310 lines
8 KiB
Plaintext
310 lines
8 KiB
Plaintext
.TH SMTPD 6
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.SH NAME
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smtpd \- SMTP listener configuration
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.SH DESCRIPTION
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The
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SMTP
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daemon
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of
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.IR mail (1)
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implements the slave side of the SMTP protocol
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to accept incoming mail on TCP port 25.
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In general,
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.IR smtpd 's
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default parameters
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are sufficient for internal systems
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on protected networks, but external or
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gateway systems require additional
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security mechanisms.
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The files
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.BR /mail/lib/smtpd.conf ,
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containing configuration parameters,
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and
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.BR /mail/lib/blocked ,
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containing
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banished addresses, provide the means to
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exercise these facilities.
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.SS Input Format
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In both files input lines
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consist of a verb followed by one or more
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parameters. These tokens are separated by white space or
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commas and all characters following a
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.B #
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are comments. A
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.B #
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cannot be escaped. Continuation lines are
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not supported, but verbs that take multiple parameters
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can be restated on many lines and the associated
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parameters accumulate into a single set.
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All token processing is case-insensitive.
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.PP
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Many parameters are
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.IR addresses ,
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either numeric IP addresses in CIDR notation
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or a
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.I "sender address"
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in UUCP-style format.
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.PP
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An IP address in CIDR notation has the form
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.PP
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.EX
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aaa.bbb.ccc.ddd/mask
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.EE
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.PP
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consisting of a four octet IP address, a slash,
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and a
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.I mask length
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specifying the number of significant high-order bits.
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The lower the mask length, the larger the
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range of addresses covered by the CIDR address;
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see RFC 1878 for a discussion of mask lengths.
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Missing low-order octets are assumed to be zero.
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If a mask length is not given, a mask length of
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16, 24, or 32 is assumed for addresses containing
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two, three, or four octets, respectively. These
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mask lengths select a class B, class C or Class D
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address block. Notice that this convention differs
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from the standard treatment, where the default mask length
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depends on the allocation class of the network
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block containing the address.
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.PP
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.I "Sender addresses"
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are specified in UUCP notation as
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follows:
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.PP
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.EX
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[domain!]...domain!user
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.EE
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.PP
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It is seldom necessary to specify more than one domain.
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When
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.I domain
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is missing or
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.BR * ,
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the address selects the specified user in all domains.
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A
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.I domain
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of the form
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.BI *. domain
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selects the domain and all of its sub-domains.
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For example,
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.B example.com!user
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only matches the account
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.I user
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in domain
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.BR example.com ,
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while
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.B *.example.com!user
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selects that account in
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.B example.com
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and all of its sub-domains.
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When
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.I user
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is omitted or
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.BR * ,
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the address selects all users in the specified domain.
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Finally, when
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.B *
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is the last character of the user name it is a wild-card
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matching all user names beginning with
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.IR user .
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This limited pattern matching capability should be used with care.
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For safety, the sender addresses
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.BR * ,
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.BR ! ,
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.BR *! ,
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.B !*
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and
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.B *!*
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are ignored.
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.SS /mail/lib/smtpd.conf
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This file contains configuration options
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and parameters describing the local domain.
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Many of the options can also be specified on the command
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line; command line options always override the values in
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this file.
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Configuration options are:
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.PD0
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.TP 10
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.BI defaultdomain " domain"
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The name of the local domain; it is appended to addresses
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lacking a domain qualification.
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This is identical to the
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.B -h
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command line option.
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.TP 10
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.BR norelay \ [ on\f1|\fPoff ]
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If
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.I on
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is specified, relaying is prohibited
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from unauthorized networks to external domains.
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Authorized networks and domains must be specified
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by the
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.B ournets
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and
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.B ourdomains
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verbs described below. Setting this option on is equivalent to specifying the
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.B -f
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command line flag, but the list of
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networks and domains can only be specified in
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this file.
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.TP 10
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.BR verifysenderdom \ [ on\f1|\fPoff ]
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When
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.IR on ,
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.I smtpd
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verifies that the first domain of the sender's address
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exists. The test is cursory; it checks only that
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there is a DNS delegation for the domain.
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Setting the option on is equivalent to specifying the
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.B -r
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command line option and
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is useful for detecting some unreturnable
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messages as well as messages with randomly
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generated domain names.
