Copyright 2004, 2024 by Anthony Howe. All rights reserved.
« We are Grey. We stand between the darkness and the light. »
-- Delenn, All Alone in the Night, Babylon 5
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.
« Gris » is French for grey and this is a variation of the grey-listing concept as originally outlined in The Next Step in the Spam Control War: Greylisting.
Basically, grey listing keeps track of tuples consisting of the connecting IP, sender's envelope, and recipient's envelope. If a tuple, does not exist in the cache, then a new entry is added and kept until it expires and the message is temporarily rejected.
A legitimate server, when temporarily rejected, is expected to queue the message and retry sending it sometime in the near future. The temporary reject policy remains in force until the temporary block period has elapsed, at which point the message will be allowed to be delivered until the cache entry expires.
There is also support for auto white listing of expected replies. If a connection, sender, or recipient is white listed in the Sendmail access.db, then any mail sent generates an auto white list entry for each recipient that might reply and also each recipient's domain for DSN messages. Normally a site will white list their local network and/or local users to send without grey-listing and with auto white listing, replies or DSN responses to those messages will by-pass grey-listing too.
milter-gris allows for fine grained control over how SMTP authenticated messages are treated. They can be globally and/or selectively white, black, or grey listed. See the milter-gris-auth: tag below.
Additional information can be found in RFC 6647.
milter-gris [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.
Do not grey-list the null address used for DSN and MDN messages. Normally bounce messages are grey listed like any other mail.
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.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.
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:[email protected]
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-gris-Connect, milter-gris-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]action Classless Inter-Domain Routing
!pattern!action Simple fast text matching.
/regex/action POSIX 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 (allow), REJECT or ERROR (deny), 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-gris-Connect:client-ip value § Can be a pattern list.
milter-gris-Connect:[client-ip] value § Can be a pattern list.
milter-gris-Connect:client-domain value § Can be a pattern list.
milter-gris-Connect: value § Can be a pattern list.
Connect:client-ip value
Connect:[client-ip] value
Connect: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-gris-Auth:auth_authen value § Can be a pattern list.
milter-gris-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-gris-Auth: allows for a milter specific default only when {auth_authen} is defined.
milter-gris-From:sender-address value § Can be a pattern list.
milter-gris-From:sender-domain value § Can be a pattern list.
milter-gris-From:sender@ value § Can be a pattern list.
milter-gris-From: value § Can be a pattern list.
From:sender-address value
From:sender-domain value
From: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-gris-To:recipient-address value § Can be a pattern list.
milter-gris-To:recipient-domain value § Can be a pattern list.
milter-gris-To:recipient@ value § Can be a pattern list.
milter-gris-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
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-gris-Connect: and milter-gris-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.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-gris-To:mx.example.com SKIP
Some additional examples:
milter-gris-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-gris-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-gris-To:example.com /^john@.+/OK /^fred\+.*@.*/OK REJECT
Accept mail to [email protected] and [email protected] when fred's address contains a plus-detail in the address. Reject everything else to example.com.
milter-gris-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-gris-To:com /@com/REJECT NEXT
To:com OK
Reject mail to places like compaq.com or com.com if the pattern matches, but resume the access.db lookups otherwise.
milter-gris-To:aol.com /^[a-zA-Z0-9!#$&'*+=?^_`{|}~.-]{3,16}@aol.com$/NEXT REJECT
To:[email protected] 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 resume lookups of white listed and/or black listed addresses.
If the initial SPF test does not yield a Pass for any reason, then we check this "best guess" TXT record (eg. v=spf1 a/24 mx/24 ptr) to see if it yields a Pass result. If the best guess passes, then the message is accepted, else the original SPF result is used. This option is disabled by default.
The amount of time in seconds that a message corresponding to the connecting IP, sender, and recipient tuple will be temporarily blocked. This value must be less than the cache-accept-ttl option. A value between 10 to 30 minutes is suggested as reasonable.
If the client connection resolves (ie. has a reverse DNS PTR record) and appears to be from a static IP address, then this value in seconds is used instead of the block-time value. The value can be 0 to pass through immediately without grey listing. The value must be less than equal to the block-time. This option is disabled with -1.
A comma separated list of what items are used for the grey-list key. While many combinations are possible, only some will be of particular interest, like mail,rcpt in case you sit behind a mail filtering service like Postini. However, some system administrators might like to experiment with other variations and its recommended that you use at least two items.
all All possible bits.
ip The connection client IP address.
helo The HELO/EHLO argument.
mail The MAIL argument.
rcpt The current RCPT argument.
This is the time-to-live in seconds for accepted and auto white list entries stored in the cache.
The file path used for BDB or flatfile cache types.
This option specifies the cache garbage collection frequency, which is based on the number of SMTP connections (not messages) handled by the milter. Every N connections, the cache is traversed to remove expired entries.
This is the time-to-live in seconds for temporary fail entries stored in the cache.
The cache type can be one of: bdb, flatfile, hash.
Start as a background daemon or foreground application.
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=''.
Write the option summary to standard output and exit. The output is suitable for use as an option file.
A socket specifier used to communicate between Sendmail and milter-gris. 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.
The sendmail/milter I/O timeout in seconds.
The file path of where to save the process-id.
Quit an already running instance of the milter and exit. This is equivalent to:
kill -QUIT `cat /var/run/milter/milter-gris.pid`.
Terminate an already running instance of the milter before starting.
The process runtime group name to be used when started by root.
The process runtime user name to be used when started by root.
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
The working directory of the process. Normally serves no purpose unless the kernel option that permits daemon process core dumps is set.
This is the list of possible SMTP responses.
-
553 5.1.0 imbalanced angle brackets in path
The path given for aMAILorRCPTcommand 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. -
450 4.7.1 try again later
The response issued during the initial temporary block period.
-
Install
SQLitefrom a package if desired. Prior to LibSnert's availability on GitHub, the oldlibsnerttarballs included SQLite, but the GitHub libsnert repository does not, so it needs to be installed separately.milter-grisdoes not require it, but other milters that need a cache will. -
If you have never built a milter for Sendmail, then please make sure that you build and install
libmilter(or install a pre-built package), which is not built by default when you build Sendmail. Please read thelibmilterdocumentation. Briefly, it should be something like this:cd (path to)/sendmail-8.13.6/libmilter sh Build -c install -
Build LibSnert first, do not disable
sqlite3support; it should find the pre-installed version of SQLite if any. -
Building
milter-grisshould be:cd com/snert/src git clone https://github.com/SirWumpus/milter-gris.git cd milter-gris ./configure --help ./configure make sudo make install -
An example
/usr/local/share/examples/milter-gris/milter-gris.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-grisand then restart Sendmail. An example startup script is provided in/usr/local/share/examples/milter-gris/milter-gris.sh.
-
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
milteris primary member of groupmilterand secondary member of groupsmmsp. Note that the milter should be started asroot, so that it can create a .pid file and .socket file in/var/run; after which it will switch process ownership tomilter:milterbefore 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-gris.cf root:root 0644 -rw-r--r-- /var/run/milter/milter-gris.pid milter:milter 0644 -rw-r--r-- /var/run/milter/milter-gris.socket milter:milter 0644 srw-r--r-- /var/db/milter-gris milter:milter 0644 -rw-r--r-- (*BSD) /var/cache/milter-gris milter:milter 0644 -rw-r--r-- (linux) /usr/local/libexec/milter-gris root:milter 0550 -r-xr-x---
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