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diff --git a/implementation-notes/MILTER b/implementation-notes/MILTER new file mode 100644 index 0000000..f67fb90 --- /dev/null +++ b/implementation-notes/MILTER @@ -0,0 +1,254 @@ +Distribution of Milter responsibility +===================================== + +Milters look at the SMTP commands as well as the message content. +In Postfix these are handled by different processes: + +- smtpd(8) (the SMTP server) focuses on the SMTP commands, strips + the SMTP encapsulation, and passes envelope information and message + content to the cleanup server. + +- the cleanup(8) server parses the message content (it understands + headers, body, and MIME structure), and creates a queue file with + envelope and content information. The cleanup server adds additional + envelope records, such as when to send a "delayed mail" notice. + +If we want to support message modifications (add/delete recipient, +add/delete/replace header, replace body) then it pretty much has +to be implemented in the cleanup server, if we want to avoid extra +temporary files. + +Network versus local submission +=============================== + +As of Sendmail 8.12, all mail is received via SMTP, so all mail is +subject to Miltering (local submissions are queued in a submission +queue and then delivered via SMTP to the main MTA, or appended to +$HOME/dead.letter). In Postfix, local submissions are received by +the pickup server, which feeds the mail into the cleanup server +after doing basic sanity checks. + +How do we set up the Milters with SMTP mail versus local submissions? + +- SMTP mail: smtpd creates Milter contexts, and sends them, including + their sockets, to the cleanup server. The smtpd is responsible + for sending the Milter abort and close messages. Both smtpd and + cleanup are responsible for closing their Milter socket. Since + smtpd and cleanup inspect mail at different times, there is no + conflict with access to the Milter socket. + +- Local submission: the cleanup server creates Milter contexts. + The cleanup server provides dummy connect and helo information, + or perhaps none at all, and provides sender and recipient events. + The cleanup server is responsible for sending the Milter abort + and close messages, and for closing the Milter socket. + +A special case of local submission is "sendmail -t". This creates +a record stream in which recipients appear after content. However, +Milters expect to receive envelope information before content, not +after. This is not a problem: just like a queue manager, the +cleanup-side Milter client can jump around through the queue file +and send the information to the Milter in the expected order. + +Interaction with XCLIENT, "postsuper -r", and external content filters +====================================================================== + +Milter applications expect that the MTA supplies context information +in the form of Sendmail-like macros (j=hostname, {client_name}=the +SMTP client hostname, etc.). Not all these macros have a Postfix +equivalent. Postfix 2.3 makes a subset available. + +If Postfix does not implement a specific macro, people can usually +work around it. But we should avoid inconsistency. If Postfix can +make macro X available at Milter protocol stage Y, then it must +also be able to make that macro available at all later Milter +protocol stages, even when some of those stages are handled by a +different Postfix process. + +Thus, when adding Milter support for a specific Sendmail-like macro +to the SMTP server: + +- We may have to update the XCLIENT protocol, so that Milter + applications can be tested with XCLIENT. If not, then we must + prominently document everywhere that XCLIENT does not provide + 100% accurate simulation for Milters. An additional complication + is that the SMTP command length is limited, and that each XCLIENT + command resets the SMTP server to the 220 stage and generates + "connect" events for anvil(8) and for Milters. + +- The SMTP server has to send the corresponding attribute to the + cleanup server. The cleanup server then stores the attribute in + the queue file, so that Milters produce consistent results when + mail is re-queued with "postsuper -r". + +But wait, there is more. If mail is filtered by an external content +filter, then it needs to preserve all the Milter attributes so that +after "postsuper -r", Milters produce the exact same result as when +mail was received originally by Postfix. Specifically, after +"postsuper -r" a signing Milter must not sign mail that it did not +sign on the first pass through Postfix, and it must not reject mail +that it accepted on the first pass through Postfix. + +Instead of trying to re-create the Milter execution environment +after "postsuper -r" we simply disable Milter processing. The +rationale for this is: if mail was Miltered before it was written +to queue file, then there is no need to Milter it again. + +We might want to take a similar approach with external (signing or +blocking) content filters: don't filter mail that has already been +filtered, and don't filter mail that didn't need to be filtered. +Such mail can be recognized by the absence of a "content_filter" +record. To make the implementation efficient, the cleanup server +would have to record the presence of a "content_filter" record in +the queue file header. + +Message envelope or content modifications +========================================= + +Milters can send modification requests after receiving the end of +the message body. If we can implement all the header/body-related +Milter operations in the cleanup server, then we can try to edit +the queue file in place, without ever having to make a temporary +copy. Once a Milter is done editing, the queue file can be used as +input for the next Milter, and so on. Finally, the cleanup server +calls fsync() and waits for successful return. + +To implement in-place queue file edits, we need to introduce +surprisingly little change to the existing Postfix queue file +structure. All we need is a way to specify a jump from one place +in the file to another. + +Postfix does not store queue files as plain text files. Instead all +information is stored in records with an explicit type and