I know this is a stupid question... But I read the man pages and googled and find nothing. The only hope seems to be nfs_vfsops.c which contains some code extracting "sec" from opt, but I failed to specify sec opt by command line( I tried several combination only getting "mount_nfs: -o sec=: option not supported" or "usage: mount_nfs blahblahblah").
Same question for server side.
Same question for server side.