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Using DTrace With a Problem Uploading Files via FTP

Derek Crudgington, May 2006

I was having a problem trying to upload something via FTP through a web application, and it wasn't uploading for some reason. The FTP daemon was receiving the commands, but the file wasn't showing up in the directory.

Here's the FTP log of the session I was trying to upload:

Oct 27 14:54:26 callacctweb ftpd[25157]: [ID 165209 daemon.info] USER pinnftp
Oct 27 14:54:26 callacctweb ftpd[25157]: [ID 125383 daemon.info] PASS password
Oct 27 14:54:26 callacctweb ftpd[25157]: [ID 201503 daemon.info] CWD /
Oct 27 14:54:26 callacctweb ftpd[25157]: [ID 313182 daemon.info] PORT
Oct 27 14:54:26 callacctweb ftpd[25157]: [ID 307055 daemon.info] TYPE Image
Oct 27 14:54:26 callacctweb ftpd[25157]: [ID 206289 daemon.info] STOR logo-left.jpg

Nothing looks suspicious here, and even to test if it was a permission error I went ahead and made the ftp directory +w by everyone. Still, I kept getting the same stuff, with no file showing up in the directory.

I decided to use DTrace to investigate. (Note: Dynamic Tracing or DTrace is a new feature in the Solaris 10 OS that is used to better understand the system and programs run on the system.) I ran the DTrace script opensnoop (as I tell the web app to upload the file):

What we are looking for here is all of the -1< FD values. This indicates there was an error somewhere. Looking at all of them, only one really sticks out, and it is the name of the image I was trying to upload, logo-left.jpg.

It has an error number of 13. Let's see what this means:

root@callacct:~/DTraceToolkit-0.83# grep 13 /usr/include/sys/errno.h
#define EACCES  13      /* Permission denied                    */
#define ECONNABORTED    130     /* Software caused connection abort */
#define ECONNRESET      131     /* Connection reset by peer */
#define ENOBUFS         132     /* No buffer space available */
#define EISCONN         133     /* Socket is already connected */
#define ENOTCONN        134     /* Socket is not connected */
/* XENIX has 135 - 142 */

It is showing that I'm getting a permission denied. This makes more sense if you look at the FTP log above: It shows it issuing CWD / then trying to upload a file, which means it's changing to the root (/) directory first then uploading the file and failing with the permission denied. The root directory doesn't have permission to let anyone upload there and the FTP web app shouldn't be issuing a cd / and then trying to upload the file unless maybe it wants the FTP daemon to run chroot'ed.

At first I overlooked the FTP application sending the CWD / command, and I didn't see this. Using DTrace, I was able to actually see this by looking at what was going on in the system, which then verified there was a permission error.

About the Author

Derek Crudgington can be reached at dacrud@gmail.com, or see his web site at: http://hell.jedicoder.net.

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