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(am-utils.info)Introduction to Hlfsd

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Introduction to Hlfsd
=====================

   Electronic mail has become one of the major applications for many
computer networks, and use of this service is expected to increase over
time, as networks proliferate and become faster.  Providing a convenient
environment for users to read, compose, and send electronic mail has
become a requirement for systems administrators (SAs).

   Widely used methods for handling mail usually require users to be
logged into a designated "home" machine, where their mailbox files
reside.  Only on that one machine can they read newly arrived mail.
Since users have to be logged into that system to read their mail, they
often find it convenient to run all of their other processes on that
system as well, including memory and CPU-intensive jobs.  For example,
in our department, we have allocated and configured several
multi-processor servers to handle such demanding CPU/memory
applications, but these were underutilized, in large part due to the
inconvenience of not being able to read mail on those machines.  (No
home directories were located on these designated CPU-servers, since we
did not want NFS service for users' home directories to have to compete
with CPU-intensive jobs.  At the same time, we discouraged users from
running demanding applications on their home machines.)

   Many different solutions have been proposed to allow users to read
their mail on any host.  However, all of these solutions fail in one or
more of several ways:

   * they introduce new single points of failure

   * they require using different mail transfer agents (MTAs) or user
     agents (UAs)

   * they do not solve the problem for all cases, i.e.  the solution is
     only partially successful for a particular environment.


   We have designed a simple filesystem, called the "Home-Link File
System", to provide the ability to deliver mail to users' home
directories, without modification to mail-related applications. We have
endeavored to make it as stable as possible.  Of great importance to us
was to make sure the HLFS daemon, `hlfsd' , would not hang under any
circumstances, and would take the next-best action when faced with
problems.  Compared to alternative methods, Hlfsd is a stable, more
general solution, and easier to install/use.  In fact, in some ways, we
have even managed to improve the reliability and security of mail
service.

   Our server implements a small filesystem containing a symbolic link
to a subdirectory of the invoking user's home directory, and named
symbolic links to users' mailbox files.

   The Hlfsd server finds out the UID of the process that is accessing
its mount point, and resolves the pathname component `home' as a
symbolic link to a subdirectory within the home directory given by the
UID's entry in the password file.  If the GID of the process that
attempts to access a mailbox file is a special one (called HLFS_GID),
then the server maps the name of the _next_ pathname component directly
to the user's mailbox.  This is necessary so that access to a mailbox
file by users other than the owner can succeed.  The server has safety
features in case of failures such as hung filesystems or home directory
filesystems that are inaccessible or full.

   On most of our machines, mail gets delivered to the directory
`/var/spool/mail'. Many programs, including UAs, depend on that path.
Hlfsd creates a directory `/mail', and mounts itself on top of that
directory.  Hlfsd implements the path name component called `home',
pointing to a subdirectory of the user's home directory.  We have made
`/var/spool/mail' a symbolic link to `/mail/home', so that accessing
`/var/spool/mail' actually causes access to a subdirectory within a
user's home directory.

   The following table shows an example of how resolving the pathname
`/var/mail/NAME' to `/users/ezk/.mailspool/NAME' proceeds.

Resolving Component   Pathname left to resolve   Value if symbolic link
/                     var/mail/NAME              
var/                  mail/NAME                  
mail@                 /mail/home/NAME            mail@ -> /mail/home
/                     mail/home/NAME             
mail/                 home/NAME                  
home@                 NAME                       home@ ->
                                                 /users/ezk/.mailspool
/                     users/ezk/.mailspool/NAME  
users/                ezk/.mailspool/NAME        
ezk/                  .mailspool/NAME            
.mailspool/           NAME                       
NAME