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+\appendix
+\section{The umap Layer} \label{sect:umap}
+
+\subsection{Introduction}
+
+Normally, the file system is expected to span a single administrative domain.
+An administrative domain, for these purposes, is a machine or set of
+machines that share common password file information, usually through
+the yellow pages mechanism.  File hierarchies that span more 
+than one domain leads to certain problems, since the same numerical 
+UID in one domain may correspond to a different user in another domain.  
+If the system administrator is very careful to ensure that both domains 
+contain identical user ID information, The umap layer can be used to
+run between those domains without changes
+
+The umap layer is a file system layer that sits on top of the normal
+file layer.  The umap layer maps Unix-style UIDs from
+one domain into the UIDs in the other domain.  By setting up the mappings
+properly, the same user with different UIDs in two domains can be seen
+as the same user, from the system point of view, or, conversely, two
+different users with the same UID in the two domains can be distinguished.
+
+First, we define some terms.  ``User'' refers to the human (or daemon) that
+has privileges to login, run programs, and access files.  ``UID''refers to
+the numerical identifier that uniquely identifies the user within a
+single domain.  ``Login name'' refers to the character string the user
+types to log into the system.  ``GID'' refers to the numerical group
+identifier used by Unix systems to identify groups of users.  ``Group
+name'' is the character string name attached to a particular GID in the
+local {\sf /etc/groups} file or the yellow pages groups file.
+
+In order for the umap layer to work properly, all users 
+in either domain must have password file entries in both domains.  
+They do not, however, have to have the same numerical UID, nor even the 
+same character string login name (the latter is highly recommended, 
+if possible, however).  Any user not having a UID in one domain will be 
+treated as the special user NOBODY by the other domain, probably with 
+undesirable consequences.  Any user not owning any files in the shared
+sub-trees need not be given a UID in the other domain.
+
+Groups work similarly.  The umap layer can translate group ID's between
+domains in the same manner as UID's.  Again, any group that wishes to
+participate must have a group ID in both domains,
+though it need not be the same GID in both.  If a group in one domain is not
+known in the other domain, that group will be treated as being NULLGROUP.
+The umap layer has no provisions for enrolling UID's from other domains
+as group members, but, since each user from each domain must have some
+UID in every domain, the UID in the local domain can be used to enroll
+the user in the local groups.  
+
+NOBODY and NULLGROUP are special reserved UID's and GID's, respectively.
+NOBODY is user 32767.  NULLGROUP is group 65534.  If the system administrator
+wants to have an appropriate text string appear when these UID's are
+encountered by programs like {\sf ls -l}, he should add these values to
+the password and {\sf /etc/groups} file, or to the appropriate yellow pages.  
+If these IDs are already in use in that domain, different values can be 
+used for NOBODY and NULLGROUP, but that will require a recompilation of 
+the umap layer code and, as a result, the entire kernel.  These 
+values are defined in the {\sf umap\_info.h} file, kept with the rest of the 
+umap source code.
+
+When the umap layer is in use, one of the participating domains is declared 
+to be the master.  All UID and GID information stored for participating files 
+will be stored in vnodes using its mappings, no matter what site the copies of 
+the files are stored at.  The master domain therefore need not run a copy 
+of the umap layer, as it already has all of the correct mappings.  All 
+other domains must run a umap layer on top of any other layers they use.
+
+\subsection{Setting Up a umap Layer}
+
+The system administrator of a system needing to use the umap layer 
+must take several actions.  
+First, he must create files containing the necessary UID
+and GID mappings.  There is a separate file for user and group IDs.  The
+format of the files is the same.  The first line contains the total number
+of entries in the file.  Each subsequent line contains one mapping.  A
+mapping line consists of two numerical UIDs, separated by white space.
+The first is the UID of a user on the local machine.  The second is the
+UID for the same user on the master machine.  The maximum number of users
+that can be mapped for a single shared sub-tree is 64.  The maximum number of
+groups that can be mapped for a single sub-tree is 16.  These constants
+are set in the {\sf umap\_info.h} file, and can be changed, but changing them
+requires recompilation.  Separate mapping files can be used for each shared 
+subtree, or the same mapping files can be shared by several sub-trees.
