Jason Rennie's Useful Information Tidbits

MIT

• Books
  mit411 hosts a book swapping bulletin board that's pretty popular with students. Great way to make $$$ off those books you forgot to return. Also, a good list of local restaurants (with menu scans).
• RSI
  Samson maintains a good page on RSI, repetetive stress injuries, something every computer scientist has to worry about.
• Navigating MIT
  MIT has a nice mapping system set up for locating buildings and other features on MIT campus.

AFS/Kerberos

• Debugging
  Here are two useful commands for attempting to debug AFS:

    tcpdump -np -i eth0 -s 1500 udp port 7001
    rxdebug -server titanic.csail.mit.edu -port 7001
  

• Cacheing
  If AFS gets into a bad state on your machine, it may not update properly when changes happen to your files elsewhere. You can clear AFS's cache of the current directory (and all other directories in the same volume) with the command

  fs flushvolume .

• NAT
  If you are behind a firewall with a Network Address Table, you will most certainly run into "connection timed out" errors with AFS. The problem is that most NATs usually drop UDP connections (the protocol AFS uses) after a short span of inactivity. By default, an AFS client pings the server every 10 minutes, so if the NAT timeout is any less than this, you'll see errors. However, it doesn't seem to be enough to simply make sure there's frequent AFS activity. I've tried opening a new file for writing every 59 seconds, but still see timeout errors (however they are relatively rare).
• pagsh
  Use pagsh to spawn a shell that will obtain its own tickets and tokens, not associated with your user id. This prevents the possibility that someone can steal your tickets/tokens by obtaining root and using su to become you. Perfect for long running jobs.
• Documentation
  TIG provides excellent introductory documentation. Openafs.org provides extensive (albeit somewhat dated) documentation.
• Groups
  Useful group commands:
  • pts membership <user/group> - show membership of user/group
  • pts listowned <user/group> - show the groups that the user/group owns
  • pts examine <user/group> - show information about user/group
  • pts cg <group> - create a group
  • pts adduser <user> <group> - add user to group
  • pts removeuser <user> <group> - remove user from group
  • pts delete <group> - delete a group
  • pts chown <group> <new owner> - change ownership of a group
  • pts rename <old name> <new name> - change the name of a group
• Long-running jobs
  If a job is to run longer than the maximum ticket life, the tickets/tokens must be renewed. One way to do this is to run an authentication loop along-side your long-running job. Two scripts are needed: one to serve as the parent process (longjob) and one to run the authentication loop (authloop). First, longjob:

  #!/bin/sh
  export KRB5CCNAME=/tmp/krb5cc_$$.longjob
  kinit -r 8d
  aklog
  nohup authloop &> /tmp/$1.auth.$$ &
  nohup $@ &> /tmp/$1.job.$$ &

  Next, authloop:

  #!/bin/sh
  while true; do
    sleep 5h
    kinit -R
    aklog
  done

  Note that '8d' should be changed to the maximum renewable lifetime for your domain. To use, put both scripts in your PATH and run

  longjob ./mylongrunningprogram

  If you're at the lab, you can just use /afs/csail/i386_linux24/local/bin/longjob.
• Quota
  All users have a disk-usage quota (20G here at CSAIL). To see your quota and see how much disk space you are using, issue this command from your home directory:

  fs lq

Linux

• Mounting Windows drives
  

    sudo mount -o uid=jrennie -t ntfs /dev/hda1 /mnt/windows
  

• dpkg -S
  Use dpkg -S to find the package associated with a file. Note that this only works for files extracted from the package. Dpkg does not know about files created via post-installation scripts.
• Tee
  The tee command lets you duplicate a stream.
• Reducing Spam at the MTA
  There is a nice write-up of various methods that can be used to stop some amount of spam at the MTA.
• Wireless
  HP sponsors the Wireless Tools for Linux project. Wireless packages for Debian include wireless-tools, pcmcia-cs, and (e.g.) pcmcia-modules-1-686.
• chkrootkit
  Debian package to check for common root exploits.
• kernel-package
  The Debian kernel-package package simplifies the process of building and installing a custom kernel on Debian. The NewbieDoc Sourceforge project provides good documentation on how to use kernel-package.
• rpm
  Most packages contain a short description of their content. To extract this description, run

  sudo rpm -q
  --queryformat="%{DESCRIPTION}\n" packagename

• TAO
  TAO is an optimization package that can be used for tasks such as convex optimization. Installation has a few "gotchas," so I made a short writeup of the installation procedure: tao/install.html.
• Python Interface to TAO
  I've created pytao, a rudimentary Python interface to TAO. It's still in its infancy and debugging can be a bit of a chore, but it's a start.
• Gamma functions for Python
  My gamma module implements the Gamma function, the Digamma function and the derivative of the Digamma function in C.
• sudo
  The sudo program lets (authorized) users run commands with super-user privileges without having to switch users. For example, to update apt's package list (as a normal user), run

  sudo apt-get update

• apt-get
  The APT HowTo
  In Debian Linux, the command

  apt-get install PACKAGENAME

  will install the package PACKAGENAME, along with any packages it relies upon (assuming the package exists). Similarly,

  apt-get remove PACKAGENAME

  will remove PACKAGENAME from the system. The command

  apt-get update

  updates the package listings (this should always be run before installing a package).
• apt-cache
  To search for an existing package, run

  apt-cache search REGEX

  where REGEX is a regular expression that matches the package name or a part of the package's description.
  To find out the current version for a package, run

  apt-cache policy PACKAGE

  where PACKAGE is the package name.
• dpkg-reconfigure
  Once a package has been installed, removing and reinstalling will not reconfigure the package. To reconfigure the package, run

  dpkg-reconfigure PACKAGENAME

• dpkg-query
  To find out the installation state of a package, run

  dpkg-query -l PATTERN

  where PATTERN is a (command-line) globbing pattern (e.g. "cups*")
• CapsLock
  Put the option

  Option "XkbOptions"
  "ctrl:nocaps"

  in the keyboard section of your /etc/X11/XF86Config-4 file in order to disable the caps lock key (it becomes a control key).
• LILO
  If lilo is set to a zero timeout, you can still get a boot prompt by holding down the left shift key during boot. Enter "linux init=/bin/sh" to get a shell (no root password necessary). "linux single" may get you into single-user mode.
• mount
  You can remount a volume read-write with the option "-o remount,rw".
• CVS
  I use CVS to maintain some of the code that I publish. A nice feature of CVS is that you can simply, seamlessly and safely use a remote CVS server. The key is to connect via ssh. Using RSA or kerberos authentication, there is no password prompt. I tell CVS to use ssh by setting the environment variable "CVS_RSH" to "ssh". I then set "CVSROOT" to ":ext:jrennie@machine.foo.com:/home/jrennie/cvsroot", where "machine.foo.com" is the machine with the remote repository and "/home/jrennie/cvsroot" is the cvs root directory. I can then issue CVS commands as though the repository were local (with minor lags depending on the network route to the root machine).