
     _________________________________________________________________
                                      
                         Design Notes On Fetchmail
                                      
   These notes are for the benefit of future hackers and maintainers. The
   following sections are both functional and narrative, read from
   beginning to end.
   
                                    History
                                       
   A direct ancestor of the fetchmail program was originally authored
   (under the name popclient) by Carl Harris . I took over development in
   June 1996 and subsequently renamed the program `fetchmail' to reflect
   the addition of IMAP support. In early November 1996 Carl officially
   ended support for the last popclient versions.
   
   Before accepting responsibility for the popclient sources from Carl, I
   had investigated and used and tinkered with every other UNIX
   remote-mail forwarder I could find, including fetchpop1.9,
   PopTart-0.9.3, get-mail, gwpop, pimp-1.0, pop-perl5-1.2, popc,
   popmail-1.6 and upop. My major goal was to get a header-rewrite
   feature like fetchmail's working so I wouldn't have reply problems
   anymore.
   
   Despite having done a good bit of work on fetchpop1.9, when I found
   popclient I quickly concluded that it offered the solidest base for
   future development. I was convinced of this primarily by the presence
   of multiple-protocol support. The competition didn't do
   POP2/RPOP/APOP, and I was already having vague thoughts of maybe
   adding IMAP. (This would advance two other goals: learn IMAP and get
   comfortable writing TCP/IP client software.)
   
   Until popclient 3.05 I was simply following out the implications of
   Carl's basic design. He already had daemon.c in the distribution, and
   I wanted daemon mode almost as badly as I wanted the header rewrite
   feature. The other things I added were bug fixes or minor extensions.
   
   After 3.1, when I put in SMTP-forwarding support (more about this
   below) the nature of the project changed -- it became a
   carefully-thought-out attempt to render obsolete every other program
   in its class. The name change quickly followed.
   
                              The rewrite option
                                       
   RFC 1123 stipulates that MTAs ought to canonicalize the addresses of
   outgoing mail so that From:, To:, Cc:, Bcc: and other address headers
   contain only fully qualified domain names. Failure to do so can break
   the reply function on many mailers.
   
   This problem only becomes obvious when a reply is generated on a
   machine different from where the message was delivered. The two
   machines will have different local username spaces, potentially
   leading to misrouted mail.
   
   Most MTAs (and sendmail in particular) do not canonicalize address
   headers in this way (violating RFC 1123). Fetchmail therefore has to
   do it. This is the first feature I added to the ancestral popclient.
   
                                Reorganization
                                       
   The second thing I did reorganize and simplify popclient a lot. Carl
   Harris's implementation was very sound, but exhibited a kind of
   unnecessary complexity common to many C programmers. He treated the
   code as central and the data structures as support for the code. As a
   result, the code was beautiful but the data structure design ad-hoc
   and rather ugly (at least to this old LISP hacker).
   
   I was able to improve matters significantly by reorganizing most of
   the program around the `query' data structure and eliminating a bunch
   of global context. This especially simplified the main sequence in
   fetchmail.c and was critical in enabling the daemon mode changes.
   
                       IMAP support and the method table
                                       
   The next step was IMAP support. I initially wrote the IMAP code as a
   generic query driver and a method table. The idea was to have all the
   protocol-independent setup logic and flow of control in the driver,
   and the protocol-specific stuff in the method table.
   
   Once this worked, I rewrote the POP3 code to use the same
   organization. The POP2 code kept its own driver for a couple more
   releases, until I found sources of a POP2 server to test against (the
   breed seems to be nearly extinct).
   
   The purpose of this reorganization, of course, is to trivialize the
   development of support for future protocols as much as possible. All
   mail-retrieval protocols have to have pretty similar logical design by
   the nature of the task. By abstracting out that common logic and its
   interface to the rest of the program, both the common and
   protocol-specific parts become easier to understand.
   
   Furthermore, many kinds of new features can instantly be supported
   across all protocols by modifying the one driver module.
   
