Ubuntu Manpage: smtpd.conf — Simple Mail Transfer Protocol daemon configuration file

NAME

     smtpd.conf — Simple Mail Transfer Protocol daemon configuration file

DESCRIPTION

smtpd.conf is the configuration file for the mail daemon smtpd(8).

When mail arrives, each “RCPT TO:” command generates a mail envelope. If an envelope
matches any of a pre-designated set of criteria (using the match directive), the message is
accepted for delivery. A copy of the message, as well as its associated envelopes, is saved
in the mail queue and later dispatched according to an associated set of actions (using the
action directive). If an envelope does not match any options, it is rejected. The match
rules are evaluated sequentially, with the first match winning.

The format of the configuration file is fairly flexible. The current line can be extended
over multiple lines using a backslash (‘\’). Comments can be put anywhere in the file using
a hash mark (‘#’), and extend to the end of the current line. Care should be taken when
commenting out multi-line text: the comment is effective until the end of the entire block.
Argument names not beginning with a letter, digit, or underscore, as well as reserved words
(such as listen, match, and port), must be quoted. Arguments containing whitespace should
be surrounded by double quotes (").

Macros can be defined that are later expanded in context. Macro names must start with a
letter, digit, or underscore, and may contain any of those characters, but may not be
reserved words. Macros are not expanded inside quotes. For example:

lan_addr = "192.168.0.1"
listen on $lan_addr
listen on $lan_addr tls auth

The syntax of smtpd.conf is described below.

action name method [options]
When the queue runner processes an envelope from the mail queue, it carries out the
action name, selected by the match ... action directive when the message was
received. The action directive provides configuration data for delivery attempts.
Required lookups are performed at the time of each delivery attempt. Consequently,
changing an action directive or the files it references and restarting the smtpd(8)
daemon causes the changes to take effect for subsequent delivery attempts for the
respective dispatcher name, even for messages that were already stuck in the queue
prior to the configuration changes.

The delivery method parameter may be one of the following:

expand-only
Only accept the message if a delivery method was specified in an aliases or
.forward file.

forward-only
Only accept the message if the recipient results in a remote address after
the processing of aliases or forward file.

lmtp destination [rcpt-to]
Deliver the message to an LMTP server at destination. The location may be
expressed as host:port or as a UNIX socket.

Optionally, rcpt-to might be specified to use the recipient email address
(after expansion) instead of the local user in the LMTP session as RCPT TO.

maildir [pathname [junk]]
Deliver the message to the maildir in pathname if specified, or by default
to ~/Maildir.

The pathname may contain format specifiers that are expanded before use (see
FORMAT SPECIFIERS).

If the junk argument is provided, the message will be moved to the Junk
folder if it contains a positive X-Spam header.

mbox Deliver the message to the user's mbox with mail.local(8).

mda command
Delegate the delivery to a command that receives the message on its standard
input.

The command may contain format specifiers that are expanded before use (see
FORMAT SPECIFIERS).

relay Relay the message to another SMTP server.

The local delivery methods support additional options:

alias <table>
Use the mapping table for aliases(5) expansion.

ttl n{s|m|h|d}
Specify how long a message may remain in the queue.

user username
Specify the username for performing the delivery, to be looked up with
getpwnam(3).

This is used for virtual hosting where a single username is in charge of
handling delivery for all virtual users.

This option is not usable with the mbox delivery method.

userbase <table>
Use the mapping table for user lookups instead of the getpwnam(3) function.

The userbase does not apply for the user option.

virtual <table>
Use the mapping table for virtual expansion. The aliasing table format is
described in table(5).

wrapper name
Use the wrapper specified in mda wrapper.

