How the NFS Service Works

The following sections describe some of the complex functions of the NFS software. Note that some of the feature descriptions in this section are exclusive to NFS version 4.

• Version Negotiation in NFS

• Features in NFS Version 4

• UDP and TCP Negotiation

• File Transfer Size Negotiation

• How File Systems Are Mounted

• Effects of the -public Option and NFS URLs When Mounting

• Client-Side Failover

• How NFS Server Logging Works

• How the WebNFS Service Works

• WebNFS Limitations With Web Browser Use

• Secure NFS System

• Secure RPC

Version Negotiation in NFS

The NFS initiation process includes negotiating the protocol levels for servers and clients. If you do not specify the version level, then the best level is selected by default. For example, if both the client and the server can support version 3, then version 3 is used. If the client or the server can only support version 2, then version 2 is used.

You can set the client_versmin, client_versmax, server_versmin and server_versmax parameters using the sharectl command. Your specified minimum and maximum values for the server and the client would replace the default values for these keywords. For both the client and the server the default minimum value is 2 and the default maximum value is 4. To find the version supported by the server, the NFS client begins with the setting for client_versmax and continues to try each version until reaching the version setting for client_versmin. As soon as the supported version is found, the process terminates. For example, if client_versmax=4 and client_versmin=2, then the client attempts version 4 first, then version 3, and finally version 2. If client_versmax and client_versmax are set to the same value, then the client always uses this version and does not attempt any other version. If the server does not offer this version, the mount fails.

For procedural information, refer to Setting Up NFS Services.

Features in NFS Version 4

Many changes have been made to NFS in version 4. This section provides descriptions of these new features.

• Unsharing and Resharing a File System in NFS Version 4

• File-System Namespace in NFS Version 4

• Volatile File Handles in NFS Version 4

• Client Recovery in NFS Version 4

• OPEN Share Support in NFS Version 4

• Delegation in NFS Version 4

• ACLs and nfsmapid in NFS Version 4

• Client-Side Failover in NFS Version 4

For procedural information related to using NFS version 4, refer to Setting Up NFS Services.

Unsharing and Resharing a File System in NFS Version 4

With both NFS version 3 and version 4, if a client attempts to access a file system that has been unshared, the server responds with an error code. However, with NFS version 3 the server maintains any locks that the clients had obtained before the file system was unshared. Thus, when the file system is reshared, NFS version 3 clients can access the file system as though that file system had never been unshared.

With NFS version 4, when a file system is unshared, all the state for any open files or file locks in that file system is destroyed. If the client attempts to access these files or locks, the client receives an error. This error is usually reported as an I/O error to the application. Note, however, that resharing a currently shared file system to change options does not destroy any of the state on the server.

For related information, refer to Client Recovery in NFS Version 4 or see the unshare_nfs(1M) man page.

File-System Namespace in NFS Version 4

NFS version 4 servers create and maintain a pseudo-file system, which provides clients with seamless access to all exported objects on the server. Prior to NFS version 4, the pseudo-file system did not exist. Clients were forced to mount each shared server file system for access. Consider the following example.

Figure 3-2 Views of the Server File System and the Client File System

Note that the client cannot see the payroll directory and the nfs4x directory, because these directories are not exported and do not lead to exported directories. However, the local directory is visible to the client, because local is an exported directory. The projects directory is visible to the client, because projects leads to the exported directory, nfs4. Thus, portions of the server namespace that are not explicitly exported are bridged with a pseudo-file system that views only the exported directories and those directories that lead to server exports.

A pseudo-file system is a structure that contains only directories and is created by the server. The pseudo-file system permits a client to browse the hierarchy of exported file systems. Thus, the client's view of the pseudo-file system is limited to paths that lead to exported file systems.

Previous versions of NFS did not permit a client to traverse server file systems without mounting each file system. However, in NFS version 4, the server namespace does the following:

• Restricts the client's file-system view to directories that lead to server exports.

• Provides clients with seamless access to server exports without requiring that the client mount each underlying file system. See the previous example. Note, however, that different operating systems might require the client to mount each server file system.

Volatile File Handles in NFS Version 4

File handles are created on the server and contain information that uniquely identifies files and directories. In NFS versions 2 and 3 the server returned persistent file handles. Thus, the client could guarantee that the server would generate a file handle that always referred to the same file. For example:

• If a file was deleted and replaced with a file of the same name, the server would generate a new file handle for the new file. If the client used the old file handle, the server would return an error that the file handle was stale.

