Access Control Mechanisms

An access control mechanism controls which clients or applications have access to the OpenWindows server. Only properly authorized clients can connect to the server. All unauthorized X clients terminate with the following error message:

Xlib: connection to hostname refused by server Xlib: Client is not authorized to connect to server

The connection attempt logs to the server console as:

AUDIT: <Date Time Year>: X: client 6 rejected from IP 129.144.152.193 port 3485 Auth name: MIT-MAGIC-COOKIE-1

The two types of access control mechanisms are: user-based and host-based. Unless the -noauth option is used with openwin, both the user-based access control mechanism and the host-based access control mechanism are active. See "Manipulating Access to the Server" on page 105 for more information.

User-Based

A user-based, or authorization-based mechanism allows you to explicitly give access to a particular user on any host. The user's client passes authorization data to the server. If the data matches the server's authorization data, the user obtains access.

Host-Based

A host-based mechanism is a general purpose mechanism. It allows you to give access to a particular host, in which all users on that host can connect to the server. This is a weak form of access control; if that host has access to the server, all users on that host can connect to the server.

OpenWindows provides the host-based mechanism for backward compatibility. Applications linked with a version of Xlib older than OpenWindows Version 2 or X11R4 do not recognize the new user-based access

control mechanism. To enable these applications to connect to the server, a user must either switch to the host-based mechanism, or relink with the newer version of Xlib.

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Note - If possible, clients linked with an older version of Xlib should be relinked with a newer version of Xlib. This enables them to connect to the server with the new user-based access control mechanism.

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Authorization Protocols

The OpenWindows environment supports two different authorization protocols: MIT-MAGIC-COOKIE-1 and SUN-DES-1. While they differ in the authorization data used, they are similar in the access control mechanism used.

The MIT-MAGIC-COOKIE-1 protocol, using the user-based mechanism, is the OpenWindows environment default.

MIT-MAGIC-COOKIE-1

The MIT-MAGIC-COOKIE-1 authorization protocol was developed by the (MIT) Massachusetts Institute of Technology. A magic cookie is a long, randomly generated binary password. At server start-up, the magic cookie is created for the server and the user who started the system. On every connection attempt, the user's client sends the magic cookie to the server as part of the connection packet. This magic cookie is compared with the servers' magic cookie. The connection is allowed if the magic cookies match, or denied if they do not match.

SUN-DES-1

The SUN-DES-1 authorization protocol was developed by Sun Microsystems. It is based on Secure RPC (Remote Procedure Call) and requires DES (Data Encryption Software) support. (See the Network Interfaces Programmer's Guide for more information). The authorization data is the machine independent netname, or network name, of a user. This data is encrypted and sent to the server as part of the connection packet. The server decrypts the data, and if the netname is known, allows the connection.

The SUN-DES-1 authorization protocol provides a higher level of security than the MIT-MAGIC-COOKIE-1 protocol. There is no way for another user to use your machine independent netname to access a server, but it is possible for another user to use the magic cookie to access a server.

This protocol is available only in libraries in the OpenWindows Version 3 and later environments. Any applications built with static libraries, in particular Xlib, in environments prior to OpenWindows Version 3 cannot use this authorization protocol.

"Allowing Access When Using SUN-DES-1" on page 107 describes how to allow another user access to your server by adding their netname to your server's access list.

Changing the Default Authorization Protocol

The default authorization protocol, MIT-MAGIC-COOKIE-1, can be changed to another supported authorization protocol or to no user-based access mechanism at all. The default is changed by supplying options with the openwin command. See the openwin(1) man page for more information.

For example, to change the default from MIT-MAGIC-COOKIE-1 to SUN-DES-1, start OpenWindows as follows:

example% openwin -auth sun-des

If you must run OpenWindows without the user-based access mechanism, use the -noauth command line option.

example% openwin -noauth

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Warning - Using -noauth weakens security. It is equivalent to running OpenWindows with only the host-based access control mechanism; the server inactivates the user-based access control mechanism. Anyone that can run applications on your local machine will be allowed access to your server.

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Manipulating Access to the Server

Unless the -noauth option is used with openwin (see "Changing the Default Authorization Protocol), both the user-based access control mechanism and the host-based access control mechanism are active. The server first checks the user-based mechanism, then the host-based mechanism. The default security configuration uses MIT-MAGIC-COOKIE-1 as the user-based mechanism, and an empty list for the host-based mechanism. Since the host-based list is empty, only the user-based mechanism is effectively active. Using the -noauth option instructs the server to inactivate the user-based access control mechanism and initializes the host-based list by adding the local host.

There are two programs that can be used to change a server's access control mechanism: xhost and xauth. For more information, see these man pages. These programs access two binary files created by the authorization protocol. These files contain session-specific authorization data. One file is for server internal use only. The other file is located in the user's $HOME directory:

.Xauthority                (Client Authority File)

Use the xhost program to change the host-based access list in the server. You can add hosts to, or delete hosts from the access list. If you are starting with the default configuration-an empty host-based access list-and use xhost to add a machine name, you lower the level of security. The server allows access to the host you added, as well as to any user specifying the default authorization protocol. See "Host-Based" on page 102 for an explanation of why the host-based access control mechanism is considered a lower level of security.

The xauth program accesses the authorization protocol data in the .Xauthority client file. You can extract this data from your .Xauthority file so that another user can merge the data into their .Xauthority file, thus allowing them access to your server, or to the server in which you connect.

See "Allowing Access When Using MIT-MAGIC-COOKIE-1" on page 107 for examples of how to use xhost and xauth.

Client Authority File

The client authority file is .Xauthority. It contains entries of the form:

connection-protocol...auth-protocol....auth-data

By default, .Xauthority contains MIT-MAGIC-COOKIE-1 as the auth-protocol, and entries for the local display only as the connection-protocol and auth-data. For example, on host anyhost, the .Xauthority file may contain the following entries:

anyhost:0         MIT-MAGIC-COOKIE-1            82744f2c4850b03fce7ae47176e75
localhost:0       MIT-MAGIC-COOKIE-1            82744f2c4850b03fce7ae47176e75
anyhost/unix:0    MIT-MAGIC-COOKIE-1            82744f2c4850b03fce7ae47176e75

When the client starts up, an entry corresponding to the connection-protocol is read from .Xauthority, and the auth-protocol and auth-data are sent to the server as part of the connection packet. In the default configuration, xhost returns an empty host-based access list and states that the authorization is enabled.

If you have changed the authorization protocol from the default to SUN-DES-1 the entries in .Xauthority contain SUN-DES-1 as the auth-protocol and the netname of the user as the auth-data. The netname is in the following form:

unix.userid@NISdomainname

For example, on host, anyhost the .Xauthority file may contain the following entries:

anyhost:0         SUN-DES-1             "unix.15339@EBB.Eng.Sun.COM"
localhost:0       SUN-DES-1             "unix.15339@EBB.Eng.Sun.COM"
anyhost/unix:0    SUN-DES-1             "unix.15339@EBB.Eng.Sun.COM"

where, unix.15339@EBB.Eng.Sun.COM is the machine independent netname of the user.

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Note - If you do not know your network name, or machine independent netname, ask your System Administrator.

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Allowing Access When Using MIT-MAGIC-COOKIE-1

If you are using the MIT-MAGIC-COOKIE-1 authorization protocol, follow these steps to allow another user access to your server:

1. On the machine running the server, use xauth to extract an entry corresponding to hostname:0 into a file.

   For this example, hostname is anyhost and the file is xauth.info:

myhost% $OPENWINHOME/bin/xauth nextract - anyhost:0 > $HOME/xauth.info

2. Send the file containing the entry to the user requesting access (using Mail Tool, rcp or some other file transfer protocol).

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Note - Mailing the file containing your authorization information is a safer method than using rcp. If you do use rcp, do not place the file in a directory that is easily accessible by another user.

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3. The other user must merge the entry into their .Xauthority file. For this example, userhost merges xauth.info into their .Xauthority file:

userhost% $OPENWINHOME/bin/xauth nmerge - < xauth.info

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Note - The auth-data is session-specific; therefore, it is valid only as long as the server is not restarted.

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Allowing Access When Using SUN-DES-1

If you are using the SUN-DES-1 authorization protocol, follow these steps to allow another user access to your server:

1. On the machine running the server, use xhost to make the new user known to the server.

   For this example, to allow new user somebody to run on myhost:

myhost% xhost + somebody@

2. The new user must use xauth to add the entry into their .Xauthority file.

   For this example, new user, somebody's machine independent netname is unix.15339@EBB.Eng.Sun.COM:

userhost% echo 'add myhost:0 SUN-DES-1 "unix.15339@EBB.Eng.Sun.COM"' | $OPENWINHOME/bin/xauth

Running Clients Remotely, or Locally as Another User

X clients use the value of the DISPLAY environment variable to get the name of the server in which they should connect.

To run clients remotely, or locally as another user, follow these steps:

1. On the machine running the server, allow another user access.

   Depending on which authorization protocol you use, follow the steps outlined in either "Allowing Access When Using MIT-MAGIC-COOKIE-1" on page 107 or "Allowing Access When Using SUN-DES-1" on page 107.

2. Set DISPLAY to the name of the host running the server. For this example, the host is remotehost:

myhost% setenv DISPLAY remotehost:0

3. Run the client program.

   The client will be displayed on the remote machine, remotehost.

myhost% client_program&