                        Implications of smtp forwarding
                                       
   The direction of the project changed radically when Harry Hochheiser
   sent me his scratch code for forwarding fetched mail to the SMTP port.
   I realized almost immediately that a reliable implementation of this
   feature would make all the other delivery modes obsolete.
   
   Why mess with all the complexity of configuring an MDA or setting up
   lock-and-append on a mailbox when port 25 is guaranteed to be there on
   any platform with TCP/IP support in the first place? Especially when
   this means retrieved mail is guaranteed to look like normal sender-
   initiated SMTP mail, which is really what we want anyway.
   
   Clearly, the right thing to do was (1) hack SMTP forwarding support
   into the generic driver, (2) make it the default mode, and (3)
   eventually throw out all the other delivery modes.
   
   I hesitated over step 3 for some time, fearing to upset long-time
   popclient users dependent on the alternate delivery mechanisms. In
   theory, they could immediately switch to .forward files or their
   non-sendmail equivalents to get the same effects. In practice the
   transition might have been messy.
   
   But when I did it (see the NEWS note on the great options massacre)
   the benefits proved huge. The cruftiest parts of the driver code
   vanished. Configuration got radically simpler -- no more grovelling
   around for the system MDA and user's mailbox, no more worries about
   whether the underlying OS supports file locking.
   
   Also, the only way to lose mail vanished. If you specified localfolder
   and the disk got full, your mail got lost. This can't happen with SMTP
   forwarding because your SMTP listener won't return OK unless the
   message can be spooled or processed.
   
   Also, performance improved (though not so you'd notice it in a single
   run). Another not insignificant benefit of this change was that the
   manual page got a lot simpler.
   
   Later, I had to bring --mda back in order to allow handling of some
   obscure situations involving dynamic SLIP. But I found a much simpler
   way to do it.
   
   The moral? Don't hesitate to throw away superannuated features when
   you can do it without loss of effectiveness. I tanked a couple I'd
   added myself and have no regrets at all. As Saint-Exupery said,
   "Perfection [in design] is achieved not when there is nothing more to
   add, but rather when there is nothing more to take away." This program
   isn't perfect, but it's trying.
   
        The most-requested features that I will never add, and why not:
                                       
1. Password encryption in .fetchmailrc

   The reason there's no facility to store passwords encrypted in the
   .fetchmailrc file is because this doesn't actually add protection.
   
   Anyone who's acquired the 0600 permissions needed to read your
   .fetchmailrc file will be able to run fetchmail as you anyway -- and
   if it's your password they're after, they'd be able to rip the
   necessary decoder out of the fetchmail code itself to get it.
   
   All .fetchmailrc encryption would do is give a false sense of security
   to people who don't think very hard.
   
2. Truly concurrent queries to multiple hosts

   Occasionally I get a request for this on "efficiency" grounds. These
   people aren't thinking either. True concurrency would do nothing to
   lessen fetchmail's total IP volume. The best it could possibly do is
   change the usage profile to shorten the duration of the active part of
   a poll cycle at the cost of increasing its demand on IP volume per
   unit time.
   
   If one could thread the protocol code so that fetchmail didn't block
   on waiting for a protocol response, but rather switched to trying to
   process another host query, one might get an efficiency gain (close to
   constant loading at the single-host level).
   
   Fortunately, I've only seldom seen a server that incurred significant
   wait time on an individual response. I judge the gain from this not
   worth the hideous complexity increase it would require in the code.
   
                         Multidrop and alias handling
                                       
   I decided to add the multidrop support partly because some users were
   clamoring for it, but mostly because I thought it would shake bugs out
   of the single-drop code by forcing me to deal with addressing in full
   generality. And so it proved.
   
   There are two important aspects of the features for handling
   multiple-drop aliases and mailing lists which future hackers should be
   careful to preserve.
   
