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Crontab basics

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cron is a unix, solaris utility that allows tasks to be automatically run in the background at regular intervals by the cron daemon. These tasks are often termed as cron jobs in unix , solaris.
Crontab (CRON TABle) is a file which contains the schedule of cron entries to be run and at specified times.


Crontab syntax


A crontab file has five fields for specifying day , date and time  followed by the command to be run at that interval.

*     *   *   *    *  command to be executed
–     –    –    –    –
|     |     |     |     |
|     |     |     |     +—– day of week (0 – 6) (Sunday=0)
|     |     |     +——- month (1 – 12)
|     |     +——— day of month (1 – 31)
|     +———– hour (0 – 23)
+————- min (0 – 59)

* in the value field above means all legal values as in braces for that column.
The value column can have a * or a list of elements separated by commas. An element is either a number in the ranges shown above or two numbers in the range separated by a hyphen (meaning an inclusive range).

Crontab Example

min      hour      day/month      month      day/week       Execution time
30     0     1     1,6,12     *     — 00:30 Hrs  on 1st of Jan, June & Dec.

0     20     *     10     1-5     –8.00 PM every weekday (Mon-Fri) only in Oct.

0     0     1,10,15     *     *     — midnight on 1st ,10th & 15th of month

5,10     0     10     *     1     — At 12.05,12.10 every Monday & on 10th of every month


By default cron jobs sends a email to the user account executing the cronjob. If this is not needed put the following command At the end of the cron job line .

>/dev/null 2>&1


To collect the cron execution execution log in a file :

30 18  *    *   *    rm /home/someuser/tmp/* > /home/someuser/cronlogs/clean_tmp_dir.log

DirectAdmin : Downgrade from apache 2 back to apache 1.3

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How to downgrade from apache 2 back to apache 1.3
If you need to go back to apache 1.3 after installing apache 2, you’ll need to do the following:

1) Restore the old httpd.conf file:

cp -f /etc/httpd/conf/httpd.conf.1.3.backup /etc/httpd/conf/httpd.conf

2) Get DirectAdmin to use the old httpd.conf files for the users. Edit the /usr/local/directadmin/conf/directadmin.conf file and change apache_ver=2.0 to apache_ver=1.3. Also, if you were using custombuild, and are going back to customapache, the apache_conf value needs to be reset to:

apacheconf=/etc/httpd/conf/httpd.conf

Then type

echo “action=rewrite&value=ips” >> /usr/local/directadmin/data/task.queue
echo “action=rewrite&value=httpd” >> /usr/local/directadmin/data/task.queue
echo “action=directadmin&value=restart” >> /usr/local/directadmin/data/task.queue

3) Now you can recompile apache 1.3

rm -f /usr/lib/apache/*
cd /usr/local/directadmin/customapache
./build clean
./build all

4) Fix the modules link:

cd /etc/httpd
rm -f modules
ln -s /usr/lib/apache modules

5) Restore the old boot script:
FreeBSD:

cp -f /usr/local/directadmin/customapache/httpd_freebsd /usr/local/etc/rc.d/httpd
chmod 755 /usr/local/etc/rc.d/httpd
/usr/local/etc/rc.d/httpd restart

RedHat:

cp -f /usr/local/directadmin/customapache/httpd /etc/init.d/httpd
chmod 755 /etc/init.d/httpd
/etc/init.d/httpd restart

How do I upgrade Apache on my Cpanel box?

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There are two ways to update your Apache version on cpanel.

The first is from within WHM. Underneath the “Software” section in the menubar is “Apache Update.” Click this link and then follow the on screen directions.

The second option is from the command line. Run “/scripts/easyapache”, which will provide an update interface including a few more options than WHM.

It is highly recommended that you create a backup of your Apache configuration before preforming an update to a newer version.

How to block ip addresses with iptables

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I have an ip address attacking my server or taking up all my httpd connections so none of my sites work.

In order to correct this you will want to use the netstat -n command to see the ip addresses connected to your server. Once you have the ip address you want to block you can use the following command to block them from accessing your server using iptables

iptables -I INPUT 1 -s IP.ADD.RES.SS -j DROP

-I INPUT 1 means to insert the rule at the top of the INPUT table (which means it will get looked at first)

-s IP.ADD.RES.SS is the source address of the packets we want to deal with

-j DROP means dump the packets into the void, and forget they ever happened.

cgi files generate Internal Server Error

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This means that the cgi script did not execute properly. There are several causes that can generate this error so a few things would need to be checked.

1) check the /var/log/httpd/suexec_log. It contains any errors that would be as a result of not having correct permissions on the file. The file needs to be in a cgi-bin and must have the owner/group as the username who owns the site. If it’s owned by anyone else, it will not run. Also, the script must have execute permission. The most common chmod permission is 755. Go through all directories from the public_html down to the directory the script is in, and make sure they’re all set to 755 (public_html can be 750 *only* if it has a group of apache).

If the suexec_log only shows the script being run, then the cause may be with the script code itself. The easiest way to figure out script coding problems is to first run the script manually from an ssh prompt.

cd /home/username/domains/domain.com/public_html/cgi-bin
./script.cgi

One common error is to use an incorrect interpreter. The 2 most common interpreters are:

#!/usr/bin/perl

and

#!/usr/local/bin/php

This code must appear on the first line of the script. Somtimes a file is uploaded in windows format so the trailing newline (return) character is formed incorrectly and the file would need to be reuploaded in a different format.

Other errors that would be generated when running the script manually from ssh would be missing perl modules, in which case you’ll need to install them yourself. Cpan is the easiest method to install new perl modules, eg:

perl -e shell -MCPAN
install Bundle::DBD::mysql

HOW-TO: Change Exim IP Address – cPanel

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Change the IP exim uses to send mail – bypass sorbs (or other useless blacklists)

On shared hosting server’s you’ll probably find yourself blocked by other mail providers because of high mail volume or one of your users spamming. To change your exim IP for outgoing mail, you can edit your /etc/exim.conf file and locate this section:

remote_smtp:
driver = smtp

And below this add:

interface – xxx.xxx.xxx.xxx

Replace the x’s with the backup mail IP. Note that the backup mail IP needs to be added to the server, and should have a valid PTR and A-record.

Note:  On newer versions of cPanel where domainkeys are installed, you will need to add another interface line:

dk_remote_smtp:
driver = smtp
dk_private_key = “/var/cpanel/domain_keys/private/${dk_domain}”
dk_canon = nofws
dk_selector = default
interface = xxx.xxx.xxx.xxx

Now restart exim.

Note: If there is an exim update when you upgrade your cpanel server you will need to re-enter the interface again.

How to forward a website to another url using PHP

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There are several ways to accomplish this task, but the simplest to understand is to use php.

To do this, you need to create the page that will do the forwarding. This can be any page, as long as it ends in “.php”. If you are trying to redirect a domain, you’d create “index.php” inside the public_html directory.

Once you decide which page you will use, then create the file and enter the following text:

<?php
header(“Location: http://whereyouwant.com/to/go.html“);
?>

Where http://whereyouwant.com/to/go.html is the location that you want the page to forward to. You can use local values, ie: /page.html, or full urls as in the above example (http://..etc.)

Apache : Showing files in a directory

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Showing files in a directory
If you want to list all files in a directory that doesn’t use an index.html (or index.php) file, you can create an .htaccess file with the following contents:

Options +Indexes

This will tell apache that you want to list all files in the directory.