Smtpd.conf (5) Linux Manual Page

smtpd.conf – Simple Mail Transfer Protocol daemon configuration file

Description

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

The current line can be extended over multiple lines using a backslash (Sq \.) Comments can be put anywhere in the file using a hash mark (Sq #,) 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 must be quoted. 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, digit, or underscore, and may contain any of those characters. Macro names may not be reserved words (for example listen, accept, port). Macros are not expanded inside quotes.

For example:

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

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

: include "/etc/opensmtpd/smtpd.conf.local"

The syntax of smtpd.conf is described below.

accept | reject: smtpd(8) accepts and rejects messages based on information gathered during the SMTP session.

For each message processed by the daemon, the 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. An exclamation mark may be specified to perform a reverse match. Following the accept/reject decision comes the optional tag matching:

tagged: [ !] tag If specified, the rule will only be matched if the client session was tagged with tag.

After that the client's IP address rule is specified:

from any: 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.
from: [ !] source Pf < table > The rule matches if the connection is made from a client whose address is declared in the table table.

In addition, finer access control may be achieved on the sender if desired:

sender: [ !] Pf < senders > If specified, the rule will only be matched if the sender email address is found in the table senders. The table may contain complete email addresses or apply to an entire domain if prefixed with Sq @.

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

for any [ alias < aliases >]

Make the rule match regardless of the domain it is sent to. If specified, the table aliases is used for looking up alternative destinations for all addresses.

for any virtual < vmap >

Make the rule match regardless of the domain it is sent to. The vmap table will be used as the virtual domain mapping.

for

[ !] domain domain [ alias < aliases >] This rule applies to mail destined for the specified domain. This parameter supports the Sq * 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, the table aliases is used for looking up alternative destinations for addresses in this domain.

for: [ !] domain Pf < domains > [ alias < aliases >] This rule applies to mail destined to domains which are part of the table domains.

If specified, the table aliases is used for looking up alternative destinations for addresses in these domains.

for

[ !] domain domain virtual < users > This rule applies to mail destined for the specified virtual domain. This parameter supports the Sq * wildcard, so that a single rule for all sub-domains can be used, for example:

: accept for domain "*.example.com" \ virtual <users> deliver to mbox

The table users holds a key-value mapping of virtual to system users. For an example of how to configure the users table, see table(5).

for: [ !] domain Pf < domains > virtual < users > This rule applies to mail destined for the virtual domains specified in the table domains.

The table users holds a key-value mapping of virtual to system users. For an example of how to configure the users table, see table(5).

for: [ !] local [ alias < aliases >] This rule applies to mail destined to ``localhost'' and to the default server name. See the FILES entry for /etc/opensmtpd/mailname below for details of how the server name is determined.
for: [ !] local virtual < vmap > This rule applies to mail destined to ``localhost'' and to the default server name. The vmap table will be used as the virtual domain mapping.

Further access control may be achieved on specific recipients if desired:

recipient: [ !] Pf < recipients > If specified, the rule will only be matched if the recipient email address is found in the table recipients. The table may contain complete email addresses or apply to an entire domain if prefixed with Sq @.

If the method of delivery is local, a user database may be specified to override the system database:

[ userbase < table >]: Look up users in the table table instead of performing system lookups using the getpwnam(3) function.

You can also accept mail just to have it forwarded elsewhere:

forward-only: Mail is accepted for local recipients ONLY if it is redirected to an external address via an alias or a ~/.forward file.

Example:

: accept for domain opensmtpd.org forward-only

Finally, the method of delivery is specified:

deliver to lmtp: [host: port | socket] [ rcpt-to] [ as user] Mail is delivered to host: port, or to the Ux socket over LMTP with the privileges of the specified user.

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.

deliver to maildir [path]: Mail is added to a maildir. Its location, path, may contain format specifiers that are expanded before use (see .B FORMAT SPECIFIERS .) If path is not provided, then ~/Maildir is assumed.
deliver to mbox: Mail is delivered to the local user's system mailbox in /var/mail.
deliver to mda program [ as user]: Mail is piped to the specified program, which is run with the privileges of the specified user or the user the message is destined to. This parameter may use conversion specifiers that are expanded before use (see .B FORMAT SPECIFIERS .)
Bk -words: relay [ backup [mx]] [ as address] [ source < source >] [ hostname name] [ hostnames < names >] [ pki pkiname] [ tls [ verify]] Ek

