Dec 112010

[Note added in August 2011: Be sure to read the comment by Dan Walsh. There is a simpler solution]

When you attempt to connect to a remote machine using freenx, you might encounter this message:

The NX service is not available or the NX access was disabled on host XXX.

This is likely due to SELinux blocking the connection. If you are using QtNX, it just hangs without any message.  Here is how to solve the issue.

(1) Disable auditd.

service auditd stop

(2) Rename /var/log/audit/audit.log or move it somewhere else.

(3) Enable auditd

service auditd start

(4) Try connection from the client. It will fail. This writes the audit.log file.

(5) Generate SELinux policy rules from the log file and install it.

cat /var/log/audit/audit.log | audit2allow -M freenx
semodule -i freenx.pp

(6) You can see the policy by reading the .te file.

cat freenx.te

module freenx 1.0;

require {
type nx_server_var_lib_t;
type sshd_t;
class file read;

#============= sshd_t ==============
allow sshd_t nx_server_var_lib_t:file read;

(7) Now, try connecting from the client again. It will fail again. Repeat the steps (1) to (5) using ‘freenx2’ instead of ‘freenx’.

(8) You will most likely need to repeat the process yet one more time until the connection finally succeeds. So, once again repeat the steps (1) to (5) but this time using ‘freenx3’ instead of ‘freenx’.

If you look at the policy files generated, you will find what was added by each action.

May 302010

I talked about getting help for CentOS in February 2009 and presented some Google search results comparing the number of hits between the mailing lists and the forums. That was 15 months ago and I thought I should update the results.  Exactly the same search was performed yesterday (May 29, 2010).

Search term  M/L Forums
============ === ======
install        0    25
installation   0     7
kernel         0    23
driver(s)      0    22
DNS            3    37
postfix       19    24
sendmail       5    30
selinux        0    23
apache         0    23
httpd          0    40
xen            3    33
kickstart      5    25
crash          2    21
panic          2    27

Google’s tendency to pick up forum posts rather than mailing list posts is much more obvious now than it was last year.  It looks as if Google is fond of the forums better than the mailing lists 😀 Or is it simply because there are more posts in the forums?

Regardless of the reason, what this means is that it is important to keep the quality of the forum contents high.  I would say that the current status is quite good; there are a number of forum regulars providing help.  Inappropriate replies, if they appear, get corrected by senior members.  Still, the forums can use more helping hands.  Why don’t you join in and make this CentOS help venue even better?

Mar 112010

If you visit the Custom kernel article on the CentOS wiki, you will see lots of warnings including the fact custom kernels are not supported by CentOS. If in doubt, just mention “custom…” in the centos IRC channel, you would be told to go away (if you are lucky enough to not get banned :-D).

In the CentOS forums, things are a little more relaxed. After usual warnings, you will receive some (or even extensive) help. As someone put it in a recent post, “… but there are *some* situations, where it [custom kernel] is desirable (usage on notebooks with HW that’s not supported by native kernels – which is e.g. my case or curious minds of young and restless geeks etc. )”.

I am far from ‘young and restless’ and I am not a geek. However, I did compile the latest mainline kernel — in an attempt to help those encountering trouble with installing newer kernels. The error was “mount: could not find filesystem”. I could reproduce it on my CentOS system.

As always, Alan and Ned engaged their brains and we tried several things to find a solution. We were getting nowhere when vanecka posted a fix in this forum thread.

Enable CONFIG_SYSFS_DEPRECATED_V2 in the kernel source configuration.
If you run “make menuconfig”, browse to:
General Setup —> enable deprecated sysfs features which may confuse old usersp, and enable this option.
Edit your .config file with a text editor and add/modify entry:

Genius! If you are interested in learning more details about this option, head for this web page.

CentOSPlus kernel and VMware Workstation6.5/Server 2.0.x

 centos  Comments Off on CentOSPlus kernel and VMware Workstation6.5/Server 2.0.x
Jan 312010

CentOS aims at being 100% binary compatible with the upstream vendor (including bugs). To accommodate users needs, the CentOS team offers the centosplus kernel which includes features that are not in the distro kernel. It has a number of options (mostly device drivers) enabled and also has patches that correct known issues.

Recently, it was reported that the centosplus kernel had a problem with certain versions of VMware products as seen in this CentOS forum thread.

Specifically, building the vmnet module on a host fails with the error:

/tmp/vmware-config2/vmnet-only/compat_netdevice.h:273: error: redefinition of ‘struct napi_struct’

Apparently, somewhere in the kernel source that was added in the centosplus kernel, there is that definition and it is conflicting with the VMware code.

