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NAME | DESCRIPTION | DESIGN | REPORT AND FILTER | PROTOCOL | CONFIGURATION | REPORT EVENTS | FILTER REQUESTS | SEE ALSO | HISTORY | COLOPHON |
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SMTPD-FILTERS(7) Miscellaneous Information Manual SMTPD-FILTERS(7)
smtpd-filters — filtering API for the smtpd daemon
The smtpd(8) daemon provides a Simple Mail Transfer Protocol
(SMTP) implementation, which allows ordinary machines to become
Mail eXchangers (MX). Some features that are commonly used by MX,
such as delivery reporting or spam filtering, are outside the
scope of SMTP and too complex to fit in smtpd(8).
Because an MX may need to provide these features, smtpd(8)
provides an API to extend its behavior through smtpd-filters.
At runtime, smtpd(8) can report events to smtpd-filters, querying
what it should answer to these events. This allows the decision
logic to rely on third-party programs.
smtpd-filters are programs that run as unique standalone
processes, they do not share smtpd(8) memory space. They are
executed by smtpd(8) at startup and expected to run in an infinite
loop, reading events and filtering requests from stdin(4), writing
responses to stdout(4) and logging to stderr(4). They are not
allowed to terminate.
Because smtpd-filters are standalone programs that communicate
with smtpd(8) through fd(4), they may run as different users than
smtpd(8) and may be written in any language. smtpd-filters must
not use blocking I/O, they must support answering asynchronously
to smtpd(8).
The API relies on two streams, report and filter.
The report stream is a one-way stream which allows smtpd(8) to
inform smtpd-filters in real-time about events. Report events do
not expect an answer from smtpd-filters; they are just meant to
provide information. A filter should be able to replicate the
smtpd(8) state for a session by gathering information coming from
report events. No decision is ever taken by the report stream.
The filter stream is a two-way stream which allows smtpd(8) to
query smtpd-filters about what it should do with a session at a
given phase. Filter requests expect an answer from smtpd-filters;
smtpd(8) will not let the session move forward until then. A
decision must always be taken by the filter stream.
It is sometimes possible to rely on filter requests to gather
information, but because a response is expected by smtpd(8), this
is more costly than using report events. The correct pattern for
writing filters is to use report events to create a local state
for a session, then use filter requests to take decisions based on
this state. The only case when using filter requests instead of
report events is correct is when a decision is required for the
filter request and there is no need for more information than that
of the event itself.
The protocol consists of human-readable lines exchanged between
smtpd-filters and smtpd(8), through fd(4).
The protocol begins with a handshake. First, smtpd(8) provides
smtpd-filters with general configuration information in the form
of key-value lines:
config|smtpd-version|7.5.0
config|protocol|0.7
config|smtp-session-timeout|300
config|subsystem|smtp-in
config|ready
Then, smtpd-filters register the stream, subsystem and event they
want to handle:
register|report|smtp-in|link-connect
register|ready
Finally, smtpd(8) emits report events and filter requests,
expecting smtpd-filters to respond or not depending on the stream:
report|0.7|1576146008.006099|smtp-in|link-connect|7641df9771b4ed00|mail.openbsd.org|pass|199.185.178.25:33174|45.77.67.80:25
report|0.7|1576147242.200225|smtp-in|link-connect|7641dfb3798eb5bf|mail.openbsd.org|pass|199.185.178.25:31205|45.77.67.80:25
report|0.7|1576148447.982572|smtp-in|link-connect|7641dfc063102cbd|mail.openbsd.org|pass|199.185.178.25:24786|45.77.67.80:25
The character “|” may only appear in the last field of a payload,
in which case it should be considered a regular character and not
a separator. No other field may contain a “|”.
The list of subsystems and events, as well as the format of
requests and responses, are documented in the sections below.
During the initial handshake, smtpd(8) emits a series of
configuration keys and values. The list is meant to be ignored by
smtpd-filters that do not require it and consumed gracefully by
filters that do.
There are currently three keys:
config|smtpd-version|7.5.0
config|protocol|0.7
config|smtp-session-timeout|300
config|subsystem|smtp-in
When smtpd(8) has sent all configuration keys, it emits the
following line:
config|ready
There is currently only one subsystem supported in the API: smtp-
in.
