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System Administration Guide: Resource Management and Network Services

How the NFS Service Works

The following sections describe some of the complex functions of the NFS software.

Version 2 and Version 3 Negotiation

NFS servers might be supporting clients that are not using the NFS version 3 software. So, part of the initiation procedure includes negotiation of the protocol level. If both the client and the server can support version 3, that version is used. If either the client or the server can only support version 2, that version is selected.

You can override the values that are determined by the negotiation by using the -vers option with the mount command. See the mount_nfs(1M) man page. Under most circumstances, you should not have to specify the version level, as the best level is selected by default.

UDP and TCP Negotiation

During initiation, the transport protocol is also negotiated. By default, the first connection-oriented transport that is supported on both the client and the server is selected. If this selection does not succeed, the first available connectionless transport protocol is used. The transport protocols that are supported on a system are listed in /etc/netconfig. TCP is the connection-oriented transport protocol that is supported by the release. UDP is the connectionless transport protocol.

When both the NFS protocol version and the transport protocol are determined by negotiation, the NFS protocol version is given precedence over the transport protocol. The NFS version 3 protocol that uses UDP is given higher precedence than the NFS version 2 protocol that is using TCP. You can manually select both the NFS protocol version and the transport protocol with the mount command. See the mount_nfs(1M) man page. Under most conditions, allow the negotiation to select the best options.

File Transfer Size Negotiation

The file transfer size establishes the size of the buffers that are used when transferring data between the client and the server. In general, larger transfer sizes are better. The NFS version 3 protocol has an unlimited transfer size. However, starting with the Solaris 2.6 release, the software bids a default buffer size of 32 Kbytes. The client can bid a smaller transfer size at mount time if needed, but under most conditions this bid is not necessary.

The transfer size is not negotiated with systems that use the NFS version 2 protocol. Under this condition, the maximum transfer size is set to 8 Kbytes.

You can use the -rsize and -wsize options to set the transfer size manually with the mount command. You might need to reduce the transfer size for some PC clients. Also, you can increase the transfer size if the NFS server is configured to use larger transfer sizes.

How File Systems Are Mounted

When a client needs to mount a file system from a server, the client must obtain a file handle from the server. The file handle must correspond to the file system. This process requires that several transactions occur between the client and the server. In this example, the client is attempting to mount /home/terry from the server. A snoop trace for this transaction follows. 


client -> server PORTMAP C GETPORT prog=100005 (MOUNT) vers=3 proto=UDP
server -> client PORTMAP R GETPORT port=33492
client -> server MOUNT3 C Null
server -> client MOUNT3 R Null 
client -> server MOUNT3 C Mount /export/home9/terry
server -> client MOUNT3 R Mount OK FH=9000 Auth=unix
client -> server PORTMAP C GETPORT prog=100003 (NFS) vers=3 proto=TCP
server -> client PORTMAP R GETPORT port=2049
client -> server NFS C NULL3
server -> client NFS R NULL3 
client -> server NFS C FSINFO3 FH=9000
server -> client NFS R FSINFO3 OK
client -> server NFS C GETATTR3 FH=9000
server -> client NFS R GETATTR3 OK

In this trace, the client first requests the mount port number from the portmap service on the NFS server. After the client receives the mount port number (33492), that number is used to ping the service on the server. After the client has determined that a service is running on that port number, the client then makes a mount request. When the server responds to this request, the server includes the file handle for the file system (9000) being mounted. The client then sends a request for the NFS port number. When the client receives the number from the server, the client pings the NFS service (nfsd). Also, the client requests NFS information about the file system that uses the file handle.

In the following trace, the client is mounting the file system with the public option. 


client -> server NFS C LOOKUP3 FH=0000 /export/home9/terry
server -> client NFS R LOOKUP3 OK FH=9000
client -> server NFS C FSINFO3 FH=9000
server -> client NFS R FSINFO3 OK
client -> server NFS C GETATTR3 FH=9000
server -> client NFS R GETATTR3 OK

By using the default public file handle (which is 0000), all of the transactions to obtain information from the portmap service and to determine the NFS port number are skipped.

