Unnamed repository; edit this file 'description' to name the repository.
Log | Files | Refs (19120B)

      1 Please note that this file is not called "Internet Mail For Dummies."
      2 It _records_ my thoughts on various issues. It does not _explain_ them.
      3 Paragraphs are not organized except by section. The required background
      4 varies wildly from one paragraph to the next.
      6 In this file, "sendmail" means Allman's creation; "sendmail-clone"
      7 means the program in this package.
     10 1. Security
     12 There are lots of interesting remote denial-of-service attacks on any
     13 mail system. A long-term solution is to insist on prepayment for
     14 unauthorized resource use. The tricky technical problem is to make the
     15 prepayment enforcement mechanism cheaper than the expected cost of the
     16 attacks. (For local denial-of-service attacks it's enough to be able to
     17 figure out which user is responsible.)
     19 qmail-send's log was originally designed for profiling. It subsequently
     20 sprouted some tracing features. However, there's no way to verify
     21 securely that a particular message came from a particular local user;
     22 how do you know the recipient is telling you the truth about the
     23 contents of the message? With QUEUE_EXTRA it'd be possible to record a
     24 one-way hash of each outgoing message, but a user who wants to send
     25 "bad" mail can avoid qmail entirely.
     27 I originally decided on security grounds not to put qmail advertisements
     28 into SMTP responses: advertisements often act as version identifiers.
     29 But this problem went away when I found a stable qmail URL.
     31 As qmail grows in popularity, the mere knowledge that rcpthosts is so
     32 easily available will deter people from setting up unauthorized MXs.
     33 (I've never seen an unauthorized MX, but I can imagine that it would be
     34 rather annoying.) Note that, unlike the bat book checkcompat() kludge,
     35 rcpthosts doesn't interfere with mailing lists.
     37 qmail-start doesn't bother with tty dissociation. On some old machines
     38 this means that random people can send tty signals to the qmail daemons.
     39 That's a security flaw in the job control subsystem, not in qmail.
     41 The resolver library isn't too bloated (before 4.9.4, at least), but it
     42 uses stdio, which _is_ bloated. Reading /etc/resolv.conf costs lots of
     43 memory in each qmail-remote process. So it's tempting to incorporate a
     44 smaller resolver library into qmail. (Bonus: I'd avoid system-specific
     45 problems with old resolvers.) The problem is that I'd then be writing a
     46 fundamentally insecure library. I'd no longer be able to blame the BIND
     47 authors and vendors for the fact that attackers can easily use DNS to
     48 steal mail. Solution: insist that the resolver run on the same host; the
     49 kernel can guarantee the security of low-numbered UDP ports.
     51 NFS is the primary enemy of security partitioning under UNIX. Here's the
     52 story. Sun knew from the start that NFS was completely insecure. It
     53 tried to hide that fact by disallowing root access over NFS. Intruders
     54 nevertheless broke into system after system, first obtaining bin access
     55 and then obtaining root access. Various people thus decided to compound
     56 Sun's error and build a wall between root and all other users: if all
     57 system files are owned by root, and if there are no security holes other
     58 than NFS, someone who breaks in via NFS won't be able to wipe out the
     59 operating system -- he'll merely be able to wipe out all user files. This
     60 clueless policy means that, for example, all the qmail users have to be
     61 replaced by root. See what I mean by "enemy"? ... Basic NFS comments:
     62 Aside from the cryptographic problem of having hosts communicate
     63 securely, it's obvious that there's an administrative problem of mapping
     64 client uids to server uids. If a host is secure and under your control,
     65 you shouldn't have to map anything. If a host is under someone else's
     66 control, you'll want to map his uids to one local account; it's his
     67 client's job to decide which of his users get to talk NFS in the first
     68 place. Sun's original map -- root to nobody, everyone else left alone -- 
     69 is, as far as I can tell, always wrong.
