[[!meta title="AFS and You"]]
  1. What is AFS?
  2. Some MIT/AFS terminology
  3. The Basics
    1. The Layout of a Typical MIT Locker
    2. Accessing Lockers
      1. From Athena
      2. From the Web
    3. Checking Quota
  4. Common Tasks
    1. Controlling Who can Access Files
    2. Creating an AFS Group
    3. Controlling Access from the Web
  5. Troubleshooting
    1. I'm trying to access my files, fs la says I should have permissions …
    2. I'd really prefer that not everyone could list my files, how should I stop …
    3. It was around 6am on a Sunday morning and suddenly I couldn't access my …
    4. It isn't Sunday and I can't get to my files
  6. Advanced Tasks
    1. Putting Software in a Locker
    2. Acquiring a new locker
  7. See Also

Mostly written by Donald Guy,
drawn from a variety of sources.
Credit goes to them, blame goes to him.

What is AFS?

The Andrew File System or AFS is a distributed network file system invented at Carnegie Mellon University as part of Project Andrew (approximately their equivalent of MIT's Project Athena). More importantly, it is the file system used to store most files on Athena today. This includes your personal home directory, the data and websites of many living groups and student groups on campus, and probably some of the software you run (if you ever use Athena clusters). (Though all user directories were migrated from NFS in the summer of 1992, some files probably still remain on NFS and, of course, various file systems are used on personal computers and servers.)

In Short: AFS is probably where some of the files you care about live

For the most part, using AFS, particularly at MIT, is well-hidden and can be used like any other UNIX file system. For some things, you will need to know a bit more. Let's start by defining some terms.

Some MIT/AFS terminology

For practical purposes, a folder. Probably the what you'll care about most of the time. Technically any directory mountable under /mit, regardless of how its stored. Today, most lockers lockers are stored in AFS.
Essentially proof to the AFS servers that you are who you say you are, thus allowing you to access files you are supposed to. Analogous to Kerberos tickets.
AFS concept of an "administrative domain of authority." Each cell has its own set of users, groups, and administrators. Analogous to a Kerberos realm. Each top-level directory in /afs corresponds to a cell. The cells you are most likely to care about are athena.mit.edu and sipb.mit.edu.

The Basics

The Layout of a Typical MIT Locker

Every Athena user has a locker (their home directory), which mounts at /mit/<username>. From a technical standpoint, it is stored in the volume user.<username> which is located at /afs/athena.mit.edu/user/<first letter>/<second letter>/<user name>. For example, the user joeuser has a home directory that mounts at /mit/joeuser, is volume user.joeuser, and is accessible at /afs/user/j/o/joeuser. Lockers for projects, software, classes, living groups, and student groups are all mounted at /mit/<lockername> and stored at various places in AFS.

Within each locker, there are (by default) 4 special subdirectories you want to care about.

By default, this directory can be read by everyone (and I do mean EVERYONE!), so put files here you want to share with the entire world.
By default, this directory can only be read and can only be listed by you (from AFS, not from web) whereas the files in other directories can (by default) only be read by you, but can be listed by anyone. This means that other people can't see the contents of your files, but they can see what files you have outside of Private
This folder is a link to a read-only copy of a backup of your files (created nightly at 3 a.m.). This copy cannot be edited and does not count towards the locker's quota. (From a technical standpoint, this is a separate volume, user.<username>.backup and is stored only as changes against the current copy.)
Where you should put a website, if you want one. There is very little special about this directory from an AFS standpoint, but it is world-readable (like Public) and is linked directly to http://www.mit.edu/~<lockername> as well as http://web.mit.edu/<lockername>.

Accessing Lockers

From Athena

On Athena, you can access a locker either as its full AFS path, if you know it (e.g. /afs/athena.mit.edu/course/6/6.01), or under /mit if it is "attached." Yet a third way is to use tilde expansion and simply cd ~~<locker> (e.g. cd ~~6.01) which will take you to the full path. Often, however, you want to attach lockers because it is easier to refer to them outside of the shell (in a script, for example) and software is set up to run with a path under /mit. There are a few ways to attach a locker:

From the Web

Generally any locker that you would access on Athena as /mit/<locker> is accessible on the web as http://web.mit.edu/<locker>. For example, the barnowl locker is at http://web.mit.edu/barnowl. As you can see, if there is no index.html (see below), the files in the directory are listed. By default, however, none of the contents are readable except in the www and Public folders.