    1. The logic path for single-recipient mailboxes doesn't involve
       header parsing or DNS lookups at all. This is important -- it
       means the code for the most common case can be much simpler and
       more robust.
    2. The multidrop handing does _not_ rely on doing the equivalent of
       passing the message to sendmail -oem -t. Instead, it explicitly
       mines members of a specified set of local usernames out of the
       header.
    3. We do _not_ attempt delivery to multidrop mailboxes in the
       presence of DNS errors. Before each multidrop poll we probe DNS to
       see if we have a nameserver handy. If not, the poll is skipped. If
       DNS crashes during a poll, the error return from the next
       nameserver lookup aborts message delivery and ends the poll. The
       daemon mode will then quietly spin until DNS comes up again, at
       which point it will resume delivering mail.
       
   When I designed this support, I was terrified of doing anything that
   could conceivably cause a mail loop (you should be too). That's why
   the code as written can only append _local_ names (never @-addresses)
   to the recipients list.
   
   The code in mxget.c is nasty, no two ways about it. But it's utterly
   necessary, there are a lot of MX pointers out there. It really ought
   to be a (documented!) entry point in the bind library.
   
                              DNS error handling
                                       
   Fetchmail's behavior on DNS errors is to suppress forwarding and
   deletion of the individual message that each occurs in, leaving it
   queued on the server for retrieval on a subsequent poll. The
   assumption is that DNS errors are transient, due to temporary server
   outages.
   
   Unfortunately this means that if a DNS error is permanent a message
   can be perpetually stuck in the server mailbox. We've had a couple bug
   reports of this kind due to subtle RFC822 parsing errors in the
   fetchmail code that resulted in impossible things getting passed to
   the DNS lookup routines.
   
   Alternative ways to handle the problem: ignore DNS errors (treating
   them as a non-match on the mailserver domain), or forward messages
   with errors to fetchmail's invoking user in addition to any other
   recipients. These would fit an assumption that DNS lookup errors are
   likely to be permanent problems associated with an address.
   
                                Lessons learned
                                       
  1. Server-side state is essential
  
   The person(s) responsible for removing LAST from POP3 deserve to
   suffer. Without it, a client has no way to know which messages in a
   box have been read by other means, such as an MUA running on the
   server.
   
   The POP3 UID feature described in RFC1725 to replace LAST is
   insufficient. The only problem it solves is tracking which messages
   have been read _by this client_ -- and even that requires tricky,
   fragile implementation.
   
   The underlying lesson is that maintaining accessible server-side
   `seen' state bits associated with Status headers is indispensible in a
   Unix/RFC822 mail server protocol. IMAP gets this right.
   
  2. Readable text protocol transactions are a Good Thing
  
   A nice thing about the general class of text-based protocols that
   SMTP, POP2, POP3, and IMAP belongs to is that client/server
   transactions are easy to watch and transaction code correspondingly
   easy to debug. Given a decent layer of socket utility functions (which
   Carl provided) it's easy to write protocol engines and not hard to
   show that they're working correctly.
   
   This is an advantage not to be despised! Because of it, this project
   has been interesting and fun -- no serious or persistent bugs, no long
   hours spent looking for subtle pathologies.
   
  3. IMAP is a Good Thing.
  
   If there were a standard IMAP equivalent of the POP3 APOP validation,
   POP3 would be completely obsolete.
   
  4. SMTP is the Right Thing
  
   In retrospect it seems clear that this program (and others like it)
   should have been designed to forward via SMTP from the beginning. This
   lesson may be applicable to other Unix programs that now call the
   local MDA/MTA as a program.
   
  5. Syntactic noise can be your friend
  
   The optional `noise' keywords in the rc file syntax started out as a
   late-night experiment. The English-like syntax they allow is
   considerably more readable than the traditional terse keyword-value
   pairs you get when you strip them all out. I think there may be a
   wider lesson here.
   
                           Motivation and validation
                                       
   It is truly written: the best hacks start out as personal solutions to
   the author's everyday problems, and spread because the problem turns
   out to be typical for a large class of users. So it was with Carl
   Harris and the ancestral popclient, and so with me and fetchmail.
   