length +for sender, recipient, arrival time, and so on. Even the content +that makes up the message header and body is stored as records with +an explicit type and length. This organization makes it very easy +to introduce pointer records, which is what we will use to jump +from one place in a queue file to another place. + +- Deleting a recipient or header record is easy - just mark the + record as killed. When deleting a recipient, we must kill all + recipient records that result from virtual alias expansion of the + original recipient address. When deleting a very long header or + body line, multiple queue file records may need to be killed. We + won't try to reuse the deleted space for other purposes. + +- Replacing header or body records involves pointer records. + Basically, a record is replaced by overwriting it with a forward + pointer to space after the end of the queue file, putting the new + record there, followed by a reverse pointer to the record that + follows the replaced information. If the replaced record is shorter + than a pointer record, we relocate the records that follow it to + the new area, until we have enough space for the forward pointer + record. See below for a discussion on what it takes to make this + safe. + + Postfix queue files are segmented. The first segment is for + envelope records, the second for message header and body content, + and the third segment is for information that was extracted or + generated from the message header and body content. Each segment + is terminated by a marker record. For now we don't want to change + their location. In particular, we want to avoid moving the start + of a segment. + + To ensure that we can always replace a header or body record by + a pointer record, without having to relocate a marker record, the + cleanup server always places a dummy pointer record at the end + of the headers and at the end of the body. + + When a Milter wants to replace an entire body, we have the option + to overwrite existing body records until we run out of space, and + then writing a pointer to space at the end of the queue file, + followed by the remainder of the body, and a pointer to the marker + that ends the message content segment. + +- Appending a recipient or header record involves pointer records + as well. This requires that the queue file already contains a + dummy pointer record at the place where we want to append recipient + or header content (Milters currently do not replace individual + body records, but we could add this if need be). To append, + change the dummy pointer into a forward pointer to space after + the end of a message, put the new record there, followed by a + reverse pointer to the record that follows the forward pointer. + + To append another record, replace the reverse pointer by a forward + pointer to space after the end of a message, put the new record + there, followed by the value of the reverse pointer that we + replace. Thus, there is no one-to-one correspondence between + forward and backward pointers! In fact, there can be multiple + forward pointers for one reverse pointer. + +When relocating a record we must not relocate the target of a jump +================================================================== + +As discussed above, when replacing an existing record, we overwrite +it with a forward pointer to the new information. If the old record +is too small we relocate one or more records that follow the record +that's being replaced, until we have enough space for the forward +pointer record. + +Now we have to become really careful. Could we end up relocating a +record that is the target of a forward or reverse pointer, and thus +corrupt the queue file? The answer is NO. + +- We never relocate end-of-segment marker records. Instead, the + cleanup server writes dummy pointer records to guarantee that + there is always space for a pointer. + +- When a record is the target of a forward pointer, it is "edited" + information that is preceded either by the end-of-queue-file + marker record, or it is preceded by the reverse pointer at the + end of earlier written "edited" information. Thus, the target of + a forward pointer will not be relocated to make space for a pointer + record. + +- When a record is the target of a reverse pointer, it is always + preceded by a forward pointer record (or by a forward pointer + record followed by some unused space). Thus, the target of a + reverse pointer will not be relocated to make space for a pointer + record. + +Could we end up relocating a pointer record? Yes, but that is OK, +as long as pointers contain absolute offsets. + +Pointer records introduce the possibility of loops +================================================== + +When a queue file is damaged, a bogus pointer value may send Postfix +into a loop. This must not happen. + +Detecting loops is not trivial: + +- A sequence of multiple forward pointers may be followed by one + legitimate reverse pointer to the location after the first forward + pointer. See above for a discussion of how to append a record to + an appended record. + +- We do know, however, that there will not be more reverse pointers + than forward pointers. But this does not help much. + +Perhaps we can include a record count at the start of the queue +file, so that the record walking code knows that it's looking at +some records more than once, and return an error indication. + +How many bytes do we need for a pointer record? +=============================================== + +A pointer record would look like this: + + type (1 byte) + offset (see below) + +Postfix uses long for queue file size/offset information, and stores +them as %15ld in the SIZE record at the start of the queue file. +This is somewhat less than a 64-bit long, but it is enough for a +some time to come, and it is easily changed without breaking forward +or backward compatibility. + +It does mean, however, that a pointer record can easily exceed the +length of a header record. This is why we go through the trouble +of record relocation and dummy records. + +In Postfix 2.4 we fixed this by adding padding to short message +header records so that we can always write a pointer record over a +message header. This immensly simplifies the code. |