• If a file was renamed, the file handle would remain the same.

• If you had to reboot the server, the file handles would remain the same.

Thus, when the server received a request from a client that included a file handle, the resolution was straightforward and the file handle always referred to the correct file.

This method of identifying files and directories for NFS operations was fine for most UNIX-based servers. However, the method could not be implemented on servers that relied on other methods of identification, such as a file's path name. To resolve this problem, the NFS version 4 protocol permits a server to declare that its file handles are volatile. Thus, a file handle could change. If the file handle does change, the client must find the new file handle.

Like NFS versions 2 and 3, the Oracle Solaris NFS version 4 server always provides persistent file handles. However, Oracle Solaris NFS version 4 clients that access non-Solaris NFS version 4 servers must support volatile file handles if the server uses them. Specifically, when the server tells the client that the file handle is volatile, the client must cache the mapping between path name and file handle. The client uses the volatile file handle until it expires. On expiration, the client does the following:

• Flushes the cached information that refers to that file handle

• Searches for that file's new file handle

• Retries the operation

Volatile file handles might expire for any of these reasons:

• When you close a file

• When the filehandle's file system migrates

• When a client renames a file

• When the server reboots

Note that if the client is unable to find the new file handle, an error message is put in the syslog file. Further attempts to access this file fail with an I/O error.

Client Recovery in NFS Version 4

The NFS version 4 protocol is a stateful protocol. A protocol is stateful when both the client and the server maintain current information about the following.

• Open files

• File locks

When a failure occurs, such as a server crash, the client and the server work together to reestablish the open and lock states that existed prior to the failure.

When a server crashes and is rebooted, the server loses its state. The client detects that the server has rebooted and begins the process of helping the server rebuild its state. This process is known as client recovery, because the client directs the process.

When the client discovers that the server has rebooted, the client immediately suspends its current activity and begins the process of client recovery. When the recovery process starts, a message, such as the following, is displayed in the system error log /var/adm/messages.

NOTICE: Starting recovery server basil.example.company.com

During the recovery process, the client sends the server information about the client's previous state. Note, however, that during this period the client does not send any new requests to the server. Any new requests to open files or set file locks must wait for the server to complete its recovery period before proceeding.

When the client recovery process is complete, the following message is displayed in the system error log /var/adm/messages.

NOTICE: Recovery done for server basil.example.company.com

Now the client has successfully completed sending its state information to the server. However, even though the client has completed this process, other clients might not have completed their process of sending state information to the server. Therefore, for a period of time, the server does not accept any open or lock requests. This period of time, which is known as the grace period, is designated to permit all the clients to complete their recovery.

During the grace period, if the client attempts to open any new files or establish any new locks, the server denies the request with the GRACE error code. On receiving this error, the client must wait for the grace period to end and then resend the request to the server. During the grace period the following message is displayed.

NFS server recovering

Note that during the grace period the commands that do not open files or set file locks can proceed. For example, the commands ls and cd do not open a file or set a file lock. Thus, these commands are not suspended. However, a command such as cat, which opens a file, would be suspended until the grace period ends.

When the grace period has ended, the following message is displayed.

NFS server recovery ok.

The client can now send new open and lock requests to the server.

Client recovery can fail for a variety of reasons. For example, if a network partition exists after the server reboots, the client might not be able to reestablish its state with the server before the grace period ends. When the grace period has ended, the server does not permit the client to reestablish its state because new state operations could create conflicts. For example, a new file lock might conflict with an old file lock that the client is trying to recover. When such situations occur, the server returns the NO_GRACE error code to the client.

If the recovery of an open operation for a particular file fails, the client marks the file as unusable and the following message is displayed.

WARNING: The following NFS file could not be recovered and was marked dead 
(can't reopen:  NFS status 70):  file :  filename

Note that the number 70 is only an example.

If reestablishing a file lock during recovery fails, the following error message is posted.

NOTICE: nfs4_send_siglost:  pid PROCESS-ID lost
lock on server SERVER-NAME

In this situation, the SIGLOST signal is posted to the process. The default action for the SIGLOST signal is to terminate the process.

For you to recover from this state, you must restart any applications that had files open at the time of the failure. Note that the following can occur.

• Some processes that did not reopen the file could receive I/O errors.

• Other processes that did reopen the file, or performed the open operation after the recovery failure, are able to access the file without any problems.

Thus, some processes can access a particular file while other processes cannot.

OPEN Share Support in NFS Version 4

The NFS version 4 protocol provides several file-sharing modes that the client can use to control file access by other clients. A client can specify the following:

• DENY_NONE mode permits other clients read and write access to a file.

