Category Archives: Scientific Linux

ELRepo project

It all started in the CentOS forums.

CentOS, being an Enterprise Class Linux distro, may not have support for new hardware, especially those found in desktops and laptops. CentOS forums get many postings from users who are looking for drivers that are needed for their network device, webcam, etc. In fact, googling for “driver” and “centos” yields much more hits from the forums than the mailing list as shown in my earlier blog.

Alan Bartlett who has been actively helping in the forum has strong background in the kernel and its modules and has been building, whenever possible, a driver module each time he sees a cry for help. In a fairly short period of time, quite an impressive number of such kernel modules were built this way and it was growing.

Because these drivers were designed to be independent of the kernel version (kABI-tracking, kABI = kernel application binary interface), they survive kernel updates. Also, they can be used in RHEL and all its rebuilds, not just CentOS. So, it was a good idea to make them available to wider communities. NedSlider proposed to establish a repository for that purpose. The ELRepo project was thus started by five members of the CentOS / Scientific Linux community.

A little more details of ELRepo can be found in the Ned’s recent blog.

yum — when all is not all

A while ago, I was talking with my buddies and the subject was about installing / updating a package using yum.  yum did not find the package that had just been added.  No problem, either yum clean metadata or yum clean all will do.  Err … it didn’t. 

As it turned out, it didn’t work because the repository in question was disabled by default.  The man page for yum clearly states:

Note that "all files" in the commands below means "all files in 
currently enabled repositories".   If you  want  to  also clean  any 
(temporarily)  disabled  repositories you need to use --enablerepo=’*’ 
option.

Getting rid of Suspend on a desktop machine

For some reason, my desktop gnome menu had a “Suspend” option.  It was right under the “Log out …” option in the System dropdown menu.  One day, when I meant to click on the “Log out” option, my mouse went too far and I ended up selecting this “Suspend” option.

After struggling for the next few minutes, I gave up and rebooted the machine to come out of the suspended state.

The Suspend option should not have been there to begin with.  Anyway, it was time to remove it.  Fortunately, that was as easy as typing [1]:

gconftool-2 --type boolean --set /apps/gnome-power-manager/can_suspend false

Likewise, if you want to get rid of the Hybernate option,

gconftool-2 --type boolean --set /apps/gnome-power-manager/can_hibernate false

[1] http://magazine.redhat.com/2008/05/30/tips-and-tricks-how-do-i-remove-the-suspendhibernate-entries-from-the-system-menu-in-the-gnome-panel/

Clean that Inbox

Like many other people, I use Linux as a backup server.  The other day, I noticed that daily incremental backup of one of the Windows machines was well over 1 GB even on the day the user was mostly idle.  The only thing the user was doing was … e-mailing.  Aha! (heard the bell?)  It must be that inflated Inbox.

Mozilla-based mail clients like Thunderbird and Seamonkey mail do not physically remove messages that user deletes.  Instead they are only tagged “deleted”.  This is true even after the Trash folder is emptied.  The [supposedly] deleted mails get [really] deleted when Inbox (or any folder for that matter) is compacted.

I went to the blasted machine and did just that and the Inbox went from > 1 GB to a fraction of its original size.

Of course, this is not just Windows.  Huge mail folders can potentially cause trouble and also degrade the performance of the client.  The best strategy to prevent this is to set up an automatic clean up.  In Thunderbird, go to Edit -> Preferences -> Advanced -> Network & Disk Space and then enable the “Compact folders when it will save over…KB” option.

Socks proxy with auto-config

OpenSSH has built-in support to act as a SOCKS proxy. In my case, there are web sites I can access only from work computers and I need to get to them from home. So, I issue the command from my home computer:

ssh -D 1080 my work IP

However, I do not want to redirect all traffic through work.  Fortunately, you can redirect only selected URLs fairly easily by using a proxy auto-config file.

In firefox, Go to Edit -> Preferences -> Advanced -> Network -> Settings

In the Connection Settings box, select “Automatic proxy configuration URL:” and enter:
file:///path/to/proxylist.pac
The proxylist.pac file may look like this:

function FindProxyForURL(url, host)
{
// Proxy direct connections to these hosts
if (
shExpMatch(url, "http://www.jbc.com/*") ||
shExpMatch(url, "*.sgmjournals.org/*") ||
shExpMatch(url, "http://www.ncbi.nih.gov/*")
) {
return "SOCKS localhost:1080; DIRECT";
}
// Otherwise go directly
else return "DIRECT";
}

For more details on the pac file and auto config, see
http://en.wikipedia.org/wiki/Proxy_auto-config

sshfs – Remote filesystem access made easy

If you often need to access files on a remote machine and do it by ssh login, there is a handy way – sshfs. Here is a simplified howto that works.

(1) Set up the rpmforge repository if not done yet (see Installing RPMForge )
(2) Either use the dkms-fuse with the stock RHEL/CentOS kernel or use the centosplus kernel that contains the fuse kernel module.

[Note 1: fuse is included in the kernel as of RHEL/CentOS/SL 5.4]
[Note 2: In RHEL/CentOS/SL 6, start with step (4)]

[root@mybox ~]# yum install dkms-fuse && modprobe fuse

(3) Also install the fuse libraries:

[root@mybox ~]# yum install fuse

(4) Then install the fuse-ssh filesystem:

[root@mybox ~]# yum install fuse-sshfs

(5) Add yourself to the group ‘fuse’:

[root@mybox ~]# usermod -a -G fuse user1

(6) Re-logon to your account
(7) Create a local directory:

[user1@mybox ~]$ mkdir remotedir/

(8) To mount (remote username=usr2):

[user1@mybox ~]$ sshfs user2@machine.example.com: remotedir/

(9) To unmount:

[user1@mybox ~]$ fusermount -u remotedir