I ran into an odd problem the other day: I was testing out some
configuration changes for a web application by dropping files into
/tmp and pointing the application configuration at the appropriate
directory. Everything worked out great when testing it by hand...but
when starting up the
httpd service, the application behaved as if it
was unable to find any of the files in
My first assumption was that had simply missed something obvious like file permissions or that I had a typo in my configuration, but after repeated checks and lots of testing it was obvious that something else was going on.
Grasping at straws I took a close look at the
systemd service file
httpd, which looks like this:
[Unit] Description=The Apache HTTP Server (prefork MPM) After=syslog.target network.target remote-fs.target nss-lookup.target [Service] Type=forking PIDFile=/var/run/httpd/httpd.pid EnvironmentFile=/etc/sysconfig/httpd ExecStart=/usr/sbin/httpd $OPTIONS -k start ExecReload=/usr/sbin/httpd $OPTIONS -t ExecReload=/bin/kill -HUP $MAINPID ExecStop=/usr/sbin/httpd $OPTIONS -k stop PrivateTmp=true [Install] WantedBy=multi-user.target
Browsing throught file the following line caught my eye:
If you know about per-process namespaces in Linux, you're probably saying "Ah-ha!". If you don't know about per-process namespaces in Linux...you should, because this is the foundation for all sorts of things including Linux Containers (LXC). Here's some good introductory reading:
In short, with this configuration in place, the service gets it's very
own version of
/tmp not shared with any other process. While the
files I placed in
/tmp were visible in my process, they didn't
exist from the point of view of Apache.
The fix in my case was to place the files somewhere other than
One could also disable the
PrivateTmp setting, but it's generally
turned on for reasons of security.