rmilter(8) milter for sendmail with many abilities designed for distributed MX network

SYNOPSIS

rmilter [-h ] [-c config_file ]

DESCRIPTION

The rmilter utility is designed to act as milter for sendmail(8) MTA. It provides several filter and mail scan features, among them are:

  • Clamav scanning (via unix or tcp socket).
  • Spamd scanning (supports rspamd and spamassassin)
  • Spf checking (via libspf2)
  • Greylisting with memcached upstream
  • Ratelimit with memcached upstream
  • Auto-whitelisting (internal and via memcached upstream)
  • Regexp checks.
  • DCC checking (optional)
  • Passing messages and/or their headers to beanstalk servers

The following additional options are supported:

-h
Show greeting message, version of rmilter and exit
-c config_file
Specify config file to use

All rmilter configuration is placed in rmilter.conf file.

CONFIGURATION FILE

The configuration file has format:

Aq name
= Aq value ;

Value may be:

  • String (may not contain spaces)
  • Quoted string (if string may contain spaces)
  • Numeric value
  • Flag (y, Yes or n, No)
  • Ip or network (eg. 127.0.0.1, 192.168.1.0/24)
  • Socket argument (eg. host:port or /path/to/socket)
  • Regexp (eg. /Match*/)
  • List (eg. value1, value2, value3)
  • Recipients list (user, user@domain, @domain)
  • Time argument (eg. 10s, 5d)

Some directives MUST be specified only in specified sections. Section definition looks like: 

section_name {
        section_directive;
        ...
};
Common sections:

  • clamav - clamav definitions
  • spamd - spamd (spamassassin or rspamd) definitions
  • limits - limits definitions
  • greylisting - greylisting definitions
  • rule - regexp rule definition (a section per rule)

Directives that can be defined in config file:

  • Global section:

    pidfile
    - specify path to pidfile
    Default: /var/run/rmilter.pid
    tempdir
    - specify path to temporary directory. For maximum performance it is recommended to put it on memory file system.
    Default: $TMPDIR
    bind_socket
    - socket credits for local bind:

    1. unix:/path/to/file - bind to local socket
    2. inet:port@host - bind to inet socket

    Default: bind_socket = unix:/var/tmp/rmilter.sock
    max_size
    - maximum size of scanned message for clamav, spamd and dcc.
    Default: 0 (no limit)
    spf_domains
    - list of domains that would be checked with spf
    Default: empty (spf disabled)
    use_dcc
    - flag that specify whether we should use dcc checks for mail
    Default: no
    whitelist
    - global recipients whitelist
    Default: no

  • Clamav section:

    servers
    - clamav socket definitions in format:

    1. /path/to/file
    2. host[:port]

    sockets are separated by ','
    Default: empty
    connect_timeout
    - timeout in miliseconds for connecting to clamav
    Default: 1s
    port_timeout
    - timeout in miliseconds for waiting for clamav port response
    Default: 4s
    results_timeout
    - timeout in miliseconds for waiting for clamav response
    Default: 20s
    error_time
    - time in seconds during which we are counting errors
    Default: 10
    dead_time
    - time in seconds during which we are thinking that server is down
    Default: 300
    maxerrors
    - maximum number of errors that can occur during error_time to make us thinking that this upstream is dead
    Default: 10

  • Spamd section:

    servers
    - spamd (or rspamd) socket definitions in format:

    1. /path/to/file
    2. host[:port]
    3. r:/path/to/file - for rspamd protocol
    4. r:host[:port] - for rspamd protocol

    sockets are separated by `,'
    Default: empty (spam checks disabled)
    connect_timeout
    - timeout in miliseconds for connecting to spamd
    Default: 1s
    results_timeout
    - timeout in miliseconds for waiting for spamd response
    Default: 20s
    error_time
    - time in seconds during which we are counting errors
    Default: 10
    dead_time
    - time in seconds during which we are thinking that server is down
    Default: 300
    maxerrors
    - maximum number of errors that can occur during error_time to make us thinking that this upstream is dead
    Default: 10
    reject_message
    - reject message for spam (quoted string)
    Default: ``Spam message rejected; If this is not spam contact abuse team ''
    spamd_soft_fail
    - if action is not reject use it for other actions (flag)
    Default: true
    spamd_greylist
    - greylist message only if action is greylist (flag)
    Default: true
    spam_header
    - add specified header if action is add_header and spamd_soft_fail os turned on
    Default: ``X-Spam''
    spam_header_value
    - add specified value for spam_header
    Default: ``yes''
    rspamd_metric
    - rspamd metric that would define whether we reject message as spam or not (quoted string)
    Default: ``default''
    whitelist
    - list of ips or nets that should be not checked with spamd
    Default: empty
    extended_spam_headers
    - add extended spamd headers to messages, is useful for debugging or private mail servers (flag)
    Default: false