     72 2. Injecting mail locally (qmail-inject, sendmail-clone)
     74 RFC 822 section 3.4.9 prohibits certain visual effects in headers, and
     75 the 822bis draft prohibits even more. qmail-inject could enforce these
     76 absurd restrictions, but why waste the time? If you will suffer from
     77 someone sending you "flash mail," go find a better mail reader.
     79 qmail-inject's "Cc: recipient list not shown: ;" successfully stops
     80 sendmail from adding Apparently-To. Unfortunately, old versions of
     81 sendmail will append a host name. This wasn't fixed until sendmail 8.7.
     82 How many years has it been since RFC 822 came out?
     84 sendmail discards duplicate addresses. This has probably resulted in
     85 more lost and stolen mail over the years than the entire Chicago branch
     86 of the United States Postal Service. The qmail system delivers messages
     87 exactly as it's told to do. Along the same lines: qmail-inject is both
     88 unable and unwilling to support anything like sendmail's (default)
     89 nometoo option. Of course, a list manager could support nometoo.
     91 There should be a mechanism in qmail-inject that does for envelope
     92 recipients what Return-Path does for the envelope sender. Then
     93 qmail-inject -n could print the recipients.
     95 Should qmail-inject bounce messages with no recipients? Should there be
     96 an option for this? If it stays as is (accept the message), qmail-inject
     97 could at least avoid invoking qmail-queue.
     99 It is possible to extract non-unique Message-IDs out of qmail-inject.
    100 Here's how: stop qmail-inject before it gets to the third line of
    101 main(), then wait until the pids wrap around, then restart qmail-inject
    102 and blast the message through, then start another qmail-inject with the
    103 same pid in the same second. I'm not sure how to fix this without
    104 system-supplied sequence numbers. (Of course, the user could just type
    105 in his own non-unique Message-IDs.)
    107 The bat book says: "Rules that hide hosts in a domain should be applied
    108 only to sender addresses." Recipient masquerading works fine with
    109 qmail. None of sendmail's pitfalls apply, basically because qmail has a
    110 straight paper path.
    112 I predicted that I would receive some pressure to make up for the
    113 failings of MUA writers who don't understand the concept of reliability.
    114 ("Like, duh, you mean I'm supposed to check the sendmail exit code?")
    115 I was right.
    118 3. Receiving mail from the network (tcp-env, qmail-smtpd)
    120 qmail-smtpd doesn't allow privacy-invading commands like VRFY and EXPN.
    121 If you really want to publish such information, use a mechanism that
    122 legitimate users actually know about, such as fingerd or httpd.
    124 RFC 1123 says that VRFY and EXPN are important to track down cross-host
    125 mailing list loops. With Delivered-To, mailing list loops do no damage,
    126 _and_ one of the list administrators gets a bounce message that shows
    127 exactly how the loop occurred. Solve the problem, not the symptom.
    129 Should dns.c make special allowances for
    131 badmailfrom (like 8BITMIME) is a waste of code space.
    133 In theory a MAIL or RCPT argument can contain unquoted LFs. In practice
    134 there are a huge number of clients that terminate commands with just LF,
    135 even if they use CR properly inside DATA.
    138 4. Adding messages to the queue (qmail-queue)
    140 Should qmail-queue try to make sure enough disk space is free in
    141 advance? When qmail-queue is invoked by qmail-local or (with ESMTP)
    142 qmail-smtpd or qmail-qmtpd or qmail-qmqpd, it could be told a size in
    143 advance. I wish UNIX had an atomic allocate-disk-space routine... 
    145 The qmail.h interface (reflecting the qmail-queue interface, which in
    146 turn reflects the current queue file structure) is constitutionally
    147 incapable of handling an address that contains a 0 byte. I can't imagine
    148 that this will be a problem.
    150 Should qmail-queue not bother queueing a message with no recipients?
    153 5. Handling queued mail (qmail-send, qmail-clean)
    155 The queue directory must be local. Mounting it over NFS is extremely
    156 dangerous -- not that this stops people from running sendmail that way!
