mimedefang-protocol (7) - Linux Manuals
mimedefang-protocol: Conventions used by mimedefang(8) to
NAME
mimedefang-protocol - Conventions used by
mimedefang(8) to communicate with filter programs.DESCRIPTION
mimedefang(8) and mimedefang-multiplexor(8) provide a simplified mechanism for hooking scripts and programs into Sendmail's milter API.The milter API is multi-threaded and written in C; mimedefang lets you write single-threaded filters written in the language of your choice. Some of the flexibility and speed of milter is sacrificed, but the ease of writing filters more than compensates for this slight loss.
This manual describes how mimedefang communicates with the filter program, and gives you enough information to write your own filter program as a replacement for mimedefang.pl if you wish.
PROTOCOL OVERVIEW
The protocol is a simple file-based protocol. For each invocation of a filter, mimedefang creates a unique working directory and populates it with files. It calls the filter, which is expected to populate the working directory with more files, which communicate the scan results back to mimedefang. This simple mechanism allows you to easily write filters in scripting languages without worrying about C-level details.
FILTER INVOCATION
The filter program may be invoked in one of five ways:
- filter_prog directory
-
If the program is invoked with a single argument which is an absolute
path name (called the working directory, the program is expected
to perform filtering in that directory and then exit.
- filter_prog -server
-
If the program is invoked with the single argument -server, it
is expected to run as a server. See SERVER MODE for details.
- filter_prog -serveru
-
If the program is invoked with the single argument -serveru, it
is expected to run as a server. In addition, anything it prints to
file descriptor 3 is used to update the "slave status" field in the
multiplexor. This lets the filter inform administrators exactly what
it is doing. (See the -Z option to mimedefang-multiplexor.)
- filter_prog -embserver
-
Similar to -server, but used by the embedded Perl code.
The program should run any initialization routines and then exit.
The multiplexor will subsequently call the Perl routine
do_main_loop when it is time for the slave to begin running
in server mode.
- filter_prog -embserveru
-
Similar to -embserver with the additional magic of updating
the slave status from data written to file descriptor 3.
INITIAL FILE LAYOUT
When the filter begins a scan, it should change directories to the working directory. In that directory, it will find the following files.
- INPUTMSG
-
A file containing the complete input e-mail message, including headers.
- HEADERS
-
A file containing just the headers, one per line. Headers which are
continued over several lines in the original message are collapsed
into a single line in this file.
- COMMANDS
-
A file containing a list of commands. Each command is a single
letter and may be followed by arguments. Each command is on its
own line.
THE COMMANDS FILE
All commands have their arguments encoded as follows: All characters outside the range 33 to 126 ASCII, as well as the characters "%", "\", "'" and double-quote, are replaced by a percent sign followed by two hex digits specifying the character's numerical value. The filter must un-escape the arguments when it reads the COMMANDS file.
The commands from the C to Perl filters are:
- Ssender
-
The sender of the message.
- sesmtp_arg
-
An ESMTP argument associated with the sender (such as SIZE=54432). There
is one s line for each ESMTP argument.
- Usubject
-
The message subject.
- Xmessage_id
-
The Message-ID.
- Rrecipient mailer host addr
-
A recipient. There is one R line for each recipient. The
mailer, host and addr parts of the line are the
values of the Sendmail {rcpt_mailer}, {rcpt_host} and {rcpt_addr}
macros if they are available, or "?" if not.
- resmtp_arg
-
An ESMTP argument associated with the most recent recipient
(such as NOTIFY=never). There is one r line for each SMTP argument.
- !
-
If this command is present, there are suspicious characters in the message
headers.
- ?
-
If this command is present, there are suspicious characters in the message
body.
- Ihost_addr
-
The SMTP relay host's IP address in dotted-quad notation.
- iidentifier
-
An identifier generated by MIMEDefang. On a given host, this identifier
is very likely to be unique over a timespan of about 24 years.
- Jhost_addr
-
The "real" SMTP relay host's IP address in dotted-quad notation.
Multi-stage MIMEDefang relays can use a special IP validation
header so that even the innermost MIMEDefang relay can see the
"original" relay's IP address.
- Hhost_name
-
The SMTP relay host name.
- Eargument
-
The argument to the SMTP "EHLO" or "HELO" command.
- Qqid
-
The message's Sendmail queue-ID.
- =macro val
-
Set the value of the specified Sendmail macro to val. Both
macro and val are percent-encoded, but the single space
character between them is not.
FILTER OPERATION
When the filter performs a scan, it can make use of all the information in the files mentioned previously. If the filter needs temporary working files, it should create a subdirectory under the working directory for its own use. In this case, you do not have to clean up your working files, because mimedefang deletes the working directory when the filter returns.
FINAL FILE LAYOUT
The filter communicates the results of the scan back to mimedefang by creating additional files in the working directory. The most important file is called RESULTS, and it contains a list of one-letter, one-line commands back to the filter. As usual, command arguments are percent-escaped. The commands from the filter back to mimedefang are:
- Bcode dsn reply_text
-
Bounce (reject) the message with the specified SMTP reply code, DSN code
and reply text.
- D
-
Silently discard the message and pretend it was delivered.
- Tcode dsn reply_text
-
Return an SMTP temporary failure code with the specified SMTP code, DSN
and reply text.
- C
-
Replace the message body. If this command is present, the file
NEWBODY must contain the new message body.
- Mheader_val
-
Replace the MIME Content-Type header with a new value. Used to change
MIME boundaries or convert non-MIME to MIME messages.