  • Memcached section:

    servers_grey
    - memcached servers for greylisting in format:
    host Bo :port Bc Bo , host Bo :port Bc Bc
    is possible to make memcached mirroring, its syntax is {server1, server2}
    Default: empty
    servers_white
    - memcached servers for whitelisting in format similar to that is used in servers_grey
    Default: empty
    servers_limits
    - memcached servers used for limits storing, can not be mirrored
    Default: empty
    connect_timeout
    - timeout in miliseconds for connecting to memcached
    Default: 1s
    error_time
    - time in seconds during which we are counting errors
    Default: 10
    dead_time
    - time in seconds during which we are thinking that server is down
    Default: 300
    maxerrors
    - maximum number of errors that can occur during error_time to make us thinking that this upstream is dead
    Default: 10
    protocol
    - protocol that is using for connecting to memcached (tcp or udp)
    Default: udp

  • Beanstalk section:

    servers
    - beanstalk servers for pushing headers in format:
    host Bo :port Bc Bo , host Bo :port Bc Bc
    Default: empty
    copy_server
    - address of server to which rmilter should send all messages copies
    Default: empty
    spam_server
    - address of server to which rmilter should send spam messages copies
    Default: empty
    connect_timeout
    - timeout in miliseconds for connecting to beanstalk
    Default: 1s
    error_time
    - time in seconds during which we are counting errors
    Default: 10
    dead_time
    - time in seconds during which we are thinking that server is down
    Default: 300
    maxerrors
    - maximum number of errors that can occur during error_time to make us thinking that this upstream is dead
    Default: 10
    id_regexp
    - regexp that defines for which messages we should put the whole message to beanstalk, not only headers, now this regexp checks only In-Reply-To headers
    Default: empty
    send_beanstalk_headers
    - defines whether we should send headers to beanstalk servers (from servers option)
    Default: no
    send_beanstalk_copy
    - defines whether we should send copy of messages to beanstalk server (from copy_server option)
    Default: no
    send_beanstalk_spam
    - defines whether we should send copy of spam messages to beanstalk server (from spam_server option)
    Default: no
    protocol
    - protocol that is using for connecting to beanstalk (tcp or udp)
    Default: tcp

  • Greylisting section:

    timeout (required)
    - time during which we mark message greylisted
    Default: 300s
    expire (required)
    - time during which we save a greylisting record
    Default: empty (greylisting disabled)
    whitelist
    - list of ip addresses or networks that should be whitelisted from greylisting
    Default: empty
    awl_enable
    - enable internal auto-whitelist mechanics
    Default: no
    awl_pool
    - size for in-memory auto whitelist
    Default: 10M
    awl_hits
    - number of messages (from this ip) that passes greylisting to put this ip into whitelist
    Default: 10
    awl_ttl
    - time to live for ip address in auto whitelist
    Default: 3600s

  • Limits section.

    Rate limits are implemented as leaked bucket, so first value is bucket burst - is peak value for messages in bucket (after reaching it bucket is counted as overflowed and new messages are rejected), second value is rate (how much messages can be removed from bucket each second). It can be schematically displayed: 

            |------------------|          <----- current value
        |                  |
        |------------------|          <----- burst
        |                  |
        |                  |
        |                  |
        |                  |
        \                  /
         ----------------- .....      <----- rate (speed of emptying)

    limit_whitelist_ip
    - don't check limits for specified ips
    Default: empty
    limit_whitelist_rcpt
    - don't check limits for specified recipients
    Default: no
    limit_bounce_addrs
    - list of address that require more strict limits
    Default: postmaster, mailer-daemon, symantec_antivirus_for_smtp_gateways, Aq , null, fetchmail-daemon
    limit_bounce_to
    - limits bucket for bounce messages (only rcpt to)
    Default: 5:0.000277778
    limit_bounce_to_ip
    - limits bucket for bounce messages (only rcpt to per one source ip)
    Default: 5:0.000277778
    limit_to
    - limits bucket for non-bounce messages (only rcpt to)
    Default: 20:0.016666667
    limit_to_ip
    - limits bucket for non-bounce messages (only rcpt to per one source ip)
    Default: 30:0.025
    limit_to_ip_from
    - limits bucket for non-bounce messages (msg from, rcpt to per one source ip)
    Default: 100:0.033333333

  • DKIM section.

