libexec/spamd/spamd.8

.\"	$OpenBSD: spamd.8,v 1.135 2019/07/24 18:41:05 mestre Exp $
.\"
.\" Copyright (c) 2002 Theo de Raadt.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
.Dd $Mdocdate: July 24 2019 $
.Dt SPAMD 8
.Os
.Sh NAME
.Nm spamd
.Nd spam deferral daemon
.Sh SYNOPSIS
.Nm spamd
.Op Fl 45bdv
.Op Fl B Ar maxblack
.Op Fl C Ar file
.Op Fl c Ar maxcon
.Op Fl G Ar passtime : Ns Ar greyexp : Ns Ar whiteexp
.Op Fl h Ar hostname
.Op Fl K Ar file
.Op Fl l Ar address
.Op Fl M Ar address
.Op Fl n Ar name
.Op Fl p Ar port
.Op Fl S Ar secs
.Op Fl s Ar secs
.Op Fl w Ar window
.Op Fl Y Ar synctarget
.Op Fl y Ar synclisten
.Sh DESCRIPTION
.Nm
is a fake mail daemon which rejects false mail.
It is designed to be very efficient so that it does not slow down the
receiving machine.
.Pp
.Nm
considers sending hosts to be of three types:
.Pp
.Em blacklisted
hosts are diverted to
.Nm
and
.Em tarpitted
i.e. they are communicated with very slowly
to consume the sender's resources.
Mail is rejected with either a 450 or 550 error message.
A blacklisted host will not be allowed to talk to a real mail server.
.Pp
.Em whitelisted
hosts do not talk to
.Nm .
Their connections are instead sent to a real mail server,
such as
.Xr smtpd 8 .
.Pp
.Em greylisted
hosts are diverted to
.Nm ,
but
.Nm
has not yet decided if they are likely spammers.
They are given a temporary failure message by
.Nm
when they try to deliver mail.
.Pp
When
.Nm
is run in default mode,
it will greylist connections from new hosts.
Depending on its configuration,
it may choose to blacklist the host or,
if the checks described below are met,
eventually whitelist it.
When
.Nm
is run in blacklist-only mode,
using the
.Fl b
flag,
it will consult a pre-defined set of blacklist addresses
to decide whether to tarpit the host or not.
.Pp
When a sending host talks to
.Nm ,
the reply will be
.Em stuttered .
That is,
the response will be sent back a character at a time, slowly.
For blacklisted hosts,
the entire dialogue is stuttered.
For greylisted hosts,
the default is to stutter for the first 10 seconds
of dialogue only.
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl 4
For blacklisted entries, return error code 450 to the spammer (default).
.It Fl 5
For blacklisted entries, return error code 550 to the spammer.
.It Fl B Ar maxblack
The maximum number of concurrent blacklisted connections to stutter at.
This value may not be greater than maxcon (see below).
The default is
.Ar maxcon
\- 100.
When this value is exceeded, new blacklisted connections will not be
stuttered at.
.It Fl b
Run in blacklist-only mode.
.It Fl C Ar file
Load the certificate for TLS from the given
.Ar file .
.It Fl c Ar maxcon
The maximum number of concurrent connections to allow.
.Ar maxcon
may not exceed
.Va kern.maxfiles
\- 200, and defaults to 800.
.It Fl d
Debug mode.
.Nm
does not
.Xr fork 2
into the background.
.It Xo
.Fl G
.Ar passtime : Ns Ar greyexp : Ns Ar whiteexp
.Xc
Adjust the three time parameters for greylisting.
.Ar passtime
defaults to 25 (minutes),
.Ar greyexp
to 4 (hours),
and
.Ar whiteexp
to 864 (hours, approximately 36 days).
.It Fl h Ar hostname
The hostname that is reported in the SMTP banner.
.It Fl K Ar file
Load the private key for TLS from the given
.Ar file .
.It Fl l Ar address
Specify the local address to which
.Nm
is to
.Xr bind 2 .
By default
.Nm
listens on the localhost address 127.0.0.1.
.It Fl M Ar address
Specify a local IP address which is listed as a low priority MX record,
used to identify and trap hosts that connect to MX hosts out of order.
See
.Sx GREYTRAPPING
below for details.