- Hheader val
-
Add a new header header with value val. The header
should not contain a colon. Each of header and val is
percent-escaped, but the single space between them is not.
- Nheader index val
-
Adds a new header header with value val in position
index. An index of zero specifies that the new header should
be prepended before all existing headers.
- Iheader index val
-
Replace the index'th occurrence of header with value
val. The index is 1-based. The header should not
contain a colon. Each of header, index and val is
percent-escaped, but the single space between them is not.
- Jheader index
-
Delete the index'th occurrence of header.
- Rrecip
-
Add a new recipient recip to the message.
- Srecip
-
Delete recip from the list of message recipients.
- fsender
-
Change the envelope sender to sender. This is only supported by
Sendmail 8.14.0 and higher.
- F
-
Indicate that we have finished issuing commands. Anything after
an F line is ignored.
SERVER MODE
In server mode, mimedefang-multiplexor runs the filter program continuously in a server loop. The filter program reads commands from standard input, and writes results to standard output. The filter program must exit shortly after it sees EOF on its standard input. If it does not exit within 10 seconds, it will be terminated with SIGTERM. If that still does not work, then after a further 10 seconds, it is killed with SIGKILL.
- SERVER COMMANDS
-
All server commands are single line commands. Each command is followed by a space-separated list of arguments; each argument is percent-encoded. The commands defined are:
- ping
-
Elicits a reply of "PONG" from the server.
- scan queue_id dir
-
Run a scan for the mail identiefied by the Sendmail queue-ID queue_id
in the directory dir. The command is terminated with a newline.
The server must write a newline-terminated "ok" if the scan completed
successfully, or "error: msg" if something went wrong.
- relayok ip_addr hostname client_port daemon_ip daemon_port
-
Test whether or not to accept a connection from the specified host.
The server must write "ok 1" if we will accept the connection, or "ok
0 error_message code dsn" if not. It can indicate a temporary failure by
writing "ok -1 error_message code dsn". Note that even if the connection is
accepted, a later scan can still reject the message based on other
criteria. "ip_addr" is the IP address of the relay and "hostname" is the
hostname (if it could be determined; otherwise, the IP address in
square brackets).
- helook ip_addr hostname helo_string client_port daemon_ip daemon_port
-
Test whether or not to accept the HELO/EHLO command. The
server must write "ok 1" if we will accept the mail attempt, or "ok 0
error_message code dsn" if not. "ok -1 error_message code dsn"
indicates a temporary failure.
helo_string is the argument to the HELO/EHLO command. The remaining
arguments have the same meaning as in relayok.
- senderok sender_addr ip_addr hostname helo_string dir queue_id [esmtp_args...]
-
Test whether or not to accept mail from the specified sender. The server must write "ok 1" if we will accept the mail attempt, or "ok 0 error_message code dsn" if not. "ok -1 error_message code dsn" indicates a temporary failure. Note that even if the sender is accepted, a later scan can still reject the message based on other criteria. "sender_addr" is the sender's e-mail address. The "ip_addr" and "hostname" arguments are as in relayok. "helo_string" is the argument to the SMTP HELO/EHLO command. "dir" is the MIMEDefang spool directory, and "queue_id" is the Sendmail queue identifier.
The optional "esmtp_args" are space-separated, percent-encoded ESMTP arguments supplied with the MAIL FROM: command.
- recipok recip_addr sender_addr ip_addr hostname first_recip helo_string dir queue_id [esmtp_args...]
-
Test whether or not to accept mail for the specified recipient. The
server must write "ok 1" if we will accept it, or "ok 0 error_message
code dsn" if not. ok -1 error_message code dsn" indicates a temporary-failure.
"recip_addr" is the argument to the RCPT TO: command, and "first_recip" is the argument to the first RCPT TO: command for this message. Other arguments are as in senderok.
- map map_name key
-
If you are using a map socket (the -N option to mimedefang-multiplexor), then the server should look up the key key in the map
map_name. The server should print a single line to standard output. The
first word on the line should be one of OK, NOTFOUND, TEMP, TIMEOUT or PERM,
indicating a successful lookup, absence of the key, a temporary failure,
a timeout or a permanent failure, respectively. This should be followed
by a space and a percent-encoded string representing the value of the key
(if it was found) or an optional error message (if something went wrong.)
- tick band
-
The filter should run filter_tick with the specified
band argument. It should print a single line to standard output:
tock band
- Additional Commands
-
The filter can define a function filter_unknown_cmd that
can extend the list of server commands. If you do this, make sure
all of your commands start with an upper-case letter to avoid
conflicts if more built-in commands are defined in the future.
- SERVER REPLIES
-
The reply codes are:
- ok [return_code] [parameters]
-
The operation completed successfully. Some operations have an
associated return code, and possibly other parameters as well.
See the source code for the gory details.
- error: Message
-
The operation failed. Message may give additional details.
In server mode, you should not write anything to standard output except reply codes, or the multiplexor will become confused. You should not terminate the program in server mode; simply echo an error: reply and return to the server loop.
When you send a reply code back to the multiplexor, be sure to terminate it with a newline, and to flush standard output. If your program uses the Standard I/O library, standard output may not be flushed immediately, and mimedefang-multiplexor will wait forever for the filter's reply, and eventually kill the filter on the assumption it has hung up.
In server mode, if the filter program receives a SIGINT signal, it must terminate. This is used by mimedefang-multiplexor to terminate slaves after they have processed a given number of e-mail messages.