Tutorial Proggraming Free

tutorial - How to disable CTRL-ALT-DEL from rebooting a Linux system

The action taken for CTRL-ALT-DEL is defined in /etc/inittab and if we will look in that file we will see a line similar to the one from bellow (taken from a Debian system):

# What to do when CTRL-ALT-DEL is pressed.
ca:12345:ctrlaltdel:/sbin/shutdown -t1 -a -r now

Here is another example: on RedHat based systems this line will look like:

ca::ctrlaltdel:/sbin/shutdown -t3 -r now

If we want to disable this action we only have to change this line. We can delete it, comment it, or even replace it with something like:

ca:12345:ctrlaltdel:/bin/echo "CTRL-ALT-DEL is disabled"

After making any changes to the inittab file, you will have to tell init to reload it, in order to activate the changes. To do that, simply run as root:

init q

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source:http://linuxpoison.blogspot.com/2007/10/1357817580327.html

tutorial - Squid Password Authentication Using PAM

We’ll be using the pam_auth module. This will allow anyone who has a shell account to also be able to use the Squid server. Search for the auth_param section in the config and add these lines:

auth_param basic program /usr/lib/squid/pam_auth
auth_param basic children 5
auth_param basic realm Squid proxy-caching web server
auth_param basic credentialsttl 2 hours

Next search for this line and uncomment it:

acl password proxy_auth REQUIRED

Now create a pam module called /etc/pam.d/squid that contains:

auth required /lib/security/pam_unix.so
account required /lib/security/pam_unix.so

Restart the squid and you are done.

source:http://linuxpoison.blogspot.com/2007/10/13578175807947.html

tutorial - How to configure ntp

You just need to add the some ntp server in your /etc/ntp.conf and /etc/ntp/step-tickers

/etc/ntp.conf:

server 192.168.0.5
server 192.168.0.6

/etc/ntp/step-tickers

192.168.0.5
192.168.0.6

Then of course make sure that ntp will start at boot time:

chkconfig –level 2345 ntpd on
chkconfig –list ntpd

And let’s start the service now:
service ntpd start

source:http://linuxpoison.blogspot.com/2007/10/135781758017091.html

tutorial - Installing Xfce on Ubuntu

Paste in this command in terminal

sudo aptitude update && sudo aptitude install xubuntu-desktop

To use Xfce after you’ve installed it:

1) Log out

2) Under “Session”, select “Xfce”

3) Log back in again.

source:http://linuxpoison.blogspot.com/2007/10/135781758017089.html

tutorial - The right way to ask queries in a Discussion Forum

No. 1 : A discussion forum is NOT a pre paid group which will answer all your queries.

No. 2 : Never ever post any query in a forum without searching (thoroughly) for the solution yourself.

No. 3 : Searching for answers involves searching a lot of places. Firstly check the archive of that particular
Discussion Forum itself since someone else might have already posted your query before you.

No. 4 : Once you done that then go to other discussion forums as well and check their archives.

No. 5 : Search at Google.com. 99.9% you will find a solution there. Since Google is simply too good and searches a hell lot of places. Even if you skip step No. 4 never ever skip searching Google.

No. 6 : Suppose you don’t get a solution at Google, check out other search engines as well, such as Lycos or Altavista. Maybe you will find a solution there.

No. 7 : Another important thing is to read the thousands of pages of reference material that is available with a default installation of Linux. The ‘man’ pages as well as ‘info’ pages. You can find a lot of information here which even users of Forums may not know.

No. 8 : Once you have searched at all these places, and if you do not get your solution do not rush to the Forum site to post your query. First check with your friends. Unless you are doing something with any cutting edge technology, some of your friend might have already faced your problem before.

No. 9 : You must be feeling its a pain to do all this before posting a query, since if you post one you would get a reply within a couple of hours. Sorry guys, you are wrong .. most of the times if you post a stupid query (stupid
query = a query which can be solved by following any of the above steps) you won’t get a reply at the Forum and if you repost your query, you are sure to get some hate mail. Never repost any of your queries if you didn’t get a reply on your first posting. No replies is an indication that your query is a simple one and a little Googling can solve your problem.