The relay delivery methods also support additional options:

backup Operate as a backup mail exchanger delivering messages to any mail exchanger
with higher priority.

backup mx name
Operate as a backup mail exchanger delivering messages to any mail exchanger
with higher priority than mail exchanger identified as name.

helo heloname
Advertise heloname as the hostname to other mail exchangers during the HELO
phase.

helo-src <table>
Use the mapping table to look up a hostname matching the source address, to
advertise during the HELO phase.

host relay-url
Do not perform MX lookups but relay messages to the relay host described by
relay-url. The format for relay-url is [proto://[label@]]host[:port]. The
following protocols are available:

smtp Normal SMTP session with opportunistic STARTTLS (the default).
smtp+tls Normal SMTP session with mandatory STARTTLS.
smtp+notls Plain text SMTP session without TLS.
lmtp LMTP session. port is required.
smtps SMTP session with forced TLS on connection, default port is 465.
Unless noted, port defaults to 25.

The label corresponds to an entry in a credentials table, as documented in
table(5). It is used with the “smtp+tls” and “smtps” protocols for
authentication. Server certificates for those protocols are verified by
default.

srs When relaying a mail resulting from a forward, use the Sender Rewriting
Scheme to rewrite sender address.

tls [no-verify]
Require TLS to be used when relaying, using mandatory STARTTLS by default.
When used with a smarthost, the protocol must not be “smtp+notls://”. If
no-verify is specified, do not require a valid certificate.

auth <table>
Use the mapping table for connecting to relay-url using credentials. This
option is usable only with host option. The credential table format is
described in table(5).

mail-from mailaddr
Use mailaddr as the MAIL FROM address within the SMTP transaction.

src address | <address>
Use the string or list table address for the source IP address. If the list
contains more than one address, all of them are used in such a way that
traffic is routed as efficiently as possible.

bounce warn-interval delay [, delay ...]
Send warning messages to the envelope sender when temporary delivery failures cause
a message to remain on the queue for longer than delay. Each delay parameter
consists of a positive decimal integer and a unit s, m, h, or d. At most four delay
parameters can be specified. The default is "bounce warn-interval 4h", sending a
single warning after four hours.

ca caname cert cafile
Associate the Certificate Authority (CA) certificate file cafile with host caname,
and use that file as the CA certificate for that host. caname is the server's name,
derived from the default hostname or set using either /etc/mailname or using the
hostname directive.

filter chain-name chain {filter-name [, ...]}
Register a chain of filters named chain-name and consisting of the filters listed
from filter-name. Filters part of a filter chain are executed in order of
declaration for each phase that they are registered for. A filter chain may be used
in place of a filter for any directive but filter chains themselves.

filter filter-name phase phase-name match conditions disconnect message
Register builtin filter filter-name matching conditions to disconnect session with
message. Phase and matching conditions are documented in a specific section, see
BUILTIN FILTERING.

filter filter-name phase phase-name match conditions junk
Register builtin filter filter-name matching conditions to mark a session or a
transaction as junk. Phase and matching conditions are documented in a specific
section, see BUILTIN FILTERING.

filter filter-name phase phase-name match conditions reject message
Register builtin filter filter-name matching conditions to reject session with
message. Phase and matching conditions are documented in a specific section, see
BUILTIN FILTERING.

filter filter-name phase phase-name match conditions report message
Register builtin filter filter-name matching conditions to report on session with
message and proceed with the transaction. Phase and matching conditions are
documented in a specific section, see BUILTIN FILTERING.

filter filter-name phase phase-name match conditions rewrite value
Register builtin filter filter-name matching conditions to rewrite phase parameter
with new value. Phase and matching conditions are documented in a specific section,
see BUILTIN FILTERING.

filter filter-name proc proc-name
Register "proc" filter filter-name backed by the proc-name process.

filter filter-name proc-exec command
Register and execute "proc" filter filter-name from command. If command starts with
a slash it is executed with an absolute path, else it will be run from
“/usr/local/libexec/smtpd/”.

include "pathname"
Replace this directive with the content of the additional configuration file at the
absolute pathname.

listen on interface [family] [options]
Listen on the interface for incoming connections, using the same syntax as for
ifconfig(8). The interface parameter may also be an interface group, an IP address,
or a domain name. Listening can optionally be restricted to a specific address
family, which can be either inet4 or inet6.

