NFS mount attempt with vers=n returns "invalid argument" or "incorrect mount option"

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Environment

SUSE Linux Enterprise Server 15

Situation

A setting for nfs version (such as "nfsvers=3") has been set in /etc/nfsmount.conf. Later, an nfs mount is attempted from the command line or /etc/fstab with a different version. This may fail depending on the syntax.

An example mount attempt with verbose output:

dp-sles15sp3:~ # mount -vvv -o vers=4.1 10.10.200.40:/export /mnt1
mount.nfs: timeout set for Wed Nov 18 14:15:25 2021
mount.nfs: trying text-based options 'vers=4.1,nfsvers=3,addr=10.10.200.40,clientaddr=10.10.200.43'
mount.nfs: mount(2): Invalid argument
mount.nfs: an incorrect mount option was specified

As shown in the output above, this resulted in 2 versions being specified in the mount command, which may not be handled well in certain combinations. Using the -s or "sloppy" mount option does not help.

Resolution

At this time, it is best not to set an NFS version in /etc/nfsmount.conf. This is expected to remain true at least through SLES 15 SP5, possibly longer. It is likely that this recommendation will not change until the next major release after SLES 15.

Cause

The code responsible for merging options together is found in mount.nfs, part of the nfs-client package, which is built from the nfs-utils source code package. Use of /etc/nfsmount.conf and the code to merge its options with others is a relatively new development.

If different NFS versions are specificed in /etc/nfsmount.conf and elsewhere (such as /etc/fstab, on the command line, or possibly autofs maps), various logic comes into play to determine which version will be used. This is further complicated by the fact that "vers=" is considered an alias of "nfsvers=" but could be handled differently by some code. There are combinations that work and some that fail, with various reasons and exceptions within the logic.

Disclaimer

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