usr.sbin/smtpd/smtpd-filters.7

.\"	$OpenBSD: smtpd-filters.7,v 1.13 2024/11/05 19:36:53 jmc Exp $
.\"
.\" Copyright (c) 2008 Janne Johansson <jj@openbsd.org>
.\" Copyright (c) 2009 Jacek Masiulaniec <jacekm@dobremiasto.net>
.\" Copyright (c) 2012 Gilles Chehade <gilles@poolp.org>
.\"
.\" Permission to use, copy, modify, and distribute this software for any
.\" purpose with or without fee is hereby granted, provided that the above
.\" copyright notice and this permission notice appear in all copies.
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.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
.\"
.\"
.Dd $Mdocdate: November 5 2024 $
.Dt SMTPD-FILTERS 7
.Os
.Sh NAME
.Nm smtpd-filters
.Nd filtering API for the smtpd daemon
.Sh DESCRIPTION
The
.Xr 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
.Xr smtpd 8 .
.Pp
Because an MX may need to provide these features,
.Xr smtpd 8
provides an API to extend its behavior through
.Nm .
.Pp
At runtime,
.Xr smtpd 8
can report events to
.Nm ,
querying what it should answer to these events.
This allows the decision logic to rely on third-party programs.
.Sh DESIGN
.Nm
are programs that run as unique standalone processes,
they do not share
.Xr smtpd 8
memory space.
They are executed by
.Xr smtpd 8
at startup and expected to run in an infinite loop,
reading events and filtering requests from
.Xr stdin 4 ,
writing responses to
.Xr stdout 4
and logging to
.Xr stderr 4 .
They are not allowed to terminate.
.Pp
Because
.Nm
are standalone programs that communicate with
.Xr smtpd 8
through
.Xr fd 4 ,
they may run as different users than
.Xr smtpd 8
and may be written in any language.
.Nm
must not use blocking I/O,
they must support answering asynchronously to
.Xr smtpd 8 .
.Sh REPORT AND FILTER
The API relies on two streams,
report and filter.
.Pp
The report stream is a one-way stream which allows
.Xr smtpd 8
to inform
.Nm
in real-time about events.
Report events do not expect an answer from
.Nm ;
they are just meant to provide information.
A filter should be able to replicate the
.Xr smtpd 8
state for a session by gathering information coming from report events.
No decision is ever taken by the report stream.
.Pp
The filter stream is a two-way stream which allows
.Xr smtpd 8
to query
.Nm
about what it should do with a session at a given phase.
Filter requests expect an answer from
.Nm ;
.Xr smtpd 8
will not let the session move forward until then.
A decision must always be taken by the filter stream.
.Pp
It is sometimes possible to rely on filter requests to gather information,
but because a response is expected by
.Xr 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.
.Sh PROTOCOL
The protocol consists of human-readable lines exchanged between
.Nm
and
.Xr smtpd 8 ,
through
.Xr fd 4 .
.Pp
The protocol begins with a handshake.
First,
.Xr smtpd 8
provides
.Nm
with general configuration information in the form of key-value lines:
.Bd -literal -offset indent
config|smtpd-version|7.5.0
config|protocol|0.7
config|smtp-session-timeout|300
config|subsystem|smtp-in
config|ready
.Ed
.Pp
Then,
.Nm
register the stream,
subsystem and event they want to handle:
.Bd -literal -offset indent
register|report|smtp-in|link-connect
register|ready
.Ed
.Pp
Finally,
.Xr smtpd 8
emits report events and filter requests,
expecting
.Nm
to respond or not depending on the stream:
.Bd -literal -offset indent
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
.Ed
.Pp
The character
.Dq |
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
.Dq | .
.Pp
The list of subsystems and events,
as well as the format of requests and responses,
are documented in the sections below.
.Sh CONFIGURATION
During the initial handshake,
.Xr smtpd 8
emits a series of configuration keys and values.
The list is meant to be ignored by
.Nm
that do not require it and consumed gracefully by filters that do.