No. 10 : A very very important thing is that when you post a query the reader of your post should get a feeling that you have really tried a lot before coming to the Forum. Noone wants people who give up in 10 minutes and come running to the forum for answers. You gotta have tried for ablest a couple of days before finally approaching a Forum. Remember merely mentioning that you tried hard is not enough.. Since the experts know very well whether the problem is one that can be solved if you had really tried or whether the problem is really tough to solve. So don’t start your posting with a “I tried a lot but I couldn’t find the solution…” unless you really really have done that.

No. 11 : People sometimes post messages asking for reference material . As in where can I find more info about this particular topic. Such postings may or may not get favourable replies. The best thing is to simply type the topic name in a Google search and you will find thousands of sites giving you info about your topic. You may have to do a little searching to get the best of the sites but that is worth the effort. But sometimes people do reply to such postings..and give you sites that they have been using. Try your luck on this one.. but I prefer Google.

No. 12 : Whenever you post your message read you message before posting it ‘as a stranger‘. What I mean is that read it as a person who has no knowledge about your machine and the stuff on it. You will realise suddenly that a lot more info about your setting and configuration is required to get a better idea of your problem. So always mention sufficient details about your machine. There is no need to mention hardware details like your chip and all generally. Its only about what sort of environment your are using and what are the related softwares that you have installed. There is no fixed details that you gotta enter. Common sense is enough to figure out what details (pertaining to your problem) should be present in a query.

No. 13 : Cut the crap. There is no need to include humour and other crap in your postings. Experts trying to help have no time for this. So get to the point and get your problem across as fast as possible (but it should be clear.. Not a hastily written message).

No. 14 : Wait for a couple of days for a reply. People may respond a little late to your query.. But don’t repost your query. A no-reply means search elsewhere.. You will definitely find the solution easily.

No. 15 : Never use words that are objectionable to others and never get into quarrels with other members even if they have hurt your sentiments.. A quarrel that carries on in a discussion forum is a waste of time for other members who have nothing to do with your problem. So its best to forget and consider it to be cheap on the other fellow’s part to have passed any comments about you.

No. 16 : Finally reply to the person who gave you a solution either thanking him or mentioning the problems you face while implementing whatever he had mentioned. This would either make him to give your further instructions (in case you faced some problems) or would make him happy that he managed to help someone. No replies from you keeps the person who replied to your query wondering about what happened to your problem. So its always nice to post a ‘Thank You’ note once your problem has been solved.

source:http://linuxpoison.blogspot.com/2007/10/135781758017673.html

tutorial - What to do if Linux refuses to boot after a power failure?

SolutionThere are a few things that you can try to get Linux back on track..

1.	While rebooting, Linux may print a message saying that there is a problem with the filesystem and it might force a check. It would do this for all Linux partitions. If the checks (they generally take around 30-60 seconds..also displays a rotating indicator) are successful then you are lucky. Linux should mostly continue booting and you should be back in business soon. If this is what happens then you may be under an impression that a direct switch off of a Linux machine leads to no problems. Please note that you were just lucky that you got away without any major problems.
2.	While rebooting, Linux may not be successful in any of the filesystem checks. In this case the booting stops abruptly with a message stating that you should run fsck manually without a few parameters. Once you reach the bash prompt (#) then run the fsck command as mentioned (fsck -a -p /dev/hdaX). The fsck command requires the partition which it has to check. So if you have installed linux on /dev/hda1 then you have to run fsck as follows : fsck -a -p /dev/hda1 When you run fsck it would mostly find some inode problems and ask you whether you want to fix them, Select the default option (yes). It would do so for all the problems found. Once this is over you can restart the machine using either “Ctlr+Alt+Del” or “shutdown -r 0″, whichever works. Now this time your Linux machine should boot properly.
3.	It has so happened that once when the power failed, Linux simple failed to boot on my machine. It gave no errors at all, but the init process would just not get initiated. It would find my partitions, mount the ext2 filesystem as read only and then would simply display a prompt. Even with this prompt I wasn’t able to do a lot. Also I noticed that a few of the default directories were missing on the native partition. The only solution that I found to this was to Reinstall Linux. Note : While reinstalling, the best and safest way for the above problem would be to insert the installation media (Redhat Linux 6.1 CD). And instead of selecting to install the OS once again, select to upgrade the existing installation. This would effectively replace all the damages areas of the OS and would also retain all your personal data and configurations in Linux (This should work in almost all cases). Hence I would always advice an upgrade to the same version (If you originally had Redhat Linux 6.1 , insert the CD and once again select to upgrade to Redhat Linux 6.1 itself). If the upgrade option doesn’t fix the problem, then you would have to do a reinstall after a reformat of the Linux partitions.
4.	In one case when Linux refused to boot I noticed that a few of the main files / directories were missing. I found that the /sbin directory (which is a very important directory) wasn’t there in the root directory. I performed a search for this directory and found it to be within another directory. After some discussion I realised that my friend has accidently moved this directory within his GUI when he was working in superuser mode. As long as he was working a few commands didn’t work but he did not bother about finding out why they didn’t work. Once he rebooted his machine the abscence of this directory hung the booting process. So I suggest that you never work in superuser mode unless absolutely necessary. Even within the superuser mode prefer the shell, since you can hardly ever do anything accidently in the shell. In my friend’s case I simply moved that directory back within the root directory and Linux booted without any problems.

