Posts Tagged ‘system boot’

Workaround “Running chkdsk in Read-Only mode” on Windows XP

Tuesday, July 16th, 2013

running chkdsk in read only mode microsoft windows  XP chkdsk schedule check drive C on next restart

I had to fix old Notebook Dell Insspiron 1501 (890 mb ram and 1.60 Ghz CPU) notebook with Windows XP SP2. The notebook looks okay but as it is an old piece of hardware I decided to check hard drives for bad sectors with Windows (Check Disk) –  chkdsk. 

Running chkdsk via -> cmd.exe does not work because file system is in use and once you run chkdsk it does spit warning and error:

"Running CHKDSK in Read-Only mode

Errors found. CHKDSK cannot continue in read-only mode

Because it checks hdd in read-only its check is not completely reliable and in case some bad block is matched it is not possible for chkdsk to write on HDD and try fix or move it to some free space blocks.


To work around this its necessary to run chkdsk with options:

chkdsk C: /v /f

This command brings out prompt like in below screenshot offering to run CHKDSK on C drive on next system boot before starting Windows services which does lock files on file system – making it unavailable for CHKDSK to read blocks on it.

Checking file system on C drive before boot windows XP screenshot

The notebook had 3  Drives C:\, D:\ and E:\ so I run above command also on D:\ and E:\ to make sure there are no physical damages on D and E partitions, i.e.:

chkdsk D: /v /f

chkdsk E: /v /f

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screen -d Fix “Must run suid root for multiuser support.” su user detach error

Thursday, March 28th, 2013

I had to run a shell script to run automatically in detached screen during Linux system boot up via /etc/rc.local. This is needed because the server uses the tiny shell script to fetch data from remote host database and fill information into local MySQL server.

My idea was to su from root to www-data (Apache) user – the script has requirements to run with Apache user, then it has to run detached using GNU screen (multi terminal emulator. The tiny one line script I imagined would do the trick is like so:

# tty=$(tty); su www-data -c 'cd /home/user/www/enetpulse; screen -d /home/user/www/enetpulse/'; chmod 0720 $tty

I run this as root user to test whether it will work or not before I put it in /etc/rc.local but for my surprise got an error:

Must run suid root for multiuser support.

After a quick investigation on what is causing it I came across the solution which is to include screen arguments (-m -S shared). The working variant that gets around the error – i.e. successfully changes user privileges to Debian Apache user (www-data) and then detach with screen is:

# tty=$(tty); chmod a+rw $tty; su www-data -c 'cd /home/user/www/enetpulse; screen -d -m -S shared /home/user/www/enetpulse/'; chmod 0720 $tty;

That's all now script works out as planned on next server reboot

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Disable bluetooth on Linux IBM / Lenovo Thinkpad laptops

Thursday, February 14th, 2013

bluetooth gnu linux disable bluetooth linux how to tux logo bluetooth thinkpad

I have a Debian GNU / Linux squeeze with bluetooth and bluetooth is started automatically on system boot. This is pretty annoying, cause I use bluetooth quite rarely.
 disable / enable bluetooth via terminal is controlled via Linux sysfs virtual filesystem. The command to disable bluetooth one time is:

debian:~# echo 0 > /sys/devices/platform/thinkpad_acpi/bluetooth_enable

It is efficient in terms of energy saving especially if you use often your notebook on battery to turn off bluetooth permanently and only enable it when needed with:

debian:~# echo 1 > /sys/devices/platform/thinkpad_acpi/bluetooth_enable

To permanently disable bluetooth on Linux boot use:

# service bluetooth stop

In /etc/rc.local before exit 0 line place:

echo 0 > /sys/devices/platform/thinkpad_acpi/bluetooth_enable

An alternative method to permanently disable bluetooth (on other non-Thinkpad – any brand laptops) is via rfkill (bluetooth device control interface), on Ubuntu rfkill is installed by default but Debian users has to explicitly install it via apt:

debian:~# apt-get install –yes rfkill

Once rfkill is installed on host put a line before exit 0 in /etc/local:

rfkill block bluetooth

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Make QMAIL with vpopmail vchkpw, courier-authlib and courier-imap auth work without MySQL on Debian Linux qmailrocks Thibs install

Friday, September 28th, 2012

How to make qmail vpopmail vchkpw courier-authlib and courier-imap work storing mails on hard disk with qmailrocks Thibs install

Recently installed a new QMAIL, following mostly Thibs Qmailrocks install guide. I didn’t followed literally Thibs good guide, cause in his guide in few of the sections like Install Vpopmail he recommends using MySQL as a Backend to store Vpopmail email data and passwords; I prefer storing all vpopmail data on the file system as I believe it is much better especially for tiny QMAIL mail servers with less than 500 mail box accounts.

