From b732a73bc773789894466b0e5320b2f1fe42c7e9 Mon Sep 17 00:00:00 2001 From: John Denker Date: Fri, 1 Jun 2012 18:58:45 -0700 Subject: original, as downloaded from http://www.qmail.org/netqmail-1.06.tar.gz --- THOUGHTS | 418 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 418 insertions(+) create mode 100644 THOUGHTS (limited to 'THOUGHTS') diff --git a/THOUGHTS b/THOUGHTS new file mode 100644 index 0000000..d6910da --- /dev/null +++ b/THOUGHTS @@ -0,0 +1,418 @@ +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. -- cgit v1.2.3