smtpd-filters(7) — Linux manual page

NAME | DESCRIPTION | DESIGN | REPORT AND FILTER | PROTOCOL | CONFIGURATION | REPORT EVENTS | FILTER REQUESTS | SEE ALSO | HISTORY | COLOPHON

FILTERS(7)        BSD Miscellaneous Information Manual        FILTERS(7)

NAME         top

     filters — filtering API for the smtpd(8) daemon

DESCRIPTION         top

     The smtpd(8) daemon provides a Simple Mail Transfer Protocol (SMTP)
     implementation that allows an ordinary machine to become Mail
     eXchangers (MX).  Many 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 needs to provide these features, smtpd(8) provides an
     API to extend its behavior through pluggable filters.

     At runtime, smtpd(8) can report events to filters and query what it
     should answer to these events.  This allows the decision logic to
     rely on third-party programs.

DESIGN         top

     The 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 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.  The filters must not
     use blocking I/O, they must support answering asynchronously to
     smtpd(8).

REPORT AND FILTER         top

     The API relies on two streams, report and filter.

     The report stream is a one-way stream which allows smtpd(8) to
     inform filters in real-time about events that are occurring in the
     daemon.  The report events do not expect an answer from filters, it
     is just meant to provide them with informations.  A filter should
     be able to replicate the smtpd(8) state for a session by gathering
     informations 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 filters about what it should do with a session at a given
     phase.  The filter requests expects an answer from 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 reponse is expected by smtpd(8), this is
     more costly than using report events.  The correct pattern for
     writing filters is to use the 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 request 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.

PROTOCOL         top

     The protocol is straightforward, it consists of a human-readable
     line exchanges between filters and smtpd(8) through fd(4).

     The protocol begins with a handshake.  First, smtpd(8) provides
     filters with general configuration information in the form of key-
     value lines:

           config|smtpd-version|6.6.1
           config|smtp-session-timeout|300
           config|subsystem|smtp-in
           config|ready

     Then, filters register the stream, subsystem and event they want to
     handle:

           register|report|smtp-in|link-connect
           register|ready

     Finally, smtpd(8) will emit report events and filter requests,
     expecting filters to react accordingly either by responding or not
     depending on the stream:

           report|0.5|1576146008.006099|smtp-in|link-connect|7641df9771b4ed00|mail.openbsd.org|pass|199.185.178.25:33174|45.77.67.80:25
           report|0.5|1576147242.200225|smtp-in|link-connect|7641dfb3798eb5bf|mail.openbsd.org|pass|199.185.178.25:31205|45.77.67.80:25
           report|0.5|1576148447.982572|smtp-in|link-connect|7641dfc063102cbd|mail.openbsd.org|pass|199.185.178.25:24786|45.77.67.80:25

     The char “|” may only appear in the last field of a payload, in
     which case it should be considered a regular char and not a
     separator.  Other fields have strict formatting excluding the
     possibility of having a “|”.

     The list of subsystems and events, as well as the format of
     requests and reponses, will be documented in the sections below.

CONFIGURATION         top

     During the initial handshake, smtpd(8) will emit a serie of
     configuration keys and values.  The list is meant to be ignored by
     filters that do not require it and consumed gracefully by filters
     that do.

     There are currently three keys:

           config|smtpd-version|6.6.1
           config|smtp-session-timeout|300
           config|subsystem|smtp-in

     When smtpd(8) has sent all configuration keys it emits the
     following line:

           config|ready

REPORT EVENTS         top

     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|0.5|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|0.5|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 are provided here
     for smtp-in:

     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 holds either the IP address and port from 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 from 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 strenght in bits.

     link-disconnect
             This event is generated upon disconnection of the client.

     link-auth: username result
             This event is generated upon authentication attempt of the
             client.

             username contains the username used for the authentication
             attempt.

             result contains the string “pass”, “fail” or “error”
             depending on the result of the authentication attempt.

     tx-reset: [message-id]
             This event is generated when a transaction is reset.

             If reset happend while in 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 protocol “<” and
             “>” are removed and so are 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 protocol “<” and
             “>” are removed and so are 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 to process the
             message, “permfail” if it has not accepted and “tempfail”
             if a transient error is preventing the processing of
             message.

     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.

FILTER REQUESTS         top

     There is currently only one subsystem supported in the API: smtp-
     in.

     The filter requests allow smtpd(8) to query filters about what to
     do with a session at a particular phase.  In addition, they allow
     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:

           filter|0.5|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 separated
     by “|” that the filter should provide in its response:

           filter|0.5|1576146008.006099|smtp-in|connect|7641df9771b4ed00|1ef1c203cc576e5d

     It is followed by a suffix containing the phase-specific parameters
     to 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 before the
     filter has instructed smtpd(8) what to do with it.

     For all phases, excepted “data-line”, the responses must follow the
     same construct, a message 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 to a “filter-result” message will be
     described below.

     For the “data-line” phase, filters are fed with a stream of lines
     corresponding to the message to filter, and terminated by a single
     dot:

           filter|0.5|1576146008.006099|smtp-in|data-line|7641df9771b4ed00|1ef1c203cc576e5d|line 1
           filter|0.5|1576146008.006103|smtp-in|data-line|7641df9771b4ed00|1ef1c203cc576e5d|line 2
           filter|0.5|1576146008.006105|smtp-in|data-line|7641df9771b4ed00|1ef1c203cc576e5d|.

     They are expected to produce an output stream similarly terminate
     by a single dot.  A filter may inject, suppress, modify or echo
     back the lines it receives.  Ultimately, smtpd(8) will assume that
     the message consists of the output from filters.

     Note that filters may be chained and the lines that are input into
     a filter are the lines that are output from 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 are provided here
     for smtp-in:

     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 a “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.

SEE ALSO         top

     smtpd(8)

HISTORY         top

     filters first appeared in OpenBSD 6.6.

COLOPHON         top

     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 2021-08-27.  (At
     that time, the date of the most recent commit that was found in the
     repository was 2021-04-28.)  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

BSD                          April 25, 2020                          BSD