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.

smtps

SMTP session with forced TLS on connection.

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.

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.

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.

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.

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.

[!] 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 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.

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.

[!] mail-from sender | <sender>

Specify that transactions's MAIL FROM should match the string or list table sender.

[!] rcpt-to recipient | <recipient>

Specify that transaction's RCPT TO should match the string or list table recipient.

[!] tag tag

Matches transactions tagged with the given tag.

[!] 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.

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 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 ‘+’.

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.

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

match for local action "local"
match for any action "relay"

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 "relay" relay

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

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

match for local action "mbox"
match tag DKIM for any action "relay"
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 "mbox" mbox alias <aliases>
action "relay" relay

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

SEE ALSO¶

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

HISTORY¶

smtpd(8) first appeared in OpenBSD 4.6.