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

smtpd.conf — Simple Mail Transfer Protocol daemon configuration file

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

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.

Arguments containing whitespace should be surrounded by double quotes (").

Macros can be defined that will later be expanded in context. Macro names must start with a letter, and may contain letters, digits and underscores. Macro names may not be reserved words (for example listen, accept, port). Macros are not expanded inside quotes.

For example:

wan_if = "fxp0"
listen on $wan_if
listen on $wan_if tls enable auth

Some configuration directives expect expansion of their parameters at runtime. Such directives (for example deliver to maildir, deliver to mda, relay via) may use format specifiers which will be expanded before delivery or relaying. The following formats are currently supported: %a expands to the user part of the email address prior to the resolution of aliases; %u expands to the user part after aliases resolution and will typically be the system account; %d expands to the domain part of the email address; %U expands to the user part of the sender email address; %D expands to the domain part of the sender email address.

Additional configuration files can be included with the include keyword, for example:

include "/etc/mail/smtpd.conf.local"

The syntax of smtpd.conf is described below.

n {s|m|h|d}
Specify how long a message can stay in the queue. The default value is 4 days. For example: 
expire 4d	# expire after 4 days
expire 10h	# expire after 10 hours
name
Specify the domain name of the server. By default the current host name is used, as returned by hostname(1).
interface [port port] [tls | smtps] [certificate name] [enable auth]
Specify an interface and port to listen on. An interface group, an IP address or a domain name may be used in place of interface.

Secured connections are provided either using STARTTLS (tls), by default on port 25, or SMTPS (smtps), by default on port 465. Host certificates may be used for these connections, and are searched for in the /etc/mail/certs directory. If certificate is specified, a certificate ⟨name⟩.crt, a key ⟨name⟩.key, a certificate authority ⟨name⟩.ca and Diffie-Hellman parameters ⟨name⟩.dh are searched for. If no certificate is specified, the default interface name is instead used, for example fxp0.crt, fxp0.key, fxp0.ca, and fxp0.dh. If no DH parameters are provided, smtpd will use built-in parameters. Creation of certificates is documented in starttls(8).

If the enable auth parameter is used, any remote sender that passed SMTPAUTH is treated as if it was the server's local user that was sending the mail. This means that filter rules using "from local" will be matched.

map { source type source }
Maps are used to provide additional configuration information for smtpd(8).

map may be named freely.

type may be one of the following:

db
Mappings are stored in a file created using makemap(8). This is the default type if none is specified.
dns
Not implemented yet.
plain
Mappings are stored in a plain text file using the same format as used to generate makemap(8) mappings.

source specifies the source of the map data. It must be an absolute path to a file for the “file” and “db” map types, or a domain name for the “dns” map type.

n
Specify a maximum message size of n bytes. The argument may contain a multiplier, as documented in scan_scaled(3).
|
smtpd(8) accepts and rejects messages based on information gathered during the SMTP session.

For each message processed by the daemon, the filter rules are evaluated in sequential order, from first to last. The first matching rule decides what action is taken. If no rule matches the message, the default action is to reject the message.

Following the accept/reject decision comes the client's IP address filter:

from all
Make the rule match regardless of the IP of connecting client.
from local
The rule matches only locally originating connections. This is the default, and may be omitted.
network
The rule matches if the connection is made from the specified network, specified in CIDR notation.

Next comes the selection based on the domain the message is sent to:

Make the rule match regardless of the domain it is sent to.
domain [alias map]
This rule applies to mail destined for the specified domain. This parameter supports the ‘*’ wildcard, so that a single rule for all sub-domains can be used, for example: 
accept for domain "*.example.com" deliver to mbox

If specified, map is used for looking up alternative destinations for addresses in this domain.

[alias map]
This rule applies to mail destined to “localhost” and to the server's fully qualified domain name, as returned by hostname(1).
map
This rule applies to mail destined for the virtual domains specified in the map map. For an example of how to configure a virtual map, see makemap(8).

Finally, the method of delivery is specified:

path
Mail is added to a maildir. Its location, path, may contain format specifiers that are expanded before use (see above). If path is not provided, then ~/Maildir is assumed.
Mail is delivered to the local user's system mailbox in /var/mail.
program
Mail is piped to the specified program, which is run with the privileges of the user the message is destined to. This parameter may use conversion specifiers that are expanded before use (see above).
[as address]
Mail is relayed. The routing decision is based on the DNS system.

If the as parameter is specified, smtpd(8) will rewrite the sender advertised in the SMTP session. address may be a user, a domain prefixed with ‘@’, or an email address, causing smtpd to rewrite the user-part, the domain-part, or the entire address, respectively.

relay via host [port port] [tls | smtps | ssl] [certificate name] [auth map] [as address]
Mail is relayed through the specified host and port.

The communication channel may be secured using the tls or smtps options. The special keyword ssl means that any of the two is acceptable: SMTPS is tried first, STARTTLS second. If a certificate name is specified and exists in the /etc/mail/certs directory with a .crt extension, it will be used if the remote server requests a client certificate. Creation of certificates is documented in starttls(8).

If an SMTPAUTH session with host is desired, use the auth parameter to specify the map that holds the credentials.

If the as parameter is specified, smtpd(8) will rewrite the sender advertised in the SMTP session. address may be a user, a domain prefixed with ‘@’, or an email address, causing smtpd to rewrite the user-part, the domain-part, or the entire address, respectively.

n {s|m|h|d}
Specify how long a message that matched this rule can stay in the queue.

/etc/mail/smtpd.conf
Default smtpd(8) configuration file.
/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 "smtp.example.com  username:password" > /etc/mail/secrets
# makemap /etc/mail/secrets

smtpd.conf would look like this:

listen on lo0
map aliases { source db "/etc/mail/aliases.db" }
map secrets { source db "/etc/mail/secrets.db" }
accept for local deliver to mbox
accept for all relay via smtp.example.com tls auth "secrets"

In this second example, the aim is to permit mail relaying for any user that can authenticate using their normal login credentials. An RSA certificate must be provided to prove the server's identity. The mail server has an external interface bnx0. Mail with a local destination should be sent to an external mda. First, the RSA certificate is created:

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

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

listen on lo0
listen on bnx0 tls certificate "mail.example.com" enable auth
map aliases { source db "/etc/mail/aliases.db" }
accept for local deliver to mda "/path/to/mda -f -"
accept from all for domain example.org deliver to mda "/path/to/mda -f -"
accept for all relay

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

smtpd(8) first appeared in OpenBSD 4.6.

December 13, 2011 OpenBSD-5.1