+++ /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