In this little article I will explain, how I made Vpopmail courier-authlib and courier-imap play nice together without storing data in SQL backend.

1. Compile vpopmail with file system data storage support

So here is how I managed to make vpopmail + courier-authlib + courier-imap, work well together:

First its necessery to compile Vpopmailin store all its users data and mail data on file system. For this in Thibs Vpopmail Intsall step compiled Vpopmail without support for MySQL, e.g. instead of using his pointed compile time ./configure, arguments I used:

# cd /downloads/vpopmail-5.4.33
# ./configure \
--enable-qmaildir=/var/qmail/ \
--enable-qmail-newu=/var/qmail/bin/qmail-newu \
--enable-qmail-inject=/var/qmail/bin/qmail-inject \
--enable-qmail-newmrh=/var/qmail/bin/qmail-newmrh \
--enable-tcprules-prog=/usr/bin/tcprules \
--enable-tcpserver-file=/etc/tcp.smtp \
--enable-clear-passwd \
--enable-many-domains \
--enable-qmail-ext \
--enable-logging=y \
--enable-auth-logging \
--enable-libdir=/usr/lib/ \
--disable-roaming-users \
--disable-passwd \
--enable-domainquotas \
# make && make install-strip
# cat > ~vpopmail/etc/vusagec.conf < < __EOF__
Disable = True;
echo 'export PATH=$PATH:/var/qmail/bin/:/home/vpopmail/bin/' > /etc/profile.d/
chmod +x /etc/profile.d/
source /etc/profile

A tiny shell script with all above options to compile (qmail) vpopmail without MySQL / PostgreSQL support is here

For other steps concerning creation of vpopmail/vchkpw – user/group just follow as Thibs suggests.

2. Compile and install courier-authlib-0.59.1

I’ve made mirror of courier-authlib.0.59.1.tar.gz cause this version includes support for vchkpw without mysql, its a pity newer versions of courier-authlib not any more have support for vpopmail to store its data directly on the hard disk.

Then on downlaod, compile && install courier-authlib:

Download authlib courier-authlib.0.59.1.tar.gz – (I made mirror of courier-authlib.0.59.1.tar.gz you can use my mirror or download it somewhere else from the net):

# cd /usr/local/src
# wget -q
# tar -zxvvf courier-authlib.0.59.1.tar.gz

Compile courier-authlib

# ./configure --prefix=/usr/local --exec-prefix=/usr/local --with-authvchkpw --without-authldap --without-authmysql --disable-root-check --with-ssl --with-authchangepwdir=/usr/local/libexec/authlib
# make && make install && make install-strip && make install-configure

On Debian Squeeze, this version of courier-authlib compiles fine, on Debian Lenny I use it too and there it is okay.

Unless above commands returns a compile error authlib will be installed inside /usr/local/libexec. If you get any errors it is most likely due to some missing header files. The error should be self explanatory enough, but just in case you have troubles to find what deb is necessery to install, please check here the complete list of installed packages I have on the host . In case of problems the quickest way (if on Debian Squeeze) is to install same packages, type:

# wget -q
# for i in $(cat list_of_all_deb_necessery_installed_packages_for_authlib.txt |awk '{ print $2 }'); do
apt-get install --yes $i;

This is for the lazy ones though it might install you some packs you don’t like to have on your host, so just install it in case you know what you’re doing 🙂

Next step is to set proper configuration for courier-authdaemon.

3. Configure courier-authlib in /usr/local/etc/authlib

Again for the lazy ones I have prepared a good config which is working 100% with vpopmail configured to store mails on the file system, to install the “good” configs, fetch mine and put them in proper location, e.g.:

# cd /usr/local/etc
# wget -q
# tar -zxvvf authlib-config-for-qmail-with-hdd-directory-stored-userdata.tar.gz

For those who prefer not to use my configuration as pointed above, here is what you will need to change manually in configs:

Edit /usr/local/etc/authlib/authdaemonrc and make sure there variable authmodulelist and authmodulelist and daemons=5
equals to:


authmodulelistorig="authuserdb authpgsql authldap authmysql authcustom authvchkpw authpipe"


Bear in mind here the setting daemons, will set how many maximum parallel connections should be possible to authdaemond on new IMAP fetch mail user requests. Setting it to 10 will allow your mail server to support up to 10 users to paralelly check your mail for a tiny mail server this setting is okay if you expect higher number of parallel mail users raise the setting to some setting fitting your needs.