The options are as follows:

auth [<authtable>]
Support SMTPAUTH: clients may only start SMTP transactions after successful
authentication. Users are authenticated against either their own normal
login credentials or a credentials table authtable, the format of which is
described in table(5).

auth-optional [<authtable>]
Support SMTPAUTH optionally: clients need not authenticate, but may do so.
This allows a listen on directive to both accept incoming mail from
untrusted senders and permit outgoing mail from authenticated users (using
match auth). It can be used in situations where it is not possible to
listen on a separate port (usually the submission port, 587) for users to
authenticate.

ca caname
For secure connections, use the CA certificate associated with caname
(declared in a ca directive) as the CA certificate when verifying client
certificates.

filter name
Apply filter name on connections handled by this listener.

hostname hostname
Use hostname in the greeting banner instead of the default server name.

hostnames <names>
Override the server name for specific addresses. The names table contains a
mapping of IP addresses to hostnames. If the address on which the
connection arrives appears in the mapping, the associated hostname is used.

mask-src
Omit the from part when prepending “Received” headers.

no-dsn Disable the DSN (Delivery Status Notification) extension.

pki pkiname
For secure connections, use the certificate associated with pkiname
(declared in a pki directive) to prove a mail server's identity.

port [port]
Listen on the given port instead of the default port 25.

proxy-v2
Support the PROXYv2 protocol, rewriting appropriately source address
received from proxy.

received-auth
In “Received” headers, report whether the session was authenticated and by
which local user.

senders <users> [masquerade]
Look up the authenticated user in the users mapping table to find the email
addresses that user is allowed to submit mail as. In addition, if the
masquerade option is provided, the From header is rewritten to match the
sender provided in the SMTP session.

smtps Support SMTPS, by default on port 465. Mutually exclusive with tls.

tag tag
Clients connecting to the listener are tagged with the given tag.

tls Support STARTTLS, by default on port 25. Mutually exclusive with smtps.

tls-require [verify]
Like tls, but force clients to establish a secure connection before being
allowed to start an SMTP transaction. With the verify option, clients must
also provide a valid certificate to establish an SMTP session.

listen on socket [mask-src]
Listen for incoming SMTP connections on the Unix domain socket /var/run/smtpd.sock.
This is done by default, even if the directive is absent. If the mask-src option is
specified, printing of the HELO name, hostname, and IP address of the originating
host is suppressed in Received: header lines.

match options action name
If at least one mail envelope matches the options of one match action directive,
receive the incoming message, put a copy into each matching envelope, and atomically
save the envelopes to the mail spool for later processing by the respective
dispatcher name.

The following matching options are supported and can all be negated:

[!] for any
Specify that session may address any destination.

[!] for local
Specify that session may address any local domain. This is the default, and
may be omitted.

[!] for domain domain | <domain>
Specify that session may address the string or list table domain.

[!] for domain regex domain | <domain>
Specify that session may address the regex or regex table domain.

[!] from any
Specify that session may originate from any source.

[!] from local
Specify that session may only originate from a local IP address, or from the
local enqueuer. This is the default, and may be omitted.

[!] from rdns
Specify that session may only originate from an IP address that resolves to
a reverse DNS.

[!] from rdns hostname | <hostname>
Specify that session may only originate from an IP address that resolves to
a reverse DNS matching string or list string hostname.

[!] from rdns regex hostname | <hostname>
Specify that session may only originate from an IP address that resolves to
a reverse DNS matching regex or list regex hostname.

[!] from socket
Specify that session may only originate from the local enqueuer.

[!] from src address | <address>
Specify that session may only originate from string or list table address
which can be a specific address or a subnet expressed in CIDR-notation.

[!] from src regex address | <address>
Specify that session may only originate from regex or regex table address
which can be a specific address or a subnet expressed in CIDR-notation.

In addition, the following transaction options:

[!] auth
Matches transactions which have been authenticated.

[!] helo helo-name | <helo-name>
Specify that session's HELO / EHLO should match the string or list table
helo-name.

[!] helo regex helo-name | <helo-name>
Specify that session's HELO / EHLO should match the regex or regex table
helo-name.

[!] mail-from sender | <sender>
Specify that transactions's MAIL FROM should match the string or list table
sender.

