nfsd mountd

Posted by admin

May 7, 2008

On the server we can decide that we don’t want to trust any requests made as root on the client. We can do that by using the root_squash option in /etc/exports:

/home slave1(rw,root_squash)

This is, in fact, the default. It should always be turned on unless you have a very good reason to turn it off. To turn it off use the no_root_squash option.

Now, if a user with UID 0 (i.e., root’s user ID number) on the client attempts to access (read, write, delete) the file system, the server substitutes the UID of the server’s ‘nobody’ account. Which means that the root user on the client can’t access or change files that only root on the server can access or change. That’s good, and you should probably use root_squash on all the file systems you export. “But the root user on the client can still use su to become any other user and access and change that users files!” say you. To which the answer is: Yes, and that’s the way it is, and has to be with Unix and NFS. This has one important implication: All important binaries and files should be owned by root, and not bin or other non-root account, since the only account the clients root user cannot access is the servers root account. In the exports(5) man page there are several other squash options listed so that you can decide to mistrust whomever you (don’t) like on the clients.

The TCP ports 1-1024 are reserved for root’s use (and therefore sometimes referred to as “secure ports”) A non-root user cannot bind these ports. Adding the secure option to an /etc/exports means that it will only listed to requests coming from ports 1-1024 on the client, so that a malicious non-root user on the client cannot come along and open up a spoofed NFS dialogue on a non-reserved port. This option is set by default.

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