I am by no means a computer security expert. But I do have a little advice for the security conscious. But be warned: This is by no means a complete list of NFS related problems and if you think you're safe once you're read and implemented all this I have a bridge I want to sell you.
This section is probably of no concern if you are on a closed network where you trust all the users, and no-one you don't trust can get access to machines on the network. I.e., there should be no way to dial into the network, and it should in no way be connected to other networks where you don't trust everyone using it as well as the security. Do you think I sound paranoid? I'm not at all paranoid. This is just basic security advice. And remember, the things I say here is just the start of it. A secure site needs a diligent and knowledgeable admin that knows where to find information about current and potential security problems.
NFS has a basic problem in that the client, if not told otherwise, will trust the NFS server and vice versa. This can be bad. It means that if the server's root account is broken into it can be quite easy to break into the client's root account as well. And vice versa. There are a couple of coping strategies for this, which we'll get back to.
Something you should read is the CERT advisories on NFS, most of the text below deals with issues CERT has written advisories about. See ftp.cert.org:/01-README for a up to date list of CERT advisories. Here are some NFS related advisories:
CA-91:21.SunOS.NFS.Jumbo.and.fsirand 12/06/91
Vulnerabilities concerning Sun Microsystems, Inc. (Sun) Network
File System (NFS) and the fsirand program. These vulnerabilities
affect SunOS versions 4.1.1, 4.1, and 4.0.3 on all architectures.
Patches are available for SunOS 4.1.1. An initial patch for SunOS
4.1 NFS is also available. Sun will be providing complete patches
for SunOS 4.1 and SunOS 4.0.3 at a later date.
CA-94:15.NFS.Vulnerabilities 12/19/94
This advisory describes security measures to guard against several
vulnerabilities in the Network File System (NFS). The advisory was
prompted by an increase in root compromises by intruders using tools
to exploit the vulnerabilities.
CA-96.08.pcnfsd 04/18/96
This advisory describes a vulnerability in the pcnfsd program (also
known as rpc.pcnfsd). A patch is included.
On the client we can decide that we don't want to trust the server
too much a couple of ways with options to mount. For example we can
forbid suid programs to work off the NFS file system with the
nosuid option. This is a good idea and you should consider using
this with all NFS mounted disks. It means that the server's root user
cannot make a suid-root program on the file system, log in to the
client as a normal user and then use the suid-root program to become
root on the client too. We could also forbid execution of files on
the mounted file system altogether with the noexec option. But
this is more likely to be impractical than nosuid since a file
system is likely to at least contain some scripts or programs
that needs to be executed. You enter these options in the options
column, with the rsize and wsize, separated by commas.
On the server we can decide that we don't want to trust the client's root account. We can do that by using the root_squash option in exports:
/mn/eris/local apollon(rw,root_squash)
Now, if a user with UID 0 on the client attempts to access (read,
write, delete) the file system the server substitutes the UID of the
servers `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 NFSd man page there are several other
squash options listed so that you can decide to mistrust whomever you
(don't) like on the clients. You also have options to squash any UID
and GID range you want to. This is described in the Linux NFSd man
page.
root_squash is in fact the default with the Linux NFSd, to grant
root access to a filesystem use no_root_squash.
Another important thing is to ensure that nfsd checks that all it's requests comes from a privileged port. If it accepts requests from any old port on the client a user with no special privileges can run a program that's is easy to obtain over the Internet. It talks nfs protocol and will claim that the user is anyone the user wants to be. Spooky. The Linux nfsd does this check by default, on other OSes you have to enable this check yourself. This should be described in the nfsd man page for the OS.
Another thing. Never export a file system to 'localhost' or 127.0.0.1. Trust me.
The basic portmapper, in combination with nfsd has a design problem that makes it possible to get to files on NFS servers without any privileges. Fortunately the portmapper that most Linux distributions use is relatively secure against this attack, and can be made more secure by configuring up access lists in two files.