[!] mail-from regex sender | <sender>
Specify that transactions's MAIL FROM should match the regex or regex table
sender.

[!] rcpt-to recipient | <recipient>
Specify that transaction's RCPT TO should match the string or list table
recipient.

[!] rcpt-to regex recipient | <recipient>
Specify that transaction's RCPT TO should match the regex or regex table
recipient.

[!] tag tag
Matches transactions tagged with the given tag.

[!] tag regex tag
Matches transactions tagged with the given tag regex.

[!] tls
Specify that transaction should take place in a TLS channel.

match options reject
Reject the incoming message during the SMTP dialogue. The same options are
supported as for the match action directive.

mda wrapper name command
Associate command with the mail delivery agent wrapper named name. When a local
delivery specifies a wrapper, the command associated with the wrapper will be
executed instead. The command may contain format specifiers (see FORMAT
SPECIFIERS).

mta max-deferred number
When delivery to a given host is suspended due to temporary failures, cache at most
number envelopes for that host such that they can be delivered as soon as another
delivery succeeds to that host. The default is 100.

pki pkiname cert certfile
Associate certificate file certfile with host pkiname, and use that file to prove
the identity of the mail server to clients. pkiname is the server's name, derived
from the default hostname or set using either /etc/mailname or using the hostname
directive. If a fallback certificate or SNI is wanted, the ‘*’ wildcard may be used
as pkiname.

A certificate chain may be created by appending one or many certificates, including
a Certificate Authority certificate, to certfile. The creation of certificates is
documented in starttls(8).

pki pkiname key keyfile
Associate the key located in keyfile with host pkiname.

pki pkiname dhe params
Specify the DHE parameters to use for DHE cipher suites with host pkiname. Valid
parameter values are none, legacy, and auto. For legacy, a fixed key length of 1024
bits is used, whereas for auto, the key length is determined automatically. The
default is none, which disables DHE cipher suites.

proc proc-name command
Register an external process named proc-name from command. Such processes may be
used to share the same instance between multiple filters. If command starts with a
slash it is executed with an absolute path, else it will be run from
“/usr/local/libexec/smtpd/”.

queue compression
Store queue files in a compressed format. This may be useful to save disk space.

queue encryption [key]
Encrypt queue files with EVP_aes_256_gcm(3). If no key is specified, it is read
with getpass(3). If the string stdin or a single dash (‘-’) is given instead of a
key, the key is read from the standard input.

queue ttl delay
Set the default expiration time for temporarily undeliverable messages, given as a
positive decimal integer followed by a unit s, m, h, or d. The default is four days
(4d).

smtp ciphers control
Set the control string for SSL_CTX_set_cipher_list(3). The default is
"HIGH:!aNULL:!MD5".

smtp limit max-mails count
Limit the number of messages to count for each session. The default is 100.

smtp limit max-rcpt count
Limit the number of recipients to count for each transaction. The default is 1000.

smtp max-message-size size
Reject messages larger than size, given as a positive number of bytes or as a string
to be parsed with scan_scaled(3). The default is "35M".

smtp sub-addr-delim character
When resolving the local part of a local email address, ignore the ASCII character
and all characters following it. The default is ‘+’.

srs key secret
Set the secret key to use for SRS, the Sender Rewriting Scheme.

srs key backup secret
Set a backup secret key to use as a fallback for SRS. This can be used to implement
SRS key rotation.

srs ttl delay
Set the time-to-live delay for SRS envelopes. After this delay, a bounce reply to
the SRS address will be discarded to limit risks of forged addresses. The default
is four days (4d).

table name [type:]pathname
Tables provide additional configuration information for smtpd(8) in the form of
lists or key-value mappings. The format of the entries depends on what the table is
used for. Refer to table(5) for the exhaustive documentation.

Each table is identified by an arbitrary, unique name.

If the type is db, information is stored in a file created with makemap(8); if it is
file or omitted, information is stored in a plain text file using the format
described in table(5). The pathname to the file must be absolute.

table name {value [, ...]}
Instead of using a separate file, declare a list table containing the given static
values. The table must contain at least one value and may declare multiple values
as a comma-separated (whitespace optional) list.

table name {key=value [, ...]}
Instead of using a separate file, declare a mapping table containing the given
static key-value pairs. The table must contain at least one key-value pair and may
declare multiple pairs as a comma-separated (whitespace optional) list.

