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NAME | DESCRIPTION | DESIGN | REPORT AND FILTER | PROTOCOL | CONFIGURATION | REPORT EVENTS | FILTER REQUESTS | SEE ALSO | HISTORY | COLOPHON |
FILTERS(7) BSD Miscellaneous Information Manual FILTERS(7)
filters — filtering API for the smtpd(8) daemon
The smtpd(8) daemon provides a Simple Mail Transfer Protocol (SMTP)
implementation that allows an ordinary machine to become Mail eXchang‐
ers (MX). Many features that are commonly used by MX, such as delivery
reporting or Spam filtering, are outside the scope of SMTP and too com‐
plex 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.
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).
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 informa‐
tion, but because a reponse is expected by smtpd(8), this is more
costly than using report events. The correct pattern for writing fil‐
ters 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 cor‐
rect, is when a decision is required for the filter request and there
is no need for more information than that of the event.
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, expect‐
ing 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.
During the initial handshake, smtpd(8) will emit a serie of configura‐
tion 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
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 tran‐
sient 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 tran‐
sient 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 transac‐
tion.
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 transac‐
tion.
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.
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 cor‐
responding 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, termi‐
nated 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 deci‐
sions 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 fail‐
ure, 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 fail‐
ure, 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)
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
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BSD April 25, 2020 BSD