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safecat(1) General Commands Manual safecat(1) NAME safecat - safely write data to a file SYNOPSIS safecat tempdir destdir INTRODUCTION safecat is a program which implements Professor Daniel Bernstein's maildir algorithm to copy stdin safely to a file in a specified directory. With safecat, the user is offered two assurances. First, if safecat returns a successful exit status, then all data is guaranteed to be saved in the des- tination directory. Second, if a file exists in the destination directory, placed there by safecat, then the file is guaranteed to be complete. When saving data with safecat, the user specifies a destination directory, but not a file name. The file name is selected by safecat to ensure that no filename collisions occur, even if many safecat processes and other pro- grams implementing the maildir algorithm are writing to the directory si- multaneously. If particular filenames are desired, then the user should rename the file after safecat completes. In general, when spooling data with safecat, a single, separate process should handle naming, collecting, and deleting these files. Examples of such a process are daemons, cron jobs, and mail readers. RELIABILITY ISSUES A machine may crash while data is being written to disk. For many pro- grams, including many mail delivery agents, this means that the data will be silently truncated. Using Professor Bernstein's maildir algorithm, every file is guaranteed complete or nonexistent. Many people or programs may write data to a common "spool" directory. Sys- tems like mh-mail store files using numeric names in a directory. Incau- tious writing to files can result in a collision, in which one write suc- ceeds and the other appears to succeed but fails. Common strategies to re- solve this problem involve creation of lock files or other synchronizing mechanisms, but such mechanisms are subject to failure. Anyone who has deleted $HOME/.netscape/lock in order to start netscape can attest to this. The maildir algorithm is immune to this problem because it uses no locks at all. THE MAILDIR ALGORITHM As described in maildir(5), safecat applies the maildir algorithm by writ- ing data in six steps. First, it stat()s the two directories tempdir and destdir, and exits unless both directories exist and are writable. Second, it stat()s the name tempdir/time.pid.host, where time is the number of sec- onds since the beginning of 1970 GMT, pid is the program's process ID, and host is the host name. Third, if stat() returned anything other than ENOENT, the program sleeps for two seconds, updates time, and tries the stat() again, a limited number of times. Fourth, the program creates tem- pdir/time.pid.host. Fifth, the program NFS-writes the message to the file. Sixth, the program link()s the file to destdir/time.pid.host. At that in- stant the data has been successfully written. In addition, safecat starts a 24-hour timer before creating tem- pdir/time.pid.host, and aborts the write if the timer expires. Upon error, timeout, or normal completion, safecat attempts to unlink() tem- pdir/time.pid.host. EXIT STATUS An exit status of 0 (success) implies that all data has been safely commit- ted to disk. A non-zero exit status should be considered to mean failure, though there is an outside chance that safecat wrote the data successfully, but didn't think so. Note again that if a file appears in the destination directory, then it is guaranteed to be complete. If safecat completes successfully, then it will print the name of the newly created file (without its path) to standard output. SUGGESTED APPLICATIONS Exciting uses for safecat abound, obviously, but a word may be in order to suggest what they are. If you run Linux and use qmail instead of sendmail, you should consider converting your inbox to maildir for its superior reliability. If your home directory is NFS mounted, qmail forces you to use maildir. On the downside, the lovely tool procmail, which filters your spam, does not know maildir. Rather than running the patched procmail, you might consider us- ing safecat to deliver to your inbox. That allows you to use the latest procmail without waiting for the maildir patches to be applied to it. (Note: the previous paragraph was written before procmail started handling maildir delivery. Since maildir delivery has been added, my point is made stronger! Procmail's maildir support does not comply with Dan's algorithm, and so does not offer the reliability promised by maildir delivery. Proc- mail plus safecat has always offered reliable maildir delivery. Another victory for modularity!) If you write CGI applications to collect data over the World Wide Web, you might find safecat useful. Web applications suffer from two major prob- lems. Their performance suffers from every stoppage or bottleneck in the internet; they cannot afford to introduce performance problems of their own. Additionally, web applications should NEVER leave the server and database in an inconsistent state. This is likely, however, if CGI scripts directly frob some database--particularly if the database is overloaded or slow. What happens when users get bored and click "Stop" or "Back"? Maybe the database activity completes. Maybe the CGI script is killed, leaving the DB in an inconsistent state. Consider the following strategy. Make your CGI script dump its request to a spool directory using