.Pp
There are currently three keys:
.Bd -literal -offset indent
config|smtpd-version|7.5.0
config|protocol|0.7
config|smtp-session-timeout|300
config|subsystem|smtp-in
.Ed
.Pp
When
.Xr smtpd 8
has sent all configuration keys, it emits the following line:
.Bd -literal -offset indent
config|ready
.Ed
.Sh REPORT EVENTS
There is currently only one subsystem supported in the API:
smtp-in.
.Pp
Each report event is generated by
.Xr smtpd 8
as a single line similar to the one below:
.Bd -literal -offset indent
report|0.7|1576146008.006099|smtp-in|link-connect|7641df9771b4ed00|mail.openbsd.org|pass|199.185.178.25:33174|45.77.67.80:25
.Ed
.Pp
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
.Dq | :
.Bd -literal -offset indent
report|0.7|1576146008.006099|smtp-in|link-connect|7641df9771b4ed00
.Ed
.Pp
It is followed by a suffix containing the event-specific parameters,
also separated by
.Dq | :
.Bd -literal -offset indent
mail.openbsd.org|pass|199.185.178.25:33174|45.77.67.80:25
.Ed
.Pp
The list of events and event-specific parameters for smtp-in are as follows:
.Bl -tag -width Ds
.It Ic link-connect : Ar rdns fcrdns src dest
This event is generated upon connection.
.Pp
.Ar rdns
contains the reverse DNS hostname for the remote end or an empty string if none.
.Pp
.Ar fcrdns
contains the string
.Dq pass
or
.Dq fail
depending on if the remote end validates FCrDNS.
.Pp
.Ar src
contains either the IP address and port of the source address,
in the format
.Dq address:port ,
or the path to a UNIX socket in the format
.Dq unix:/path .
.Pp
.Ar dest
holds either the IP address and port of the destination address,
in the format
.Dq address:port ,
or the path to a UNIX socket in the format
.Dq unix:/path .
.It Ic link-greeting : Ar hostname
This event is generated upon display of the server banner.
.Pp
.Ar hostname
contains the hostname displayed in the banner.
.It Ic link-identify : Ar method identity
This event is generated upon
.Dq HELO
or
.Dq EHLO
command from the client.
.Pp
.Ar method
contains the string
.Dq HELO
or
.Dq EHLO
indicating the method used by the client.
.Pp
.Ar identity
contains the identity provided by the client.
.It Ic link-tls : Ar tls-string
This event is generated upon successful negotiation of TLS.
.Pp
.Ar 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.
.It Ic link-disconnect
This event is generated upon disconnection of the client.
.It Ic link-auth : Ar result username
This event is generated upon an authentication attempt by the client.
.Pp
.Ar result
contains the string
.Dq pass ,
.Dq fail
or
.Dq error
depending on the result of the authentication attempt.
.Pp
.Ar username
contains the username used for the authentication attempt.
.It Ic tx-reset : Op message-id
This event is generated when a transaction is reset.
.Pp
If reset took place during a transaction,
.Ar message-id
contains the identifier of the transaction being reset.
.It Ic tx-begin : Ar message-id
This event is generated when a transaction is initiated.
.Pp
.Ar message-id
contains the identifier for the transaction.
.It Ic tx-mail : Ar message-id Ar result address
This event is generated when client emits
.Dq MAIL FROM .
.Pp
.Ar message-id
contains the identifier for the transaction.
.Pp
.Ar result
contains
.Dq ok
if the sender was accepted,
.Dq permfail
if it was rejected
or
.Dq tempfail
if it was rejected for a transient error.
.Pp
.Ar address
contains the e-mail address of the sender.
The address is normalized and sanitized,
the characters
.Dq <
and
.Dq >
are removed,
along with any parameters to
.Dq MAIL FROM .
.It Ic tx-rcpt : Ar message-id Ar result address
This event is generated when client emits
.Dq RCPT TO .
.Pp
.Ar message-id
contains the identifier for the transaction.
.Pp
.Ar result
contains
.Dq ok
if the recipient was accepted,
.Dq permfail
if it was rejected
or
.Dq tempfail
if it was rejected for a transient error.
.Pp
.Ar address
contains the e-mail address of the recipient.