Mail is relayed. The routing decision is based on the DNS system. If the backup parameter is specified, the current server will act as a backup server for the target domain. Accepted mails are only relayed through servers with a lower preference value in the MX record for the domain than the one specified in mx. If mx is not specified, the default server name will be assumed. 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 Sq @, or an email address, causing smtpd(8) to rewrite the user-part, the domain-part, or the entire address, respectively. If the source parameter is specified, smtpd(8) will explicitly bind to an address found in the table referenced by source when connecting to the relay. If the table contains more than one address, they are picked in turn each time a new connection is opened. By default, when connecting to a remote server, smtpd(8) advertises its default server name. A hostname parameter may be specified to advertise the alternate hostname name. If the source parameter is used, the hostnames parameter may be specified to advertise a hostname based on the source address. Table names contains a mapping of IP addresses to hostnames and smtpd(8) will automatically select the name that matches its source address when connected to the remote server. The hostname and hostnames parameters are mutually exclusive. When relaying, STARTTLS is always attempted if available on remote host and smtpd(8) will try to present a certificate matching the outgoing hostname if one is registered in the pki. If pki is specified, the certificate registered for pkiname is used instead. If tls is specified, smtpd(8) will refuse to relay unless the remote host provides STARTTLS. If tls verify is specified, smtpd(8) will refuse to relay unless the remote host provides STARTTLS and the certificate it presented has been verified. Note that the tls and tls verify options should only be used in private networks as they will prevent proper relaying on the Internet.

relay via: host [ auth < auth >] [ as address] [ source < source >] [ hostname name] [ hostnames < names >] [ pki pkiname] [ verify]

Mail is relayed through the specified host expressed as a URL. For example:

: smtp://mx1.example.org # use SMTP smtp://mx1.example.org:4321 # use SMTP \ # with port 4321 lmtp://localhost:2026 # use LMTP \ # with port 2026

The communication channel may be secured using one of the secure schemas. For example:

: tls://mx1.example.org # use TLS smtps://mx1.example.org # use SMTPS secure://mx1.example.org # try SMTPS and \ # fallback to TLS

In addition, credentials for authenticated relaying may be provided when using a secure schema. For example:

: tls+auth://label@mx.example.org # over TLS smtps+auth://label@mx.example.org # over SMTPS secure+auth://label@mx.example.org # over either \ # SMTPS or TLS

If a pki entry exists for the outgoing hostname, or one is provided with pkiname, the associated certificate will be sent to the remote server. If an SMTPAUTH session with host is desired, the auth parameter is used to specify the auth table that holds the credentials. Credentials will be looked up using the label provided in the URL. 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 Sq @, or an email address, causing smtpd(8) to rewrite the user-part, the domain-part, or the entire address, respectively. If the source parameter is specified, smtpd(8) will explicitly bind to an address found in the table referenced by Pf < source > when connecting to the relay. If the table contains more than one address, they are picked in turn each time a new connection is opened. By default, when connecting to a remote server, smtpd(8) advertises its default server name. A hostname parameter may be specified to advertise the alternate hostname name. If the source parameter is used, the hostnames parameter may be specified to advertise a hostname based on the source address. Table names contains a mapping of IP addresses to hostnames and smtpd(8) will automatically select the name that matches its source address when connected to the remote server. The hostname and hostnames parameters are mutually exclusive. If verify is specified, smtpd(8) will refuse to relay unless the remote host provides STARTTLS and the certificate it presented has been verified. The relay URL must specify TLS for this option to be valid.

Additional per-rule adjustments are available:

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

bounce-warn

Sm off n {s | m | h | d} [, Sm on ... ] Specify the delays for which temporary failure reports must be generated when messages are stuck in the queue. For example:

: bounce-warn 1h, 6h, 2d

will generate a failure report when an envelope is in the queue for more than one hour, six hours and two days. The default is 4h.

ca hostname certificate cafile: Associate a custom CA certificate located in cafile with hostname.
ciphers cipher-list: Specify an alternate list of ciphers to use when establishing TLS sessions. It is highly recommended to avoid making use of this option unless there is a good understanding of the implications.

When not specified, only ciphers considered safe are chosen.

expire

Sm off n {s | m | h | d} Sm on Specify how long a message can stay in the queue. The default value is 4d. For example:

: expire 4d # expire after 4 days expire 10h # expire after 10 hours

filter name filter [arguments]

Specify a filter with the given name and the program filter using the given filter arguments. Filters are used to hook into the SMTP dialog and provide additional filtering options for smtpd(8).

filter name chain filter ...

