# Milter API

## Contents

- Library Control Functions
- Data Access Functions
- Message Modification Functions
- Callbacks
- Miscellaneous

## Library Control Functions

Before handing control to libmilter (by calling smfi_main), a filter may call the following functions to set libmilter parameters. In particular, the filter must call smfi_register to register its callbacks. Each function will return either MI_SUCCESS or MI_FAILURE to indicate the status of the operation.

None of these functions communicate with the MTA. All alter the library's state, some of which is communicated to the MTA inside smfi_main.

Function | Description
--- | ---
smfi_opensocket | Try to create the interface socket.
smfi_register | Register a filter.
smfi_setconn | Specify socket to use.
smfi_settimeout | Set timeout.
smfi_setbacklog | Define the incoming `listen(2)` queue size.
smfi_setdbg | Set the milter library debugging (tracing) level.
smfi_stop | Cause an orderly shutdown.
smfi_main | Hand control to libmilter.

## Data Access Functions

The following functions may be called from within the filter-defined callbacks to access information about the current connection or message.

Function | Description
--- | ---
smfi_getsymval | Return the value of a symbol.
smfi_getpriv | Get the private data pointer.
smfi_setpriv | Set the private data pointer.
smfi_setreply | Set the specific reply code to be used.
smfi_setmlreply | Set the specific multi-line reply to be used.

## Message Modification Functions

The following functions change a message's contents and attributes. They may only be called in xxfi_eom. All of these functions may invoke additional communication with the MTA. They will return either MI_SUCCESS or MI_FAILURE to indicate the status of the operation. Message data (senders, recipients, headers, body chunks) passed to these functions via parameters is copied and does not need to be preserved (i.e., allocated memory can be freed).

A filter which might call a message modification function must set the appropriate flag (listed below), either in the description passed to smfi_register or via xxfi_negotiate. Failure to do so will cause the MTA to treat a call to the function as a failure of the filter, terminating its connection.

Note that the status returned indicates only whether or not the filter's message was successfully sent to the MTA, not whether or not the MTA performed the requested operation. For example, smfi_addheader, when called with an illegal header name, will return MI_SUCCESS even though the MTA may later refuse to add the illegal header.

Function | Description | SMFIF_* flag
--- | --- | ---
smfi_addheader | Add a header to the message. | SMFIF_ADDHDRS
smfi_chgheader | Change or delete a header. | SMFIF_CHGHDRS
smfi_insheader | Insert a header into the message. | SMFIF_ADDHDRS
smfi_chgfrom | Change the envelope sender address. | SMFIF_CHGFROM
smfi_addrcpt | Add a recipient to the envelope. | SMFIF_ADDRCPT
smfi_addrcpt_par | Add a recipient including ESMTP parameter to the envelope. | SMFIF_ADDRCPT_PAR
smfi_delrcpt | Delete a recipient from the envelope. | SMFIF_DELRCPT
smfi_replacebody | Replace the body of the message. | SMFIF_CHGBODY

## Other Message Handling Functions

The following functions provide special case handling instructions for milter or the MTA, without altering the content or status of the message. They too may only be called in xxfi_eom. All of these functions may invoke additional communication with the MTA. They will return either MI_SUCCESS or MI_FAILURE to indicate the status of the operation.

Note that the status returned indicates only whether or not the filter's message was successfully sent to the MTA, not whether or not the MTA performed the requested operation.

Function | Description
--- | ---
smfi_progress | Report operation in progress.
smfi_quarantine | Quarantine a message.

## Callbacks

The filter should implement one or more of the following callbacks, which are registered via smfi_register:

Function | Description
--- | ---
xxfi_connect | connection info
xxfi_helo | SMTP HELO/EHLO command
xxfi_envfrom | envelope sender
xxfi_envrcpt | envelope recipient
xxfi_data | DATA command
xxfi_unknown | Unknown SMTP command
xxfi_header | header
xxfi_eoh | end of header
xxfi_body | body block
xxfi_eom | end of message
xxfi_abort | message aborted
xxfi_close | connection cleanup
xxfi_negotiate | option negotiation

The above callbacks should all return one of the following return values, having the indicated meanings. Any return other than one of the below values constitutes an error, and will cause sendmail to terminate its connection to the offending filter.

Milter distinguishes between recipient-, message-, and connection-oriented routines. Recipient-oriented callbacks may affect the processing of a single message recipient; message-oriented callbacks, a single message; connection-oriented callbacks, an entire connection (during which multiple messages may be delivered to multiple sets of recipients). xxfi_envrcpt is recipient-oriented. xxfi_negotiate, xxfi_connect, xxfi_helo and xxfi_close are connection-oriented. All other callbacks are message-oriented.

Return value | Description
--- | ---
SMFIS_CONTINUE | Continue processing the current connection, message, or recipient.
SMFIS_REJECT | For a connection-oriented routine, reject this connection; call xxfi_close. For a message-oriented routine (except xxfi_abort), reject this message. For a recipient-oriented routine, reject the current recipient (but continue processing the current message).
SMFIS_DISCARD | For a message- or recipient-oriented routine, accept this message, but silently discard it. SMFIS_DISCARD should not be returned by a connection-oriented routine.
SMFIS_ACCEPT | For a connection-oriented routine, accept this connection without further filter processing; call xxfi_close. For a message- or recipient-oriented routine, accept this message without further filtering.
SMFIS_TEMPFAIL | Return a temporary failure, i.e., the corresponding SMTP command will return an appropriate 4xx status code. For a message-oriented routine (except xxfi_envfrom), fail for this message. For a connection-oriented routine, fail for this connection; call xxfi_close. For a recipient-oriented routine, only fail for the current recipient; continue message processing.
SMFIS_SKIP | Skip further callbacks of the same type in this transaction. Currently this return value is only allowed in xxfi_body(). It can be used if a milter has received sufficiently many body chunks to make a decision, but still wants to invoke message modification functions that are only allowed to be called from xxfi_eom(). Note: the milter must negotiate this behavior with the MTA, i.e., it must check whether the protocol action `SMFIP_SKIP` is available and if so, the milter must request it.
SMFIS_NOREPLY | Do not send a reply back to the MTA. The milter must negotiate this behavior with the MTA, i.e., it must check whether the appropriate protocol action `SMFIP_NR_*` is available and if so, the milter must request it. If you set the `SMFIP_NR_*` protocol action for a callback, that callback must always reply with SMFIS_NOREPLY. Using any other reply code is a violation of the API. If in some cases your callback may return another value (e.g., due to some resource shortages), then you must not set `SMFIP_NR_*` and you must use SMFIS_CONTINUE as the default return code. (Alternatively you can try to delay reporting the problem to a later callback for which `SMFIP_NR_*` is not set.)

## Miscellaneous

Function | Description
--- | ---
smfi_version | libmilter (runtime) version info
smfi_setsymlist | Set the list of macros that the milter wants to receive from the MTA for a protocol stage.

Constant | Description
--- | ---
SMFI_VERSION | libmilter (compile time) version info

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