+
+Below is a sample UID mapping file.  There are four entries.  UID 5 is mapped
+to 5, 521 to 521, and 7000 to 7000.  UID 2002 is mapped to 604.  On this
+machine, the UID's for users 5, 521, and 7000 are the same as on the master,
+but UID 2002 is for a user whose UID on the master machine is 604.  All
+files in the sub-tree belonging to that user have UID 604 in their inodes,
+even on this machine, but the umap layer will ensure that anyone running
+under UID 2002 will have all files in this sub-tree owned by 604 treated as if 
+they were owned by 2002.  An {\sf ls -l} on a file owned by 604 in this sub-tree
+will show the login name associated with UID 2002 as the owner.
+
+\noindent4\newline
+5 5\newline
+521 521\newline
+2002 604\newline
+7000 7000\newline
+
+The user and group mapping files should be owned by the root user, and
+should be writable only by that user.  If they are not owned by root, or
+are writable by some other user, the umap mounting command will abort.
+
+Normally, the sub-treeis grafted directly into the place in
+the file hierarchy where the it should appear to users.Using the umap
+layer requires that the sub-tree be grafted somewhere else, and
+the umap layer be mounted in the desired position in the file hierarchy.
+Depending on the situation, the underlying sub-tree can be wherever is
+convenient.
+
+\subsection{Troubleshooting umap Layer Problems}
+
+The umap layer code was not built with special convenience or
+robustness in mind, as it is expected to be superseded with a better
+user ID mapping strategy in the near future.  As a result, it is not
+very forgiving of errors in being set up.  Here are some possible
+problems, and what to do about them.
+
+\begin{itemize}
+
+
+\item{Problem: A file belongs to NOBODY, or group NULLGROUP.
+
+Fixes: The mapping files don't know about this file's real user or group.  
+Either they are not in the mapping files, or the counts on the number of 
+entries in the mapping files are too low, so entries at the end (including 
+these) are being ignored.  Add the entries or fix the counts, and either
+unmount and remount the sub-tree, or reboot.}
+
+\item{Problem: A normal operation does not work.
+
+Fixes: Possibly, some mapping has not been set properly.  Check to
+see which files are used by the operation and who they appear to be
+owned by.  If they are owned by NOBODY or some other suspicious user,
+there may be a problem in the mapping files.  Be sure to check groups,
+too.  As above, if the counts of mappings in the mapping files are lower 
+than the actual numbers of pairs, pairs at the end of the file will be 
+ignored.  If any changes are made in the mapping files, you will need to 
+either unmount and remount or reboot before they will take effect.
+
+Another possible problem can arise because not all Unix utilities
+rely exclusively on numeric UID for identification.  For instance, 
+SCCS saves the login name in files.  If a user's login name on two machines
+isn't the same, SCCS may veto an operation even though Unix file permissions,
+as checked by the umap layer, may say it's OK.  There's not much to be
+done in such cases, unless the login name can be changed or one fiddles
+improperly with SCCS information.  There may be other, undiscovered cases
+where similar problems arise, some of which may be even harder to handle.}
+
+\item{Problem: Someone has access permissions he should not have.
+
+Fixes: This is probably caused by a mistake in the mapping files.  Check 
+both user and group mapping files.  If any changes are made in the mapping 
+files, you will need to unmount and remount the sub-tree or reboot before they 
+will take effect.}
+
+\item{Problem: {\sf ls -l} (or a similar program) shows the wrong user for a file.
+
+Fixes: Probably a mistake in the mapping files.  In particular, if
+two local UIDs are mapped to a single master UID, stat calls will assign
+ownership to the first local UID occurring in the file, which may or may
+not be what was intended.  (Generally speaking, mapping two local UIDs to
+a single master UID is a bad idea, but the software will not prevent it.
+Similarly, mapping a single local UID to two master UIDs is a bad idea,
+but will not be prevented.  In this case, only the first mapping of the
+local UID will be done.  The second, and all subsequent ones, will be 
+ignored.) If any changes are made in the mapping files, you will need to 
+unmount and remount the sub-tree or reboot before they will take effect.}
+
+\end{itemize}
+
+\end{document}