Specify a filter chain with the given name and filters.

limit session

{max-rcpt | max-mails} num Instruct smtpd(8) to accept a maximum number of recipients or emails at once in the receiving queue. Defaults are 100 for max-mails and 1000 for max-rcpt.

limit mta

[ for domain domain] family Instruct smtpd(8) to only use the specified address family for outgoing connections. Accepted values are inet4 and inet6. If a domain is specified, the restriction only applies when connecting to MXs for this domain.

limit scheduler max-inflight num

Suspend the scheduling of envelopes for deliver/relay until the number of inflight envelopes falls below num. Changing the default value might degrade performance.

listen on socket

[ filter name] [ mask-source] br Bk -words listen on interface [family] [ port port] [ filter name] [ tls | tls-require | tls-require verify | smtps | secure] [ pki pkiname] [ ca caname] [ auth | auth-optional [< authtable >]] [ tag tag] [ hostname hostname] [ hostnames < names >] [ senders < users >[masquerade]] [ mask-source] [ received-auth] [ no-dsn] Ek

Specify a socket or an interface to listen on for incoming connections. The listen on socket directive is used to modify the behavior of the listener handling messages submitted through the local enqueuer, for example via the mail(1) utility. This is an optional directive: if unspecified then smtpd(8) will simply listen for connections on the socket as if it was configured with no option. Clients connecting through the socket will always be tagged with the 'local' tag. To listen on a specific network interface, specify an interface and an optional port. An interface group, an IP address or a domain name may be used in place of interface. The family parameter can be used to listen only on specific address family. Accepted values are inet4 and inet6. A filter may be specified to use a filter or filter chain with the given name on SMTP transactions. Secured connections are provided either using STARTTLS ( tls,) by default on port 25, or SMTPS ( smtps,) by default on port 465. tls-require may be used to force clients to establish a secure connection before being allowed to start an SMTP transaction. If tls-require verify is specified, the client must provide a valid certificate to be able to establish an SMTP session. secure may be specified to provide both STARTTLS and SMTPS services. Host certificates may be used for these connections, and must be previously declared using the pki directive. If pki is specified, a certificate matching name is searched for. Moreover, a previously declared ca directive may be specified to use a custom CA certificate. If the auth parameter is used, then a client may only start an SMTP transaction after a successful authentication. 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. If auth-optional is specified, then SMTPAUTH is not required to establish an SMTP transaction. This is only useful to let a listener accept incoming mail from untrusted senders and outgoing mail from authenticated users in situations where it is not possible to listen on the submission port. Both auth and auth-optional accept an optional table as a parameter. When provided, credentials are looked up in this table. The credentials format is described in table(5). If the tag parameter is used, then clients connecting to the listener will be tagged tag. If the hostname parameter is used, then it will be used in the greeting banner instead of the default server name. The hostnames parameter overrides the server name for specific addresses. Table names contains a mapping of IP addresses to hostnames and smtpd(8) will use the hostname that matches the address on which the connection arrives if it is found in the mapping. If the senders parameter is used, then smtpd(8) will look up a mapping of username to email addresses to see whether the authenticated user is allowed to submit mail as the sender that was provided in the SMTP session. In addition, if the masquerade option is provided, the From header will be rewritten to match the sender provided in the SMTP session. If the mask-source parameter is used, then the listener will skip the from part when prepending the ``Received'' header. If the received-auth parameter is used, the ``Received'' header will display if the session was authenticated and by which local user. If the no-dsn parameter is used, DSN (Delivery Status Notification) extension will not be enabled.

max-message-size n: Specify a maximum message size of n bytes. The argument may contain a multiplier, as documented in scan_scaled(3). The default maximum message size is 35MB if none is specified.
pki hostname certificate certfile: Associate the certificate located in certfile with hostname.

