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.
- 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.
- 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.
- 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.
- 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?
- 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.
- 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.
- 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.
- 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!
- Miscellany
sendmail-clone is too bletcherous to be documented.
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.