(This document was generated from design-notes.html)

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                           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 <ceharris@mal.com>. I took over
   development in June 1996 and subsequently renamed the program `fetchmail' to
   reflect the addition of IMAP support and SMTP delivery. 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

   MTAs ought to canonicalize the addresses of outgoing non-local 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.
   (Sendmail has an option to do this.)

   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:

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.

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.

Multiple concurrent instances of fetchmail

   Fetchmail locking is on a per-invoking-user because finer-grained locks
   would be really hard to implement in a portable way. The problem is that you
   don't want two fetchmails querying the same site for the same remote user at
   the same time.

   To handle this optimally, multiple fetchmails would have to associate a
   system-wide semaphore with each active pair of a remote user and host
   canonical address. A fetchmail would have to block until getting this
   semaphore at the start of a query, and release it at the end of a query.

   This would be way too complicated to do just for an "it might be nice"
   feature. Instead, you can run a single root fetchmail polling for multiple
   users in either single-drop or multidrop mode.

   The fundamental problem here is how an instance of fetchmail polling host
   foo can assert that it's doing so in a way visible to all other fetchmails.
   System V semaphores would be ideal for this purpose, but they're not
   portable.

   I've thought about this a lot and roughed up several designs. All are
   complicated and fragile, with a bunch of the standard problems (what happens
   if a fetchmail aborts before clearing its semaphore, and how do we recover
   reliably?).

   I'm just not satisfied that there's enough functional gain here to pay for
   the large increase in complexity that adding these semaphores would entail.

                         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 -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.

                                IPv6 and IPSEC

   The IPv6 support patches are really more protocol-family independence
   patches. Because of this, in most places, "ports" (numbers) have been
   replaced with "services" (strings, that may be digits). This allows us to
   run with certain protocols that use strings as "service names" where we in
   the IP world think of port numbers. Someday we'll plumb strings all over and
   then, if inet6 is not enabled, do a getservbyname() down in SocketOpen. The
   IPv6 support patches use getaddrinfo(), which is a POSIX p1003.1g mandated
   function. So, in the not too distant future, we'll zap the ifdefs and just
   let autoconf check for getaddrinfo. IPv6 support comes pretty much
   automatically once you have protocol family independence.

                             Internationalization

   Internationalization is handled using GNU gettext (see the file ABOUT_NLS in
   the source distribution). This places some minor constraints on the code.

   Strings that must be subject to translation should be wrapped with GT_() or
   N_() -- the former in function arguments, the latter in static initializers
   and other non-function-argument contexts.

                         Checklist for Adding Options

   Adding a control option is not complicated in principle, but there are a lot
   of fiddly details in the process. You'll need to do the following minimum
   steps.
     * Add a field to represent the control in struct run, struct query, or
       struct hostdata.
     * Go to rcfile_y.y. Add the token to the grammar. Don't forget the %token
       declaration.
     * Pick an actual string to declare the option in the .fetchmailrc file.
       Add the token to rcfile_l.
     * Pick a long-form option name, and a one-letter short option if any are
       left. Go to options.c. Pick a new LA_ value. Hack the longoptions table
       to set up the association. Hack the big switch statement to set the
       option. Hack the `?' message to describe it.
     * If the default is nonzero, set it in def_opts near the top of
       load_params in fetchmail.c.
     * Add code to dump the option value in fetchmail.c:dump_params.
     * For a per-site or per-user option, add proper FLAG_MERGE actions in
       fetchmail.c's optmerge() function. For a global option, add an override
       at the end of load_params; this will involve copying a "cmd_run." field
       to a corresponding "run." field, see the existing code for models.
     * Document the option in fetchmail.man. This will require at least two
       changes; one to the collected table of options, and one full text
       description of the option.
     * Hack fetchmailconf to configure it. Bump the fetchmailconf version.
     * Hack conf.c to dump the option so we won't have a version-skew problem.
     * Add an entry to NEWS.
     * If the option implements a new feature, add a note to the feature list.

   There may be other things you have to do in the way of logic, of course.

   Before you implement an option, though, think hard. Is there any way to make
   fetchmail automatically detect the circumstances under which it should
   change its behavior? If so, don't write an option. Just do the check!

                                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.

   Now that there is a standard IMAP equivalent of the POP3 APOP validation in
   CRAM-MD5, POP3 is 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.

                                    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), Chris Hanson (Kerberos V4
   support), and Craig Metz (OPIE, IPv6, IPSEC).

                                  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

   RFC1508
          Generic Security Service Application Program Interface

   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

   RFC1892
          The Multipart/Report Content Type for the Reporting of Mail System
          Administrative Messages

   RFC1894
          An Extensible Message Format 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

   RFC1957
          Some Observations on Implementations of the Post Office Protocol
          (POP3)

   RFC1985
          SMTP Service Extension for Remote Message Queue Starting

   RFC2033
          Local Mail Transfer Protocol

   RFC2060
          Internet Message Access Protocol - Version 4rev1

   RFC2061
          IMAP4 Compatibility With IMAP2bis

   RFC2062
          Internet Message Access Protocol - Obsolete Syntax

   RFC2195
          IMAP/POP AUTHorize Extension for Simple Challenge/Response

   RFC2177
          IMAP IDLE command

   RFC2449
          POP3 Extension Mechanism

   RFC2554
          SMTP Service Extension for Authentication

   RFC2595
          Using TLS with IMAP, POP3 and ACAP

   RFC2645
          On-Demand Mail Relay: SMTP with Dynamic IP Addresses

   RFC2683
          IMAP4 Implementation Recommendations

   RFC2821
          Simple Mail Transfer Protocol

   RFC2822
          Internet Message Format
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    Eric S. Raymond <esr@snark.thyrsus.com>