Upstream qmail 1.01

[qmail] / THOUGHTS

Please note that this file is not called ``Internet Mail For Dummies.''
It _records_ my thoughts on various issues. It does not _explain_ them.
Paragraphs are not organized except by section. The required background
varies wildly from one paragraph to the next.

In this file, ``sendmail'' means Allman's creation; ``sendmail-clone''
means the program in this package.


1. Security

There are lots of interesting remote denial-of-service attacks on any
mail system. A long-term solution is to insist on prepayment for
unauthorized resource use. The tricky technical problem is to make the
prepayment enforcement mechanism cheaper than the expected cost of the
attacks. (For local denial-of-service attacks it's enough to be able to
figure out which user is responsible.)

qmail-send's log was originally designed for profiling. It subsequently
sprouted some tracing features. However, there's no way to verify
securely that a particular message came from a particular local user;
how do you know the recipient is telling you the truth about the
contents of the message? With QUEUE_EXTRA it'd be possible to record a
one-way hash of each outgoing message, but a user who wants to send
``bad'' mail can avoid qmail entirely.

I originally decided on security grounds not to put qmail advertisements
into SMTP responses: advertisements often act as version identifiers.
But this problem went away when I found a stable qmail URL.

As qmail grows in popularity, the mere knowledge that rcpthosts is so
easily available will deter people from setting up unauthorized MXs.
(I've never seen an unauthorized MX, but I can imagine that it would be
rather annoying.) Note that, unlike the bat book checkcompat() kludge,
rcpthosts doesn't interfere with mailing lists.

qmail-start doesn't bother with tty dissociation. On some old machines
this means that random people can send tty signals to the qmail daemons.
That's a security flaw in the job control subsystem, not in qmail.

The resolver library isn't too bloated (before 4.9.4, at least), but it
uses stdio, which _is_ bloated. Reading /etc/resolv.conf costs lots of
memory in each qmail-remote process. So it's tempting to incorporate a
smaller resolver library into qmail. (Bonus: I'd avoid system-specific
problems with old resolvers.) The problem is that I'd then be writing a
fundamentally insecure library. I'd no longer be able to blame the BIND
authors and vendors for the fact that attackers can easily use DNS to
steal mail. Possible solution: replace dns.c with something that passes
requests (reliably!) to a local daemon; call the original resolver
library from that daemon.

NFS is the primary enemy of security partitioning under UNIX. Here's the
story. Sun knew from the start that NFS was completely insecure. It
tried to hide that fact by disallowing root access over NFS. Intruders
nevertheless broke into system after system, first obtaining bin access
and then obtaining root access. Various people thus decided to compound
Sun's error and build a wall between root and all other users: if all
system files are owned by root, and if there are no security holes other
than NFS, someone who breaks in via NFS won't be able to wipe out the
operating system---he'll merely be able to wipe out all user files. This
clueless policy means that, for example, all the qmail users have to be
replaced by root. See what I mean by ``enemy''? ... Basic NFS comments:
Aside from the cryptographic problem of having hosts communicate
securely, it's obvious that there's an administrative problem of mapping
client uids to server uids. If a host is secure and under your control,
you shouldn't have to map anything. If a host is under someone else's
control, you'll want to map his uids to one local account; it's his
client's job to decide which of his users get to talk NFS in the first
place. Sun's original map---root to nobody, everyone else left alone---
is, as far as I can tell, always wrong.


2. Injecting mail locally (qmail-inject, sendmail-clone)

RFC 822 section 3.4.9 prohibits certain visual effects in headers.
qmail-inject doesn't waste the time to enforce this absurd restriction.
If you will suffer from someone sending you ``flash mail,'' go find a
better mail reader.

qmail-inject's ``Cc: recipient list not shown: ;'' successfully stops
sendmail from adding Apparently-To. Unfortunately, old versions of
sendmail will append a host name. This wasn't fixed until sendmail 8.7.
How many years has it been since RFC 822 came out?

sendmail discards duplicate addresses. This has probably resulted in
more lost and stolen mail over the years than the entire Chicago branch
of the United States Postal Service. The qmail system delivers messages
exactly as it's told to do. Along the same lines: qmail-inject is both
unable and unwilling to support anything like sendmail's (default)
nometoo option. Of course, a list manager could support nometoo.

There should be a mechanism in qmail-inject that does for envelope
recipients what Return-Path does for the envelope sender. Then
qmail-inject -n could print the recipients.

Should qmail-inject bounce messages with no recipients? Should there be
an option for this? If it stays as is (accept the message), qmail-inject
could at least avoid invoking qmail-queue.

