr.hananet.net!REJECT
If you know that an ISP's mail and web servers follow a standard naming convention, you might prefer to only accept mail from those instead. We include web servers here to handle the case where a web server might have to send a mail response based on a form being filled in.
Connect:hananet.net !smtp*.hananet.net!OK !www*.hananet.net!OK REJECT
Note that SPF was designed to help mail servers identify originators of mail, so creating patterns as shown in the above two examples is not normally required. However, SPF is still considered experimental and not as widely deployed as one might hope.
The next example, /regex/, uses Extended Regular Expressions to validate the
format of the local-part of an AOL address, which must be between 3 and 16
characters long, can contain dots and RFC 2822 ``atext'' characters except % and /.
The SKIP word allows the one regular expression to validate the format of the address and continue if the
pattern matches; otherwise if the regular expression failed to match, REJECT the suspect aol.com address.
From:grandma@aol.com OK
From:aol.com /^[a-zA-Z0-9!#$&'*+=?^_`{|}~.-]{3,16}@aol.com$/SKIP REJECT
The discussion of Extended Regular Expressions is vast and complex, well beyond the scope of this document. There are many on-line tutorials and references available and the book Mastering Regular Expressions, 3e from O'Reilly covers the topic in depth.
If you need to use a pattern list, then try and follow these suggestions:
- A pattern cannot be used as the key in an access-db lookup. Key-value tables work with constants for the keys using a predefined lookup order as outlined above.
- Use the key lookup as a selector to find a pattern list.
- Use simple !pattern! matching where possible, as it will be faster than Extended Regular Expressions, /regex/.
- Avoid using pattern lists with bare tag variants that specify a global default. It will more often than not cause a lot of unnecessary attempts to match a pattern.
- Keep your pattern lists short & simple.
The milter-link-Connect:, milter-link-From:, and milter-link-To: tags provide a milter specific means to override the Sendmail and Postfix 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 or Postfix to relay for your network or white listing for other milters. So for example if you have tags like:
To:mx.example.com RELAY
You might have to add milter specific overrides in order to make sure the mail still gets filtered:
To:mx.example.com RELAY milter-link-To:mx.example.com SKIP
Some additional examples:
milter-link-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-link-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-link-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-link-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 it is assumed to be SKIP.
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. discard Discard the message.
MAIL FROM:
address and mail addresses found in select headers and the message are MD5
hashed, which are then checked against these black lists.
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. discard Discard the message.
milter-link.
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.
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. discard Discard the message.
kill -QUIT `cat /var/run/milter/milter-link.pid`milter-link-auth: tag (access-db=)
for finer granularity of control.
.zen.spamhaus.org
The host or domain name found in a URI is used to find its DNS A record
and IP address, which is then checked against these IP DNS black lists.
.multi.surbl.org.
The domain name found in a URI is checked against these DNS black lists.
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. discard Discard the message.
.multi.surbl.org
or .black.uribl.com, first test the registered domain, then any
sub-domains from right-to-left. Typically sub-domains are not listed.
§ 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
SMTP Responses
This is the list of possible SMTP responses.
- 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.
- 550 5.7.1 connection .+ [.+] blocked
- The connection is blacklisted in the sendmail access.db.
- 550 5.7.1 sender blocked
- The sender is blacklisted in the sendmail access.db.
- 550 5.7.1 recipient blocked
- The recipient is blacklisted in the sendmail access.db.
- 550 5.7.1 broken URL ".+": .+
- A URL found in the message fails to connect, return a valid document, or causes a redirection loop.
- 550 5.7.1 black listed URL host .+ by .+
- A URL found in the message is black listed by the stated DNSBL service.
- 550 5.7.1 rejected URL host .+
- A URL found in the message is locally black listed in the access.db using a milter-link-body: tag.
Installation
Download:
milter-link/2.0 md5sum Change Log LibSnert md5sum Change Log Sendmail 8.14 http://www.sendmail.org/ Berkeley DB http://www.sleepycat.com/ -
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 thelibmilterdocumentation. Briefly, it should be something like this:cd (path to)/sendmail-8.14.0/libmilter sh Build -c install
-
The build process for
libsnertandmilter-linkis pretty straight forward once you havelibmilterinstalled:cd (path to)/com/snert/src/lib ./configure --without-sqlite3 make build cd ../milter-link ./configure make build make install
SQLite support is not required in SnertSoft milters that do not use a cache. If you have compiled LibSnert for a mix of SnertSoft milters, some that require a cache, then you can build them all with the SQLite support. Will not hurt, just produce larger binaries in those that do not need it.
Both configuration scripts have some options that allow you to override defaults. Those options are listed with:
./configure --help
-
An example
/usr/local/share/examples/milter-link/milter-link.mcis supplied. This file should be reviewed and the necessary elements inserted into your Sendmail.mcfile andsendmail.cfrebuilt. Please note the comments on the general milter flags. -
Once installed and configured, start
milter-linkand then restart Sendmail. An example startup script is provided in/usr/local/share/examples/milter-link/milter-link.sh. The default options can be altered by specifying them on the command-line or within a/etc/mail/milter-link.cf. Themilter-link.cfis 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:smmsp 0750 drwxr-x--- /etc/mail/access.db root:smmsp 0640 -rw-r----- /etc/mail/sendmail.cf root:smmsp 0640 -rw-r----- /etc/mail/milter-link.cf root:root 0644 -rw-r--r-- /var/run/milter/milter-link.pid milter:milter 0644 -rw-r--r-- /var/run/milter/milter-link.socket milter:milter 0644 srw-r--r-- /var/db/milter-link milter:milter 0644 -rw-r--r-- (*BSD) /var/cache/milter-link milter:milter 0644 -rw-r--r-- (linux) /usr/local/libexec/milter-link root:milter 0550 -r-xr-x---
License Agreement 1.9
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