    157 Diskless hosts should use mini-qmail instead.
    159 Queue reliability demands that single-byte writes be atomic. This is
    160 true for a fixed-block filesystem such as UFS, and for a logging
    161 filesystem such as LFS.
    163 qmail-send uses 8 bytes of memory per queued message. Double that for
    164 reallocation. (Fix: use a small forest of heaps; i.e., keep several
    165 prioqs.) Double again for buddy malloc()s. (Fix: be clever about the
    166 heap sizes.) 32 bytes is worrisome, but not devastating. Even on my
    167 disk-heavy memory-light machine, I'd run out of inodes long before
    168 running out of memory.
    170 Some mail systems organize the queue by host. This is pointless as a
    171 means of splitting up the queue directory. The real issue is what to do
    172 when you suddenly find out that a host is up. For local SLIP/PPP links
    173 you know in advance which hosts need this treatment, so you can handle
    174 them with virtualdomains and serialmail.
    176 For the old queue structure I implemented recipient list compression:
    177 if mail goes out to a giant mailing list, and most of the recipients are
    178 delivered, make a new, compressed, todo list. But this really isn't
    179 worth the effort: it saves only a tiny bit of CPU time.
    181 qmail-send doesn't have any notions of precedence, priority, fairness,
    182 importance, etc. It handles the queue in first-seen-first-served order.
    183 One could put a lot of work into doing something different, but that
    184 work would be a waste: given the triggering mechanism and qmail's
    185 deferral strategy, it is exceedingly rare for the queue to contain more
    186 than one deliverable message at any given moment.
    188 Exception: Even with all the concurrency tricks, qmail-send can end up
    189 spending a few minutes on a mailing list with thousands of remote
    190 entries. A user might send a new message to a remote address in the
    191 meantime. The simplest way to handle this would be to put big messages
    192 on a separate channel.
    194 qmail-send will never start a pass for a job that it already has. This
    195 means that, if one delivery takes longer than the retry interval, the
    196 next pass will be delayed. I implemented the opposite strategy for the
    197 old queue structure. Some hassles: mark() had to understand how job
    198 input was buffered; every new delivery had to check whether the same
    199 mpos in the same message was already being done.
    201 Some things that qmail-send does synchronously: queueing a bounce
    202 message; doing a cleanup via qmail-clean; classifying and rewriting all
    203 the addresses in a new message. As usual, making these asynchronous
    204 would require some housekeeping, but could speed things up a bit.
    205 (I'm willing to assume POSIX waitpid() for asynchronous bounces; putting
    206 an unbounded buffer into wait_pid() for the sake of NeXTSTEP 3 is not
    207 worthwhile.)
    209 Disk I/O is a bottleneck; UFS is reliable but it isn't fast. A good
    210 logging filesystem offers much better performance, but logging
    211 filesystems aren't widely available. Solution: Keep a journal, separate
    212 from the queue, adequate to rebuild the queue (with at worst some
    213 duplicate deliveries). Compress the journal. This would dramatically
    214 reduce total disk I/O.
    216 Bounce aggregation is a dubious feature. Bounce records aren't
    217 crashproof; there can be a huge delay between a failure and a bounce;
    218 the resulting bounce format is unnecessarily complicated. I'm tempted to
    219 scrap the bounce directory and send one bounce for each failing
    220 recipient, with appropriate modifications in the accompanying text.
    222 qmail-stop implementation: setuid to UID_SEND; kill -TERM -1. Or run
    223 qmail-start under an external service controller, such as supervise;
    224 that's why it runs in the foreground.
    226 The readdir() interface hides I/O errors. Lower-level interfaces would
    227 lead me into a thicket of portability problems. I'm really not sure what
    228 to do about this. Of course, a hard I/O error means that mail is toast,
    229 but a soft I/O error shouldn't cause any trouble.