P.S. On some qmail installations this value has created weird problems and took me hours to debug the whole mess is caused by this setting, make sure you plan it now unless you don’t to loose some time in future.

4. Stop debian courier-authdaemon and start custom compiled one

Now all is ready and authdaemond can be started, but before that if you have installed courier-authlib as a debian package you need to stop it via init script and only when completely sure old default Debian courier-authdaemon is stopped launch the new installed one:

# /etc/init.d/courier-authdaemon stop
# s ax |grep -i authdaemond |grep -v grep
# /usr/local/sbin/authdaemond start

To make the newly custom source installed courier-authdaemon to load itself on system boot instead of the debian installed package

# dpkg -l |grep -i courier-authdaemon
ii courier-authdaemon 0.63.0-3 Courier authentication daemon

open /etc/init.d/courier-authdaemond, after line:

. /lib/lsb/init-functions


/usr/local/sbin/authdaemond start
exit 0

This will make the script exit once launches cmd /usr/local/sbin/authdaemond start

5. Compile and Install courier-imap

You will also have to install from courier-imap archive source, I have tested it and know Qmail + Vpopmail + Courier-Imap works for sure with version courier-imap-4.1.2.tar.bz2

As of time of writing this post courier-imap-4.11.0.tar.bz2 is the latest available for download from Courier-imap download site unfortunately this version requires higher version of >= courier-authlib-0.63

In order install courier-imap-4.1.2.tar.bz2

# cd /usr/local/src
# wget -q
# tar -jxvvf courier-imap-4.1.2.tar.bz2
# chown -R hipo:hipo courier-imap-4.1.2
# su hipo
$ cd courier-imap-4.1.2/
$ export COURIERAUTHCONFIG=/usr/local/bin/courierauthconfig
$ export CPPFLAGS=-I/usr/local/courier-authlib/include
$ ./configure --prefix=/usr/local/courier-imap --disable-root-check
$ exit
# make
# make install
# make install configure

It is recommended courier-imap to be compiled with non root username. In above code I use my username hipo, other people have to use any non-root user.

6. Set proper configuration and new init script for courier-imap

In /usr/lib/courier-imap, download following working configs (for convenience I’ve made tar with my configs):

# cd /usr/lib/courier-imap
# rm -rf etc
# wget -q

Then you will have to overwrite default courier-imap init script in /etc/init.d/courier-imap with another one to start the custom compiled one instead of debian default installed courier-imap

# mv /etc/init.d/courier-imap /root
# cd /etc/init.d
# wget -q
# mv debian-courier-imap courier-imap
# chmod +x courier-imap

This init script is written use /var/lock/subsys/courier-imap, so you will have to also create /var/lock/subsys/

# mkdir -p /var/lock/subsys

7. Start custom installed courier-imap

The start/stop init script of newly installed courier-imap is /usr/lib/courier-imap/libexec/imapd.rc

/usr/lib/courier-imap/libexec/imapd.rc start

Since a new /etc/init.dcourier-imap is installed too, it can be also used to control courier-imap start/stop.

Well thats should be enough for Courier-authlib and Courier-Authlib to communicate fine between each other and be able to connect and fetch e-mail stored in file system by vpopmail.