   It's gratifying that fetchmail has become so popular. Until just
   before 1.9 I was designing strictly to my own taste. The multi-drop
   mailbox support and the new --limit option were the first features to
   go in that I didn't need myself.
   
   By 1.9, four months after I started hacking on popclient and a month
   after the first fetchmail release, there were literally a hundred
   people on the fetchmail-friends contact list. That's pretty powerful
   motivation. And they were a good crowd, too, sending fixes and
   intelligent bug reports in volume. A user population like that is a
   gift from the gods, and this is my expression of gratitude.
   
   The beta testers didn't know it at the time, but they were also the
   subjects of a sociological experiment. The results are described in my
   paper, _The Cathedral And The Bazaar_, available on the Fetchmail home
   page.
   
                                    Credits
                                       
   Special thanks go to Carl Harris, who built a good solid code base and
   then tolerated me hacking it out of recognition. And to Harry
   Hochheiser, who gave me the idea of the SMTP-forwarding delivery mode.
   
   Other significant contributors to the code have included Dave
   Bodenstab (error.c code and --syslog), George Sipe (--monitor and
   --interface), Gordon Matzigkeit (netrc.c), Al Longyear (UIDL support),
   and Nalin Dahyabhai (Kerberos V4 support).
   
                                  Conclusion
                                       
   At this point, the fetchmail code appears to be pretty stable. It will
   probably undergo substantial change only if and when support for a new
   retrieval protocol or authentication method is added.
   
                                 Relevant RFCS
                                       
   Not all of these describe standards explicitly used in fetchmail, but
   they all shaped the design in one way or another.
   
   RFC821
          SMTP protocol
          
   RFC822
          Mail header format
          
   RFC937
          Post Office Protocol - Version 2
          
   RFC974
          MX routing
          
   RFC976
          UUCP mail format
          
   RFC1081
          Post Office Protocol - Version 3
          
   RFC1123
          Host requirements (modifies 821, 822, and 974)
          
   RFC1176
          Interactive Mail Access Protocol - Version 2
          
   RFC1203
          Interactive Mail Access Protocol - Version 3
          
   RFC1225
          Post Office Protocol - Version 3
          
   RFC1344
          Implications of MIME for Internet Mail Gateways
          
   RFC1413
          Identification server
          
   RFC1428
          Transition of Internet Mail from Just-Send-8 to 8-bit SMTP/MIME
          
   RFC1460
          Post Office Protocol - Version 3
          
   RFC1521
          MIME: Multipurpose Internet Mail Extensions
          
   RFC1869
          SMTP Service Extensions (ESMTP spec)
          
   RFC1652
          SMTP Service Extension for 8bit-MIMEtransport
          
   RFC1725
          Post Office Protocol - Version 3
          
   RFC1730
          Interactive Mail Access Protocol - Version 4
          
   RFC1731
          IMAP4 Authentication Mechanisms
          
   RFC1732
          IMAP4 Compatibility With IMAP2 And IMAP2bis
          
   RFC1734
          POP3 AUTHentication command
          
   RFC1870
          SMTP Service Extension for Message Size Declaration
          
   RFC1891
          SMTP Service Extension for Delivery Status Notifications
          
   RFC1893
          Enhanced Mail System Status Codes
          
   RFC1894
          An Extensible Message Format for Delivery Status Notifications
          
   RFC1938
          A One-Time Password System
          
   RFC1939
          Post Office Protocol - Version 3
          
   RFC1985
          SMTP Service Extension for Remote Message Queue Starting
          
   RFC2060
          Internet Message Access Protocol - Version 4rev1
          
   RFC2061
          IMAP4 Compatibility With IMAP2bis
          
   RFC2062
          Internet Message Access Protocol - Obsolete Syntax
          
     _________________________________________________________________
                                      
   
   
    Eric S. Raymond <esr@snark.thyrsus.com>