Not all Linux distributions were created equal. Some seemingly
up-to-date distributions does not include a securable portmapper,
even today, many years since the vulnerability became common
knowledge. At least one distribution even contains the manpage for a
securable portmapper but the actual portmapper is not
secureable. The easy way to check if your portmapper is good
or not is to run strings(1) and see if it reads the relevant files,
/etc/hosts.deny and /etc/hosts.allow. Assuming your
portmapper is /usr/sbin/portmap you can check it with this
command: strings /usr/sbin/portmap | grep hosts. On my
machine it comes up with this:
/etc/hosts.allow /etc/hosts.deny @(#) hosts_ctl.c 1.4 94/12/28 17:42:27 @(#) hosts_access.c 1.20 96/02/11 17:01:27
First we edit /etc/hosts.deny. It should contain the line
portmap: ALL
which will deny access to everyone. While it is closed thus run
rpcinfo -p just to check that your portmapper really reads
and obeys this file. rpcinfo should give no output, or possebly a
errormessage. Restarting the portmapper should not be
necessary.
Closing the portmapper for everyone is a bit drastic, so we open it
again by editing /etc/hosts.allow. But first we need to
figure out what to put in it. It should basically list all machines
that should have access to your portmapper. On a run of the mill
Linux system there are very few machines that need any access for any
reason. The portmapper administrates nfsd, mountd, ypbind/ypserv,
pcnfsd, and 'r' services like ruptime and rusers. Of these only nfsd,
mountd, ypbind/ypserv and perhaps pcnfsd are of any consequence. All
machines that needs to access services on your machine should be
allowed to do that. Let's say that your machines address is
129.240.223.254 and that it lives on the subnet 129.240.223.0 should
have access to it (those are terms introduced by the networking HOWTO,
go back and refresh your memory if you need to). Then we write
portmap: 129.240.223.0/255.255.255.0
in hosts.allow. This is the same as the network address you give
to route and the subnet mask you give to ifconfig. For the device
eth0 on this machine ifconfig should show
...
eth0 Link encap:10Mbps Ethernet HWaddr 00:60:8C:96:D5:56
inet addr:129.240.223.254 Bcast:129.240.223.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:360315 errors:0 dropped:0 overruns:0
TX packets:179274 errors:0 dropped:0 overruns:0
Interrupt:10 Base address:0x320
...
and netstat -rn should show
Kernel routing table Destination Gateway Genmask Flags Metric Ref Use Iface ... 129.240.223.0 0.0.0.0 255.255.255.0 U 0 0 174412 eth0 ...
(Network address in first column).
The hosts.deny and hosts.allow files are described in the
manual pages of the same names.
IMPORTANT: Do not put anything but IP NUMBERS in the portmap lines of these files. Host name lookups can indirectly cause portmap activity which will trigger host name lookups which can indirectly cause portmap activity which will trigger...
The above things should make your server tighter. The only remaining problem (Yeah, right!) is someone breaking root (or boot MS-DOS) on a trusted machine and using that privilege to send requests from a secure port as any user they want to be.
It's a very good idea to firewall the nfs and portmap ports in your
router or firewall. The nfsd operates at port 2049, both udp and tcp
protocols. The portmapper at port 111, tcp and udp, and mountd at
port 745 and and 747, tcp and udp. Normally. You should check the
ports with the rpcinfo -p command.
If on the other hand you want NFS to go through a firewall there are options for newer NFSds and mountds to make them use a specific (nonstandard) port which can be open in the firewall.
If you use the hosts.allow/deny, root_squash, nosuid and privileged
port features in the portmapper/nfs software you avoid many of the
presently known bugs in nfs and can almost feel secure about that
at least. But still, after all that: When an intruder has access to
your network, s/he can make strange commands appear in your
.forward or read your mail when /home or
/var/spool/mail is NFS exported. For the same reason,
you should never access your PGP private key over nfs. Or at least
you should know the risk involved. And now you know a bit of it.
NFS and the portmapper makes up a complex subsystem and therefore it's not totally unlikely that new bugs will be discovered, either in the basic design or the implementation we use. There might even be holes known now, which someone is abusing. But that's life. To keep abreast of things like this you should at least read the newsgroups comp.os.linux.announce and comp.security.announce at a absolute minimum.