If a fallback certificate or SNI is wanted, the Sq * wildcard may be used as hostname. A certificate chain may be created by appending one or many certificates, including a Certificate Authority certificate, to certfile. Creation of certificates is documented in starttls(8).

pki hostname key keyfile

Associate the key located in keyfile with hostname.

pki hostname dhe params

Specify the DHE parameters to use for DHE cipher suites with hostname. 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

Enable transparent compression of envelopes and messages. The only supported algorithm at the moment is gzip. Envelopes and messages may be inspected using the smtpctl(8) or gzcat(1) utilities.

queue encryption [key key]

Enable transparent encryption of envelopes and messages. key must be a 16-byte random key in hexadecimal representation. It can be obtained using the openssl(1) utility as follow:

: $ openssl rand -hex 16

If the key parameter is not specified, it is read with getpass(3) at startup. If key is stdin, then it is read from the standard input at startup. The only supported algorithm is AES-256 in GCM mode. Envelopes and messages may be inspected using the smtpctl(8) utility. Queue encryption can be used with queue compression and will always perform compression before encryption.

table name [type:] config: Tables are used to 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.

The table is identified using table name name; the name itself is arbitrarily chosen. type specifies the table backend, and should be one of the following:

db: Information is stored in a file created using makemap(8).
file: Information is stored in a plain text file using the same format as used to generate makemap(8) mappings. This is the default.

config specifies a configuration file for the table data. It must be an absolute path to a file for the ``file'' and ``db'' table types.

table name {value [, ...]}: Tables containing list of static values may be declared using an inlined notation.

The table is identified using table name name; the name itself is arbitrarily chosen. The table must contain at least one value and may declare many values as a list of comma-separated strings.

table name {key=value [, ...]}: Tables containing static key-value mappings may be declared using an inlined notation.

The table is identified using table name name; the name itself is arbitrarily chosen. The table must contain at least one key-value mapping and may declare many mappings as a list of comma-separated key=value descriptions.

Format Specifiers

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

%{sender} Ta sender email address
%{sender.user} Ta user part of the sender email address
%{sender.domain} Ta domain part of the sender email address
%{rcpt} Ta recipient email address
%{rcpt.user} Ta user part of the recipient email address
%{rcpt.domain} Ta domain part of the recipient email address
%{dest} Ta recipient email address after expansion
%{dest.user} Ta user part after expansion
%{dest.domain} Ta domain part after expansion
%{user.username} Ta local user
%{user.directory} Ta home directory of the local user

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

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

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

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

For security concerns, expanded values are sanitized and potentially dangerous characters are replaced with Sq :. In situations where they are desirable, the “raw” modifier may be applied. For example, with recipient “user+t?g@example.org :”

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

Files

/etc/opensmtpd/smtpd.conf: Default smtpd(8) configuration file.
/etc/opensmtpd/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/spool/smtpd/: Spool directories for mail during processing.

Examples

The default smtpd.conf file 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/opensmtpd/secrets
# chmod 640 /etc/opensmtpd/secrets
# chown root:_smtpd /etc/opensmtpd/secrets
# echo "label username:password" > /etc/opensmtpd/secrets

smtpd.conf would look like this:

: table aliases file:/etc/opensmtpd/aliases table secrets file:/etc/opensmtpd/secrets listen on lo0 accept for local alias <aliases> deliver to mbox accept for any relay via tls+auth://label@smtp.example.com \ 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 listens on all interfaces the default route(s) point to. Mail with a local destination should be 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 certificate "/etc/ssl/mail.example.com.crt" pki mail.example.com key "/etc/ssl/private/mail.example.com.key" table aliases file:/etc/opensmtpd/aliases listen on lo0 listen on egress tls pki mail.example.com auth accept for local alias <aliases> deliver to mda "/path/to/mda -f -" accept from any for domain example.com \ deliver to mda "/path/to/mda -f -" accept for any 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/opensmtpd/aliases listen on lo0 listen on lo0 port 10028 tag DKIM accept for local alias <aliases> deliver to mbox accept tagged DKIM for any relay accept from local for any relay via smtp://127.0.0.1:10027

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 table other-relays can be used to specify the IP addresses of relays that may legitimately originate mail with your domain as the sender.

: table aliases file:/etc/opensmtpd/aliases table other-relays file:/etc/opensmtpd/other-relays listen on lo0 listen on egress accept for local alias <aliases> deliver to mbox accept from local for any relay reject from ! source <other-relays> sender "@example.com" for any accept from any for domain example.com \ alias <aliases> deliver to mbox

History

smtpd(8) first appeared in OpenBSD 4.6.

smtpd.conf (5) Linux Manual Page

Description

Format Specifiers

Files

Examples

See Also

History

Leave a Reply Cancel reply