Category: General IT Info

Sendmail Hangs When Starting Up or When Using It with PHP

If you notice Sendmail hanging up for many minutes while starting up at boot or while being used such as sending an email from a PHP script, it is probably a DNS lookup problem. Ensure your /etc/hosts file has the appropriate entries for your server that sendmail is running on:

127.0.0.1       localhost localhost.localdomain
192.168.1.80    www.foo.com www
192.168.1.80    www.foo.com.

Replace “www.foo.com” and “www” with your server’s fully qualified domain name and host name respectively. Replace “192.168.1.80” with the IP address of your server. The last entry with the “.” at the end is critical so don’t forget it.

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Mysqldump Error 29
	


	

		
If you try to backup a MySQL database with a lot of tables using mysqldump, you may get an error similar to the following:


mysqldump: Got error: 29: File './test/test.MYD' not found (Errcode: 24) when using LOCK TABLES


To fix the problem you can add the –skip-lock-tables option to your mysqldump command line like so:


mysqldump --skip-lock-tables -u root -p db_foo > db_foo_backup_13FEB09.sql


Or you can permanently fix the problem by increasing the number of open files allowed by your MySQL service by specifying a sufficiently large value for the open_files_limit setting in your MySQL server configuration file. The open_files_limit needs to be placed under the [mysqld] section of the MySQL server configuration file. On CentOS 5 the default location of the MySQL service configuration file is /etc/my.cnf. Try specifying a value of 8192 like so:


[mysqld]
open_files_limit = 8192


After you make the change to the MySQL server configuration file you’ll need to restart the MySQL service to put the changes into effect. On CentOS 5 you can restart the MySQL service with the following command:


service mysqld restart


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How to Mirror A Directory Locally Using Rsync
	


	

		
Most of the instructions for mirroring a directory with Rsync involve mirroring the data to a different server. You can also use Rsync to sync or mirror a directory on the same computer as the source directory.


Here is an example command on how to mirror two directories locally. In this example, /home/apache/public_html is the source directory and /home/apache/public_html_bak is the destination directory. Forward slashes at the end of the directory names in the rsync command are important so don’t forget them. The sync is one-way in that the destination will mirror the source:




rsync -avr --delete --links /home/apache/public_html/ /home/apache/public_html_bak/




If you need to preserve ACLs and Extended attributes then you need to add the -A and -X switches:




rsync -avrAX --delete --links /home/apache/public_html/ /home/apache/public_html_bak/




If you want to just do a dry run to see what will happen when you run Rsync, then add the -n switch:




rsync -avrAXn --delete --links /home/apache/public_html/ /home/apache/public_html_bak/



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How to Block an IP Address Using Firestarter
	


	

		
Firestarter is a GNOME program which offers a GUI interface to the IP Tables firewall.


If you have Firestarter configured to allow all IPs addresses to all ports or a particular port on your server, you can’t block a specific IP from accessing those ports using the GUI interface. However, Firestarter does allow you to manually specify IP Tables rules to either load up BEFORE or AFTER the Firestarter firewall rules by editing configuration files. On CentOS 5, the file to put the rules you want to load before Firestarter’s rules is /etc/firestarter/user-pre. For rules you want to load after Firestarter loads its firewall rules, edit the file /etc/firestarter/user-post. When you add your rules, instead of using the command “iptables” you need to use the variable name “$IPT” instead. To block an IP address from accessing any of your ports, you will need to add the IP Tables rule to the user-post file.


For example, say you want to block the IP address 123.11.112.1 from accessing your server on any port. The normal IPTables rules for this is:


iptables -I INPUT -s 123.11.112.1 -j DROP


To make this rule work with Firestarter, add the following line to the /etc/firestarter/user-post file:




$IPT -I INPUT -s 123.11.112.1 -j DROP




After you edit the user-pre or user-post files, you have to restart the Firestarter service to put them into effect. On CentOS 5 you can run the following terminal command to restart the Firestarter service:




service firestarter restart




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