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Description of problem:
tshark does not create a new file, if file specified with -w option does not exist, instead exits with 

tshark: The file to which the capture would be saved ("<file>") could not be opened: Permission denied.

and returns 1

Version-Release number of selected component (if applicable):
wireshark-1.6.8-1.el7.x86_64


How reproducible:
always

Steps to Reproduce:
1. tshark -i <interface> -w <file that does not yet exist>

Expected results:
File with the given name is created

dumpcap utility gives up root privileges too early, before opening the output file. I sent a patch upstream: https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=7672

I added a newer version of the patch to upstream bugzilla: https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=7672

Patch: https://bugs.wireshark.org/bugzilla/attachment.cgi?id=9797&action=diff&context=patch&collapsed=&headers=1&format=raw

I suspect that you're trying to create a file in /root, and /root has 550 permissions.  That needs DAC override capabilities even with EUID=0, and we don't want to keep these (because they're strongly root-equivalent).

Workaround is to use:

# tshark -w - > /root/foo

I think this should be closed with WONTFIX.

In theory, it should be possible to use setresuid/setresgid to swap real and effective IDs, open the output file, swap them back (possibly regaining root privileges), and the continue as before (drop the unused capabilities etc.).