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.TP 10
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.BR saveblockedmsg \ [ on\f1|\fPoff ]
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When
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.IR on ,
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causes copies of blocked messages to be saved
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in subdirectories of
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.BR /mail/queue.dump .
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Directories are named with the date and file names
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are random numbers.
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If this option is
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.I off
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blocked messages are discarded.
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Setting this option on is equivalent to specifying the
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.B -s
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command line option.
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.TP 10
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.BR ournets " \fIIP address\fP [, \fIIP address\fP, ..., \fIIP address\fP]"
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This option specifies trusted
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source networks that are allowed to relay mail to external domains.
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These are usually the internal networks of the local domain, but they
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can also include friendly
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external networks. Addresses
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are in CIDR notation.
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.TP 10
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.BR ourdomains " \fIdomain\fP [, \fIdomain\fP, ..., \fIdomain\fP]"
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This option specifies destination domains that are allowed
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to receive relayed mail. These are usually the domains
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served by a gateway system.
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Domain specifications conform to the format
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for sender addresses given above.
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.PD
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.PP
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When the
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.B norelay
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option is enabled or the
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.B -f
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command line option given,
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relaying is allowed only if the source IP address is in
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.B ournets
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or the destination domain is specified in
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.BR ourdomains .
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.SS Blocked Addresses
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.I Smtpd
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consults
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.B /mail/ratify
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(see
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.IR ratfs (4))
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for a list of banned addresses.
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Messages received from these addresses are
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rejected with a 5\fIxx\fP-series SMTP error code.
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There is no option
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to turn blocking on or off; if
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.B /mail/ratify
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is mounted,
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.I smtpd
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will use it, even for connections from trusted networks.
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.PP
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The command line format and address specifications
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conform to the notation described above. If the parameters
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of the verb is sender addresses in UUCP format, the line
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must begin with an
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.B *
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character; if the parameters are one or more IP addresses,
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the
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.B *
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must precede the verb. Most
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verbs cause messages to be rejected; verbs
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of this class generally select different error
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messages. The remaining verbs specify addresses that
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are always accepted, in effect overriding blocked addresses.
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The file is processed in order, so an override must
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precede its associated blocked address.
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Supported verbs are:
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.PD0
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.TP 10
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.BR dial " \fIIP address\fP [,..., \fIIP address\fP]"
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The parameters are IP addresses associated with
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dial-up ports. The rejection message states
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that connections from dial-up ports are not accepted. Copies
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of messages are never saved.
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.TP 10
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.BR block " \fIaddress\fP [, ... \fIaddress\fP]"
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Messages from addresses
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matching the parameters
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are rejected with an error message saying
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that spam is not accepted. The message is saved if
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the option is enabled.
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.TP 10
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.BR relay " \fIaddress\fP [, ... \fIaddress\fP]"
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This verb is identical to
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.BR block ,
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but the error message states that
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the message is rejected because the sending
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system is being used as a spam relay.
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.TP
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.BR deny " \fIaddress\fP [, ... \fIaddress\fP]"
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The
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.B deny
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command rejects a message when the
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sender address matches one of its parameters.
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The rejection message asks the sender to
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contact
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.BR postmaster @
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.I hostdomain
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for further information.
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This verb is usually used to block
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inadvertently abusive traffic, for example,
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mail loops and stuck senders. Messages are
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never saved.
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.TP
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.BR allow " \fIaddress\fP [, ... \fIaddress\fP]"
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The
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.B allow
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verb negates the effect of subsequent blocking commands.
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It is useful when a large range of addresses contains
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a few legitimate addresses, for example, when
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a mail server is in a Class C network block
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of modem ports. Rather than enumerate the dial ports, it is
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easier to block the entire Class C with a
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.B dial
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command, and precede it with an override for
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the address of the mail server. Similarly,
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it is possible to block mail from an entire
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domain while accepting mail from a few friendly
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senders in the domain.
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The verb
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.B accept
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is a synonym for
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.BR allow .
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.PD
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.PP
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.IR Scanmail (8)
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describes spam detection
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software that works well with
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the capabilities described here
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and
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.IR mail (1)
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defines additional
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.I smtpd
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command line arguments applicable
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to exposed systems.
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.SH "SEE ALSO"
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.IR mail (1),
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.IR ratfs (4),
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.IR scanmail (8)
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