It turned out that the problem exists in the VM guest as well: building of the vmxnet modules fails with the same error. The fix, in both cases, is to untar:

/usr/lib/vmware/modules/source/vmnet.tar (host)
/usr/lib/vmware-tools/modules/source/vmxnet.tar (guest)

and comment out the following block in compat_netdevice.h:

struct napi_struct {
    int dummy;

More details can be found in the CentOS bug tracker.

Go green with newer AMD processors

 centos, linux, RHEL  Comments Off on Go green with newer AMD processors
Aug 062009

Not long ago, Steve, one of the founders of the ELRepo project, built new systems with the AMD II X4 Phenom processor. After hearing his positive comments, I replaced my old desktop with a new one equipped with a Phenom cpu.

Steve soon noticed that the machine consumed more power when it was running CentOS compared to when running Fedora. Turns out that, in CentOS, there is no per-core control — meaning when the system needs a higher processor power, all cores will shoot up to the maximum frequency. In Fedora, each core gets attenuated independently.

This issue was noted by a CentOS forum user, AlexAT, here. He not only reported it in the upstream bugzilla but came up with a fix later.

Using the patch provided by AlexAT, we built a kernel module kmod-powernow-k8 and released it through ELRepo. After installing kmod-powernow-k8, Steve saw that the system was now measured drawing ~110W at idle from the wall outlet, similar to the power consumption observed under Fedora 10 and under CentOS 4.7. So without kmod-powernow-k8 installed, the system was consuming ~40W (36%) more power at idle and the core temperature was running 8-10°C hotter than with kmod-powernow-k8 installed, making this a very environmentally friendly kmod.

So, if you have newer Opterons, the Phenoms or Phenoms II (or Kuma core Athlons X2), you should give this driver a try. Also, you would want a backported AMD K10 core temperature monitor driver module (kmod-k10temp) from ELRepo.

Let’s go green!

ELRepo project

 centos, RHEL, Scientific Linux  Comments Off on ELRepo project
Jun 302009

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 — priorities matters

 centos  Comments Off on yum — priorities matters
May 292009

One of the most commonly asked questions in the CentOS forums is concerned with 3rd party repositories. In addition to referring to the Repositories wiki article, we need to make sure people use the yum priorities plugin appropriately.

Often times, we want to know how they set up the priority scores but there is no easy way to display them. yum repolist all does not include this info. A forum helper jlehtone posted a one-liner that does just that:

cat /etc/yum.repos.d/*.repo | sed -n -e "/^\[/h; /priority *=/{ G; s/\n/ /; s/ity=/ity = /; p }"  | sort -k3n

The output may look like this:

priority = 10 [base]
priority = 10 [updates]
priority = 30 [extras]
priority = 50 [centosplus]
priority = 60 [c5-testing]
priority = 70 [rpmforge]
priority = 75 [kbs-CentOS-Testing]
priority = 90 [kbs-CentOS-Misc-Testing]
priority = 94 [epel]

Almost immediately after the above post, Alan the perfectionist, a cat person himself, opposed the use of over-working cats and came up with his version of the one-liner:

sed -n -e "/^\[/h; /priority *=/{ G; s/\n/ /; s/ity=/ity = /; p }" /etc/yum.repos.d/*.repo | sort -k3n

Personally … I want to see the cats working for the community … 😀

yum — when all is not all

 centos, RHEL, Scientific Linux  Comments Off on yum — when all is not all
May 292009

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=’*’ 

Getting rid of Suspend on a desktop machine

 centos, RHEL, Scientific Linux  Comments Off on Getting rid of Suspend on a desktop machine
Apr 122009

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


Mar 192009

My first encounter with kernel modules was when my systems running Fedora Core 5 started crashing after the kernel update to 2.6.18.  Long story short, it was due to a bug in the cifs kernel module which was fixed after more than 100 e-mail exchanges with samba developers.  The problem was solved, but there was something I never understood; the patched cifs module I built was much larger than the distro’s.  This is always true.  A custom-built version can be more than 10-times larger than its counter part in the distro kernel.

I never pursued this issue until the other day when wolfy and Ned started a conversation on IRC about this size difference.  It is not due to stripping because the distro version is “not stripped” and, in fact, if you strip it the module no longer loads.

Then later, Alan asked in his clever voice, “have you checked the options to strip?  It might be that a total strip is verboten but a selective strip works?”

That was it! 🙂

I tried a strip --strip-unneeded command and that reduced the size of my module file to something close to the distro’s AND the file remained “not stripped“.  Soon after, Ned found the --strip-debug option does the trick, too.  This is probably what is used when the kernel is built and modules compiled.

Mystery solved.