    231 job_open() or pass_dochan() could be paranoid about the same id,channel
    232 already being open; but, since messdone() is so paranoid, the worst
    233 possible effect of a bug along these lines would be double delivery.
    235 Mathematical amusement: The optimal retry schedule is essentially,
    236 though not exactly, independent of the actual distribution of message
    237 delay times. What really matters is how much cost you assign to retries
    238 and to particular increases in latency. qmail's current quadratic retry
    239 schedule says that an hour-long delay in a day-old message is worth the
    240 same as a ten-minute delay in an hour-old message; this doesn't seem so
    241 unreasonable.
    243 Insider information: AOL retries their messages every five minutes for
    244 three days straight. Hmmm.
    247 6. Sending mail through the network (qmail-rspawn, qmail-remote)
    249 Are there any hosts, anywhere, whose mailers are bogged down by huge
    250 messages to multiple recipients at a single host? For typical hosts,
    251 multiple RCPTs per SMTP aren't an "efficiency feature"; they're a
    252 _slowness_ feature. Separate SMTP transactions have much lower latency.
    254 I've heard three complaints about bandwidth use from masochists sending
    255 messages through a modem through a smarthost to thousands of users -- 
    256 without sublists! They can get much better performance with QMQP.
    258 In the opposite direction: It's tempting to remove the @host part of the
    259 qmail-remote recip argument. Or at least avoid double-dns_cname.
    261 There are lots of reasons that qmail-rspawn should take a more active
    262 role in qmail-remote's activities. It should call separate programs to
    263 do (1) MX lookups, (2) SMTP connections, (3) QMTP connections. (But this
    264 wouldn't be so important if the DNS library didn't burn so much memory.)
    266 I bounce ambiguous MXs. (An "ambiguous MX" is a best-preference MX
    267 record sending me mail for a host that I don't recognize as local.)
    268 Automatically treating ambiguous MXs as local is incompatible with my
    269 design decision to keep local delivery working when the network goes
    270 down. It puts more faith in DNS than DNS deserves. Much better: Have
    271 your MX records generated automatically from control/locals.
    273 If I successfully connect to an MX host but it temporarily refuses to
    274 accept the message, I give up and put the message back into the queue.
    275 But several documents seem to suggest that I should try further MX
    276 records. What are they thinking? My approach deals properly with downed
    277 hosts, hosts that are unreachable through a firewall, and load
    278 balancing; what else do people use multiple MX records for?
    280 Currently qmail-remote sends data in 1024-byte buffers. Perhaps it
    281 should try to take account of the MTU.
    283 Perhaps qmail-remote should allocate a fixed amount of DNS/connect()
    284 time across any number of MXs; this idea is due to Mark Delany.
    286 RFC 821 doesn't say what it means by "text." qmail-remote assumes that
    287 the server's reply text doesn't contain bare LFs.
    289 RFC 821 and RFC 1123 prohibit host names in MAIL FROM and RCPT TO from
    290 being aliases. qmail-remote, like sendmail, rewrites aliases in RCPT;
    291 people who don't list aliases in control/locals or sendmail's Cw are
    292 implicitly relying on this conversion. It is course quite silly for an
    293 internal DNS detail to have such an effect on mail delivery, but that's
    294 how the Internet works. On the other hand, the compatibility arguments
    295 do not apply to MAIL FROM. qmail-remote no longer bothers with CNAME
    296 lookups for the envelope sender host.
    299 7. Delivering mail locally (qmail-lspawn, qmail-local)
    301 qmail-local doesn't support comsat. comsat is a pointless abomination.
    302 Use qbiff if you want that kind of notification.
    304 The getpwnam() interface hides I/O errors. Solution: qmail-pw2u.
    307 8. sendmail V8's new features
    309 sendmail-8.8.0/doc/op/ includes a list of big improvements of
    310 sendmail 8.8.0 over sendmail 5.67. Here's how qmail stacks up against
    311 each of those improvements. (Of course, qmail has its own improvements,
    312 but that's not the point of this list.)