source:http://linuxpoison.blogspot.com/2007/10/13578175807432.html

tutorial - Setup Quotas

1) Enter Single user Mode:

# init 1

2) Edit your /etc/fstab file

The /etc/fstab file lists all the partitions that need to be auto-mounted when the system boots. Edit and add “usrquota” option to the partition that you want to have quota enable

LABEL=/home       /home          ext3    defaults,usrquota  1 2

3) Remount your partition

# mount -o remount /home

4) Come out of single user mode

# Exit

5) Create quota file

The uppermost directory of the filesystem needs to have an aquota.user file (defines quotas by user) and an aquota.group file (defines quotas by group), or both.

# touch /home/aquota.user

# chmod 600 /home/aquota.user

5) Initialize the user quota

# quotacheck -vagum

6) Edit the quota for a user

# edquota -u nikesh

The command will invoke the vi editor.

Disk quotas for user nikesh (uid 503):
 Filesystem  blocks       soft       hard     inodes     soft     hard
 /dev/hda3       24          0          0          7        0        0

source:http://linuxpoison.blogspot.com/2007/10/135781758014340.html

tutorial - Recover lost root password

1) Reboot your system.

2) When GRUB comes up type e .

3) Now select entry that begins with kernel.

4) Add then end of the kernel entry type either s or single, now hit enter.

5) Type b, the box will boot up and now you can type in the cmd prompt passwd root.

This will reset the password to whatever you like.

source:http://linuxpoison.blogspot.com/2007/10/135781758015930.html

tutorial - How to use procmail + spamassassin

The /etc/procmailrc file is used by procmail to determine the procmail helper programs that should be used to filter mail. This file isn’t created by default.

spamassassin has a template you can use called /etc/mail/spamassassin/spamassassin-spamc.rc. Copy the template to the /etc directory.

# cp /etc/mail/spamassassin/spamassassin-spamc.rc /etc/procmailrc

source:http://linuxpoison.blogspot.com/2007/10/13578175801409.html

tutorial - How to use SPAM Blacklists (Public) With Sendmail

There are many publicly available lists of known open mail relay servers and spam generating mail servers on the Internet. Some are maintained by volunteers, others are managed by public companies

You can configure sendmail to use its dnsbl feature to both query these lists and reject the mail if a match is found. Here are some sample entries you can add to your /etc/sendmail.mc file; they should all be on one line.

FEATURE(`dnsbl', `ipwhois.rfc-ignorant.org',`"550 Mail from " $&{client_addr} " refused. Rejected for bad WHOIS info on IP of your SMTP server - see http://www.rfc-ignorant.org/“‘)

FEATURE(`dnsbl', `proxies.blackholes.easynet.nl', `"550 5.7.1 ACCESS DENIED to OPEN PROXY SERVER "$&{client_name}" by easynet.nl DNSBL  (http://proxies.blackholes.easynet.nl/errors.html)”‘, `’)dnl

FEATURE(`dnsbl', `relays.ordb.org', `"550 Email rejected due to sending server misconfiguration - see http://www.ordb.org/faq/#why_rejected“‘)dnl