Each report event is generated by smtpd(8) as a single line
similar to the one below:
report|0.7|1576146008.006099|smtp-in|link-connect|7641df9771b4ed00|mail.openbsd.org|pass|199.185.178.25:33174|45.77.67.80:25
The format consists of a protocol prefix containing the stream,
the protocol version, the timestamp, the subsystem, the event and
the unique session identifier, separated by “|”:
report|0.7|1576146008.006099|smtp-in|link-connect|7641df9771b4ed00
It is followed by a suffix containing the event-specific
parameters, also separated by “|”:
mail.openbsd.org|pass|199.185.178.25:33174|45.77.67.80:25
The list of events and event-specific parameters for smtp-in are
as follows:
link-connect: rdns fcrdns src dest
This event is generated upon connection.
rdns contains the reverse DNS hostname for the remote end
or an empty string if none.
fcrdns contains the string “pass” or “fail” depending on
if the remote end validates FCrDNS.
src contains either the IP address and port of the source
address, in the format “address:port”, or the path to a
UNIX socket in the format “unix:/path”.
dest holds either the IP address and port of the
destination address, in the format “address:port”, or the
path to a UNIX socket in the format “unix:/path”.
link-greeting: hostname
This event is generated upon display of the server banner.
hostname contains the hostname displayed in the banner.
link-identify: method identity
This event is generated upon “HELO” or “EHLO” command from
the client.
method contains the string “HELO” or “EHLO” indicating the
method used by the client.
identity contains the identity provided by the client.
link-tls: tls-string
This event is generated upon successful negotiation of
TLS.
tls-string contains a colon-separated list of TLS
properties including the TLS version, the cipher suite
used by the session and the cipher strength in bits.
link-disconnect
This event is generated upon disconnection of the client.
link-auth: result username
This event is generated upon an authentication attempt by
the client.
result contains the string “pass”, “fail” or “error”
depending on the result of the authentication attempt.
username contains the username used for the authentication
attempt.
tx-reset: [message-id]
This event is generated when a transaction is reset.
If reset took place during a transaction, message-id
contains the identifier of the transaction being reset.
tx-begin: message-id
This event is generated when a transaction is initiated.
message-id contains the identifier for the transaction.
tx-mail: message-id result address
This event is generated when client emits “MAIL FROM”.
message-id contains the identifier for the transaction.
result contains “ok” if the sender was accepted,
“permfail” if it was rejected or “tempfail” if it was
rejected for a transient error.
address contains the e-mail address of the sender. The
address is normalized and sanitized, the characters “<”
and “>” are removed, along with any parameters to “MAIL
FROM”.
tx-rcpt: message-id result address
This event is generated when client emits “RCPT TO”.
message-id contains the identifier for the transaction.
result contains “ok” if the recipient was accepted,
“permfail” if it was rejected or “tempfail” if it was
rejected for a transient error.
address contains the e-mail address of the recipient. The
address is normalized and sanitized, the characters “<”
and “>” are removed, along with any parameters to “RCPT
TO”.
tx-envelope: message-id envelope-id
This event is generated when an envelope is accepted.
envelope-id contains the unique identifier for the
envelope.
tx-data: message-id result
This event is generated when client has emitted “DATA”.
message-id contains the unique identifier for the
transaction.
result contains “ok” if server accepted the message for
processing, “permfail” if it has not been accepted and
“tempfail” if a transient error prevented message
processing.
tx-commit: message-id message-size
This event is generated when a transaction has been
accepted by the server.
message-id contains the unique identifier for the SMTP
transaction.
message-size contains the size of the message submitted in
the “DATA” phase of the SMTP transaction.
tx-rollback: message-id
This event is generated when a transaction has been
rejected by the server.
message-id contains the unique identifier for the SMTP
transaction.
protocol-client: command
This event is generated for every command submitted by the
client. It contains the raw command as received by the
server.
command contains the command emitted by the client to the
server.
protocol-server: response
This event is generated for every response emitted by the
server. It contains the raw response as emitted by the
server.
response contains the response emitted by the server to
the client.
filter-report: filter-kind name message
This event is generated when a filter emits a report.
filter-kind may be either “builtin” or “proc” depending on
if the filter is an smtpd(8) builtin filter or a proc
filter implementing the API.
name is the name of the filter that generated the report.
message is a filter-specific message.
filter-response: phase response [param]
This event is generated when a filter responds to a
filtering request.
phase contains the phase name for the request. The phases
are documented in the next section.
response contains the response of the filter to the
request, it is either one of “proceed”, “report”,
“reject”, “disconnect”, “junk or” “rewrite”.
If specified, param is the parameter to the response.
timeout
This event is generated when a timeout happens for a
session.
There is currently only one subsystem supported in the API: smtp-
in.
Filter requests allow smtpd(8) to query smtpd-filters about what
to do with a session at a particular phase. In addition, they
allow smtpd-filters to alter the content of a message by adding,
modifying, or suppressing lines of input in a way that is similar
to what program like sed(1) or grep(1) would do.