Effects of the -public Option and NFS URLs When Mounting

Using the -public option can create conditions that cause a mount to fail. Adding an NFS URL can also confuse the situation. The following list describes the specifics of how a file system is mounted when you use these options.

Client-Side Failover

By using client-side failover, an NFS client can switch to another server if the server that supports a replicated file system becomes unavailable. The file system can become unavailable under one of the following circumstances.

The failover, under these conditions, is normally transparent to the user. Thus, the failover can occur at any time without disrupting the processes that are running on the client.

Failover requires that the file system be mounted read-only. The file systems must be identical for the failover to occur successfully. See What Is a Replicated File System? for a description of what makes a file system identical. A static file system or a file system that is not changed often is the best candidate for failover.

You cannot use file systems that are mounted by using CacheFS with failover. Extra information is stored for each CacheFS file system. This information cannot be updated during failover, so only one of these two features can be used when mounting a file system.

The number of replicas that need to be established for every file system depends on many factors. Ideally, you should have a minimum of two servers. Each server should support multiple subnets. This setup is better than having a unique server on each subnet. The process requires that each listed server be checked. Therefore, if more servers are listed, each mount is slower.

Failover Terminology

To fully comprehend the process, you need to understand two terms.

What Is a Replicated File System?

For the purposes of failover, a file system can be called a replica when each file is the same size and has the same vnode type as the original file system. Permissions, creation dates, and other file attributes are not considered. If the file size or vnode types are different, the remap fails and the process hangs until the old server becomes available.

You can maintain a replicated file system by using rdist, cpio, or another file transfer mechanism. Because updating the replicated file systems causes inconsistency, follow these suggestions for best results:

Failover and NFS Locking

Some software packages require read locks on files. To prevent these products from breaking, read locks on read-only file systems are allowed but are visible to the client side only. The locks persist through a remap because the server does not “know” about the locks. Because the files should not change, you do not need to lock the file on the server side.

Large Files

Starting with 2.6, the Solaris release supports files that are over 2 Gbytes. By default, UFS file systems are mounted with the -largefiles option to support the new capability. Previous releases cannot handle files of this size. See How to Disable Large Files on an NFS Server for instructions.

If the server's file system is mounted with the -largefiles option, a Solaris 2.6 NFS client can access large files without the need for changes. However, not all 2.6 commands can handle these large files. See largefile(5) for a list of the commands that can handle the large files. Clients that cannot support the NFS version 3 protocol with the large file extensions cannot access any large files. Although clients that run the Solaris 2.5 release can use the NFS version 3 protocol, large file support was not included in that release.

How NFS Server Logging Works

NFS server logging provides records of NFS reads and writes, as well as operations that modify the file system. This data can be used to track access to information. In addition, the records can provide a quantitative way to measure interest in the information.

When a file system with logging enabled is accessed, the kernel writes raw data into a buffer file. This data includes the following:

The nfslogd daemon converts this raw data into ASCII records that are stored in log files. During the conversion, the IP addresses are modified to host names and the UIDs are modified to logins if the name service that is enabled can find matches. The file handles are also converted into path names. To accomplish the conversion, the daemon tracks the file handles and stores information in a separate file handle-to-path table. That way, the path does not have to be re-identified each time a file handle is accessed. Because no changes to the mappings are made in the file handle-to-path table if nfslogd is turned off, you must keep the daemon running.

How the WebNFS Service Works

The WebNFS service makes files in a directory available to clients by using a public file handle. A file handle is an address that is generated by the kernel that identifies a file for NFS clients. The public file handle has a predefined value, so the server does not need to generate a file handle for the client. The ability to use this predefined file handle reduces network traffic by eliminating the MOUNT protocol. This ability should also accelerate processes for the clients.

