# Milter API

## Contents

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

## 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_register | Register a filter.
smfi_setconn | Specify socket to use.
smfi_settimeout | Set timeout.
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.

## 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.

A filter must have set the appropriate flag (listed below) in the description passed to smfi_register to call any message modification function. 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_addrcpt | Add a recipient to the envelope. | SMFIF_ADDRCPT
smfi_delrcpt | Delete a recipient from the envelope. | SMFIF_DELRCPT
smfi_replacebody | Replace the body of the message. | SMFIF_CHGBODY

## 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_header | header
xxfi_eoh | end of header
xxfi_body | body block
xxfi_eom | end of message
xxfi_abort | message aborted
xxfi_close | connection cleanup

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_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_eom or 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.

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