• DENY_READ mode denies other clients read access to a file.

• DENY_WRITE mode denies other clients write access to a file.

• DENY_BOTH mode denies other clients read and write access to a file.

The Oracle Solaris NFS version 4 server fully implements these file-sharing modes. Therefore, if a client attempts to open a file in a way that conflicts with the current share mode, the server denies the attempt by failing the operation. When such attempts fail with the initiation of the open or create operations, the NFS version 4 client receives a protocol error. This error is mapped to the application error EACCES.

Even though the protocol provides several sharing modes, currently the open operation in Oracle Solaris does not offer multiple sharing modes. When opening a file, a Oracle Solaris NFS version 4 client can only use the DENY_NONE mode.

Also, even though the fcntl system call has an F_SHARE command to control file sharing, the fcntl commands cannot be implemented correctly with NFS version 4. If you use these fcntl commands on an NFS version 4 client, the client returns the EAGAIN error to the application.

Delegation in NFS Version 4

NFS version 4 provides both client support and server support for delegation. Delegation is a technique by which the server delegates the management of a file to a client. For example, the server could grant either a read delegation or a write delegation to a client. Read delegations can be granted to multiple clients at the same time, because these read delegations do not conflict with each other. A write delegation can be granted to only one client, because a write delegation conflicts with any file access by any other client. While holding a write delegation, the client would not send various operations to the server because the client is guaranteed exclusive access to a file. Similarly, the client would not send various operations to the server while holding a read delegation. The reason is that the server guarantees that no client can open the file in write mode. The effect of delegation is to greatly reduce the interactions between the server and the client for delegated files. Therefore, network traffic is reduced, and performance on the client and the server is improved. Note, however, that the degree of performance improvement depends on the kind of file interaction used by an application and the amount of network and server congestion.

The decision about whether to grant a delegation is made entirely by the server. A client does not request a delegation. The server makes decisions about whether to grant a delegation, based on the access patterns for the file. If a file has been recently accessed in write mode by several different clients, the server might not grant a delegation. The reason is that this access pattern indicates the potential for future conflicts.

A conflict occurs when a client accesses a file in a manner that is inconsistent with the delegations that are currently granted for that file. For example, if a client holds a write delegation on a file and a second client opens that file for read or write access, the server recalls the first client's write delegation. Similarly, if a client holds a read delegation and another client opens the same file for writing, the server recalls the read delegation. Note that in both situations, the second client is not granted a delegation because a conflict now exists. When a conflict occurs, the server uses a callback mechanism to contact the client that currently holds the delegation. On receiving this callback, the client sends the file's updated state to the server and returns the delegation. If the client fails to respond to the recall, the server revokes the delegation. In such instances, the server rejects all operations from the client for this file, and the client reports the requested operations as failures. Generally, these failures are reported to the application as I/O errors. To recover from these errors, the file must be closed and then reopened. Failures from revoked delegations can occur when a network partition exists between the client and the server while the client holds a delegation.

Note that one server does not resolve access conflicts for a file that is stored on another server. Thus, an NFS server only resolves conflicts for files that it stores. Furthermore, in response to conflicts that are caused by clients that are running various versions of NFS, an NFS server can only initiate recalls to the client that is running NFS version 4. An NFS server cannot initiate recalls for clients that are running earlier versions of NFS.

The process for detecting conflicts varies. For example, unlike NFS version 4, because version 2 and version 3 do not have an open procedure, the conflict is detected only after the client attempts to read, write, or lock a file. The server's response to these conflicts varies also. For example:

• For NFS version 3, the server returns the JUKEBOX error, which causes the client to halt the access request and try again later. The client prints the message File unavailable.

• For NFS version 2, because an equivalent of the JUKEBOX error does not exist, the server makes no response, which causes the client to wait and then try again. The client prints the message NFS server not responding.

These conditions clear when the delegation conflict has been resolved.

By default, server delegation is enabled. You can disable delegation by setting the server_delegation parameter to none. For procedural information, refer to How to Select Different Versions of NFS on a Server.

No keywords are required for client delegation. The NFS version 4 callback daemon, nfs4cbd, provides the callback service on the client. This daemon is started automatically whenever a mount for NFS version 4 is enabled. By default, the client provides the necessary callback information to the server for all Internet transports that are listed in the /etc/netconfig system file. Note that if the client is enabled for IPv6 and if the IPv6 address for the client's name can be determined, then the callback daemon accepts IPv6 connections.