By default, the public file handle on an NFS server is established on the root file system. This default provides WebNFS access to any clients that already have mount privileges on the server. You can change the public file handle to point to any file system by using the share command.

When the client has the file handle for the file system, a LOOKUP is run to determine the file handle for the file to be accessed. The NFS protocol allows the evaluation of only one path name component at a time. Each additional level of directory hierarchy requires another LOOKUP. A WebNFS server can evaluate an entire path name with a single multicomponent lookup transaction when the LOOKUP is relative to the public file handle. Multicomponent lookup enables the WebNFS server to deliver the file handle to the desired file without exchanging the file handles for each directory level in the path name.

In addition, an NFS client can initiate concurrent downloads over a single TCP connection. This connection provides quick access without the additional load on the server that is caused by setting up multiple connections. Although web browser applications support concurrent downloading of multiple files, each file has its own connection. By using one connection, the WebNFS software reduces the overhead on the server.

If the final component in the path name is a symbolic link to another file system, the client can access the file if the client already has access through normal NFS activities.

Normally, an NFS URL is evaluated relative to the public file handle. The evaluation can be changed to be relative to the server's root file system by adding an additional slash to the beginning of the path. In this example, these two NFS URLs are equivalent if the public file handle has been established on the /export/ftp file system.


nfs://server/junk
nfs://server//export/ftp/junk

How WebNFS Security Negotiation Works

The Solaris 8 release includes a new protocol so a WebNFS client can negotiate a selected security mechanism with a WebNFS server. The new protocol uses security negotiation multicomponent lookup, which is an extension to the multicomponent lookup that was used in earlier versions of the WebNFS protocol.

The WebNFS client initiates the process by making a regular multicomponent lookup request by using the public file handle. Because the client has no knowledge of how the path is protected by the server, the default security mechanism is used. If the default security mechanism is not sufficient, the server replies with an AUTH_TOOWEAK error. This reply indicates that the default mechanism is not valid. The client needs to use a stronger default mechanism.

When the client receives the AUTH_TOOWEAK error, the client sends a request to the server to determine which security mechanisms are required. If the request succeeds, the server responds with an array of security mechanisms that are required for the specified path. Depending on the size of the array of security mechanisms, the client might have to make more requests to obtain the complete array. If the server does not support WebNFS security negotiation, the request fails.

After a successful request, the WebNFS client selects the first security mechanism from the array that the client supports. The client then issues a regular multicomponent lookup request by using the selected security mechanism to acquire the file handle. All subsequent NFS requests are made by using the selected security mechanism and the file handle.

WebNFS Limitations With Web Browser Use

Several functions that a web site that uses HTTP can provide are not supported by the WebNFS software. These differences stem from the fact that the NFS server only sends the file, so any special processing must be done on the client. If you need to have one web site configured for both WebNFS and HTTP access, consider the following issues:

Secure NFS System

The NFS environment is a powerful way and convenient way to share file systems on a network of different computer architectures and operating systems. However, the same features that make sharing file systems through NFS operation convenient also pose some security problems. Historically, most NFS implementations have used UNIX (or AUTH_SYS) authentication, but stronger authentication methods such as AUTH_DH have also been available. When using UNIX authentication, an NFS server authenticates a file request by authenticating the computer that makes the request, but not the user. Therefore, a client user can run su and impersonate the owner of a file. If DH authentication is used, the NFS server authenticates the user, making this sort of impersonation much harder.

With root access and knowledge of network programming, anyone can introduce arbitrary data into the network and extract any data from the network. The most dangerous attacks are those attacks that involve the introduction of data. An example is the impersonation of a user by generating the right packets or by recording “conversations” and replaying them later. These attacks affect data integrity. Attacks that involve passive eavesdropping—merely listening to network traffic without impersonating anybody—are not as dangerous, as data integrity is not compromised. Users can protect the privacy of sensitive information by encrypting data that is sent over the network.