safecat. Immediately return a receipt to the browser. Now the browser has a complete guarantee that their submission is received, and the perceived performance of your web application is optimal. Meanwhile, a spooler daemon notices the fresh request, snatches it and up- dates the database. Browsers can be informed that their request will be fulfilled in X minutes. The result is optimal performance despite a capri- cious internet. In addition, users can be offered nearly 100% reliability. EXAMPLES To convince sendmail to use maildir for message delivery, add the following line to your .forward file: |SAFECAT HOME/Maildir/tmp HOME/Maildir/new || exit 75 #USERNAME where SAFECAT is the complete path of the safecat program, HOME is the com- plete path to your home directory, and USERNAME is your login name. Making this change is likely to pay off; many campuses and companies mount user home directories with NFS. Using maildir to deliver to your inbox folder helps ensure that your mail will not be lost due to some NFS error. Of course, if you are a System Administrator, you should consider switching to qmail. To run a program and catch its output safely into some directory, you can use a shell script like the following. #!/bin/bash MYPROGRAM=cat # The program you want to run TEMPDIR=/tmp # The name of a temporary directory DESTDIR=$HOME/work/data # The directory for storing information try() { $* 2>/dev/null || echo NO 1>&2 } set `( try $MYPROGRAM | try safecat $TEMPDIR $DESTDIR ) 2>&1` test "$?" = "0" || exit -1 test "$1" = "NO" && { rm -f $DESTDIR/$2; exit -1; } This script illustrates the pitfalls of writing secure programs with the shell. The script assumes that your program might generate some output, but then fail to complete. There is no way for safecat to know whether your program completed successfully or not, because of the semantics of the shell. As a result, safecat might create a file in the data directory which is "complete" but not useful. The shell script deletes the file in that case. More generally, the safest way to use safecat is from within a C program which invokes safecat with fork() and execve(). The parent process can the simply kill() the safecat process if any problems develop, and optionally can try again. Whether to go to this trouble depends upon how serious you are about protecting your data. Either way, safecat will not be the weak link in your data flow. BUGS In order to perform the last step and link() the temporary file into the destination directory, both directories must reside in the same file sys- tem. If they do not, safecat will quietly fail every time. In Professor Bernstein's implementation of maildir, the temporary and destination direc- tories are required to belong to the same parent directory, which essen- tially avoids this problem. We relax this requirement to provide some flexibility, at the cost of some risk. Caveat emptor. Although safecat cleans up after itself, it may sometimes fail to delete the temporary file located in tempdir. Since safecat times out after 24 hours, you may freely delete any temporary files older than 36 hours. Files newer than 36 hours should be left alone. A system of data flow in- volving safecat should include a cron job to clean up temporary files, or should obligate consumers of the data to do the cleanup, or both. In the case of qmail, mail readers using maildir are expected to scan and clean up the temporary directory. The guarantee of safe delivery of data is only "as certain as UNIX will al- low." In particular, a disk hardware failure could result in safecat con- cluding that the data was safe, when it was not. Similarly, a successful exit status from safecat is of no value if the computer, its disks and backups all explode at some subsequent time. In other words, if your data is vital to you, then you won't just use safe- cat. You'll also invest in good equipment (possibly including a RAID disk), a UPS for the server and drives, a regular backup schedule, and com- petent system administration. For many purposes, however, safecat can be considered 100% reliable. Also note that safecat was designed for spooling email messages; it is not the right tool for spooling large files--files larger than 2GB, for exam- ple. Some operating systems have a bug which causes safecat to fail silently when spooling files larger than 2GB. When building safecat, you can take advantage of conditional support for large files on Linux; see conf-cc for further information. CREDITS The maildir algorithm was devised by Professor Daniel Bernstein, the author of qmail. Parts of this manpage borrow directly from maildir(5) by Profes- sor Bernstein. In particular, the section "THE MAILDIR ALGORITHM" trans- plants his explanation of the maildir algorithm in order to illustrate that safecat complies with it. The original code for safecat was written by the present author, but was since augmented with heavy borrowings from qmail code. However, under no circumstances should the author of qmail be contacted concerning safecat bugs; all are the fault, and the responsibility, of the present author. Copyright (c) 2000, Len Budney. All rights reserved. SEE ALSO mbox(5), qmail-local(8), maildir(5) safecat(1)
NAME | SYNOPSIS | INTRODUCTION | RELIABILITY ISSUES | THE MAILDIR ALGORITHM | EXIT STATUS | SUGGESTED APPLICATIONS | EXAMPLES | BUGS | CREDITS | SEE ALSO
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