The address is normalized and sanitized,
the characters
.Dq <
and
.Dq >
are removed,
along with any parameters to
.Dq RCPT TO .
.It Ic tx-envelope : Ar message-id Ar envelope-id
This event is generated when an envelope is accepted.
.Pp
.Ar envelope-id
contains the unique identifier for the envelope.
.It Ic tx-data : Ar message-id Ar result
This event is generated when client has emitted
.Dq DATA .
.Pp
.Ar message-id
contains the unique identifier for the transaction.
.Pp
.Ar result
contains
.Dq ok
if server accepted the message for processing,
.Dq permfail
if it has not been accepted and
.Dq tempfail
if a transient error prevented message processing.
.It Ic tx-commit : Ar message-id Ar message-size
This event is generated when a transaction has been accepted by the server.
.Pp
.Ar message-id
contains the unique identifier for the SMTP transaction.
.Pp
.Ar message-size
contains the size of the message submitted in the
.Dq DATA
phase of the SMTP transaction.
.It Ic tx-rollback : Ar message-id
This event is generated when a transaction has been rejected by the server.
.Pp
.Ar message-id
contains the unique identifier for the SMTP transaction.
.It Ic protocol-client : Ar command
This event is generated for every command submitted by the client.
It contains the raw command as received by the server.
.Pp
.Ar command
contains the command emitted by the client to the server.
.It Ic protocol-server : Ar response
This event is generated for every response emitted by the server.
It contains the raw response as emitted by the server.
.Pp
.Ar response
contains the response emitted by the server to the client.
.It Ic filter-report : Ar filter-kind Ar name message
This event is generated when a filter emits a report.
.Pp
.Ar filter-kind may be either
.Dq builtin
or
.Dq proc
depending on if the filter is an
.Xr smtpd 8
builtin filter or a proc filter implementing the API.
.Pp
.Ar name
is the name of the filter that generated the report.
.Pp
.Ar message
is a filter-specific message.
.It Ic filter-response : Ar phase response Op param
This event is generated when a filter responds to a filtering request.
.Pp
.Ar phase
contains the phase name for the request.
The phases are documented in the next section.
.Pp
.Ar response
contains the response of the filter to the request,
it is either one of
.Dq proceed ,
.Dq report ,
.Dq reject ,
.Dq disconnect ,
.Dq junk
or
.Dq rewrite .
.Pp
If specified,
.Ar param
is the parameter to the response.
.It Ic timeout
This event is generated when a timeout happens for a session.
.El
.Sh FILTER REQUESTS
There is currently only one subsystem supported in the API:
smtp-in.
.Pp
Filter requests allow
.Xr smtpd 8
to query
.Nm
about what to do with a session at a particular phase.
In addition,
they allow
.Nm
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
.Xr sed 1
or
.Xr grep 1
would do.
.Pp
Each filter request is generated by
.Xr smtpd 8
as a single line similar to the one below.
Fields are separated by the
.Dq |
character.
.Bd -literal -offset indent
filter|0.7|1576146008.006099|smtp-in|connect|7641df9771b4ed00|1ef1c203cc576e5d|mail.openbsd.org|pass|199.185.178.25:33174|45.77.67.80:25
.Ed
.Pp
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:
.Bd -literal -offset indent
filter|0.7|1576146008.006099|smtp-in|connect|7641df9771b4ed00|1ef1c203cc576e5d
.Ed
.Pp
It is followed by a suffix containing the phase-specific parameters of the
filter request,
also separated by
.Dq | :
.Bd -literal -offset indent
mail.openbsd.org|pass|199.185.178.25:33174|45.77.67.80:25
.Ed
.Pp
Unlike with report events,
.Xr smtpd 8
expects answers from filter requests and will not allow a session to move
forward until the filter has instructed
.Xr smtpd 8
how to treat it.
