From: www.itworld.com

A third in fourths: An NFS potpourri

October 15, 2001 —

 

We've gently eased into the summer months (or
winter doldrums for our Southern readers) with a two-month discussion
of the automounter and its configuration. By now we hope that you can
build maps, install and customize them to your heart's content, and
perhaps you've justified the need to change every single
client configuration to reach administrative harmonic convergence.
Then again, if you're like the 99 percent of our readers who inhabit the real
world (we're convinced there are some 'bots who digest each month's
offering) you've run into some last little detail separating
automounted bliss from true nirvana.

In order to enable you with powers to perform the more spiritual
acts of system management -- those that actually elicit kind words from
impressed users -- we'll take this third pass through the automounter
and NFS. We'll start by getting stopped, looking at the various ways
the automounter can get stuck, and what you can do to repair the
situations. From automounters that don't work we'll move on to those
that work too well, giving root-enabled users access to home
directories and mail spools of unsuspecting users. We'll introduce NFS
authentication to get you started on the path to a solution and take a
peek at WebNFS, Sun's filesystem for the Internet. Our final section
deals with auto-sharing and automounting removable media like CD-ROMs.
Armed with a four-corner offensive arsenal of expert tricks, you should
be prepared for anything the NFS-ready masses throw your way.

Tracing the mystery: Debugging mount attempts

Any system with hidden moving parts is bound to break. Create some
complex automounter maps and you're likely to find unexpected behavior
or get unintended filesystem mounts showing up in less than desirable
places. The automounter has two debugging flags: -T and
-v, enabling request tracing and verbose mount requests,
respectively. When the -v flag is supplied on the
automountd command line, each request is sent to syslog. With a single
-T on the command line, you'll get significant information
about the inner workings of the automounter:

	MOUNT REQUEST: name=/var/mail map=auto.direct
	opts=ro,intr,nosuid path=/var/mail
  		PUSH /etc/auto.direct
  		POP /etc/auto.direct
  		mapname = auto.direct, key = /var/mail
     			(nfs,nfs) / 	-rw,intr,nosuid,actimeo=0	sunrise:/var/mail 
  		nfsmount: sunrise:/var/mail /var/mail rw,intr,nosuid,actimeo=0
  		ping sunrise OK
  		mount sunrise:/var/mail /var/mail (rw,intr,nosuid,actimeo=0)
  		mount sunrise:/var/mail OK
	MOUNT REPLY    : status=0

Supply a double trace flag (-T -T) and you get detail
targeted at those with source code.

Killing the automounter and restarting it with a plethora of flags
isn't conducive to rapid problem solving or user satisfaction. If you
have a variety of volumes mounted, it may be hard to kill the
automounter gracefully and have it start up again without side
effects. Fortunately, there's a back door into setting the debug
options. When the automounter sees a lookup request for a file name
starting with an equal sign (=), it parses the name to see if it
matches one of the following settings:

	=v	Toggle -v on/off
	=0	Turn off all tracing
	=1	Set -T (simple trace)
	=2	Set -T -T (advanced trace)

This trick only works for indirect mount points, where the
automounter would match the file name component to a map key. For
example, turn on verbose mount request logging and simple tracing by
performing two "ls" operations on the appropriate files in
/net (a default indirect map):

	# ls /net/=v
	# ls /net/=1

Only the superuser can toggle the debugging flags. If you're using
/home as an indirect automounter mount point, you could just
as easily have done:

	# ls /home/=v /home/=1

What kind of problems are you likely to uncover? One of the more
typical misbehaviors stems from the * wildcard key being substituted
into the server specification, producing an illegal server name or an
invalid server path name. For example, you use a default entry in the
auto.home map like:

	*	&:/export/home

If a user accidentally uses /home/fred instead of
/home/bigboy/fred, an attempt is made to mount
fred:/export/home. You may have a machine with that name,
producing unexpected mount results, or the automounter may fail trying
to resolve the host name.

If you see messages complaining "Can't remount," check for
overlapping map entries. You may be mounting one filesystem on top of
part of another, making it impossible to unmount the the underlying
directory after it has reached quiescence. Check hierarchical maps
and those with variables or wildcards carefully to make sure the name
spaces of each map remain independent. This error is also common with
subdirectory mounts where the actual mount point changes depending
upon the order in which the subdirectories are accessed.

