SMTPD(6) SMTPD(6)
NAME
smtpd - SMTP listener configuration
DESCRIPTION
The SMTP daemon of mail(1) implements the slave side of the
SMTP protocol to accept incoming mail on TCP port 25. In
general, smtpd's default parameters are sufficient for
internal systems on protected networks, but external or
gateway systems require additional security mechanisms. The
files /mail/lib/smtpd.conf, containing configuration parame-
ters, and /mail/lib/blocked, containing banished addresses,
provide the means to exercise these facilities.
Input Format
In both files input lines consist of a verb followed by one
or more parameters. These tokens are separated by white
space or commas and all characters following a # are com-
ments. A # cannot be escaped. Continuation lines are not
supported, but verbs that take multiple parameters can be
restated on many lines and the associated parameters accumu-
late into a single set. All token processing is case-
insensitive.
Many parameters are addresses, either numeric IP addresses
in CIDR notation or a sender address in UUCP-style format.
An IP address in CIDR notation has the form
aaa.bbb.ccc.ddd/mask
consisting of a four octet IP address, a slash, and a mask
length specifying the number of significant high-order bits.
The lower the mask length, the larger the range of addresses
covered by the CIDR address; see RFC 1878 for a discussion
of mask lengths. Missing low-order octets are assumed to be
zero. If a mask length is not given, a mask length of 16,
24, or 32 is assumed for addresses containing two, three, or
four octets, respectively. These mask lengths select a
class B, class C or Class D address block. Notice that this
convention differs from the standard treatment, where the
default mask length depends on the allocation class of the
network block containing the address.
Sender addresses are specified in UUCP notation as follows:
[domain!]...domain!user
It is seldom necessary to specify more than one domain.
When domain is missing or *, the address selects the speci-
fied user in all domains. A domain of the form *.domain
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selects the domain and all of its sub-domains. For example,
example.com!user only matches the account user in domain
example.com, while *.example.com!user selects that account
in example.com and all of its sub-domains. When user is
omitted or *, the address selects all users in the specified
domain. Finally, when * is the last character of the user
name it is a wild-card matching all user names beginning
with user. This limited pattern matching capability should
be used with care. For safety, the sender addresses *, !,
*!, !* and *!* are ignored.
/mail/lib/smtpd.conf
This file contains configuration options and parameters
describing the local domain. Many of the options can also
be specified on the command line; command line options
always override the values in this file. Configuration
options are:
defaultdomain domain
The name of the local domain; it is appended to
addresses lacking a domain qualification. This is
identical to the -h command line option.
norelay [on|off]
If on is specified, relaying is prohibited from
unauthorized networks to external domains. Autho-
rized networks and domains must be specified by
the ournets and ourdomains verbs described below.
Setting this option on is equivalent to specifying
the -f command line flag, but the list of networks
and domains can only be specified in this file.
verifysenderdom [on|off]
When on, smtpd verifies that the first domain of
the sender's address exists. The test is cursory;
it checks only that there is a DNS delegation for
the domain. Setting the option on is equivalent
to specifying the -r command line option and is
useful for detecting some unreturnable messages as
well as messages with randomly generated domain
names.
saveblockedmsg [on|off]
When on, causes copies of blocked messages to be
saved in subdirectories of /mail/queue.dump.
Directories are named with the date and file names
are random numbers. If this option is off blocked
messages are discarded. Setting this option on is
equivalent to specifying the -s command line
option.
ournets IP address [, IP address, ..., IP address]
This option specifies trusted source networks that
are allowed to relay mail to external domains.
These are usually the internal networks of the
local domain, but they can also include friendly
external networks. Addresses are in CIDR
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notation.
ourdomains domain [, domain, ..., domain]
This option specifies destination domains that are
allowed to receive relayed mail. These are usu-
ally the domains served by a gateway system.
Domain specifications conform to the format for
sender addresses given above.
When the norelay option is enabled or the -f command line
option given, relaying is allowed only if the source IP
address is in ournets or the destination domain is specified
in ourdomains.
Blocked Addresses
Smtpd consults /mail/ratify (see ratfs(4)) for a list of
banned addresses. Messages received from these addresses
are rejected with a 5xx-series SMTP error code. There is no
option to turn blocking on or off; if /mail/ratify is
mounted, smtpd will use it, even for connections from
trusted networks.
The command line format and address specifications conform
to the notation described above. If the parameters of the
verb is sender addresses in UUCP format, the line must begin
with an * character; if the parameters are one or more IP
addresses, the * must precede the verb. Most verbs cause
messages to be rejected; verbs of this class generally
select different error messages. The remaining verbs spec-
ify addresses that are always accepted, in effect overriding
blocked addresses. The file is processed in order, so an
override must precede its associated blocked address. Sup-
ported verbs are:
dial IP address [,..., IP address]
The parameters are IP addresses associated with
dial-up ports. The rejection message states that
connections from dial-up ports are not accepted.
Copies of messages are never saved.
block address [, ... address]
Messages from addresses matching the parameters
are rejected with an error message saying that
spam is not accepted. The message is saved if the
option is enabled.
relay address [, ... address]
This verb is identical to block, but the error
message states that the message is rejected
because the sending system is being used as a spam
relay.
deny address [, ... address]
The deny command rejects a message when the sender
address matches one of its parameters. The rejec-
tion message asks the sender to contact
postmaster@ hostdomain for further information.
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SMTPD(6) SMTPD(6)
This verb is usually used to block inadvertently
abusive traffic, for example, mail loops and stuck
senders. Messages are never saved.
allow address [, ... address]
The allow verb negates the effect of subsequent
blocking commands. It is useful when a large
range of addresses contains a few legitimate
addresses, for example, when a mail server is in a
Class C network block of modem ports. Rather than
enumerate the dial ports, it is easier to block
the entire Class C with a dial command, and pre-
cede it with an override for the address of the
mail server. Similarly, it is possible to block
mail from an entire domain while accepting mail
from a few friendly senders in the domain. The
verb accept is a synonym for allow.
Scanmail(8) describes spam detection software that works
well with the capabilities described here and mail(1)
defines additional smtpd command line arguments applicable
to exposed systems.
SEE ALSO
mail(1), ratfs(4), scanmail(8)
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