It is possible to extract non-unique Message-IDs out of qmail-inject.
Here's how: stop qmail-inject before it gets to the third line of
main(), then wait until the pids wrap around, then restart qmail-inject
and blast the message through, then start another qmail-inject with the
same pid in the same second. I'm not sure how to fix this. (Of course,
the user could just type in his own non-unique Message-IDs.)

The bat book says: ``Rules that hide hosts in a domain should be applied
only to sender addresses.'' Recipient masquerading works fine with
qmail. None of sendmail's pitfalls apply, basically because qmail has a
straight paper path.

I expect to receive some pressure to make up for the failings of MUA
writers who don't understand the concept of reliability. (``Like, duh,
you mean I was supposed to check the sendmail exit code?'')


3. Receiving mail from the network (tcp-env, qmail-smtpd)

RFC 1123 requires VRFY support, but says that it's okay if an
implementation can be configured to not allow VRFY. qmail-smtpd doesn't
allow VRFY. If you desperately want your SMTP server (i.e., inetd) to
provide useful information for VRFY, just compile and install sendmail.
Were the RFC 1123 writers aware of the as-if principle of interface
specification? ... They say that VRFY and EXPN are important for
tracking down cross-host mailing list loops. Catch up to the 1990s,
guys: with Delivered-To, mailing list loops do absolutely no damage,
_and_ one of the list administrators gets a bounce that shows exactly
how the loop occurred. Solve the problem, not the symptom. ... There's a
vastly superior alternative to EXPN. Hint: finger postmaster@ai.mit.edu.

Should dns.c make special allowances for 127.0.0.1/localhost?

badmailfrom (like 8BITMIME) is a waste of code space.


4. Adding messages to the queue (qmail-queue)

Should qmail-queue try to make sure enough disk space is free in
advance? When qmail-queue is invoked by qmail-local or (with ESMTP)
qmail-smtpd or qmail-qmtpd, it could be told a size in advance. I wish
UNIX had an atomic allocate-disk-space routine... 

The qmail.h interface (reflecting the qmail-queue interface, which in
turn reflects the current queue file structure) is constitutionally
incapable of handling an address that contains a 0 byte. I can't imagine
that this will be a problem.

Should qmail-queue not bother queueing a message with no recipients?


5. Handling queued mail (qmail-send, qmail-clean)

The queue directory must be local. Mounting it over NFS is extremely
dangerous---not that this stops people from running sendmail that way!
Perhaps it is worth putting together a diskless-host qmail package with
just qmail-inject and an SMTP client in place of qmail-queue. Sending
mail to the server via SMTP is of course vastly better than trying to do
anything over NFS. If the NFS server is up but the mail server is down,
users will just have to wait.

Queue reliability demands that single-byte writes be atomic. This is
true for a fixed-block filesystem such as UFS, and for a logging
filesystem such as LFS.

qmail-send uses 8 bytes of memory per queued message. Double that for
reallocation. (Fix: use a small forest of heaps; i.e., keep several
prioqs.) Double again for buddy malloc()s. (Fix: be clever about the
heap sizes.) 32 bytes is worrisome, but not devastating. Even on my
disk-heavy memory-light machine, I'd run out of inodes long before
running out of memory.

Some mail systems organize the queue by host. This is pointless as a
means of splitting up the queue directory. The real issue is what to do
when you suddenly find out that a host is up. For local SLIP/PPP links
you know in advance which hosts need this treatment, so you can handle
them with virtualdomains and serialmail.

For the old queue structure I implemented recipient list compression:
if mail goes out to a giant mailing list, and most of the recipients are
delivered, make a new, compressed, todo list. But this really isn't
worth the effort: it saves only a tiny bit of CPU time.

qmail-send doesn't have any notions of precedence, priority, fairness,
importance, etc. It handles the queue in first-seen-first-served order.
One could put a lot of work into doing something different, but that
work would be a waste: given the triggering mechanism and qmail's
deferral strategy, it is exceedingly rare for the queue to contain more
than one deliverable message at any given moment.

Exception: Even with all the concurrency tricks, qmail-send can end up
spending a few minutes on a mailing list with thousands of remote
entries. A user might send a new message to a remote address in the
meantime. Perhaps qmail-send should limit its time per message to,
say, thirty recipients. This will require some way to mark recipients
who were already done on this pass. Possible approach: Maintain two todo
lists (for both L and R). Always work on the earlier todo list. Move
deferrals to the other todo list.

qmail-send will never start a pass for a job that it already has. This
means that, if one delivery takes longer than the retry interval, the
next pass will be delayed. I implemented the opposite strategy for the
old queue structure. Some hassles: mark() had to understand how job
input was buffered; every new delivery had to check whether the same
mpos in the same message was already being done.