.Pp
For all phases except
.Dq data-line ,
responses must follow the same construct:
a message of type
.Dq filter-result ,
followed by the unique session id,
the opaque token,
a decision and optional decision-specific parameters:
.Bd -literal -offset indent
filter-result|7641df9771b4ed00|1ef1c203cc576e5d|proceed
filter-result|7641df9771b4ed00|1ef1c203cc576e5d|reject|550 nope
.Ed
.Pp
The possible decisions for a
.Dq filter-result
message are documented below.
.Pp
For the
.Dq data-line
phase,
.Nm
are fed a stream of lines corresponding to the message to filter,
terminated by a single dot:
.Bd -literal -offset indent
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|.
.Ed
.Pp
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,
.Xr smtpd 8
assumes that the message consists of the output from
.Nm .
.Pp
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.
.Pp
The response to
.Dq data-line
requests use their own construct.
A
.Dq filter-dataline
prefix,
followed by the unique session identifier,
the opaque token and the output line as follows:
.Bd -literal -offset indent
filter-dataline|7641df9771b4ed00|1ef1c203cc576e5d|line 1
filter-dataline|7641df9771b4ed00|1ef1c203cc576e5d|line 2
filter-dataline|7641df9771b4ed00|1ef1c203cc576e5d|.
.Ed
.Pp
The list of events and event-specific parameters for smtp-in are as follows:
.Bl -tag -width Ds
.It Ic connect : Ar rdns fcrdns src dest
This request is emitted after connection,
before the banner is displayed.
.It Ic helo : Ar identity
This request is emitted after the client has emitted
.Dq HELO .
.It Ic ehlo : Ar identity
This request is emitted after the client has emitted
.Dq EHLO .
.It Ic starttls : Ar tls-string
This request is emitted after the client has requested
.Dq STARTTLS .
.It Ic auth : Ar auth
This request is emitted after the client has requested
.Dq AUTH .
.It Ic mail-from : Ar address
This request is emitted after the client has requested
.Dq MAIL FROM .
.It Ic rcpt-to : Ar address
This request is emitted after the client has requested
.Dq RCPT TO .
.It Ic data
This request is emitted after the client has requested
.Dq DATA .
.It Ic data-line : Ar line
This request is emitted for each line of input in the
.Dq DATA
phase.
The lines are raw dot-escaped SMTP DATA input,
terminated with a single dot.
.It Ic commit
This request is emitted after the final single dot is received.
.El
.Pp
For every filtering phase,
excepted
.Dq data-line ,
the following decisions may be taken by a filter:
.Bl -tag -width Ds
.It Ic proceed
No action is taken,
session or transaction may be passed to the next filter.
.It Ic junk
The session or transaction is marked as spam.
.Xr smtpd 8
will prepend an
.Dq X-Spam
header to the message.
.It Ic reject Ar error
The command is rejected with the message
.Ar error .
The message must be a valid SMTP message including status code,
5xx or 4xx.
.Pp
Messages starting with a 5xx status result in a permanent failure,
those starting with a 4xx status result in a temporary failure.
.Pp
Messages starting with a 421 status will result in a client disconnect.
.It Ic disconnect Ar error
The client is disconnected with the message
.Ar error .
The message must be a valid SMTP message including status code,
5xx or 4xx.
.Pp
Messages starting with a 5xx status result in a permanent failure,
those starting with a 4xx status result in a temporary failure.
.It Ic rewrite Ar parameter
The command parameter is rewritten.
.Pp
This decision allows a filter to perform a rewrite of client-submitted
commands before they are processed by the SMTP engine.
.Ar parameter
is expected to be a valid SMTP parameter for the command.
.It Ic report Ar parameter
Generates a report with
.Ar parameter
for this filter.
.El
.\".Sh EXAMPLES
.\"This example filter written in
.\".Xr sh 1
.\"will echo back...
.\".Bd -literal -offset indent
.\"XXX
.\".Ed
.\".Pp
.\"This example filter will filter...
.\".Bd -literal -offset indent
.\"XXX
.\".Ed
.\".Pp
.\"Note that libraries may provide a simpler interface to
.\".Nm
.\"that does not require implementing the protocol itself.
.\".Ed
.Sh SEE ALSO
.Xr smtpd 8
.Sh HISTORY
.Nm
first appeared in
.Ox 6.6 .