A common approach to network security problems is to leave the solution to each application. A better approach is to implement a standard authentication system at a level that covers all applications.

The Solaris operating environment includes an authentication system at the level of remote procedure call (RPC)—the mechanism on which NFS operation is built. This system, known as Secure RPC, greatly improves the security of network environments and provides additional security to services such as the NFS system. When the NFS system uses the facilities that are provided by Secure RPC, it is known as a Secure NFS system.

Secure RPC

Secure RPC is fundamental to the Secure NFS system. The goal of Secure RPC is to build a system that is at minimum as secure as a time-sharing system. In a time-sharing system all users share a single computer. A time-sharing system authenticates a user through a login password. With data encryption standard (DES) authentication, the same authentication process is completed. Users can log in on any remote computer just as users can log in on a local terminal. The users' login passwords are their passports to network security. In a time-sharing environment, the system administrator has an ethical obligation not to change a password to impersonate someone. In Secure RPC, the network administrator is trusted not to alter entries in a database that stores public keys.

You need to be familiar with two terms to understand an RPC authentication system: credentials and verifiers. Using ID badges as an example, the credential is what identifies a person: a name, address, birthday, and so on. The verifier is the photo that is attached to the badge. You can be sure the badge has not been stolen by checking the photo on the badge against the person who is carrying the badge. In RPC, the client process sends both a credential and a verifier to the server with each RPC request. The server sends back only a verifier because the client already “knows” the server's credentials.

RPC's authentication is open ended, which means that a variety of authentication systems can be plugged into it, such as UNIX, DH, and KERB.

When UNIX authentication is used by a network service, the credentials contain the client's host name, UID, GID, and group-access list. However, the verifier contains nothing. Because no verifier exists, a superuser could falsify appropriate credentials by using commands such as su. Another problem with UNIX authentication is that UNIX authentication assumes all computers on a network are UNIX computers. UNIX authentication breaks down when applied to other operating systems in a heterogeneous network.

To overcome the problems of UNIX authentication, Secure RPC uses DH authentication.

DH Authentication

DH authentication uses the Data Encryption Standard (DES) and Diffie-Hellman public-key cryptography to authenticate both users and computers in the network. DES is a standard encryption mechanism. Diffie-Hellman public-key cryptography is a cipher system that involves two keys: one public and one secret. The public keys and secret keys are stored in the namespace. NIS stores the keys in the public-key map. These maps contain the public key and secret key for all potential users. See the System Administration Guide: Naming and Directory Services (DNS, NIS, and LDAP) for more information on how to set up the maps.

The security of DH authentication is based on a sender's ability to encrypt the current time, which the receiver can then decrypt and check against its own clock. The timestamp is encrypted with DES. The requirements for this scheme to work are as follows:

If a network runs a time-synchronization program, the time on the client and the server is synchronized automatically. If a time-synchronization program is not available, timestamps can be computed by using the server's time instead of the network time. The client asks the server for the time before starting the RPC session, then computes the time difference between its own clock and the server's. This difference is used to offset the client's clock when computing timestamps. If the client and server clocks get out of synchronization to the point where the server begins to reject the client's requests, the DH authentication system on the client resynchronizes with the server.

The client and server arrive at the same encryption key by generating a random conversation key, also known as the session key, and by using public-key cryptography to deduce a common key. The common key is a key that only the client and server are capable of deducing. The conversation key is used to encrypt and decrypt the client's timestamp. The common key is used to encrypt and decrypt the conversation key.

KERB Authentication

Kerberos is an authentication system that was developed at MIT. Encryption in Kerberos is based on DES. Kerberos support is no longer supplied as part of Secure RPC, but a server-side and client-side implementation is included with the Solaris 9 release. See “Introduction to SEAM” in System Administration Guide: Security Services for more information about the Solaris 9 implementation of Kerberos Authentication.

Using Secure RPC With NFS

Be aware of the following points if you plan to use Secure RPC: