Why does tcpdump fail to write files with -C option despite root privileges?

Here is what I'm trying to do (CentOS 7): root@serv02 # /usr/sbin/tcpdump -nn -i eno16780032 -s 0 udp port 53 -w /var/opt/A.pcap -C 100 -W 30 tcpdump: /var/opt/A.pcap00: Permission denied I'm running this as root and the file system is not full. I just don't understand why tcpdump does not create the file. The command worked fine without -C and looked like -C caused tcpdump to drop privileges, but why?