Each filter request is generated by smtpd(8) as a single line
similar to the one below. Fields are separated by the “|”
character.
filter|0.7|1576146008.006099|smtp-in|connect|7641df9771b4ed00|1ef1c203cc576e5d|mail.openbsd.org|pass|199.185.178.25:33174|45.77.67.80:25
The format consists of a protocol prefix containing the stream,
the protocol version, the timestamp, the subsystem, the filtering
phase, the unique session identifier and an opaque token that the
filter should provide in its response:
filter|0.7|1576146008.006099|smtp-in|connect|7641df9771b4ed00|1ef1c203cc576e5d
It is followed by a suffix containing the phase-specific
parameters of the filter request, also separated by “|”:
mail.openbsd.org|pass|199.185.178.25:33174|45.77.67.80:25
Unlike with report events, smtpd(8) expects answers from filter
requests and will not allow a session to move forward until the
filter has instructed smtpd(8) how to treat it.
For all phases except “data-line”, responses must follow the same
construct: a message of type “filter-result”, followed by the
unique session id, the opaque token, a decision and optional
decision-specific parameters:
filter-result|7641df9771b4ed00|1ef1c203cc576e5d|proceed
filter-result|7641df9771b4ed00|1ef1c203cc576e5d|reject|550 nope
The possible decisions for a “filter-result” message are
documented below.
For the “data-line” phase, smtpd-filters are fed a stream of lines
corresponding to the message to filter, terminated by a single
dot:
filter|0.7|1576146008.006099|smtp-in|data-line|7641df9771b4ed00|1ef1c203cc576e5d|line 1
filter|0.7|1576146008.006103|smtp-in|data-line|7641df9771b4ed00|1ef1c203cc576e5d|line 2
filter|0.7|1576146008.006105|smtp-in|data-line|7641df9771b4ed00|1ef1c203cc576e5d|.
They are expected to return an output stream similarly terminated
by a single dot. A filter may add to, suppress, modify or echo
back the lines it receives. Ultimately, smtpd(8) assumes that the
message consists of the output from smtpd-filters.
Note that filters may be chained, and the lines that are input
into a subsequent filter are the lines that are output from a
previous filter.
The response to “data-line” requests use their own construct. A
“filter-dataline” prefix, followed by the unique session
identifier, the opaque token and the output line as follows:
filter-dataline|7641df9771b4ed00|1ef1c203cc576e5d|line 1
filter-dataline|7641df9771b4ed00|1ef1c203cc576e5d|line 2
filter-dataline|7641df9771b4ed00|1ef1c203cc576e5d|.
The list of events and event-specific parameters for smtp-in are
as follows:
connect: rdns fcrdns src dest
This request is emitted after connection, before the
banner is displayed.
helo: identity
This request is emitted after the client has emitted
“HELO”.
ehlo: identity
This request is emitted after the client has emitted
“EHLO”.
starttls: tls-string
This request is emitted after the client has requested
“STARTTLS”.
auth: auth
This request is emitted after the client has requested
“AUTH”.
mail-from: address
This request is emitted after the client has requested
“MAIL FROM”.
rcpt-to: address
This request is emitted after the client has requested
“RCPT TO”.
data This request is emitted after the client has requested
“DATA”.
data-line: line
This request is emitted for each line of input in the
“DATA” phase. The lines are raw dot-escaped SMTP DATA
input, terminated with a single dot.
commit This request is emitted after the final single dot is
received.
For every filtering phase, excepted “data-line”, the following
decisions may be taken by a filter:
proceed
No action is taken, session or transaction may be passed
to the next filter.
junk The session or transaction is marked as spam. smtpd(8)
will prepend an “X-Spam” header to the message.
reject error
The command is rejected with the message error. The
message must be a valid SMTP message including status
code, 5xx or 4xx.
Messages starting with a 5xx status result in a permanent
failure, those starting with a 4xx status result in a
temporary failure.
Messages starting with a 421 status will result in a
client disconnect.
disconnect error
The client is disconnected with the message error. The
message must be a valid SMTP message including status
code, 5xx or 4xx.
Messages starting with a 5xx status result in a permanent
failure, those starting with a 4xx status result in a
temporary failure.
rewrite parameter
The command parameter is rewritten.
This decision allows a filter to perform a rewrite of
client-submitted commands before they are processed by the
SMTP engine. parameter is expected to be a valid SMTP
parameter for the command.
report parameter
Generates a report with parameter for this filter.
smtpd(8)
smtpd-filters first appeared in OpenBSD 6.6.
This page is part of the OpenSMTPD (a FREE implementation of the
server-side SMTP protocol) project. Information about the project
can be found at https://www.opensmtpd.org/. If you have a bug
report for this manual page, see
⟨https://github.com/OpenSMTPD/OpenSMTPD/issues⟩. This page was
obtained from the project's upstream Git repository
⟨https://github.com/OpenSMTPD/OpenSMTPD.git⟩ on 2025-02-02. (At
that time, the date of the most recent commit that was found in
the repository was 2024-10-20.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
GNU May 14, 2024 SMTPD-FILTERS(7)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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