80 Mail special header at the beginning of the mail file, Unix assumes that any line beginning with the letters F-r-o-m followed by a space (“ ”) marks the beginning of a new mail message. Using bits that might be contained by e-mail messages to represent infor- mation about e-mail messages is called inband communication, and any- body who has ever taken a course on telecommunications knows that it is a bad idea. The reason that inband communication is bad is that the commu- nication messages themselves sometimes contain these characters. For this reason, sendmail searches out lines that begin with “From and changes them to “From.” Now, you might think this is a harmless little behavior, like someone burp- ing loudly in public. But sometimes those burps get enshrined in public papers whose text was transmitted using sendmail. The recipient believes that the message was already proofread by the sender, so it gets printed ver- batim. Different text preparation systems do different things with the “” character. For example, LaTeX turns it into an upside question mark (¿). If you don't believe us, obtain the paper “Some comments on the assumption- commitment framework for compositional verification of distributed pro- grams” by Paritosh Pandya, in “Stepwise Refinement of Distributed Sys- tems,” Springer-Verlag, Lecture Notes in Computer Science no. 430, pages 622–640. Look at pages 626, 630, and 636—three paragraphs start with a “From” that is prefixed with a ¿. Sendmail even mangles mail for which it isn’t the “final delivery agent”— that is, mail destined for some other machine that is just passing through some system with a sendmail mailer. For example, just about everyone at Microsoft uses a DOS or Windows program to send and read mail. Yet internal mail gets goosed with those “Froms” all over the place. Why? Because on its hop from one DOS box to another, mail passes through a Unix-like box and is scarred for life. So what happens when you complain to a vendor of electronic mail ser- vices (whom you pay good money to) that his machine doesn’t follow pro- tocol—what happens if it is breaking the law? Jerry Leichter complained to his vendor and got this response: Date: Tue, 24 Mar 92 22:59:55 EDT From: Jerry Leichter leichter@lrw.com To: UNIX-HATERS Subject: That wonderful “From” From: A customer service representative5
From: MAILER-DAEMON@berkeley.edu 81 I don’t and others don’t think this is a bug. If you can come up with an RFC that states that we should not be doing this I’m sure we will fix it. Until then this is my last reply. I have brought this to the attention of my supervisors as I stated before. As I said before, it appears it is Unix’s way of handling it. I have sent test messages from machines running the latest software. As my final note, here is a section from rfc976: [deleted] I won’t include that wonderful quote, which nowhere justifies a mail forwarding agent modifying the body of a message—it simply says that “From” lines and “From” lines, wherever they might have come from, are members of the syntactic class From_Lines. Using typical Unix reasoning, since it doesn’t specifically say you can’t do it, and it mentions that such lines exist, it must be legal, right? I recently dug up a July 1982 RFC draft for SMTP. It makes it clear that messages are to be delivered unchanged, with certain docu- mented exceptions. Nothing about ’s. Here we are 10 years later, and not only is it still wrong—at a commercial system that charges for its services—but those who are getting it wrong can’t even SEE that it’s wrong. I think I need to scream. uuencode: Another Patch, Another Failure You can tell those who live on the middle rings of Unix Hell from those on lower levels. Those in the middle levels know about From lossage but think that uuencode is the way to avoid problems. Uuencode encodes a file that uses only 7-bit characters, instead of 8-bit characters that Unix mailers or network systems might have difficulty sending. The program uudecode decodes a uuencoded file to produce a copy of the original file. A uuen- coded file is supposedly safer to send than plain text for example, “From” distortion can’t occur to such a file. Unfortunately, Unix mailers have other ways of screwing users to the wall: 5This message was returned to a UNIX-HATER subscriber by a technical support representative at a major Internet provider. We’ve omitted that company’s name, not in the interest of protecting the guilty, but because there was no reason to single out this particular company: the notion that “sendmail is always right” is endemic among all of the Internet service providers.
Previous Page Next Page