BUILTIN FILTERING
In a regular workflow, smtpd(8) may accept or reject a message based only on the content of
envelopes. Its decisions are about the handling of the message, not about the handling of
an active session.

Filtering extends the decision making process by allowing smtpd(8) to stop at each phase of
an SMTP session, check that conditions are met, then decide if a session is allowed to move
forward.

With filtering, a session may be interrupted at any phase before an envelope is complete. A
message may also be rejected after being submitted, disregarding if the envelope was
accepted or not.

The following phases are currently supported:

connect upon connection, before a banner is displayed
helo after HELO command is submitted
ehlo after EHLO command is submitted
mail-from after MAIL FROM command is submitted
rcpt-to after RCPT TO command is submitted
data after DATA command is submitted
commit after message is fully is submitted

At each phase, multiple criteria may be checked:

fcrdns forward-confirmed reverse DNS is valid
rdns session has a reverse DNS
rdns <table> session has a reverse DNS in table
src <table> source address is in table
helo <table> helo name is in table
mail-from <table> sender address is in table
rcpt-to <table> recipient address is in table

All criteria from previous phases are available to subsequent phases, so while the helo
criteria is not available before the helo or ehlo phase, the fcrdns criteria is available in
all phases.

Criteria may all be negated by prefixing them with an exclamation mark:

! fcrdns forward-confirmed reverse DNS is invalid

Any criteria using a table may indicate that tables hold regex by prefixing the table name
with the keyword regex.

helo regex <table> helo name matches a regex in table
Finally, four decisions may be taken:

disconnect message the session is disconnected with message
junk the session or transaction is junked
reject message the command is rejected with message
rewrite value the command parameter is rewritten with value

Decisions that involve a message require that the message be RFC valid, meaning that they
should either start with a 4xx or 5xx status code. Descisions can be taken at any phase,
however junking can only happen before a message is committed.

FORMAT SPECIFIERS
Some configuration directives support expansion of their parameters at runtime. Such
directives (for example action maildir, action mda) may use format specifiers which are
expanded before delivery or relaying. The following formats are currently supported:

%{sender} sender email address, may be empty string
%{sender.user} user part of the sender email address, may be empty
%{sender.domain} domain part of the sender email address, may be empty
%{rcpt} recipient email address
%{rcpt.user} user part of the recipient email address
%{rcpt.domain} domain part of the recipient email address
%{dest} recipient email address after expansion
%{dest.user} user part after expansion
%{dest.domain} domain part after expansion
%{user.username} local user
%{user.directory} home directory of the local user
%{mbox.from} name used in mbox From separator lines
%{mda} mda command, only available for mda wrappers

Expansion formats also support partial expansion using the optional bracket notations with
substring offset. For example, with recipient domain “example.org”:

%{rcpt.domain[0]} expands to “e”
%{rcpt.domain[1]} expands to “x”
%{rcpt.domain[8:]} expands to “org”
%{rcpt.domain[-3:]} expands to “org”
%{rcpt.domain[0:6]} expands to “example”
%{rcpt.domain[0:-4]} expands to “example”

In addition, modifiers may be applied to the token. For example, with recipient
“User+Tag@Example.org”:

%{rcpt:lowercase} expands to “user+tag@example.org”
%{rcpt:uppercase} expands to “USER+TAG@EXAMPLE.ORG”
%{rcpt:strip} expands to “User@Example.org”
%{rcpt:lowercase|strip} expands to “user@example.org”

For security concerns, expanded values are sanitized and potentially dangerous characters
are replaced with ‘:’. In situations where they are desirable, the “raw” modifier may be
applied. For example, with recipient “user+t?g@example.org”:

%{rcpt} expands to “user+t:g@example.org”
%{rcpt:raw} expands to “user+t?g@example.org”

FILES

     /etc/smtpd.conf     Default smtpd(8) configuration file.
     /etc/mailname       If this file exists, the first line is used as the server name.
                         Otherwise, the server name is derived from the local hostname returned
                         by gethostname(3), either directly if it is a fully qualified domain
                         name, or by retrieving the associated canonical name through
                         getaddrinfo(3).
     /var/run/smtpd.sock
                         Unix domain socket for incoming SMTP connections.
     /var/spool/smtpd/   Spool directories for mail during processing.

