Can't mount nfs4 filesystem with kerberos auth (sec=krb5)

[Elena Grandi]

Under raspbian jessie trying to mount a nfs4 filesystem with kerberos auth results in "mount.nfs4: an incorrect mount option was specified":

  # mount -v -t nfs4 -o sec=krb5 192.168.0.1:/home /mnt
  mount.nfs4: timeout set for Thu Oct 19 14:29:12 2017
  mount.nfs4: trying text-based options 'sec=krb5,addr=192.168.0.1,clientaddr=192.168.0.72'
  mount.nfs4: mount(2): Invalid argument
  mount.nfs4: an incorrect mount option was specified

The same command works on a regular debian jessie (amd64 and i386) with a similar client configuration, so I'm quite confident in the server configuration and I expect that any difference in the required client configuration should be caused by a raspbian/debian difference.

I've checked dmesg, /var/log/daemon.log and journalctl for more hints, but nothing was printed after the mount command, so I have no hint to what could be missing.

The following is the kernel in use, including what I believe are the relevant modules, that seem to be correctly loaded / present.

  # uname -a
  Linux raspberrypi 4.9.35-v7+ #1014 SMP Fri Jun 30 14:47:43 BST 2017 armv7l GNU/Linux
  # lsmod | grep krb
  rpcsec_gss_krb5 23752 0
  # grep nfs /lib/modules/4.9.35-v7+/modules.builtin
  kernel/fs/nfs/nfs.ko
  kernel/fs/nfs/nfsv2.ko
  kernel/fs/nfs/nfsv3.ko
  kernel/fs/nfs/nfsv4.ko
  kernel/fs/nfs_common/nfs_acl.ko
  kernel/fs/nfs_common/grace.ko

Please let me know if you need any other information that can help to find the issue.

[Elena Grandi (valhalla-d)]

Apparently the issue is no longer present in raspbian stretch, where mounting works as expected.