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SMTPD.CONF(5) File Formats Manual SMTPD.CONF(5)

smtpd.confSimple Mail Transfer Protocol daemon configuration file

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.

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:

Only accept the message if a delivery method was specified in an aliases or .forward file.
Only accept the message if the recipient results in a remote address after the processing of aliases or forward file.
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.

[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.

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

The local delivery methods support additional options:

<table>
Use the mapping table for aliases(5) expansion.
n{s|m|h|d}
Specify how long a message may remain in the queue.
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.

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

The userbase does not apply for the user option.

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

The relay delivery methods also support additional options:

Operate as a backup mail exchanger delivering messages to any mail exchanger with higher priority.
name
Operate as a backup mail exchanger delivering messages to any mail exchanger with higher priority than mail exchanger identified as name.
heloname
Advertise heloname as the hostname to other mail exchangers during the HELO phase.
<table>
Use the mapping table to look up a hostname matching the source address, to advertise during the HELO phase.
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.

[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.
<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).
mailaddr
Use mailaddr as the MAIL FROM address within the SMTP transaction.
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.
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.
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/mail/mailname or using the hostname directive.
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-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-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-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-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-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-name proc proc-name
Register “proc” filter filter-name backed by the proc-name process.
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/”.
pathname
Replace this directive with the content of the additional configuration file at the absolute pathname.
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:

[<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).
[<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.
caname
For secure connections, use the CA certificate associated with caname (declared in a ca directive) as the CA certificate when verifying client certificates.
name
Apply filter name on connections handled by this listener.
hostname
Use hostname in the greeting banner instead of the default server name.
<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.
Omit the from part when prepending “Received” headers.
Disable the DSN (Delivery Status Notification) extension.
pkiname
For secure connections, use the certificate associated with pkiname (declared in a pki directive) to prove a mail server's identity.
[port]
Listen on the given port instead of the default port 25.
Support the PROXYv2 protocol, rewriting appropriately source address received from proxy.
In “Received” headers, report whether the session was authenticated and by which local user.
<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.
Support SMTPS, by default on port 465. Mutually exclusive with tls.
tag
Clients connecting to the listener are tagged with the given tag.
Support STARTTLS, by default on port 25. Mutually exclusive with smtps.
[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.
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.
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.
options reject
Reject the incoming message during the SMTP dialogue. The same options are supported as for the match action directive.
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).
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.
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/mail/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).

pkiname key keyfile
Associate the key located in keyfile with host pkiname.
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-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/”.
compression
Store queue files in a compressed format. This may be useful to save disk space.
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.
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).
ciphers control
Set the control string for SSL_CTX_set_cipher_list(3). The default is “HIGH:!aNULL:!MD5”.
max-mails count
Limit the number of messages to count for each session. The default is 100.
max-rcpt count
Limit the number of recipients to count for each transaction. The default is 1000.
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”.
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 ‘+’.
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.

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.
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.

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 criterias 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 criterias 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.

Criterias 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.

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”

/etc/mail/smtpd.conf
Default smtpd(8) configuration file.
/etc/mail/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.

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

smtpd.conf would look like this:

table aliases file:/etc/mail/aliases
table secrets file:/etc/mail/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/mail/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/mail/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/mail/aliases
table other-relays file:/etc/mail/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"

mailer.conf(5), table(5), makemap(8), smtpd(8)

smtpd(8) first appeared in OpenBSD 4.6.

September 6, 2019 OpenBSD-current