8. Test if Qmail IMAP proto finally works

# telnet localhost 143
Connected to localhost.
Escape character is '^]'.
a login my-username-password
a LIST "" "*"
* LIST (\HasNoChildren) "." "INBOX.Sent"
* LIST (\Marked \HasChildren) "." "INBOX"
* LIST (\HasNoChildren) "." "INBOX.Drafts"
* LIST (\HasNoChildren) "." "INBOX.Trash"
a OK LIST completed
* FLAGS (\Draft \Answered \Flagged \Deleted \Seen \Recent)
* OK [PERMANENTFLAGS ()] No permanent flags permitted
* 6683 EXISTS
* 471 RECENT
* OK [UIDVALIDITY 1272460837] Ok
* OK [MYRIGHTS "acdilrsw"] ACL
* 1 FETCH (BODY[] {2619}
Received: (qmail 22304 invoked by uid 1048); 24 Apr 2012 14:49:49 -0000
Received: from unknown (HELO localhost) (
by with SMTP; 24 Apr 2012 14:49:49 -0000
Received: from localhost []

That’s all it works. Enjoy 🙂

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How to disable PC Speaker on FreeBSD / Mute PC-Speaker on BSD kernels

Wednesday, May 16th, 2012


old school personal computer pc speaker / freebsd disable Pc-Speaker picture

After finding out How PC Speaker is muted on Linux , I've decided to also disable the annoying beeps on BSD. This is in tandem with the minimalistic philosophy I try to apply to every server I manage.

Also on BSD Desktop machines it is quite annoying especially if csh (C Shell) is used, everytime you press TAB you get the beep sound. On BSD beep sound produced on tab completion is louder than in Linux and that makes it even more annoying …

Disabling pc-speaker beeps on BSDs is done via a sysctl kernel variable:

freebsd# sysctl hw.syscons.bell=0
hw.syscons.bell: 0 -> 0

To further permanently disable on system boot add hw.syscons.bell=0 to /etc/sysctl.conf, e.g.:

freebsd# echo 'hw.syscons.bell=0' >> /etc/sysctl.conf


Well that's it no more mind drilling beeps :)


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How to configure NTP server (ntpd) to synchronize server clock over the Internet on FreeBSD

Friday, February 10th, 2012


FreeBSD ntpd logo / How to configure ntpd to synchronize with internet time servers on FreeBSD

On FreeBSD ntpd , ntpdc , ntpdate , ntpq doesn't need to be installed via a specific package like on GNU/Linux as they're part of the FreeBSD world (binary standardly shipped with FreeBSD basis system).

The FreeBSD handbook has a chapter explaining thoroughfully on ntp on FreeBSD ,however for the lazy ones here is a short few steps tutorial on how to install and configure ntpd on bsd :

1. Copy sample ntp.conf file to /etc/

freebsd# cp -rpf /usr/src/etc/ntp.conf /etc/ntp/

No need for any modifications if you don't want to apply some specific restrictions on whom can access the ntpd server. If you update regularly the FreeBSD system with freebsd-update or directly by rebuilding the FreeBSD kernel / world adding restrictions might be not necessery..

If you check /usr/src/etc/ntp.conf you will notice freebsd project people are running their own ntp servers , by default ntpd will use this servers to fetch timing information. The exact server hosts which as of time of writting are used can be seen in ntp.conf and are:

server iburst maxpoll 9
server iburst maxpoll 9
server iburst maxpoll 9

2. Add ntpd daemon to load on system boot via /etc/rc.conf

By default ntpd is disabled on FreeBSD, you can see if it is disabled or enabled by invoking:

freebsd# /etc/rc.d/ntpd rcvar
# ntpd

To Enable ntpd to get loaded each time it boots , following 3 lines has to be added in /etc/rc.conf .


Quick way to add them is to use echo :

echo 'ntpdate_enable="YES" >> /etc/rc.conf
echo 'ntpdate_flags="" >> /etc/rc.conf
echo 'ntpd_enable="YES" >> /etc/rc.conf

Now as the 3 rc.conf vars are set to "YES", the ntpd can be started. Without having this variables in /etc/rc.conf , "/etc/rc.d/ntpd start" will refuse to start ntpd.

3. Start the ntpd service

freebsd# /etc/rc.d/ntpd start

One interesting note to make is ntpd can also operate without specifying any config file (/etc/ntp.conf), the only requirement for the server to start is to have a properly set ntpdate server, like lets say (ntpdate_flags="")

4. Permit only certain host or localhost to "talk" to the ntpd server

If you want to imply some ntp server restrictions, the configuration directives are same like on Linux:

To allow only a a host inside a local network with IP as well as localhost, to be able to fetch time information via ntpd server put inside /etc/ntp.conf:

restrict mask nomodify notrap

If you want to prohibit ntpd to serve as a Network Time Server, to any other host except localhost, add in /etc/ntp.conf :

restrict default ignore

Allowing and denying certain hosts can be also done on pf (packet filter) or ipfw firewall level, and in my view is easier (and less confusing), than adding restrictions through ntp.conf. Besides that using directly the server firewall to apply restrictions is more secure. If for instance a remote exploit vulnerability is discovered affecting your ntpd server. this will not affect you externally as access to the UDP port 123 will be disabled on a firewall level.
Something good to mention is NTP servers communicate between each other using the UDP source/destination (port 123). Hence if the NTPD server has to be publicly accessible and there is a firewall already implemented, access to source/dest port 123 should be included in the configured firewall …

5. Check if the ntp server is running properly / ntp server query operations

[root@pcfreak /home/hipo]# ps axuww|grep -i ntp
root 15647 0.0 0.2 4672 1848 ?? Ss 2:49PM 0:00.04 /usr/sbin/ntpd -c /etc/ntp.conf -p /var/run/ -f /var/db/ntpd.drift

To query the now running ntpd server as well as set various configuration options "on the fly" (e.g. without need for ntp.conf edits and init script restart), a tool called ntpdc exists. ntpdc tool could be used to connect to localhost running ntpd as well as to connect and manage remotely a ntpd server.
The most basic use of ntpdc is to check (server peers).:
freebsd# ntpdc localhost
ntpdc> peers
remote local st poll reach delay offset disp
=================================================== 2 64 377 0.00282 -0.050575 0.06059
*billing.easy-la 2 64 377 0.01068 -0.057400 0.06770
=ns2.novatelbg.n 2 64 377 0.01001 -0.055290 0.06058

ntpdc has also a non-interactive interface, handy if there is a need for requests to a ntpd to be scripted. To check ntpd server peers non-interactively:

freebsd# ntpdc -p localhost
=================================================== 2 64 377 0.00284 -0.043157 0.06184
=billing.easy-la 2 64 377 0.01059 -0.042648 0.05811
*ns2.novatelbg.n 2 64 377 0.00996 -0.041097 0.06094

ntpdc has plenty of other ntpd query options, e.g. :

ntpdc> help
ntpdc commands:
addpeer controlkey fudge keytype quit timeout
addrefclock ctlstats help listpeers readkeys timerstats
addserver debug host loopinfo requestkey traps
addtrap delay hostnames memstats reset trustedkey
authinfo delrestrict ifreload monlist reslist unconfig
broadcast disable ifstats passwd restrict unrestrict
clkbug dmpeers iostats peers showpeer untrustedkey
clockstat enable kerninfo preset sysinfo version
clrtrap exit keyid pstats sysstats

ntpdc is an advanced query tool for ntpd , servers. Another tool exists called ntpq which syntax is almost identical to ntpdc . The main difference between the two is ntpq is a monitoring tool mostly used just for monitoring purposes, where ntpdc can also change plenty of things in the server configuration.

For people who want to learn more on ntpd the man page is a great reading , containing chapters describing thoroughfully exactly how NTPD time servers operate, etc.

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How to install VirtualBox Virtual Machine to run Windows XP on Ubuntu Linux (11.10)

Tuesday, January 17th, 2012

My beloved sister was complaining games were failing to properly be played with wine emulator , therefore I decided to be kind and help her by installing a Windows XP to run inside a Virtual Machine.My previous install experiments with running MS Windows XP on Linux was on Debian using QEMU virtualmachine emulator.
However as Qemu is a bit less interactive and slower virtualmachine for running Windows (though I prefer it for being completely free software), this time I decided to install the Windows OS with Virtualbox.

My hope was using VirtualBox would be a way easier but I was wrong… I've faced few troubles and I thought many people who initially try to install Virtualbox VM to run Windows on Ubuntu and other Debian based Linux distros will probably experience the same problems as mine, so here is how this article was born.

Here is what I did to have a VirtualBox OS emulator to run Windows XP SP2 on Ubuntu 11.10 Linux

1. Install Virtualbox required packages with apt

root@ubuntu:~# apt-get install virtualbox virtualbox-dkms virtualbox-guest-dkms root@ubuntu:~# apt-get install virtualbox-ose-dkms virtualbox-guest-utils virtualbox-guest-x11

If you prefer more GUI or lazy to type commands, the Software Package Manager can also be used to straight install the same packages.
virtualbox-dkms virtualbox-guest-dkms packages are the two which are absolutely necessery in order to enable VirtualBox to support installing Microsoft Windows XP. DKMS modules are also necessery to be able to emulate some other proprietary (non-free) operating systems.
The DKMS packages provide a source for building Vbox guest (OS) additional kernel modules. They also require the kernel source to be install otherwise they fail to compile.

