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I'm aware its best to create temporary files with mktemp, but what about named pipes?

I prefer things to be as POSIX compliant as possible, but Linux only is acceptable. Avoiding Bashisms is my only hard criteria, as I write in dash.

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5 Answers 5

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52 
tmppipe=$(mktemp -u)
mkfifo -m 600 "$tmppipe"

Unlike regular file creation, which is prone to being hijacked by an existing file or a symbolic link, the creation of a name pipe through mkfifo or the underlying function either creates a new file in the specified place or fails. Something like : >foo is unsafe because if the attacker can predict the output of mktemp then the attacker can create the target file for himself. But mkfifo foo would fail in such a scenario.

If you need full POSIX portability, mkfifo -m 600 /tmp/myfifo is safe against hijacking but prone to a denial of service; without access to a strong random file name generator, you would need to manage retry attempts.

If you don't care for the subtle security problems around temporary files, you can follow a simple rule: create a private directory, and keep everything in there.

tmpdir=
cleanup () {
  trap - EXIT
  if [ -n "$tmpdir" ] ; then rm -rf "$tmpdir"; fi
  if [ -n "$1" ]; then trap - $1; kill -$1 $$; fi
}
tmpdir=$(mktemp -d)
trap 'cleanup' EXIT
trap 'cleanup HUP' HUP
trap 'cleanup TERM' TERM
trap 'cleanup INT' INT
mkfifo "$tmpdir/pipe"
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  • I actually already chose this as my solution earlier yesterday, I was waiting to see if someone posted it, or a better one. I also felt the dir was the way to go, in my case. Regardless... Thanks for the help, I appreciate it.
    – J. M. Becker
    Jan 25, 2012 at 2:48
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    Shouldn't the trap command be trap "rm -rf '$tempdir'" EXIT HUP INT TERM? Can trap do it's own variable expansion?
    – Six
    May 9, 2015 at 5:27
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    @Gilles ... as long as $tempdir value does not change either, which is acceptable for basically all purposes. Just be careful not to use the same name to create multiple temp files/dirs...
    – MestreLion
    Jan 15, 2016 at 15:50
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    @gilles Why do you trap INT HUP TERM? Doesn't EXIT cover all of those?
    – Sukima
    Oct 31, 2017 at 13:59
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    @RobinA.Meade Yes, if the shell has readonly. But if that's an issue, you have bigger problems. If your script is so complex that you can't keep track of all the variables that it uses, you should probably use a different language.
    – Gilles 'SO- stop being evil'
    Apr 30, 2020 at 6:36
8

A safer alternative is to use mktemp to create a directory safely, then put your named pipe inside that directory, do an rm -R $dir to get rid of it in the end.

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1
  • I opted to create the FIFO in a mktemp directory, as it was the really the only acceptable answer, Just in case you didn't notice, @Gilles already posted this answer in-depth.
    – J. M. Becker
    Dec 9, 2012 at 19:14
2

Use the "dry-run" option:

mkfifo $(mktemp -ut pipe.XXX)
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4
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    According to the man pages, the use of the -u option "is not encouraged".
    – dogbane
    Jan 24, 2012 at 10:04
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    @dogbane So is the use of -t, but as long as it works reliably, I'd go with it.
    – polemon
    Jan 24, 2012 at 10:07
  • sorry, where does it say that -t is discouraged?
    – dogbane
    Jan 24, 2012 at 10:11
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    @dogbane If it is in any way critical, I'd make a tiny C application, calling the mkstemp() function (linux.die.net/man/3/mkstemp). The -t switch isn't discouraged, it is -p, my bad.
    – polemon
    Jan 24, 2012 at 10:12
1

You can use mktemp to create a temporary file, then delete it and create a named pipe with the same name.

For example:

TMPPIPE=$(mktemp -t pipe.XXX) && {
    rm -f $TMPPIPE
    mkfifo $TMPPIPE
}
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5
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    Does deleting the $TMPPIPE before the mkfifo avoid the 'unsafe' problem associated from doing TMPPIPE=`mktemp -u` ; mkfifo $TMPPIPE ?
    – J. M. Becker
    Jan 24, 2012 at 9:46
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    @TechZilla mkfifo is actually safe, unlike usual regular file creation from the shell. If it wasn't, creating a file then deleting it would not help at all (in fact it would greatly facilitate the attacker's job by not requiring him to guess the file name). So dogbane's answer works, but the intermediate file creation is a useless complication.
    – Gilles 'SO- stop being evil'
    Jan 24, 2012 at 17:46
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    @Gilles, Do you know in what regard the mktemp man page called the -u option 'unsafe'?
    – J. M. Becker
    Jan 25, 2012 at 2:39
  • @dogbane, I voted up on your answer, and your comments which I felt were solid. Although, I already completed this yesterday.. I was just waiting to see when someone posted either my solution, or hopefully a better one. So even though I used mktemp -d you still got some points for your input. Thank you for all your help, I really appreciate it!
    – J. M. Becker
    Jan 25, 2012 at 2:46
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    @TechZilla mktemp -u is unsafe when creating a regular file, because it provides the protection against denial of service (if the name it generates is sufficiently unpredictable) but does not prevent an attacker from creating the file under the program's nose. Creating a fifo instead of a regular file is a rare use case that the man page doesn't address.
    – Gilles 'SO- stop being evil'
    Jan 25, 2012 at 9:41
0

Use mkfifo or mknod in Unix, where by two separate processes can access the pipe by name — one process can open it as a reader, and the other as a writer.

mkfifo my_pipe
gzip -9 -c < my_pipe > out.gz
cat file > my_pipe

The named pipe can be deleted just like any file:

rm my_pipe

mkfifo --mode=0666 /tmp/namedPipe
gzip --stdout -d file.gz > /tmp/namedPipe

NamedPipe can be used a regular file for only reading once.

http://www.linuxjournal.com/article/2156

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    I'm fully aware about named pipes, and mkfifo. It doesn't address my question about temporary named pipes, anymore than mkdir would address creating temporary directories.
    – J. M. Becker
    Jan 24, 2012 at 7:16
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    How does this address the issues that mktemp addresses to safely create a named pipe?
    – camh
    Jan 24, 2012 at 7:17
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    oh ok, best is to create them under /tmp, they are temporary files by definition and will clear off once the system reboots. Or better yet, have a shell function which will create the namedpipe out of the mktemp result itself (ofcourse by deleting the temp file first and then running mkfifo on the same). mktemp can also be used to create a temporary directory, try with -t -d switch.
    – Nikhil Mulley
    Jan 24, 2012 at 7:24

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