.It Fl n Ar name
The SMTP version banner that is reported upon initial connection.
.It Fl p Ar port
Specify a different port number from the default port that
.Nm
should listen for diverted SMTP connections on.
The default port is found by looking for the named service
.Dq spamd
using
.Xr getservbyname 3 .
.It Fl S Ar secs
Stutter at greylisted connections for the specified amount
of seconds, after which the connection is not stuttered at.
The default is 10; maximum is 90.
.It Fl s Ar secs
Delay each character sent to the client by the specified
amount of seconds.
The default is 1; maximum is 10.
.It Fl v
Enable verbose logging.
By default
.Nm
logs connections, disconnections and blacklist matches to
.Xr syslogd 8
at
.Dv LOG_INFO
level.
With verbose logging enabled, message detail
including subject and recipient information is logged at
.Dv LOG_INFO ,
along with the message body and SMTP dialogue being logged at
.Dv LOG_DEBUG
level.
.It Fl w Ar window
Set the socket receive buffer to this many bytes, adjusting the window size.
.It Fl Y Ar synctarget
Add target
.Ar synctarget
to receive synchronisation messages.
.Ar synctarget
can be either an IPv4 address for unicast messages
or a network interface and optional TTL value for multicast messages
to the group 224.0.1.240.
If the multicast TTL is not specified, a default value of 1 is used.
This option can be specified multiple times.
See also
.Sx SYNCHRONISATION
below.
.It Fl y Ar synclisten
Listen on
.Ar synclisten
network interface for incoming synchronisation messages.
This option can be specified only once.
See also
.Sx SYNCHRONISATION
below.
.El
.Pp
When run in default mode,
connections receive the pleasantly innocuous temporary failure of:
.Bd -literal -offset 4n
451 Temporary failure, please try again later.
.Ed
.Pp
This happens in the SMTP dialogue
immediately after the DATA command is received from the client.
.Nm
will use the db file in
.Pa /var/db/spamd
to track these connections to
.Nm
by connecting IP address, HELO/EHLO, envelope-from, and envelope-to, or
.Em tuple
for short.
Hosts which connect but do not attempt to deliver mail
will not generate a tuple and always be ignored.
.Pp
A previously unseen tuple is added to the
.Pa /var/db/spamd
database, recording the time an initial connection attempt was seen.
After
.Em passtime
minutes if
.Nm
sees a retried attempt to deliver mail for the same tuple,
.Nm
will whitelist the connecting address by adding it as a
whitelist entry to
.Pa /var/db/spamd .
.Pp
.Nm
regularly scans the
.Pa /var/db/spamd
database and configures all whitelist addresses as the
.Xr pf 4
<spamd-white>
table,
allowing connections to pass to the real MTA.
Any addresses not found in
<spamd-white>
are diverted to
.Nm .
.Pp
An example
.Xr pf.conf 5
fragment is given below.
In the example, the file
.Pa /etc/mail/nospamd
contains addresses of hosts who should be passed directly
to the SMTP agent (thus bypassing
.Nm ) .
.Bd -literal -offset 4n
table <spamd-white> persist
table <nospamd> persist file "/etc/mail/nospamd"
pass in on egress proto tcp to any port smtp \e
    divert-to 127.0.0.1 port spamd
pass in on egress proto tcp from <nospamd> to any port smtp
pass in log on egress proto tcp from <spamd-white> to any port smtp
pass out log on egress proto tcp to any port smtp
.Ed
.Pp
.Nm
removes tuple entries from the
.Pa /var/db/spamd
database if delivery has not been retried within
.Em greyexp
hours from the initial time a connection is seen.
The default is 4 hours as this is the most common setting after which
MTAs will give up attempting to retry delivery of a message.
.Pp
.Nm
removes whitelist entries from the
.Pa /var/db/spamd
database if no mail delivery activity has been seen from the
whitelisted address by
.Xr spamlogd 8
within
.Em whiteexp
hours from the initial time an address
is whitelisted.
The default is 36 days to allow for the delivery of
monthly mailing list digests without greylist delays every time.
.Pp
.Xr spamd-setup 8
should be run periodically by
.Xr cron 8
to update the blacklists configured in
.Xr spamd.conf 5 .
Use
.Xr crontab 1
to uncomment the entry in root's crontab.
When run in blacklist-only mode,
the
.Fl b
flag should be specified.
.Pp
.Xr spamlogd 8
should be used to update the whitelist entries in
.Pa /var/db/spamd
when connections are seen to pass to the real MTA on the
.Em smtp
port.
.Pp
.Xr spamdb 8
can be used to examine and alter the contents of
.Pa /var/db/spamd .
See
.Xr spamdb 8
for further information.
.Pp
.Nm
sends log messages to
.Xr syslogd 8
using
.Em facility
daemon and, with increasing verbosity,
.Em level
err, warn, info, and debug.
The following
.Xr syslog.conf 5
section can be used to log connection details to a dedicated file:
.Bd -literal -offset indent
!spamd
daemon.info	/var/log/spamd
.Ed
.Pp
A typical entry shows the time of the connection and
the IP address of the connecting host.
When a host connects,
the total number of active connections and
the number of connections from blacklisted hosts is shown
.Pq connected (xx/xx) .
When a host disconnects,
the amount of time spent talking to
.Nm
is shown.
.Sh GREYTRAPPING
When running
.Nm
in default mode,
it may be useful to define
.Em spamtrap
destination addresses to catch spammers as they send mail from greylisted
hosts.
Such spamtrap addresses affect only greylisted connections to
.Nm
and are used to temporarily blacklist a host that is obviously sending spam.
Unused email addresses or email addresses on spammers' lists are very
useful for this.
When a host that is currently greylisted attempts to send mail to a
spamtrap address,
it is blacklisted for 24 hours by adding the host to the
.Nm
blacklist
<spamd-greytrap>.
Spamtrap addresses are added to the
.Pa /var/db/spamd
database with the following
.Xr spamdb 8
command:
.Pp
.Dl # spamdb -T -a 'spamtrap@mydomain.org'
.Pp
See
.Xr spamdb 8
for further details.
.Pp
The file
.Pa /etc/mail/spamd.alloweddomains
can be used to specify a list of domainname suffixes, one per line, one of
which must match each destination email address in the greylist.
Any destination address which does not match one of the suffixes listed in
.Pa spamd.alloweddomains
will be trapped, exactly as if it were sent to a spamtrap address.
Comment lines beginning with
.Sq #
and empty lines are ignored.
.Pp
For example, if
.Pa spamd.alloweddomains
contains:
.Bd -literal -offset indent
@humpingforjesus.com
obtuse.com
.Ed
.Pp
The following destination addresses
.Em would not
cause the sending host to be trapped:
.Bd -literal -offset indent
beardedclams@humpingforjesus.com
beck@obtuse.com
beck@snouts.obtuse.com
.Ed
.Pp
However the following addresses
.Em would
cause the sending host to be trapped:
.Bd -literal -offset indent
peter@apostles.humpingforjesus.com
bigbutts@bofh.ucs.ualberta.ca
.Ed
.Pp
A low priority MX IP address may be specified with the
.Fl M
option.
When
.Nm
has such an address specified, no host may enter new greylist
tuples when connecting to this address; only existing entries
may be updated.
Any host attempting to make new deliveries to
the low priority MX for which a tuple has not previously
been seen will be trapped.
.Pp
Note that it is important to ensure that a host running
.Nm
with the low priority MX address active must see all the greylist
changes for a higher priority MX host for the same domains.
This is best done by the host itself receiving the connections to
the higher priority MX on another IP address (which may be an IP alias).
This will ensure that hosts are not trapped erroneously if the higher
priority MX is unavailable.
For example, on a host which is an existing MX record for a domain of
value 10, a second IP address with MX of value 99 (a higher number, and
therefore lower priority) would ensure that any RFC conformant client
would attempt delivery to the IP address with the MX value of 10
first, and should not attempt to deliver to the address with MX value 99.
.Sh BLACKLIST-ONLY MODE
When running in default mode, the
.Xr pf.conf 5
rules described above are sufficient.
However when running in blacklist-only mode,
a slightly modified
.Xr pf.conf 5
ruleset is required,
diverting any addresses found in the <spamd> table to
.Nm .
Any other addresses
are passed to the real MTA.