Also, you may access something in one of the MIT AFS cells by taking its full AFS path after web.mit.edu (http://web.mit.edu/afs/athena.mit.edu/activity/c/chess-club). (That link also shows that if you have a text file named README readable, as a link to Public/README for example, its contents will be displayed below the directory listing).

Checking Quota

You are only allocated a certain amount of disk space by MIT. To check the quota of the locker you are presently in run fs listquota. You will see output similar to the following:

user@host:/mit/chess-club$ fs listquota
Volume Name                   Quota      Used %Used   Partition
activity.chess-club         1500000     13163    1%         90% 

If this information is good enough for you, then you are done. If not, read on.

Common Tasks

Controlling Who can Access Files

You may be familiar with Unix permissions. Sad to say, but that knowledge is basically useless here. Whereas Unix permissions, are per-file, AFS permissions are controlled by Access Control Lists (ACLs) on a per-directory basis.

To view the ACL for a given directory (where you have permission to do so), run fs listacl or fs la, for short. For a typical user locker, the ACL in the top level will look like this

user@host:~$ fs la
Access list for . is
Normal rights:
  system:expunge ld
  system:anyuser l
  user rlidwka

This is a list of users or AFS groups and their permissions in this directory (and subdirectories that don't have their own ACL modifications). In AFS there are seven permissions as follows

r read user or members of group can read files in the directory (i.e. see the contents of files)
l list user or members of group can list files in the directory (i.e. see the names of files)
i insert user or members of group can create files (or subdirectories) in the directory
d delete user or members of group can delete files in the directory
w write user or members of group can modify files in the directory (i.e. change the contents of files)
k lock user or members of group can lock files in the directory (you will likely never use this)
a admin user or members of group can see and change permissions. It does not affect pre-existing subdirectories.

To add a user or group to the ACL for a given directory simply run fs setacl or fs sa as follows:

fs sa <directory> <user or group> <permissions>
can be an absolute or relative path, usually you will want .
<user or group>
can be any user or group. Some you probably want to know: system:anyuser means EVERYONE (the entire AFS-using world plus the entire world wide web), system:expunge is the daemon for MIT's delete/lsdel/undelete utilities to work in a given directory.
can be a string of the above letters (in any order) or any of the words read, write, all and none which are equivalent to rl, rlidwk, rlidwka and the empty string, respectively

For example, if user wants his friends sipbtest and jarandom to be able to read and write files and anyone to be able to read files in his awesome_project directory, he might have a session that looks like this

user@host:~$ cd awesome_project/
user@host:~/awesome_project$ fs sa . system:anyuser read
user@host:~/awesome_project$ fs sa . jarandom write
user@host:~/awesome_project$ fs sa . sipbtest write
user@host:~/awesome_project$ fs la
Access list for . is
Normal rights:
  system:expunge ld
  system:anyuser rl
  sipbtest rlidwk
  jarandom rlidwk
  user rlidwka

See also: man 1 fs, fs help <command>

Creating an AFS Group

The "normal" way to make an AFS group would be with a command similar to pts creategroup <your user name>:<group name> and then add people with pts adduser <user> <full group name>(e.g. If Donald Guy wanted to created a group for people to edit his www directory (including sipbtest and jflorey, he might use the following chain of commands pts creategroup fawkes:www ; pts adduser sipbtest fawkes:www; pts adduser jflorey fawkes:www; fs sa ~/www fawkes:www write

This method will work, but at MIT, it is much more common to use moira lists as a group. To create a new list, use the web interface at https://wserv.mit.edu:444/fcgi-bin/lc? to create a moira list, NOT A MAILMAN LIST and be sure to check the box for "Should this list be an AFS Group?" (to make an already existing moira list into an AFS group simply blanche -G <list>). After the servers update (which may take anywhere between 1 second and 10 minutes depending on the number of similar requests), the AFS group system:<list name> will exist in the athena.mit.edu cell.

