NFS Howto Security

From Linux NFS

Revision as of 12:35, 6 April 2006

Security and NFS

This list of security tips and explanations will not make your site completely secure. NOTHING will make your site completely secure. Reading this section may help you get an idea of the security problems with NFS. This is not a comprehensive guide and it will always be undergoing changes. If you have any tips or hints to give us please send them to the HOWTO maintainer.

If you are on a network with no access to the outside world (not even a modem) and you trust all the internal machines and all your users then this section will be of no use to you. However, its our belief that there are relatively few networks in this situation so we would suggest reading this section thoroughly for anyone setting up NFS.

With NFS, there are two steps required for a client to gain access to a file contained in a remote directory on the server. The first step is mount access. Mount access is achieved by the client machine attempting to attach to the server. The security for this is provided by the /etc/exports file. This file lists the names or IP addresses for machines that are allowed to access a share point. If the client's ip address matches one of the entries in the access list then it will be allowed to mount. This is not terribly secure. If someone is capable of spoofing or taking over a trusted address then they can access your mount points. To give a real-world example of this type of "authentication": This is equivalent to someone introducing themselves to you and you believing they are who they claim to be because they are wearing a sticker that says "Hello, My Name is ...." Once the machine has mounted a volume, its operating system will have access to all files on the volume (with the possible exception of those owned by root; see below) and write access to those files as well, if the volume was exported with the rw option.

The second step is file access. This is a function of normal file system access controls on the client and not a specialized function of NFS. Once the drive is mounted the user and group permissions on the files determine access control.

An example: bob on the server maps to the UserID 9999. Bob makes a file on the server that is only accessible the user (the equivalent to typing chmod 600 filename). A client is allowed to mount the drive where the file is stored. On the client mary maps to UserID 9999. This means that the client user mary can access bob's file that is marked as only accessible by him. It gets worse: If someone has become superuser on the client machine they can su - username and become any user. NFS will be none the wiser.

Its not all terrible. There are a few measures you can take on the server to offset the danger of the clients. We will cover those shortly.

If you don't think the security measures apply to you, you're probably wrong. In Section 6.1 we'll cover securing the portmapper, server and client security in Section 6.2 and Section 6.3 respectively. Finally, in Section 6.4 we'll briefly talk about proper firewalling for your nfs server.

Finally, it is critical that all of your nfs daemons and client programs are current. If you think that a flaw is too recently announced for it to be a problem for you, then you've probably already been compromised.

A good way to keep up to date on security alerts is to subscribe to the bugtraq mailinglists. You can read up on how to subscribe and various other information about bugtraq here: http://www.securityfocus.com/forums/bugtraq/faq.html

Additionally searching for NFS at securityfocus.com's search engine will show you all security reports pertaining to NFS.

You should also regularly check CERT advisories. See the CERT web page at www.cert.org.

The Portmapper

The portmapper keeps a list of what services are running on what ports. This list is used by a connecting machine to see what ports it wants to talk to access certain services.

The portmapper is not in as bad a shape as a few years ago but it is still a point of worry for many sys admins. The portmapper, like NFS and NIS, should not really have connections made to it outside of a trusted local area network. If you have to expose them to the outside world - be careful and keep up diligent monitoring of those systems.

Not all Linux distributions were created equal. Some seemingly up-to-date distributions do not include a securable portmapper. 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 /sbin/portmap you can check it with this command. The output that follows would look something like this on a securable machine:

strings /sbin/portmap | grep hosts
/etc/hosts.allow
   /etc/hosts.deny
   @(#) hosts_ctl.c 1.4 94/12/28 17:42:27
   @(#) hosts_access.c 1.21 97/02/12 02:13:22

First we edit /etc/hosts.deny. It should contain the line

portmap: ALL

which will deny access to everyone. While it is closed run:

rpcinfo -p

just to check that your portmapper really reads and obeys this file. Rpcinfo should give no output, or possibly an error message. The files /etc/hosts.allow and /etc/hosts.deny take effect immediately after you save them. No daemon needs to be restarted.

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 administers nfsd, mountd, ypbind/ypserv, rquotad, lockd (which shows up as nlockmgr), statd (which shows up as status) and 'r' services like ruptime and rusers. Of these only nfsd, mountd, ypbind/ypserv and perhaps rquotad,lockd and statd are of any consequence. All machines that need to access services on your machine should be allowed to do that. Let's say that your machine's address is 192.168.0.254 and that it lives on the subnet 192.168.0.0, and that all machines on the subnet should have access to it (for an overview of those terms see the the Networking-Overview-HOWTO). Then we write:

portmap: 192.168.0.0/255.255.255.0

in /etc/hosts.allow. If you are not sure what your network or netmask are, you can use the ifconfig command to determine the netmask and the netstat command to determine the network. For, example, for the device eth0 on the above machine ifconfig should show:

...
   eth0   Link encap:Ethernet  HWaddr 00:60:8C:96:D5:56
          inet addr:192.168.0.254  Bcast:192.168.0.255 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
   ...
   192.168.0.0     0.0.0.0         255.255.255.0   U     0      0   174412 eth0
   ...

(The network address is in the first column).

The /etc/hosts.deny and /etc/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... Versions 0.2.0 and higher of the nfs-utils package also use the hosts.allow and hosts.deny files, so you should put in entries for lockd, statd, mountd, and rquotad in these files too. For a complete example, see Section 3.2.2.

The above things should make your server tighter. The only remaining problem is if someone gains administrative access to one of your trusted client machines and is able to send bogus NFS requests. The next section deals with safeguards against this problem.

Server security: nfsd and mountd

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:

# cat /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.

Client Security

NFS and firewalls (ipchains and netfilter)

Tunneling NFS through SSH

Summary