Failing to build the DKMS modules will give you error every time you try to create new VirtualMachine container for installing a fresh Windows XP.
The error happens if the two packages do not properly build the vboxdrv extra Vbox kernel module while the Windows XP installer is loaded from a CD or ISO. The error to pop up is:

Kernel driver not installed (rc=-1908)

The VirtualBox Linux kernel driver (vboxdrv) is either not loaded or there is a permission problem with /dev/vboxdrv. Please reinstall the kernel module by executing

VirtualBox vboxdrv not loaded error Ubuntu Screen

To fix the error:

2. Install latest Kernel source that corresponds to your current kernel version

root@ubuntu:~# apt-get install linux-headers-`uname -r`

Next its necessery to rebuild the DKMS modules using dpkg-reconfigure:

3. Rebuild VirtualBox DKMS deb packages

root@ubuntu:~# dpkg-reconfigure virtualbox-dkms
root@ubuntu:~# dpkg-reconfigure virtualbox-guest-dkms
root@ubuntu:~# dpkg-reconfigure virtualbox-ose-dkms

Hopefully the copilation of vboxdrv kernel module should complete succesfully.
To test if all is fine just load the module:

4. Load vboxdrv virtualbox kernel module

root@ubuntu:~# modprobe vboxdrv

If you get some error during loading, this means vboxdrv failed to properly compile, try read thoroughfully what the error is and fix it) ;).

As a next step the vboxdrv has to be set to load on every system boot.

5. Set vboxdrv to load on every Ubuntu boot

root@ubuntu:~# echo 'vboxdrv' >> /etc/modules

I am not sure if this step is required, it could be /etc/init.d/virtualbox init script automatically loads the module, anyways putting it to load on boot would do no harm, so better do it.

That's all now, you can launch VirtualBox and use the New button to initiate a new Virtual Machine, I will skip explaining how to do the configurations for a Windows XP as most of the configurations offered by default would simply work without any tampering.

After booting the Windows XP installer I simply followed the usual steps to install Windows and all went smoothly.
Below you see a screenshot showing the installed Windows XP Virtualbox saved VM session. The screenshot letters are in Bulgarian as my sisters default lanaguage for Ubuntu is bulgarian 😉

VirtualBox installed MS Windows VM screenshot

I hope this article helps someone out there. Please drop me a comment if you experience any troubles with it. Cya 🙂

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How to disable IPv6 on Debian / Ubuntu / CentOS and RHEL Linux

Friday, December 9th, 2011

I have few servers, which have automatically enabled IPv6 protocols (IPv6 gets automatically enabled on Debian), as well as on most latest Linux distribituions nowdays.

Disabling IPv6 network protocol on Linux if not used has 2 reasons:

1. Security (It’s well known security practice to disable anything not used on a server)
Besides that IPv6 has been known for few criticil security vulnerabilities, which has historically affected the Linux kernel.
2. Performance (Sometimes disabling IPv6 could have positive impact on IPv4 especially on heavy traffic network servers).
I’ve red people claiming disabling IPv6 improves the DNS performance, however since this is not rumors and did not check it personally I cannot positively confirm this.

Disabling IPv6 on all GNU / Linuces can be achieved by changing the kernel sysctl settings net.ipv6.conf.all.disable_ipv6 by default net.ipv6.conf.all.disable_ipv6 equals 1 which means IPv6 is enabled, hence to disable IPv6 I issued:

server:~# sysctl net.ipv6.conf.all.disable_ipv6=0

To set it permanently on system boot I put the setting also in /etc/sysctl.conf :

server:~# echo 'net.ipv6.conf.all.disable = 1 >> /etc/sysctl.conf

The aforedescribed methods should be working on most Linux kernels version > 2.6.27 in that number it should work 100% on recent versions of Fedora, CentOS, Debian and Ubuntu.

To disable IPv6 protocol on Debian Lenny its necessery to blackist the ipv6 module in /etc/modprobe.d/blacklist by issuing:

echo 'blacklist ipv6' >> /etc/modprobe.d/blacklist

On Fedora / CentOS there is a another universal “Redhat” way disable IPv6.

