--- /dev/null
+
+
+ A Sample Authorization Protocol for X
+
+
+Overview
+
+The following note describes a very simple mechanism for providing individual
+access to an X Window System display. It uses existing core protocol and
+library hooks for specifying authorization data in the connection setup block
+to restrict use of the display to only those clients that show that they
+know a server-specific key called a "magic cookie". This mechanism is *not*
+being proposed as an addition to the Xlib standard; among other reasons, a
+protocol extension is needed to support more flexible mechanisms. We have
+implemented this mechanism already; if you have comments, please send them
+to us.
+
+This scheme involves changes to the following parts of the sample release:
+
+ o xdm
+ - generate random magic cookie and store in protected file
+ - pass name of magic cookie file to server
+ - when user logs in, add magic cookie to user's auth file
+ - when user logs out, generate a new cookie for server
+
+ o server
+ - a new command line option to specify cookie file
+ - check client authorization data against magic cookie
+ - read in cookie whenever the server resets
+ - do not add local machine to host list if magic cookie given
+
+ o Xlib
+ - read in authorization data from file
+ - find data for appropriate server
+ - send authorization data if found
+
+ o xauth [new program to manage user auth file]
+ - add entries to user's auth file
+ - remove entries from user's auth file
+
+This mechanism assumes that the superuser and the transport layer between
+the client and the server is secure. Organizations that desire stricter
+security are encouraged to look at systems such as Kerberos (at Project
+Athena).
+
+
+Description
+
+The sample implementation will use the xdm Display Manager to set up and
+control the server's authorization file. Sites that do not run xdm will
+need to build their own mechanisms.
+
+Xdm uses a random key (seeded by the system time and check sum of /dev/kmem)
+to generate a unique sequence of characters at 16 bytes long. This sequence
+will be written to a file which is made readable only by the server. The
+server will then be started with a command line option instructing it to use
+the contents of the file as the magic cookie for connections that include
+authorization data. This will also disable the server from adding the local
+machine's address to the initial host list. Note that the actual cookie must
+not be stored on the command line or in an environment variable, to prevent
+it from being publicly obtainable by the "ps" command.
+
+If a client presents an authorization name of "MIT-MAGIC-COOKIE-1" and
+authorization data that matches the magic cookie, that client is allowed
+access. If the name or data does not match and the host list is empty,
+that client will be denied access. Otherwise, the existing host-based access
+control will be used. Since any client that is making a connection from a
+machine on the host list will be granted access even if their authorization
+data is incorrect, sites are strongly urged not to set up any default hosts
+using the /etc/X*.hosts files. Granting access to other machines should be
+done by the user's session manager instead.
+
+Assuming the server is configured with an empty host list, the existence of the
+cookie is sufficient to ensure there will be no unauthorized access to the
+display. However, xdm will (continue to) work to minimize the chances of
+spoofing on servers that do not support this authorization mechanism. This
+will be done by grabbing the server and the keyboard after opening the display.
+This action will be surrounded by a timer which will kill the server if the
+grabs cannot be done within several seconds. [This level of security is now
+implemented in patches already sent out.]
+
+After the user logs in, xdm will add authorization entries for each of the
+server machine's network addresses to the user's authorization file (the format
+of which is described below). This file will usually be named .Xauthority in
+the users's home directory; will be owned by the user (as specified by the
+pw_uid and pw_gid fields in the user's password entry), and will be accessible
+only to the user (no group access). This file will contain authorization data
+for all of the displays opened by the user.
+
+When the session terminates, xdm will generate and store a new magic cookie
+for the server. Then, xdm will shutdown its own connection and send a
+SIGHUP to the server process, which should cause the server to reset. The
+server will then read in the new magic cookie.
+
+To support accesses (both read and write) from multiple machines (for use in
+environments that use distributed file systems), file locking is done using
+hard links. This is done by creat'ing (sic) a lock file and then linking it
+to another name in the same directory. If the link-target already exists,
+the link will fail, indicating failure to obtain the lock. Linking is used
+instead of just creating the file read-only since link will fail even for
+the superuser.
+
+Problems and Solutions
+
+There are a few problems with .Xauthority as described. If no home directory
+exists, or if xdm cannot create a file there (disk full), xdm stores the
+cookie in a file in a resource-specified back-up directory, and sets an
+environment variable in the user's session (called XAUTHORITY) naming this
+file. There is also the problem that the locking attempts will need to be
+timed out, due to a leftover lock. Xdm, again, creates a file and set an
+environment variable. Finally, the back-up directory might be full. Xdm,
+as a last resort, provides a function key binding that allows a user to log
+in without having the authorization data stored, and with host-based access
+control disabled.
+
+Xlib
+
+XOpenDisplay in Xlib was enhanced to allow specification of authorization
+information. As implied above, Xlib looks for the data in the
+.Xauthority file of the home directory, or in the file pointed at by the
+XAUTHORITY environment variable instead if that is defined. This required
+no programmatic interface change to Xlib. In addition, a new Xlib routine
+is provided to explicitly specify authorization.
+
+ XSetAuthorization(name, namelen, data, datalen)
+ int namelen, datalen;
+ char *name, *data;
+
+There are three types of input:
+
+ name NULL, data don't care - use default authorization mechanism.
+ name non-NULL, data NULL - use the named authorization; get
+ data from that mechanism's default.
+ name non-NULL, data non-NULL - use the given authorization and data.
+
+This interface is used by xdm and might also be used by any other
+applications that wish to explicitly set the authorization information.
+
+Authorization File
+
+The .Xauthority file is a binary file consisting of a sequence of entries
+in the following format:
+
+ 2 bytes Family value (second byte is as in protocol HOST)
+ 2 bytes address length (always MSB first)
+ A bytes host address (as in protocol HOST)
+ 2 bytes display "number" length (always MSB first)
+ S bytes display "number" string
+ 2 bytes name length (always MSB first)
+ N bytes authorization name string
+ 2 bytes data length (always MSB first)
+ D bytes authorization data string
+
+The format is binary for easy processing, since authorization information
+usually consists of arbitrary data. Host addresses are used instead of
+names to eliminate potentially time-consuming name resolutions in
+XOpenDisplay. Programs, such as xdm, that initialize the user's
+authorization file will have to do the same work as the server in finding
+addresses for all network interfaces. If more than one entry matches the
+desired address, the entry that is chosen is implementation-dependent. In
+our implementation, it is always the first in the file.
+
+The Family is specified in two bytes to allow out-of-band values
+(i.e. values not in the Protocol) to be used. In particular,
+two new values "FamilyLocal" and "FamilyWild" are defined. FamilyLocal
+refers to any connections using a non-network method of connetion from the
+local machine (Unix domain sockets, shared memory, loopback serial line).
+In this case the host address is specified by the data returned from
+gethostname() and better be unique in a collection of machines
+which share NFS directories. FamilyWild is currently used only
+by xdm to communicate authorization data to the server. It matches
+any family/host address pair.
+
+For FamilyInternet, the host address is the 4 byte internet address, for
+FamilyDecnet, the host address is the byte decnet address, for FamilyChaos
+the address is also two bytes.
+
+The Display Number is the ascii representation of the display number
+portion of the display name. It is in ascii to allow future expansion
+to PseudoRoots or anything else that might happen.
+
+A utility called "xauth" will be provided for editing and viewing the
+contents of authorization files. Note that the user's authorization file is
+not the same as the server's magic cookie file.
projects / rdpsrv / blobdiff