The callback daemon uses a transient program number and a dynamically assigned port number. This information is provided to the server, and the server tests the callback path before granting any delegations. If the callback path does not test successfully, the server does not grant delegations, which is the only externally visible behavior.

Note that because callback information is embedded within an NFS version 4 request, the server is unable to contact the client through a device that uses Network Address Translation (NAT). Also, the callback daemon uses a dynamic port number. Therefore, the server might not be able to traverse a firewall, even if that firewall enables normal NFS traffic on port 2049. In such situations, the server does not grant delegations.

ACLs and nfsmapid in NFS Version 4

An access control list (ACL) provides better file security by enabling the owner of a file to define file permissions for the file owner, the group, and other specific users and groups. On ZFS file systems, ACLs are set on the server and the client by using the chmod command. For UFS file systems, use the setfacl command. See the chmod(1) and the setfacl(1) man pages for more information. In NFS version 4, the ID mapper, nfsmapid, is used to map user or group IDs in ACL entries on a server to user or group IDs in ACL entries on a client. The reverse is also true. The user and group IDs in the ACL entries must exist on both the client and the server.

Reasons for ID Mapping to Fail

The following situations can cause ID mapping to fail:

• If the user or group that exists in an ACL entry on the server cannot be mapped to a valid user or group on the client, the user can read the ACL, but some of the users or groups will be shown as “unknown”.

  For example, when you issue the ls -lv or ls -lV command, some of the ACL entries will have the group or user be displayed as “unknown”. For more information about this command, see the ls(1) man page.

• If the user or group ID in any ACL entry that is set on the client cannot be mapped to a valid user or group ID on the server, the setfacl or the chmod command can fail and return the Permission denied error message.

• If the client and server have mismatched nfsmapid_domain values, ID mapping fails. For more information, see nfsmapid Daemon.

Avoiding ID Mapping Problems With ACLs

To avoid ID mapping problems, do the following:

• Make sure that the value for nfsmapid_domain is set correctly.

• Make sure that all user and group IDs in the ACL entries exist on both the NFS version 4 client and server.

Checking for Unmapped User or Group IDs

To determine if any user or group cannot be mapped on the server or client, use the following script:

#! /usr/sbin/dtrace -Fs

sdt:::nfs4-acl-nobody
{
     printf("validate_idmapping: (%s) in the ACL could not be mapped!", 
stringof(arg0));
}

Additional Information About ACLs or nfsmapid

See the following:

• Chapter 7, Using ACLs and Attributes to Protect Oracle Solaris ZFS Files, in Oracle Solaris 11.1 Administration: ZFS File Systems

• nfsmapid Daemon

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. 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

The following description applies to NFS version 3 mounts. The NFS version 4 mount process does not include the portmap service nor does it include the MOUNT protocol.

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 test the availability of 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 tests the availability of 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 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.

• Public option with NFS URL – Forces the use of the public file handle. The mount fails if the public file handle is not supported.

• Public option with regular path – Forces the use of the public file handle. The mount fails if the public file handle is not supported.

• NFS URL only – Use the public file handle if this file handle is enabled on the NFS server. If the mount fails when using the public file handle, then try the mount with the MOUNT protocol.

• Regular path only – Do not use the public file handle. The MOUNT protocol is used.

Client-Side Failover

By using client-side failover, an NFS client can be aware of multiple servers that are making the same data available and can switch to an alternate server when the current server is unavailable. The file system can become unavailable if one of the following occurs.

• If the file system is connected to a server that crashes

• If the server is overloaded

• If a network fault occurs

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 CacheFS and client-side failover on the same NFS mount. 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.

• failover – The process of selecting a server from a list of servers that support a replicated file system. Normally, the next server in the sorted list is used, unless it fails to respond.

• remap – To use a new server. Through normal use, the clients store the path name for each active file on the remote file system. During the remap, these path names are evaluated to locate the files on the new server.

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 file size or file type as the original file system. Permissions, creation dates, and other file attributes are not considered. If the file size or file types are different, the remap fails and the process hangs until the old server becomes available. In NFS version 4, the behavior is different. See Client-Side Failover in NFS Version 4.

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

• Renaming the old version of the file before installing a new version of the file

• Running the updates at night when client usage is low

• Keeping the updates small

• Minimizing the number of copies

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.

Client-Side Failover in NFS Version 4

In NFS version 4, if a replica cannot be established because the file sizes are different or the file types are not the same, then the following happens.

