| SPAMD(8) | System Manager's Manual | SPAMD(8) |
spamd — spam
deferral daemon
spamd |
[-45bdv] [-B
maxblack] [-C
file] [-c
maxcon] [-G
passtime:greyexp:whiteexp]
[-h hostname]
[-K file]
[-l address]
[-M address]
[-n name]
[-p port]
[-S secs]
[-s secs]
[-w window]
[-Y synctarget]
[-y synclisten] |
spamd 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.
spamd considers sending hosts to be of
three types:
blacklisted
hosts are diverted to spamd and
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.
whitelisted
hosts do not talk to spamd. Their connections are
instead sent to a real mail server, such as
smtpd(8).
greylisted
hosts are diverted to spamd, but
spamd has not yet decided if they are likely
spammers. They are given a temporary failure message by
spamd when they try to deliver mail.
When spamd 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 spamd is run in
blacklist-only mode, using the -b flag, it will
consult a pre-defined set of blacklist addresses to decide whether to tarpit
the host or not.
When a sending host talks to
spamd, the reply will be
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.
The options are as follows:
-4-5-B
maxblack-b-C
file-c
maxcon-dspamd does not
fork(2) into the
background.-G
passtime:greyexp:whiteexp-h
hostname-K
file-l
addressspamd is to
bind(2). By default
spamd listens on the localhost address
127.0.0.1.-M
address-n
name-p
portspamd should listen for diverted SMTP connections
on. The default port is found by looking for the named service
“spamd” using
getservbyname(3).-S
secs-s
secs-vspamd logs
connections, disconnections and blacklist matches to
syslogd(8) at
LOG_INFO level. With verbose logging enabled,
message detail including subject and recipient information is logged at
LOG_INFO, along with the message body and SMTP
dialogue being logged at LOG_DEBUG level.-w
window-Y
synctarget-y
synclistenWhen run in default mode, connections receive the pleasantly innocuous temporary failure of:
451 Temporary failure, please try again later.
This happens in the SMTP dialogue immediately after the
DATA command is received from the client. spamd will
use the db file in /var/db/spamd to track these
connections to spamd by connecting IP address,
HELO/EHLO, envelope-from, and envelope-to, or
tuple for short.
Hosts which connect but do not attempt to deliver mail will not generate a
tuple and always be ignored.
A previously unseen tuple is added to the
/var/db/spamd database, recording the time an
initial connection attempt was seen. After
passtime minutes
if spamd sees a retried attempt to deliver mail for
the same tuple, spamd will whitelist the connecting
address by adding it as a whitelist entry to
/var/db/spamd.
spamd regularly scans the
/var/db/spamd database and configures all whitelist
addresses as the pf(4)
<spamd-white> table, allowing connections to pass to the real MTA. Any
addresses not found in <spamd-white> are diverted to
spamd.
An example
pf.conf(5) fragment is given
below. In the example, the file /etc/mail/nospamd
contains addresses of hosts who should be passed directly to the SMTP agent
(thus bypassing spamd).
table <spamd-white> persist
table <nospamd> persist file "/etc/mail/nospamd"
pass in on egress proto tcp to any port smtp \
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
spamd removes tuple entries
from the /var/db/spamd database if delivery has not
been retried within
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.
spamd removes whitelist
entries from the /var/db/spamd database if no mail
delivery activity has been seen from the whitelisted address by
spamlogd(8) within
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.
spamd-setup(8)
should be run periodically by
cron(8) to update the
blacklists configured in
spamd.conf(5). Use
crontab(1) to uncomment the
entry in root's crontab. When run in blacklist-only mode, the
-b flag should be specified.
spamlogd(8) should be used to update the whitelist entries in /var/db/spamd when connections are seen to pass to the real MTA on the smtp port.
spamdb(8) can be used to examine and alter the contents of /var/db/spamd. See spamdb(8) for further information.
spamd sends log messages to
syslogd(8) using
facility daemon
and, with increasing verbosity,
level
err, warn, info, and debug. The following
syslog.conf(5) section
can be used to log connection details to a dedicated file:
!spamd daemon.info /var/log/spamd
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 (connected (xx/xx)). When a host disconnects, the amount of time spent
talking to spamd is shown.
When running spamd in default mode, it may
be useful to define
spamtrap
destination addresses to catch spammers as they send mail from greylisted
hosts. Such spamtrap addresses affect only greylisted connections to
spamd 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 spamd
blacklist <spamd-greytrap>. Spamtrap addresses are added to the
/var/db/spamd database with the following
spamdb(8) command:
# spamdb -T -a
'spamtrap@mydomain.org'See spamdb(8) for further details.
The file /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 spamd.alloweddomains will be trapped, exactly as if it were sent to a spamtrap address. Comment lines beginning with ‘#’ and empty lines are ignored.
For example, if spamd.alloweddomains contains:
@humpingforjesus.com obtuse.com
The following destination addresses would not cause the sending host to be trapped:
beardedclams@humpingforjesus.com beck@obtuse.com beck@snouts.obtuse.com
However the following addresses would cause the sending host to be trapped:
peter@apostles.humpingforjesus.com bigbutts@bofh.ucs.ualberta.ca
A low priority MX IP address may be specified with the
-M option. When spamd 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.
Note that it is important to ensure that a host running
spamd 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.
When running in default mode, the
pf.conf(5) rules described
above are sufficient. However when running in blacklist-only mode, a
slightly modified pf.conf(5)
ruleset is required, diverting any addresses found in the <spamd>
table to spamd. Any other addresses are passed to
the real MTA.
table <spamd> persist
pass in on egress inet proto tcp from <spamd> to any port smtp \
divert-to 127.0.0.1 port spamd
Addresses can be loaded into the table, like:
# pfctl -q -t spamd -T replace -f /usr/local/share/spammers
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 spamd over a localhost socket, giving
spamd 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
spamd 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.
spamd listens for configuration
connections on the port identified by the named service
“spamd-cfg” (see
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 (;):
tag;"rejection message";aaa.bbb.ccc.ddd/mm;aaa.bbb.ccc.ddd/mm
The rejection message must be inside double quotes. A \" will
produce a double quote in the output. \n 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. \\ will produce a single \.
spamd will reject mail by displaying all the
messages from all blacklists in which a connecting address is matched.
spamd-setup(8) is
normally used to configure this information.
spamd supports realtime synchronisation of
spamd databases between a number of spamd daemons running on multiple
machines, using the -Y and
-y options. The databases are synchronised for
greylisted and trapped entries; whitelisted entries and entries made
manually using spamdb(8) are
not updated.
The following example will accept incoming multicast and unicast synchronisation messages, and send outgoing multicast messages through the network interface em0:
# /usr/libexec/spamd -y em0 -Y em0
The second example will increase the multicast TTL to a value of 2, add the unicast targets foo.somewhere.org and bar.somewhere.org, and accept incoming unicast messages received on bge0 only.
# /usr/libexec/spamd -y bge0 -Y em0:2 \ -Y foo.somewhere.org -Y bar.somewhere.org
If the file /etc/mail/spamd.key exists,
spamd 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:
# dd if=/dev/random of=/etc/mail/spamd.key bs=2048 count=1
The file needs to be copied to all hosts sending or receiving synchronisation messages.
pf.conf(5), services(5), spamd.conf(5), syslog.conf(5), pfctl(8), spamd-setup(8), spamdb(8), spamlogd(8), syslogd(8)
The spamd command first appeared in
OpenBSD 3.3.
| July 24, 2019 | OpenBSD-6.8 |