    Dkim can be used to sign messages by . Dkim support must be provided with opendkim library and rmilter must be configured with --enable-dkim option.

    header_canon
    - canonization of headers (simple or relaxed)
    Default: simple
    body_canon
    - canonization of body (simple or relaxed)
    Default: simple
    sign_alg
    - signature algorithm (sha1 for rsa-sha1 and sha256 for rsa-sha256)
    Default: sha1
    auth_only
    - sign mail for authorized users only
    Default: yes
    fold_header
    - fold the resulting header (can break some bad MTA)
    Default: no
    domain
    - domain entry must be enclosed in braces {}

    • key - path to private key
    • domain - domain to be used for signing (this matches with SMTP FROM data)
    • selector - dkim DNS selector (e.g. for selector dkim and domain example.com DNS TXT record should be for dkim._domainkey.example.com).

EXAMPLE CONFIG

# pidfile - path to pid file
# Default: pidfile = /var/run/rmilter.pid
pidfile = /var/run/rmilter/rmilter.pid;
clamav {
        # servers - clamav socket definitions in format:
        # /path/to/file
        # host[:port]
        # sockets are separated by ','
        # Default: empty
        servers = clamav.test.ru, clamav.test.ru, clamav.test.ru;
        # connect_timeout - timeout in miliseconds for connecting to clamav
        # Default: 1s
        connect_timeout = 1s;
        # port_timeout - timeout in miliseconds for waiting for clamav port response
        # Default: 4s
        port_timeout = 4s;
        # results_timeout - timeout in miliseconds for waiting for clamav response
        # Default: 20s
        results_timeout = 20s;
        # error_time - time in seconds during which we are counting errors
        # Default: 10
        error_time = 10;
        # dead_time - time in seconds during which we are thinking that server is down
        # Default: 300
        dead_time = 300;
        # maxerrors - maximum number of errors that can occur during error_time to make us thinking that 
        # this upstream is dead
        # Default: 10
        maxerrors = 10;
};
spamd {
        # servers - spamd socket definitions in format:
        # /path/to/file
        # host[:port]
        # sockets are separated by ','
        # Default: empty
        servers = clamav.test.ru, clamav.test.ru, clamav.test.ru;
        # connect_timeout - timeout in miliseconds for connecting to spamd
        # Default: 1s
        connect_timeout = 1s;
        # results_timeout - timeout in miliseconds for waiting for spamd response
        # Default: 20s
        results_timeout = 20s;
        # error_time - time in seconds during which we are counting errors
        # Default: 10
        error_time = 10;
        # dead_time - time in seconds during which we are thinking that server is down
        # Default: 300
        dead_time = 300;
        # maxerrors - maximum number of errors that can occur during error_time to make us thinking that 
        # this upstream is dead
        # Default: 10
        maxerrors = 10;
        # reject_message - reject message for spam
        # Default: "Spam message rejected; If this is not spam contact abuse team"
        reject_message = "Spam message rejected; If this is not spam contact abuse at example.com";
        # whitelist - list of ips or nets that should be not checked with spamd
        # Default: empty
        whitelist = 127.0.0.1/32, 192.168.0.0/16;
};
memcached {
        # servers_grey - memcached servers for greylisting in format:
        # host[:port][, host[:port]]
        # It is possible to make memcached mirroring, its syntax is {server1, server2}
        servers_grey = {localhost, memcached.test.ru}, memcached.test.ru:11211;
        # servers_white - memcached servers for whitelisting in format similar to that is used
        # in servers_grey
        # servers_white = {localhost, memcached.test.ru}, memcached.test.ru:11211;
        
