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

Provided by: opensmtpd_6.0.3p1-1ubuntu0.2_amd64

NAME

     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 (‘\’). 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 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/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 matching of optional session related
properties:

[!] authenticated
If specified, the rule will only be matched if the client session was
authenticated either by requesting authentication over the network or
because the message was submitted over the local enqueuer.

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 <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 [!] <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 ‘@’.

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 ‘*’ 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 <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 ‘*’ 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 <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 (the FILES entry for /etc/mailname details how the server name is
determined). This is the default, and may be omitted.

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

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 [!] <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 ‘@’.

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 UNIX 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 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 FORMAT
SPECIFIERS).

relay [backup [mx]] [as address] [source <source>] [hostname name]
[hostnames <names>] [pki pkiname] [tls [verify]]

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 ‘@’, 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 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.

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

bounce-warn n{s|m|h|d}[, ...]
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 n{s|m|h|d}
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

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 interface [family] [port port] [tls | tls-require | tls-require verify | smtps]
[pki pkiname] [ca caname] [auth | auth-optional [<authtable>]] [tag tag]
[hostname hostname] [hostnames <names>] [senders <users> [masquerade]] [mask-source]
[received-auth] [no-dsn]
Specify an interface and optional port to listen on for incoming connections. 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.

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.

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.

listen on socket [mask-source]
Modify behaviour for the listener which handles messages submitted through the local
enqueuer, such as the mail(1) utility. Clients connecting in this manner are tagged
with the "local" tag.

Parameters available are:

mask-source Skip the from part when prepending the “Received” header.

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 ‘*’ 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.

subaddressing-delimiter delimiter
Redefine the subaddressing delimiter from the default ‘+’ to delimiter.

Any printable character valid in an email address is allowed, except spaces and ‘@’.

The first character in the user-part of an email address that matches delimiter is
considered to be the subaddressing delimiter.

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} sender email address
%{sender.user} user part of the sender email address
%{sender.domain} domain part of the sender email address
%{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

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

     smtpd.conf would look like this:

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

NAME

DESCRIPTION

FILES

EXAMPLES

SEE ALSO

HISTORY