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.

Debian                                            July 11, 2017                                    SMTPD.CONF(5)

NAME

DESCRIPTION

FILES

EXAMPLES

SEE ALSO

HISTORY