Chuck Lever wrote:
RFC 2623 suggests how the server should sort the returned flavor
list. However I don't think there's a consistent algorithm the client
can use with that list to determine a good default for that mount.
So, I would argue that any client that uses the "first" or "last"
entry in that list as the mount's auth flavor is probably broken; it
should pick a sec= default for all mounts, and if it's not on the
returned list, fail the mount. That is, incidentally, what the kernel
MNT client does now.
The MOUNT Version 3 protocol, associated with NFS Version 3, solves
the problem by having the response to the MNT procedure include a
list of flavors in the MNT procedure. Note that because some NFS
servers will export file systems to specific lists of clients, with
different access (read-only versus read-write), and with different
security flavors, it is possible a client might get back multiple
security flavors in the list returned in the MNT response. The use of
one flavor instead of another might imply read-only instead of read-
write access, or perhaps some other degradation of access. For this
reason, a NFS client SHOULD use the first flavor in the list that it
supports, on the assumption that the best access is provided by the
first flavor. NFS servers that support the ability to export file
systems with multiple security flavors SHOULD either present the best
accessing flavor first to the client, or leave the order under the
control of the system administrator.
It sounds pretty clear, the server SHOULD order them in some fashion and
the client SHOULD
pick the first one it supports in the list. It is not 'MUST', but if all
servers and clients follow the same
algorithm, it becomes accepted practice.
Having said that, our nfssec(5) states that a client can pick any of the
modes in the list.
But our server returns them in the order entered in the share by the admin.
The client either:
1) Uses the explicit flavor set in the mount command.
or
2) Uses the first supported one in the list.
or
3) Fails the mount.
With OpenSolaris NFSv3, there is no autonegotiation. With NFSv4, we
support the autonegotiation
as defined in the protocol.
We just went through a regression with this algorithm.
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