        # servers_limits - memcached servers used for limits storing, can not be mirrored
        servers_limits = memcached.test.ru, memcached.test.ru:11211;
        # connect_timeout - timeout in miliseconds for waiting for memcached
        # Default: 1s
        connect_timeout = 1s;
        # error_time - time in seconds during which we are counting errors
        # Default: 10
        error_time = 10;
        # dead_time - time in seconds during which we are thinking that server is down
        # Default: 300
        dead_time = 300;
        # maxerrors - maximum number of errors that can occur during error_time to make us thinking that 
        # this upstream is dead
        # Default: 10
        maxerrors = 10;
        # protocol - protocol that is using for connecting to memcached (tcp or udp)
        # Default: udp
        protocol = tcp;
};
# bind_socket - socket credits for local bind:
# unix:/path/to/file - bind to local socket
# inet:port@host - bind to inet socket
# Default: bind_socket = unix:/var/tmp/rmilter.sock;
bind_socket = unix:/var/run/rmilter/rmilter.sock;
# tempdir - path to directory that contains temporary files
# Default: $TMPDIR
tempdir = /spool/tmp;
# max_size - maximum size of scanned mail with clamav and dcc
# Default: 0 (no limit)
max_size = 10M;
# spf_domains - path to file that contains hash of spf domains
# Default: empty
spf_domains = rambler.ru, mail.ru;
# use_dcc - whether use or not dcc system
# Default: no
use_dcc = yes;
# whitelisted recipients
# domain are whitelisted as @example.com
whitelist = postmaster, abuse;
# rule definition:
# rule {
#       accept|discard|reject|tempfail|quarantine "[message]"; <- action definition
#       [not] connect <regexp> <regexp>; <- conditions
#       helo <regexp>;
#       envfrom <regexp>;
#       envrcpt <regexp>;
#       header <regexp> <regexp>;
#       body <regexp>;
# };
# limits section
limits {
        # Whitelisted ip
        limit_whitelist_ip = 194.67.45.4;
        # Whitelisted recipients
        limit_whitelist_rcpt =  postmaster, mailer-daemon;
        # Addrs for bounce checks
        limit_bounce_addrs = postmaster, mailer-daemon, symantec_antivirus_for_smtp_gateways, <>, null, fetchmail-daemon;
        # Limit for bounce mail
        limit_bounce_to = 5:0.000277778;
        # Limit for bounce mail per one source ip
        limit_bounce_to_ip = 5:0.000277778;
        # Limit for all mail per recipient
        limit_to = 20:0.016666667;
        # Limit for all mail per one source ip
        limit_to_ip = 30:0.025;
        # Limit for all mail per one source ip and from address
        limit_to_ip_from = 100:0.033333333;
};
beanstalk {
        # List of beanstalk servers, random selected
        servers = bot01.example.com:3132;
        # Beanstalk protocol
        protocol = tcp;
        # Time to live for task in seconds
        lifetime = 172800;
        # Regexp that define for which messages we should put the whole message to beanstalk
        # now only In-Reply-To headers are checked
        id_regexp = "/^SomeID.*$/";
};
dkim {
    domain {
        key = /usr/local/etc/dkim_example.key;
        domain = "example.com";
        selector = "dkim";
    };
    domain {
        key = /usr/local/etc/dkim_test.key;
        domain = "test.com";
        selector = "dkim";
    };
    header_canon = relaxed;
    body_canon = relaxed;
    sign_alg = sha256;
};

NOTES

There are several things that might be useful to notice.

The order of checks:

  1. DKIM test from and create signing context (MAIL FROM)
  2. Ratelimit (RCPT TO)
  3. Greylisting (DATA)
  4. Ratelimit (EOM, set bucket value)
  5. Rules (EOM)
  6. SPF (EOM)
  7. Message size (EOM) if failed, skip clamav, dcc and spamd checks
  8. DCC (EOM)
  9. Spamd (EOM)
  10. Clamav (EOM)
  11. Beanstalk (EOM)
  12. DKIM add signature (EOM)

Keys used in memcached:

  • rcpt - bucket for rcpt filter
  • rcpt:ip - bucket for rcpt_ip filter
  • rcpt:ip:from - bucket for rcpt_ip_from filter
  • rcpt:<> - bucket for bounce_rcpt filter
  • rcpt:ip:<> - bucket for bounce_rcpt_ip filter
  • md5(from . ip . to) - key for greylisting triplet (hexed string of md5 value)

Postfix settings

There are several useful settings for postfix to work with this milter: 
smtpd_milters = unix:/var/run/rmilter/rmilter.sock
milter_mail_macros =  i {mail_addr} {client_addr} {client_name}
milter_protocol = 4

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