Some things that qmail-send does synchronously: queueing a bounce
message; doing a cleanup via qmail-clean; classifying and rewriting all
the addresses in a new message. As usual, making these asynchronous
would require some housekeeping, but could speed things up a bit.
(Making bounces asynchronous, without POSIX waitpid(), means that
wait_pid() has to keep a buffer of previous wait()s. Ugh.)

fsync() is a bottleneck. To make this asynchronous would require gobs of
dedicated output processes whose only purpose in life is to watch data
get written to the disk. Inconceivable! (``You keep using that word. I
do not think that word means what you think it means.'')

On the other hand, I could survive without fsync()ing the local and
remote and info files as long as I don't unlink todo. This would require
redefining the queue states. I need to see how much speed can be gained.

Currently qmail-send sends at most one bounce message for each incoming
message. This means that the sender doesn't get flooded with copies of
his own message. On the other hand, a single slow address can hold up
bounces for a bunch of fast addresses. It would be easy to call
injectbounce() more often. What is the best strategy? This feels like
the TCP-buffering issue... don't want to pepper the other guy with
little packets, but do want to get the data across.

qmail-stop implementation: setuid to UID_SEND; kill -TERM -1. Given how
simple this is, I'm not inclined to set up some tricky locking solution
where qmail-send records its pid etc. But I just know that, if I provide
this qmail-stop program, someone will screw himself by making another
uid the same as UID_SEND, or making UID_SEND be root, or whatever.
Aargh. Maybe use another named pipe... New solution: Run qmail-start
under an external service controller---it runs in the foreground now.

Bounce messages could include more statistical information in the first
paragraph: when I received the message, how many recipients I was
supposed to handle, how many I successfully dealt with, how many I
already told you about, how many are still in the queue. Have to
emphasize that the number of recipients _here_ is perhaps less than the
number of recipients on the original message.

The readdir() interface hides I/O errors. Lower-level interfaces would
lead me into a thicket of portability problems. I'm really not sure what
to do about this. Of course, a hard I/O error means that mail is toast,
but a soft I/O error shouldn't cause any trouble.

job_open() or pass_dochan() could be paranoid about the same id,channel
already being open; but, since messdone() is so paranoid, the worst
possible effect of a bug along these lines would be double delivery.

Mathematical amusement: The optimal retry schedule is essentially,
though not exactly, independent of the actual distribution of message
delay times. What really matters is how much cost you assign to retries
and to particular increases in latency. qmail's current quadratic retry
schedule says that an hour-long delay in a day-old message is worth the
same as a ten-minute delay in an hour-old message; this doesn't seem so
unreasonable.

Insider information: AOL retries their messages every five minutes for
three days straight. Hmmm.


6. Sending mail through the network (qmail-rspawn, qmail-remote)

Are there any hosts, anywhere, whose mailers are bogged down by huge
messages to multiple recipients at a single host? For typical hosts,
multiple RCPTs per SMTP aren't an ``efficiency feature''; they're a
_slowness_ feature. Separate SMTP transactions have much lower latency.

The multiple-RCPT bandwidth gain _might_ be noticeable for a machine
that sends most messages to a smarthost. It would be easy to have
qmail-rspawn supply qmail-remote with all the addresses at once, as long
as qmail-send says when it's about to block... Putting recipients into
the right order is clearly the UA's job. One multiple-RCPT pitfall is
that a remote host might not be able to deal with (say) 10 recipients,
even though RFC 821 says everyone has to be able to handle 100;
qmail-rspawn would have to notice this and back off. (Not that other
mailers do. Sometimes I'm amazed Internet mail works at all.)

In the opposite direction: It's tempting to remove the @host part of the
qmail-remote recip argument. Or at least avoid double-dns_cname.

There are lots of reasons that qmail-rspawn should take a more active
role in qmail-remote's activities. It should call separate programs to
do (1) MX lookups, (2) SMTP connections, (3) QMTP connections.

I bounce ambiguous MXs. (An ``ambiguous MX'' is a best-preference MX
record sending me mail for a host that I don't recognize as local.)
Automatically treating ambiguous MXs as local is incompatible with my
design decision to keep local delivery working when the network goes
down. It puts more faith in DNS than DNS deserves. Much better: Have
your MX records generated automatically from control/locals.