On them disabling IPv6 is done by editting /etc/sysconfig/network and adding:


I would be happy to hear how people achieved disabling the IPv6, since on earlier and (various by distro) Linuxes the way to disable the IPv6 is probably different.

Alto to stop Iptables IPV6 on CentOS / Fedora and RHEL issue:

# service ip6tables stop

# service ip6tables off

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How to make GRE tunnel iptables port redirect on Linux

Saturday, August 20th, 2011

I’ve recently had to build a Linux server with some other servers behind the router with NAT.
One of the hosts behind the Linux router was running a Window GRE encrypted tunnel service. Which had to be accessed with the Internet ip address of the server.
In order < б>to make the GRE tunnel accessible, a bit more than just adding a normal POSTROUTING DNAT rule and iptables FORWARD is necessery.

As far as I’ve read online, there is quite of a confusion on the topic of how to properly configure the GRE tunnel accessibility on Linux , thus in this very quick tiny tutorial I’ll explain how I did it.

1. Load the ip_nat_pptp and ip_conntrack_pptp kernel module

linux-router:~# modprobe ip_nat_pptp
linux-router:~# modprobe ip_conntrack_pptp

These two modules are an absolutely necessery to be loaded before the remote GRE tunnel is able to be properly accessed, I’ve seen many people complaining online that they can’t make the GRE tunnel to work and I suppose in many of the cases the reason not to be succeed is omitting to load this two kernel modules.

2. Make the ip_nat_pptp and ip_nat_pptp modules to load on system boot time

linux-router:~# echo 'ip_nat_pptp' >> /etc/modules
linux-router:~# echo 'ip_conntrack_pptp' >> /etc/modules

3. Insert necessery iptables PREROUTING rules to make the GRE tunnel traffic flow

linux-router:~# /sbin/iptables -A PREROUTING -d -p tcp -m tcp --dport 1723 -j DNAT --to-destination
linux-router:~# /sbin/iptables -A PREROUTING -p gre -j DNAT --to-destination

In the above example rules its necessery to substitute the ip address withe the external internet (real IP) address of the router.

Also the IP address of is the internal IP address of the host where the GRE host tunnel is located.

Next it’s necessery to;

4. Add iptables rule to forward tcp/ip traffic to the GRE tunnel

linux-router:~# /sbin/iptables -A FORWARD -p gre -j ACCEPT

Finally it’s necessery to make the above iptable rules to be permanent by saving the current firewall with iptables-save or add them inside the script which loads the iptables firewall host rules.
Another possible way is to add them from /etc/rc.local , though this kind of way is not recommended as rules would add only after succesful bootup after all the rest of init scripts and stuff in /etc/rc.local is loaded without errors.

Afterwards access to the GRE tunnel to the local IP using the port 1723 and host IP is possible.
Hope this is helpful. Cheers 😉

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How to load custom Kernel (tun) module in CentOS and RHEL Linux

Thursday, July 14th, 2011

kernel module load on boot in CentOS and Fedora

Just recently it was necessery to load up a tun kernel module on few CentOS Linux servers.

I’m using Debian on daily basis, and everybody that had even little of experience with Debian should already be aware about the existence of the handy:
/etc/modules file.
On Debian to enable a certain kernel module to load up on Linux boot, all necessery is to just place the kernel module name in /etc/modules.
For example loading the tun tunneling kernel module I issue the command:

debian:~# echo tun >> /etc/modules

I wondered if CentOS, also supports /etc/modules as it was necessery now to add this tun module to load up on CentOS’s boot.
After a bit of research I’ve figured out CentOS does not have support for adding modules names in /etc/modules , anyhow after consulting CentOS documentation on , I found CentOS and RHEL use /etc/rc.modules instead of Debian’s /etc/modules to load up any custom kernel modules not loaded by default during system boot.

Therefore instructing the RHEL Linux to load up my desired tun module in kernel on next boot was as easy as executing:

[root@centos ~]# echo 'modprobe tun' >> /etc/rc.modules
[root@centos ~]# chmod +x /etc/rc.modules

Now on next boot CentOS will load up the tun module in kernel. Achiving the same module load up is also possible through /etc/rc.local , but it’s not recommended way as /etc/rc.local would load up the kernel module after all of the rest init boot scripts complete and therefore will load up the module slightly later, at the final boot stage.

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