Auto Pileups: Unsticking the automounter

By far the most common problem is when the automounter gets stuck
waiting for a server that has crashed, or when the daemon is unable to
resolve a mount point due to name server, network loading, or map
syntax problems. The automounter emulates an NFS server, so when it
gets stuck or stops processing requests, the user-visible symptoms are
the same as for a crashed NFS server: "server not responding" messages
in the console window and a general lock up of processes accessing the
failed server. Instead of seeing a server name, however, you'll see
the process id of the automounter daemon:

	pid203@/home not responding, still trying

The automounter is single threaded, so it only processes one mount
request at a time. Subsequent requests for the automounter's services
are routinely ignored until the current mount operation completes. If
the mount in progress makes no progress, other processes talking to the
automounter have their requests dropped, just as an overloaded server
drops incoming NFS RPC packets.

From the point of view of the process that tickled the automounter,
an NFS server (the one running on the local machine) has crashed, so
the standard NFS warning is displayed. It may be having trouble
talking to a server that crashed, or it may be encountering NIS/DNS
problems that lead to high latency in host name resolution, routing
problems that prevent mount requests from reaching the server, or other
variations of lossy networks.

You can immediately tell what map is causing the problem. But if you
want to see the key in the map that is leading to the hang, you'll need
to turn on debugging before you coax the automounter into a
compromising position. It's possible that the automounter has gotten
wedged because it can't get enough CPU to process the map files and
send off an NFS mount RPC. Remember that the automounter daemon is a
user-level process, although it spends most of its life making system
calls. Be sure your system isn't suffering from some runaway
real-time processes or sources of high-level interrupts, like serial
line input, that could prevent a user-level process from completing a
series of system calls. In most cases, however, the automounter
pileups are caused by an unresponsive server or name service problems
that prevent the daemon from converting map entries into valid server
IP address and path name specifications.

Early versions of the automounter could be lulled to sleep faster
than some infant children. In response to bugs, and the need for
non-Sun automounter functionality, Jan-Simon Pendry wrote the publicly
available amd. A code archive is available for ftp from ftp.acl.lanl.gov:/pub/amd,
and an
archive of amd notes, questions and bug fixes is available at the University
of Maryland. One of the nicer features of amd is that
of a keep-alive mount point: The automounter pings the currently
mounted server from a replicated set, until it finds that it no longer
gets a response. When the server ostensibly has died, amd
renames the old mount point so that future references cause a new mount
(of another server) to be completed. amd tries its best
to never block a user-level process.

Does this make amd better than the Solaris
automounter? The keep-alive feature helps when you need to start a new
series of processes that rely on a dead mount point. The new requests
cause new mounts to be completed on the new staging area, while the old
mount point (and processes waiting on it) spin quietly in the
background. Neither amd nor the automounter help you if
you need to recover processes waiting on a crashed server. Those
processes go into disk wait and can't be cleaned up easily. To get an
inventory of processes using a particular mount point, use the
lsof utility to locate processes with open files on the
automounted directory hierarchy.

Averting the evil eyes: Using stronger authentication

Sometimes the automounter works too well. A user has root access on
her own machine, so she creates another login (for you) on her machine
with a trivial password. A quick "su" to root, and then
an "su fred", and voila! -- she's now browsing your home
directory, reading your mail and generally enjoying the privileges of
membership in your e-life. How do you prevent users from accessing
others' files when they can basically do what they want as root? Gain
a stronger measure of control using Secure NFS, Sun's layering of
strong authentication on top of the NFS V2 and V3 protocols.

Secure NFS uses Secure RPC, the same mechanism used by NIS+ to
authenticate client requests. Secure RPC uses a verifier attached to
the credentials for each RPC request. The verifier includes the user's
credentials and a timestamp to prevent request replays. The verifier
is encrypted with a session key unique to each user-host pair. How do
the NFS server and user's client determine the session key? A large
random value is chosen as the key and is passed from the client to the
server using public key encryption. The server and user keys are
maintained in an NIS or NIS+ table. The public key is left is
plaintext, and the private key is encrypted using the user's (or root's
in the case of a server) login password.

To complete the key exchange, the user needs to decrypt her private
key using her login password. If the correct login password isn't
provided, the secret key can't be decrypted, and the user cannot be
authenticated to the server. Nosy users that create duplicate, fake
logins can't gain access to the spoofed users' NFS-mounted files
because they can't decrypt the user's private key. See the man pages
for publickey, newkey, and chkey
for more information on generating the public key pairs, building the
NIS and NIS+ tables, and setting up server keys.

Turn on Secure NFS using the secure mount option in
/etc/vfstab or in the automounter map and by sharing (or
exporting) the filesystem with the secure option as well. On the
client side, the secure flag tells the kernel RPC code that it needs to
supply encrypted verifiers and credentials with each request. On the
server side, the option ensures that credentials are verified before
allowing access to the file. User credentials that have no verifier or
can't be decrypted properly are treated as null credentials, giving the
user the same permissions as nobody.