    314 Connection caching, MX piggybacking: Nope. (Profile. Don't speculate.)
    316 Response to RCPT command is fast: Yup.
    318 IP addresses show up in Received lines: Yup.
    320 Self domain literal is properly handled: Yup.
    322 Different timeouts for QUIT, RCPT, etc.: No, just a single timeout.
    324 Proper <> handling, route-address pruning: Yes, but not configurable.
    326 ESMTP support: Yup. (Server-side, including PIPELINING.)
    328 8-bit clean: Yup. (Including server-side 8BITMIME support; same as
    329 sendmail with the 8 option.)
    331 Configurable user database: Yup.
    333 BIND support: Yup.
    335 Keyed files: Yes, in fastforward.
    337 931/1413/Ident/TAP: Yup.
    339 Correct 822 address list parsing: Yup. (Note that sendmail still has
    340 some major problems with quoting.)
    342 List-owner handling: Yup.
    344 Dynamic header allocation: Yup.
    346 Minimum number of disk blocks: Yes, via tunefs -m. (Or quotas; the right
    347 setup has qmailq with a small quota, qmails with a larger quota, so that
    348 qmail-send always has room to work.)
    350 Checkpointing: Yes, but not configurable -- qmail always checkpoints.
    352 Error message configuration: Nope.
    354 GECOS matching: Not directly, but easy to hook in.
    356 Hop limit configuration: No. (qmail's limit is 100 hops. qmail offers
    357 automatic loop protection much more advanced than hop counting.)
    359 MIME error messages: No. (qmail uses QSBMF error messages, which are
    360 much easier to parse.)
    362 Forward file path: Yes, via /etc/passwd.
    364 Incoming SMTP configuration: Yes, via inetd or tcpserver.
    366 Privacy options: Yes, but they're not options.
    368 Best-MX mangling: Nope. See section 6 for further discussion.
    370 7-bit mangling: Nope. qmail always uses 8 bits.
    372 Support for up to 20 MX records: Yes, and more. qmail has no limits
    373 other than memory.
    375 Correct quoting of name-and-address headers: Yup.
    377 VRFY and EXPN now different: Nope. qmail always hides this information.
    379 Multi-word classes, deferred macro expansion, separate envelope/header
    380 $g processing, separate per-mailer envelope and header processing, new
    381 command line flags, new configuration lines, new mailer flags, new
    382 macros: These are sendmail-specific; they wouldn't even make sense for
    383 qmail. For example, _of course_ qmail handles envelopes and headers
    384 separately; they're almost entirely different objects!
    387 9. Miscellany
    389 sendmail-clone is too bletcherous to be documented.
    391 I've considered making install atomic, but this is very difficult to do
    392 right, and pointless if it isn't done right.
    394 RN suggests automatically putting together a reasonable set of lines for
    395 /etc/passwd. I perceive this as getting into the adduser business, which
    396 is worrisome: I'll be lynched the first time I screw up somebody's
    397 passwd file. This should be left to OS-specific installation scripts.
    399 The BSD 4.2 inetd didn't allow a username. I think I can safely forget
    400 about this. (DS notes that the username works under Ultrix even though
    401 it's undocumented.)
    403 I should clean up the bput/put choices.
    405 Some of the stralloc_0()s indicate that certain lower-level routines
    406 should grok stralloc.
    408 qmail assumes that all times are positive; that pid_t, time_t and ino_t
    409 fit into unsigned long; that gid_t fits into int; that the character set
    410 is ASCII; and that all pointers are interchangeable. Do I care?
    412 The bat book justifies sendmail's insane line-splitting mechanism by
    413 pointing out that it might be useful for "a 40-character braille
    414 print-driving program." C'mon, guys, is that your best excuse?
    416 qmail's mascot is a dolphin.