.Bd -literal -offset 4n
table <spamd> persist
pass in on egress inet proto tcp from <spamd> to any port smtp \e
    divert-to 127.0.0.1 port spamd
.Ed
.Pp
Addresses can be loaded into the
.Em table ,
like:
.Bd -literal -offset 4n
# pfctl -q -t spamd -T replace -f /usr/local/share/spammers
.Ed
.Pp
.Xr spamd-setup 8
can also be used to load addresses into the <spamd> table.
It has the added benefit of being able to remove addresses from
blacklists, and will connect to
.Nm
over a localhost socket, giving
.Nm
information about each source of blacklist addresses, as well as custom
rejection messages for each blacklist source
that can be used to let any real person whose mail
is deferred by
.Nm
know why their address has been listed
from sending mail.
This is important as it allows legitimate mail
senders to pressure spam sources into behaving properly so that they
may be removed from the relevant blacklists.
.Sh CONFIGURATION CONNECTIONS
.Nm
listens for configuration connections on the port identified by the
named service
.Dq spamd-cfg
(see
.Xr services 5 ) .
The configuration socket listens only on the INADDR_LOOPBACK
address.
Configuration of spamd is done by connecting to the configuration
socket, and sending blacklist information, one blacklist per line.
Each blacklist consists of a name, a message to reject mail
with, and addresses in CIDR format, all separated by semicolons (;):
.Bd -literal -offset indent
tag;"rejection message";aaa.bbb.ccc.ddd/mm;aaa.bbb.ccc.ddd/mm
.Ed
.Pp
The rejection message must be inside double quotes.
A \e" will produce a double quote in the output.
\en will produce a newline.
%A will expand to the connecting IP address in dotted quad format.
%% may be used to produce a single % in the output.
\e\e will produce a single \e.
.Nm
will reject mail by displaying all the messages from all blacklists in which
a connecting address is matched.
.Xr spamd-setup 8
is normally used to configure this information.
.Sh SYNCHRONISATION
.Nm
supports realtime synchronisation of spamd databases between
a number of spamd
daemons running on multiple machines,
using the
.Fl Y
and
.Fl y
options.
The databases are synchronised for greylisted and trapped entries;
whitelisted entries and entries made manually using
.Xr spamdb 8
are not updated.
.Pp
The following example will accept incoming multicast and unicast
synchronisation messages, and send outgoing multicast messages through
the network interface
.Ar em0 :
.Bd -literal -offset indent
# /usr/libexec/spamd -y em0 -Y em0
.Ed
.Pp
The second example will increase the multicast TTL to a value of 2,
add the unicast targets
.Ar foo.somewhere.org
and
.Ar bar.somewhere.org ,
and accept incoming unicast messages received on
.Ar bge0
only.
.Bd -literal -offset indent
# /usr/libexec/spamd -y bge0 -Y em0:2 \e
	-Y foo.somewhere.org -Y bar.somewhere.org
.Ed
.Pp
If the file
.Pa /etc/mail/spamd.key
exists,
.Nm
will calculate the message-digest fingerprint (checksum) for the file
and use it as a shared key to authenticate the synchronisation messages.
The file itself can contain any data.
For example, to create a secure random key:
.Bd -literal -offset indent
# dd if=/dev/random of=/etc/mail/spamd.key bs=2048 count=1
.Ed
.Pp
The file needs to be copied to all hosts
sending or receiving synchronisation messages.
.Sh FILES
.Bl -tag -width "/etc/mail/spamd.alloweddomainsXX" -compact
.It Pa /etc/mail/spamd.alloweddomains
Required suffixes for greytrapping.
.It Pa /etc/mail/spamd.conf
Default configuration file.
.It Pa /etc/mail/spamd.key
Authentication key for synchronisation messages.
.It Pa /var/db/spamd
Greylisting database.
.El
.Sh SEE ALSO
.Xr pf.conf 5 ,
.Xr services 5 ,
.Xr spamd.conf 5 ,
.Xr syslog.conf 5 ,
.Xr pfctl 8 ,
.Xr spamd-setup 8 ,
.Xr spamdb 8 ,
.Xr spamlogd 8 ,
.Xr syslogd 8
.Sh HISTORY
The
.Nm
command first appeared in
.Ox 3.3 .