Here are some other warnings and advisories to help you ward off
evil prowling eyes on your network:

Secure NFS is only part of a security strategy, not a panacea. It
slows down the most obvious su-to-someone-important sneak
through, but it doesn't prevent more onerous systematic attacks. NFS
was designed to run on a local area network, with some measure of
internal security -- that is, you have some level of trust built up for
the hosts and users on that network. When you make NFS filesystems
available to unknown hosts or users, you need to think carefully about
the value of the data you're exposing or opening up for damage.

Why then would anyone use NFS over the Internet? The TCP/IP
protocol for Web page exchange, HTTP, is fairly inefficient at moving
multiple files, opening a new TCP connection for each transfer. It's
also hard to browse through directories of files with a protocol
designed for pure file transfer. There are times when you'd like to
bring together the transparent access capabilities of HTTP with the
file transfer and browsing efficiency of a distributed filesystem,
which is what Sun has done with WebNFS.

Take a quick glance at the WebNFS information on Sun's Web server.

WebNFS provides a general purpose file transfer mechanism for the
Internet. It differs from the generic NFS protocol in a two subtle
ways. First, the mount protocol is eliminated, so only NFS requests
pass between client and server. To get an initial file handle (needed
for all lookup, directory reading and file operations), the client
sends a request with a zero-length file handle, which the WebNFS server
interprets as a request for the handle to be filled in. Normally, the
NFS mount protocol returns the file handle of the filesystem's
top-level exported (shared) directory to the client. The second
difference is that clients can ask for multiple pathname components to
be resolved in a single call, instead of one at a time. This multiple
component lookup reduces the number of NFS RPC calls needed to start
moving data.

WebNFS isn't restricted to making data available to the public.
Running NFS through a firewall -- internal or external -- is difficult
because the mount protocol uses UDP to reach the portmapper. You need
to let through UDP packets to the portmapper, UDP traffic to the
rpc.mountd daemon, and UDP or TCP requests to the NFS
server port. Creating a firewall configuration to pass the required
packets through may leave you with a significantly weaker perimeter.
WebNFS eliminates the mount protocol and can restrict traffic to NFS
over TCP, letting you connect clients and servers through a firewall
with only minimal port exposures.

Is WebNFS a good idea? Again, it's not a cure-all for data sharing
over insecure networks, but it's an improvement over existing
protocols. The October 11, 1995 edition of the New York
Times ran a front-page story entitled "Discovery of Internet
Flaws Is Setback for On-Line Trade," in which the security flaws of NFS
were blamed for the lack of growth in electronic commerce. Despite the
mismatch between technical facts and conclusions, the article
highlighted the key point raised in the previous section: If you're
sharing data over a public network, you need to make sure it's not
sensitive (if you're on the server side), and you need stronger
guarantees about the quality and authenticity of the data you pick up
on the client side. If you use WebNFS with some selected group of net
surfers, consider making digital signatures available for files that
you'll want authenticated.

Sharing without caring: Automating access to removable media

So far we've looked at ways of improving transparent, secure access to
filesystems, without worrying about whether the filesystems are there
or not. What happens when you want to have a client automount a
CD-ROM? You need to make the volume available for sharing after it's
been inserted and mounted. The Solaris volume manager,
vold provides the foundation for making CD-ROMs and NFS
work together.

The volume manager uses /etc/vold.confto determine what
devices it manages and where to mount them. We'll look at the volume
manager and its configuration in more detail in a coming month.
Sitting underneath vold is the real workhorse -- the
removable media mounting tool rmmount. It uses its own
configuration file, /etc/rmmount.conf, to guide it through
media changes such as inserts and ejects. Add the following line to
/etc/rmmount.conf to have all CD-ROM devices automatically
shared as soon as they are mounted:

	share  cdrom*

Have the clients mount the server CD-ROMs using the
server:/cdrom path specification, and you'll be able to
automatically access each server's CD-ROM as it is loaded. A typical
automounter map, auto_cdrom, for a network of CD-ROM drives
looks like this:

	*	&:/cdrom

The auto_master entry for this indirect map is:

	 /cds	auto_cdrom

You can't drop the network of CD-ROMs onto /cdrom on the
clients, or you'll collide with the local volume manager. The maps
above mount server-side CD-ROMs on /cds/fred,
/cds/bigboy, and so on.

As usual, there are things to keep in mind:

Having gone through most variations on the NFS and automounter
theme, you should be in a position to make nearly any kind of
filesystem available just about anywhere. Doing the difficult under
adverse conditions is de rigeur for system management. It also frees
time for more important things -- like the USENIX Security Symposium.
When you have your users sliding from volume to volume without your
involvement, you can make your own transparent access to events much
more entertaining.


Unix Insider