FEATURE(`dnsbl', `bl.spamcop.net', `"450 Mail from " $`'&{client_addr} " refused - see http://spamcop.net/bl.shtml“‘)

FEATURE(`dnsbl',`sbl.spamhaus.org',`Rejected - see http://spamhaus.org/’)dnl

Be sure to visit the URLs listed to learn more about the individual services.

source:http://linuxpoison.blogspot.com/2007/10/13578175808731.html

tutorial - Lock User Accounts After Too Many Login Failures

Add the following two lines highlighted in blue to the /etc/pam.d/system-auth file as shown below:

auth        required      /lib/security/$ISA/pam_env.so
auth        required      /lib/security/$ISA/pam_tally.so onerr=fail no_magic_root
auth        sufficient    /lib/security/$ISA/pam_unix.so likeauth nullok
auth        required      /lib/security/$ISA/pam_deny.so
account     required      /lib/security/$ISA/pam_unix.so
account     required  /lib/security/$ISA/pam_tally.so per_user deny=5 no_magic_root reset
account     sufficient    /lib/security/$ISA/pam_succeed_if.so uid < 100 quiet
account     required      /lib/security/$ISA/pam_permit.so
password    requisite     /lib/security/$ISA/pam_cracklib.so retry=3
password    sufficient    /lib/security/$ISA/pam_unix.so nullok use_authtok md5 shadow
password    required      /lib/security/$ISA/pam_deny.so
session     required      /lib/security/$ISA/pam_limits.so
session     required      /lib/security/$ISA/pam_unix.so

The first added line counts failed login and failed su attempts for each user. The default location for attempted accesses is recorded in /var/log/faillog.

source:http://linuxpoison.blogspot.com/2007/10/135781758016862.html

tutorial - How to scan a host

You can use nmap to determine all the TCP/IP ports on which a remote server is listening. It isn’t usually an important tool in the home environment, but it can be used in a corporate environment to detect vulnerabilities in your network, such as servers running unauthorized network applications. It is a favorite tool of malicious surfers and therefore should be used to test external as well as internal servers under your control.

Here is an example of trying to do a scan using valid TCP connections (-sT) in the extremely slow “insane” mode (-T 5) from ports 1 to 5000.

# nmap -sT -T 5 -p 1-5000 192.168.1.153

source:http://linuxpoison.blogspot.com/2007/10/13578175809575.html

tutorial - Block mp3, mpg, mpeg, exe files using Squid proxy server

First open squid.conf file /etc/squid/squid.conf:

# vi /etc/squid/squid.conf

Now add following lines to your squid ACL section:

acl blockfiles urlpath_regex “/etc/squid/multimedia.files.acl”

Now create the the file

# vi /etc/squid/multimedia.files.acl

\.[Ee][Xx][Ee]$ \.[Aa][Vv][Ii]$ \.[Mm][Pp][Gg]$ \.[Mm][Pp][Ee][Gg]$ \.[Mm][Pp]3$

Save and close the file and Restart Squid:

# /etc/init.d/squid restart

source:http://linuxpoison.blogspot.com/2007/10/135781758018004.html

tutorial - Block Ads by using squid and Ad Zapper

It has been observed that 30% of your bandwidth is consume by Ads so if you block these ads you can save this 30% bandwidth and can use to some other things. How we can do this …

Make sure your squid is configured and working fine
Get ad-zapper using following commands:

# cd /etc/squid
# wget http://voidmain.is-a-geek.net/files/scripts/update-zapper
# chown root:root update-zapper
# chmod 700 update-zapper

Now run the update-zapper which should automatically download the latest adzapper Perl script from sourceforge: # ./update-zapper

If the script ran properly you should now see an executable Perl script named squid_redirect
# ls squid_redirect

Edit squid_redirect file and make sure the perl path ( command - # whereis perl) is defined according to your system

#!/usr/bin/perl

Now you want to edit the squid.conf file with your favorite text editor and search for the section containing the tag redirect_program. This is where we tell Squid to use the ad zapper script.

Edit squid.conf file and insert the following line

redirect_program /path/to/squid_redirect

Restart the squid - # service squid reload

source:http://linuxpoison.blogspot.com/2007/10/135781758014124.html

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