EXAMPLES

     The default smtpd.conf file which ships with OpenBSD listens on the loopback network
     interface (lo0) and allows for mail from users and daemons on the local machine, as well as
     permitting email to remote servers.  Some more complex configurations are given below.

     This first example is the same as the default configuration, but all outgoing mail is
     forwarded to a remote SMTP server.  A secrets file is needed to specify a username and
     password:

           # touch /etc/secrets
           # chmod 640 /etc/secrets
           # chown root:_smtpd /etc/secrets
           # echo "bob username:password" > /etc/secrets

     smtpd.conf would look like this:

           table aliases file:/etc/aliases
           table secrets file:/etc/secrets

           listen on lo0

           action "local_mail" mbox alias <aliases>
           action "outbound" relay host smtp+tls://bob@smtp.example.com \
                   auth <secrets>

           match for local action "local_mail"
           match for any action "outbound"

     In this second example, the aim is to permit mail delivery and relaying only for users that
     can authenticate (using their normal login credentials).  An RSA certificate must be
     provided to prove the server's identity.  The mail server listens on all interfaces the
     default routes point to.  Mail with a local destination is sent to an external MDA.  First,
     the RSA certificate is created:

           # openssl genrsa -out /etc/ssl/private/mail.example.com.key 4096
           # openssl req -new -x509 -key /etc/ssl/private/mail.example.com.key \
                   -out /etc/ssl/mail.example.com.crt -days 365
           # chmod 600 /etc/ssl/mail.example.com.crt
           # chmod 600 /etc/ssl/private/mail.example.com.key

     In the example above, a certificate valid for one year was created.  The configuration file
     would look like this:

           pki mail.example.com cert "/etc/ssl/mail.example.com.crt"
           pki mail.example.com key "/etc/ssl/private/mail.example.com.key"

           table aliases file:/etc/aliases

           listen on lo0
           listen on egress tls pki mail.example.com auth

           action mda_with_aliases mda "/path/to/mda -f -" alias <aliases>
           action mda_without_aliases mda "/path/to/mda -f -"
           action "outbound" relay

           match for local action mda_with_aliases
           match from any for domain example.com action mda_without_aliases
           match for any action "outbound"
           match auth from any for any action "outbound"

     For sites that wish to sign messages using DKIM, the dkimproxy package may be used as a
     filter.  The following example is the same as the default configuration, but all outgoing
     mail is passed to dkimproxy_out on port 10027 for signing.  The signed messages are received
     on port 10028 and tagged for relaying.

           table aliases file:/etc/aliases

           listen on lo0
           listen on lo0 port 10028 tag DKIM

           action "local_mail" mbox alias <aliases>
           action "outbound" relay
           action "relay_dkim" relay host smtp://127.0.0.1:10027

           match for local action "local_mail"
           match tag DKIM for any action "outbound"
           match for any action "relay_dkim"

     Sites that accept non-local messages may be able to cut down on the volume of spam received
     by rejecting forged messages that claim to be from the local domain.  The following example
     uses a list table other-relays to specify the IP addresses of relays that may legitimately
     originate mail with the owner's domain as the sender.

           table aliases file:/etc/aliases
           table other-relays file:/etc/other-relays

           listen on lo0
           listen on egress

           action "local_mail" mbox alias <aliases>
           action "outbound" relay

           match for local action "local_mail"
           match for any action "outbound"
           match !from src <other-relays> mail-from "@example.com" for any \
                 reject
           match from any for domain example.com action "local_mail"

HISTORY

     smtpd(8) first appeared in OpenBSD 4.6.

NAME

DESCRIPTION

FILES

EXAMPLES

SEE ALSO

HISTORY