• The file is marked dead.

• A warning is printed.

• The application receives a system call failure.

In NFS version 4, you no longer receive replication errors for directories of different sizes. In prior versions of NFS, this condition was treated as an error and would impede the remapping process.

Furthermore, in NFS version 4, if a directory read operation is unsuccessful, the operation is performed by the next listed server. In previous versions of NFS, unsuccessful read operations would cause the remap to fail and the process to hang until the original server was available.

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:

• A timestamp

• The client IP address

• The UID of the requester

• The file handle of the file or directory object that is being accessed

• The type of operation that occurred

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 identified again 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 multi-component lookup transaction when the LOOKUP is relative to the public file handle. Multi-component 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 NFS service includes a protocol that enables a WebNFS client to negotiate a selected security mechanism with a WebNFS server. The new protocol uses security negotiation multi-component lookup, which is an extension to the multi-component lookup that was used in earlier versions of the WebNFS protocol.

The WebNFS client initiates the process by making a regular multi–component 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 multi-component 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:

• NFS browsing does not run CGI scripts. So, a file system with an active web site that uses many CGI scripts might not be appropriate for NFS browsing.

• The browser might start different viewers to handle files in different file formats. Accessing these files through an NFS URL starts an external viewer if the file type can be determined by the file name. The browser should recognize any file name extension for a standard MIME type when an NFS URL is used. The WebNFS software does not check inside the file to determine the file type. So, the only way to determine a file type is by the file name extension.

• NFS browsing cannot utilize server-side image maps (clickable images). However, NFS browsing can utilize client-side image maps (clickable images) because the URLs are defined with the location. No additional response is required from the document server.

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, which is merely listening to network traffic without impersonating anybody, are not as dangerous, because 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 Oracle Solaris operating system includes an authentication system at the level of the remote procedure call (RPC), which is the mechanism on which the 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 assurance of 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, and birthday. The verifier is the photo that is attached to the badge. You can be sure that 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 Oracle Solaris Administration: Naming and Directory Services for more information about 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:

• The two agents must agree on the current time.

• The sender and receiver must be using the same encryption key.

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 become unsynchronized 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. Kerberos offers a variety of encryption types, including DES. Kerberos support is no longer supplied as part of Secure RPC, but a server-side and client-side implementation is included in the release. See Chapter 19, Introduction to the Kerberos Service, in Oracle Solaris 11.1 Administration: Security Services for more information about the implementation of Kerberos authentication.

Using Secure RPC With NFS

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

• If a server crashes when no one is around (after a power failure, for example), all the secret keys that are stored on the system are deleted. Now no process can access secure network services or mount an NFS file system. The important processes during a reboot are usually run as root. Therefore, these processes would work if root's secret key were stored away, but nobody is available to type the password that decrypts it. keylogin -r allows root to store the clear secret key in /etc/.rootkey, which keyserv reads.

• Some systems boot in single-user mode, with a root login shell on the console and no password prompt. Physical security is imperative in such cases.

• Diskless computer booting is not totally secure. Somebody could impersonate the boot server and boot a devious kernel that, for example, makes a record of your secret key on a remote computer. The Secure NFS system provides protection only after the kernel and the key server are running. Otherwise, no way exists to authenticate the replies that are given by the boot server. This limitation could be a serious problem, but the limitation requires a sophisticated attack, using kernel source code. Also, the crime would leave evidence. If you polled the network for boot servers, you would discover the devious boot server's location.

• Most setuid programs are owned by root. If the secret key for root is stored in /etc/.rootkey, these programs behave as they always have. If a setuid program is owned by a user, however, the setuid program might not always work. For example, suppose that a setuid program is owned by dave and dave has not logged into the computer since it booted. The program would not be able to access secure network services.

• If you log in to a remote computer (using login, rlogin, or telnet) and use keylogin to gain access, you give access to your account. The reason is that your secret key is passed to that computer's key server, which then stores your secret key. This process is only a concern if you do not trust the remote computer. If you have doubts, however, do not log in to a remote computer if the remote computer requires a password. Instead, use the NFS environment to mount file systems that are shared by the remote computer. As an alternative, you can use keylogout to delete the secret key from the key server.

• If a home directory is shared with the -o sec=dh option, remote logins can be a problem. If the /etc/hosts.equiv or ~/.rhosts files are not set to prompt for a password, the login succeeds. However, the users cannot access their home directories because no authentication has occurred locally. If the user is prompted for a password, the user has access to his or her home directory if the password matches the network password.