Thanks once more, I figured it out, at least to some extent. The strange MIME type put me on the track:
First, I tried to find its origin:
root@platte:~# find /usr/local/modules/ -xdev -type f -print0 | xargs -0 grep "text/plan"
Binary file /usr/local/modules/cgi/app_mgr/app_mgr.cgi matches
Binary file /usr/local/modules/cgi/mydlink.cgi matches
Binary file /usr/local/modules/cgi/system_mgr.cgi matches
Binary file /usr/local/modules/cgi/webfile_mgr.cgi matches
(searching in other mounts yielded no results)
Next, I looked at the most likely candidate:
strings /usr/local/modules/cgi/system_mgr.cgi|less
and found, amongst lots of other stuff
config_set -b >/dev/null
Content-Disposition: attachment;filename="backup";
Connection: Keep-Alive
Content-Type: text/plan
rm -rf /tmp/backup
rm -r /tmp/backup.tgz
This looked like it could be related to the config file saving, so I did a
root@platte:~# config_set --help
config_set: invalid option -- '-'
Usage:
-b bakcup config to /tmp/config.tgz
-r restore config from /tmp/config.tgz
-s save restore config
-t [compress type]: zip/tgz -d enable debug
and tried a
config_set -b
which led to a "no space left on device" error.
A df showed that my root file system was indeed full and some further searching via du found a file "/mnt/HD_b4/.systemfile/.upnpav-db/upnpav.db" with a size of 11 MB dating back to January 2013. After I moved that out of the way, saving of the configuration settings worked again, yielding a gzipped tar file called backup weighing in at around 8kb. The contents of that file look very similar to what ivan posted, with some differences which are probably due to either different configuration settings or the different firmware version.
I had briefly toyed with using the DNS-320L as a UPNP server in the past but have long since disabled that feature. I guess the large DB file was a remnant of these attempts.
Thanks again to ivan and FurryNutz for taking an interest and putting me on the right track!
All the best,
Manuel