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How to work around “sorry, you must have a tty to run sudo” without sacrificing security


While working on $client‘s Linux server last week, I found myself installing a cron job that ran as root. The cron job called a custom bash script that, in turn, called out to various custom maintenance tasks client had already written. One task in particular had to run as a different user.

During testing, I discovered that the odd-ball task failed to run, and found the following error in the system log:

sudo: sorry, you must have a tty to run sudo

I traced this error to a line trying to invoke a perl command as a user called dynamic:

sudo -u dynamic /usr/bin/perl run-periodic-tasks --load 5 --randomly

A simple Google search turned up an obvious solution to the error: use visudo to disable sudo’s tty requirement, allowing sudo to be invoked from any shell lacking a tty (including cron). This would have solved my problem, but it just felt wrong, dirty, and most troublingly insecure.

One reason why sudo ships with the requiretty option enabled by default is, among other reasons, to prevent remote users from exposing the root password over SSH. Disabling this security precaution for a simple maintenance task already running as root seemed totally unnecessary, not to mention irresponsible. Moreover, client‘s script didn’t even need a tty.

Thankfully, there’s a better way: use su --session-command and send the whole job to the background.

su --session-command="/usr/bin/perl run-periodic-tasks --load 5 --randomly" dynamic &

This line launches a new, non-login shell (typically bash) as the other user in a separate, background process and runs the command you passed using the shell’s -c option. Sending the command to the background (using &) continues execution of the rest of the cron job.

A process listing would look like this:

root     28109     1  0 17:10 ?        00:00:00 su --session-command=/usr/bin/perl run-periodic-tasks --load 5 --randomly dynamic
dynamic  28110 28109  0 17:10 ?        00:00:00 bash -c /usr/bin/perl run-periodic-tasks --load 5 --randomly

Note the parent process (PID 28109) is owned by root but the actual perl process (PID 28110) is being run as dynamic.

This in-script solution that replaces sudo -u user cmd with su --session-command=cmd user seems much better than relying on a change in sudo‘s default (and more secure) configuration to me.

Written by Meitar

March 17th, 2010 at 8:21 pm