If I successfully connect to an MX host but it temporarily refuses to
accept the message, I give up and put the message back into the queue.
But several documents seem to suggest that I should try further MX
records. What are they thinking? My approach deals properly with downed
hosts, hosts that are unreachable through a firewall, and load
balancing; what else do people use multiple MX records for?

Currently qmail-remote sends data in 1024-byte buffers. Perhaps it
should try to take account of the MTU.

Perhaps qmail-remote should allocate a fixed amount of DNS/connect()
time across any number of MXs; this idea is due to Mark Delany.

RFC 821 doesn't say what it means by ``text.'' qmail-remote assumes that
the server's reply text doesn't contain bare LFs.


7. Delivering mail locally (qmail-lspawn, qmail-local)

qmail-local doesn't support comsat. comsat is a pointless abomination.
Use qbiff if you want that kind of notification.

The getpwnam() interface hides I/O errors. Solution: qmail-pw2u.


8. sendmail V8's new features

sendmail-8.8.0/doc/op/op.me includes a list of big improvements of
sendmail 8.8.0 over sendmail 5.67. Here's how qmail stacks up against
each of those improvements. (Of course, qmail has its own improvements,
but that's not the point of this list.)

Connection caching, MX piggybacking: Nope. (Profile. Don't speculate.)

Response to RCPT command is fast: Yup.

IP addresses show up in Received lines: Yup.

Self domain literal is properly handled: Yup.

Different timeouts for QUIT, RCPT, etc.: No, just a single timeout.

Proper <> handling, route-address pruning: Yes, but not configurable.

ESMTP support: Yup. (Server-side, including PIPELINING.)

8-bit clean: Yup. (Including server-side 8BITMIME support; same as
sendmail with the 8 option.)

Configurable user database: Yup.

BIND support: Yup.

Keyed files: Yes, in qmsmac.

931/1413/Ident/TAP: Yup.

Correct 822 address list parsing: Yup. (Note that sendmail still has
some major problems with quoting.)

List-owner handling: Yup.

Dynamic header allocation: Yup.

Minimum number of disk blocks: Yes, via tunefs -m.

Checkpointing: Yes, but not configurable---qmail always checkpoints.

Error message configuration: Nope.

GECOS matching: Not directly, but easy to hook in.

Hop limit configuration: No. (qmail's limit is 100 hops. qmail offers
automatic loop protection much more advanced than hop counting.)

MIME error messages: No. (qmail uses QSBMF error messages, which are
much easier to parse.)

Forward file path: Yes, via /etc/passwd.

Incoming SMTP configuration: Yes, via inetd or tcpserver.

Privacy options: Yes, but they're not options.

Best-MX mangling: Nope. See section 6 for further discussion.

7-bit mangling: Nope. qmail always uses 8 bits.

Support for up to 20 MX records: Yes, and more. qmail has no limits
other than memory.

Correct quoting of name-and-address headers: Yup.

VRFY and EXPN now different: Nope. qmail always hides this information.

Multi-word classes, deferred macro expansion, separate envelope/header
$g processing, separate per-mailer envelope and header processing, new
command line flags, new configuration lines, new mailer flags, new
macros: These are sendmail-specific; they wouldn't even make sense for
qmail. For example, _of course_ qmail handles envelopes and headers
separately; they're almost entirely different objects!


9. Miscellany

sendmail-clone and qsmhook are too bletcherous to be documented. (The
official replacement for qsmhook is preline, together with the
qmail-command environment variables.)

I've considered making install atomic, but this is very difficult to do
right, and pointless if it isn't done right.

RN suggests automatically putting together a reasonable set of lines for
/etc/passwd. I perceive this as getting into the adduser business, which
is worrisome: I'll be lynched the first time I screw up somebody's
passwd file. This should be left to OS-specific installation scripts.

The BSD 4.2 inetd didn't allow a username. I think I can safely forget
about this. (DS notes that the username works under Ultrix even though
it's undocumented.)

I should clean up the bput/put choices.

Some of the stralloc_0()s indicate that certain lower-level routines
should grok stralloc.

RN suggests having qlist smash the case of the incoming host name.

K1J suggests that mailing list subscription managers should have
a three-way handshake, to prevent person A from subscribing person B to
a mailing list. qlist doesn't do this, but ezmlm does.

qmail assumes that all times are positive; that pid_t, time_t and ino_t
fit into unsigned long; that gid_t fits into int; that the character set
is ASCII; and that all pointers are interchangeable. Do I care?

The bat book justifies sendmail's insane line-splitting mechanism by
pointing out that it might be useful for ``a 40-character braille
print-driving program.'' C'mon, guys, is that your best excuse?

qmail's mascot is a dolphin.