A feature I wish the Linux NFS client had: remapping UIDs and GIDs
My office workstation is a completely standalone machine, deliberately not dependent on, say, our NFS fileserver infrastructure. As a sysadmin's primary machine, there are obvious motivations for this and it's not something I'm ever going to change, but at the same time it does have its drawbacks and it'd be nice to sometimes NFS mount my fileserver home directory on my workstation.
Unfortunately there is a little but important obstacle. For peculiar historical reasons, the UID and GID of my local workstation account are not the same as on our fileservers (and thus our Ubuntu servers and so on). My login name is the same so things like SSH work fine, but NFS cares about the actual UID and GID and I wind up out of luck as far as NFS mounts go.
In theory the solution to this is NFS v4. In practice we don't use NFS v4 now, we're very unlikely to add NFS v4 any time soon for general use, and there is exactly zero chance that we'll add NFS v4 to our environment just so that I can NFS mount my fileserver home directory on my office workstation. Put bluntly, there are much easier solutions to that particular problem, ones that put all the work on my head where it rightfully belongs.
Hence my wish that the NFS client would support remapping UIDs and GIDs between the NFS server's view and the local Unix's view. In my particular situation I'd even be happy with a mount option that said 'always tell the server that we're UID X and GID Y', because that's all I need.
There's a pro and a con argument for doing this in the NFS client instead of the NFS server. The pro argument is that it's easier to scale this administratively if you can do it in the client. If it's done on the client, only the person running the client has to care; if it's done on the server, the server administrators have to be involved every time another client needs another UID or GID remapping.
The con argument is that NFS v3 'security' is already leaky enough without handing people a totally convenient client side way of subverting it totally (well, if you have root on a client). Yes, sure, you can already do this in a number of ways if you have client root, but all of those ways take at least some work. This feature would make it trivially easy, and there's a benefit to avoiding that.
(I expect that the real answer here is 'the Linux NFS maintainers have no interest in adding NFS v3 UID and GID mapping code in either the client or the server; use NFS v4 if you need this'. On the other hand, they did add NFS v3 server support for more than 16 GIDs.)