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. Solution: insist that the resolver run on the same host; the

kernel can guarantee the security of low-numbered 127.0.0.1 UDP ports.

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, and

the 822bis draft prohibits even more. qmail-inject could enforce these

absurd restrictions, but why waste the time? 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 without

system-supplied sequence numbers. (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 predicted that I would 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'm supposed to check the sendmail exit code?'')

I was right.

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

qmail-smtpd doesn't allow privacy-invading commands like VRFY and EXPN.

If you really want to publish such information, use a mechanism that

legitimate users actually know about, such as fingerd or httpd.

RFC 1123 says that VRFY and EXPN are important to track down cross-host

mailing list loops. With Delivered-To, mailing list loops do no damage,

_and_ one of the list administrators gets a bounce message that shows

exactly how the loop occurred. Solve the problem, not the symptom.

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

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

In theory a MAIL or RCPT argument can contain unquoted LFs. In practice

there are a huge number of clients that terminate commands with just LF,

even if they use CR properly inside DATA.

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 or qmail-qmqpd, 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!

Diskless hosts should use mini-qmail instead.

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. The simplest way to handle this would be to put big messages

on a separate channel.

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.

(I'm willing to assume POSIX waitpid() for asynchronous bounces; putting

an unbounded buffer into wait_pid() for the sake of NeXTSTEP 3 is not

worthwhile.)

Disk I/O is a bottleneck; UFS is reliable but it isn't fast. A good

logging filesystem offers much better performance, but logging

filesystems aren't widely available. Solution: Keep a journal, separate

from the queue, adequate to rebuild the queue (with at worst some

duplicate deliveries). Compress the journal. This would dramatically

reduce total disk I/O.

Bounce aggregation is a dubious feature. Bounce records aren't

crashproof; there can be a huge delay between a failure and a bounce;

the resulting bounce format is unnecessarily complicated. I'm tempted to

scrap the bounce directory and send one bounce for each failing

recipient, with appropriate modifications in the accompanying text.

qmail-stop implementation: setuid to UID_SEND; kill -TERM -1. Or run

qmail-start under an external service controller, such as supervise;

that's why it runs in the foreground.

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.

I've heard three complaints about bandwidth use from masochists sending

messages through a modem through a smarthost to thousands of users---

without sublists! They can get much better performance with QMQP.

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. (But this

wouldn't be so important if the DNS library didn't burn so much memory.)

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.

RFC 821 and RFC 1123 prohibit host names in MAIL FROM and RCPT TO from

being aliases. qmail-remote, like sendmail, rewrites aliases in RCPT;

people who don't list aliases in control/locals or sendmail's Cw are

implicitly relying on this conversion. It is course quite silly for an

internal DNS detail to have such an effect on mail delivery, but that's

how the Internet works. On the other hand, the compatibility arguments

do not apply to MAIL FROM. qmail-remote no longer bothers with CNAME

lookups for the envelope sender host.

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 fastforward.

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. (Or quotas; the right

setup has qmailq with a small quota, qmails with a larger quota, so that

qmail-send always has room to work.)

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.

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.