This is useful because one often wants the same certain people who can operate on files in a folder to be a mailing list. Thus, for example, it is possible to send mail to gnu@mit.edu and use system:gnu as an AFS group on ACLs. (it is also possible to make moira lists that are AFS groups, but not mailing lists).

see also: man 1 pts

Controlling Access from the Web

If you make a directory listable and readable by system:anyuser then it can be viewed by any user on the web via the urls mentioned above

Unfortunately, just because you add specific users to an AFS ACL does not mean they can see the folder when the access from the web. IS&T, however, does provide a solution to this. First, make sure that the wanted directory is not readable by system:anyuser. Next fs sa <dir> system:htaccess.mit read . Then create a file named .htaccess.mit in that directory. In that file you can do three things,

There after the users should be able to get to the folders at http'''s'''://web.mit.edu/<locker>/<path to folder> if they have certificates and no one should be able to reach it via http. Make sure to add yourself if you are going to be accessing it.

see also: http://web.mit.edu/is/web/reference/web-resources/https.html


I'm trying to access my files, fs la says I should have permissions here, but it still says : Permission denied

There are two likely possibilities. First, its likely that your tokens may have expired. To get new tokens, make sure you have valid kerberos tickets and then run aklog. Another possibility is that you have tokens but not for the correct cell. tokens will tell you what tokens you already have. In all likelihood, if you are reading this, you probably want aklog athena sipb. Finally, a third possibility is that your group membership has changed since you acquired tokens. Try running aklog -force

I'd really prefer that not everyone could list my files, how should I stop this?

What you don't want to do is take away the l permission from system:anyuser because then no one will be able to get to your Public or www which you probably don't want. All told, there is no way to stop people on Athena from listing your files (unless you keep everything under Private or a similarly restricted directory). It is rather trivial to stop web users, however. A solution I recommend, if you have a website in www, is to make a page redirect.html (or similar) with the contents like:

  <meta http-equiv="Refresh" content="0; url=http://www.mit.edu/~<lockername>/">
  <p>Please go to my <a href="http://www.mit.edu/~<lockername>/">www</a>!</p>


and then doing a ln -s www/redirect.html index.html in the top level of the locker. Alternatively you can link index.html to a blank file in your Public or www (you can't simply it in the top level because its not readable there, this will result in a 403 Forbidden error).

It was around 6am on a Sunday morning and suddenly I couldn't access my files

Yeah, most AFS servers restart weekly at 6 AM on Sunday.

It isn't Sunday and I can't get to my files

There may be a non-scheduled AFS outage. Check 3down, hopefully it will be back up soon :-(.

Advanced Tasks

Putting Software in a Locker

The Athena environment was designed to allow software to run on several architectures on the same network. On modern Athena, this means 32-bit x86s running Linux, 64-bit x86s running Linux, and SPARCs running Solaris. To accommodate these these various architectures AFS (at least on Athena) has a notion of what systems are compatible with the operating system. You can find these by running fs sysname.

The special variable @sys in a path corresponds to the first of these that matches. When a user runs add, /mit/<locker>/arch/@sys/bin is added to their PATH and /mit/<locker>/man or /mit/<locker>/arch/@sys/man is added to their MANPATH.

Primarily because of this fact, a typical locker is set up with the following sort of layout:

user@host:/mit/<some locker>$ tree -dL 2
|-- arch
|   |-- i386_linux22
|   |-- i386_linux24 -> i386_rhel4/
|   |-- i386_rh9 -> i386_linux24
|   |-- i386_rhel3 -> i386_rhel4
|   |-- i386_rhel4
|   |-- sgi_65
|   |-- sun4x_510
|   |-- sun4x_57
|   |-- sun4x_58 -> sun4x_510/
|   `-- sun4x_59 -> sun4x_58
|-- bin -> arch/@sys/bin
|-- include -> arch/@sys/include
|-- info
|-- lib -> arch/@sys/lib
|-- man
   `-- man1

Please note that the @sys variable should really be used ONLY in these convenience in symlinks. To make the folder in arch you usually want to either pick a more general architecture and set it up yourself or simply use the following command in the top level of the locker: mkdir -p arch/$ATHENA_SYS/{bin,lib,include}. After making this folder, make the shortcut symlinks with commands similar to ln -s arch/@sys/bin bin (again, this is the ONLY time you should use @sys yourself). Other than that, its mainly just a matter of making sure to run configure with options like --prefix=/mit/<lockername> and --with-manpath=/mit/<lockername>/man.

See also: man lockers

Acquiring a new locker

While it is easily possible to make an AFS group for yourself, it is harder to get a new locker. If you want a locker for something official like a student group or a piece of software, send an email to accounts@mit.edu explaining what you need it for and someone at IS&T will let you know. If you feel it is something less official or you would just rather consider it a SIPB project (perhaps get other SIPB people involved) send an email to sipb-afsreq@mit.edu instead. In either case, if a mount point under /mit fails to get set up for you, let hesreq@mit.edu know.

See Also

SIPB's older guide, Inessential AFS
OpenAFS documentation at http://www.openafs.org/