How to force Linux cifs mount to default to smb3?

Graphical file managers do not establish 'cifs' kernel mounts; they use Samba's libsmbclient.

The corresponding /etc/samba/smb.conf options are:

[global]

client min protocol = SMB2_02
; default is 'CORE' (MS-DOS era)

client max protocol = SMB3
; default was 'NT1' (aka CIFS) until Samba v4.6
; finally includes SMB3 as of Samba v4.7

As soon as you raise the maximum to SMBv3, the client will negotiate the best supported version. It is not necessary to block CIFS/SMBv1 via client min protocol.

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Note that the "SMB2" and "SMB3" keywords map to the highest supported sub-version of that protocol (currently "SMB2_10" and "SMB3_11"). This means that setting the minimum protocol to "SMB3" would actually exclude Windows 8.1 (which uses SMB3_02).

See the smb.conf manual page for more details and the full protocol list.