If you want to block a range of IP addresses from accessing your CentOS server you can issue the following IPTables firewall command:

/sbin/iptables -I INPUT -m iprange --src-range 192.168.1.1-192.168.1.2.100 -j DROP

Replace “192.168.1.1-192.168.1.2.100″ with the IP range you want to block. This command only works with the IPTables firewall so if your operating system is using a different firewall then this command will not work.

Here is another example which uses CIDR notation to specify the IP range:

/sbin/iptables -I INPUT -s 192.168.1.1/25 -j DROP

If you restart your server, the drop command will be removed. If you want this command to persist through reboots, then you’ll need to add it to a startup script.

Command to recursively chmod only directories:

find . -type d -exec chmod 755 {} \;

How to recursively set the execute bit on every directory:

chmod -R a+X *

The +X flag sets the execute bit on directories only

How to recursively chmod only files:

find . -type f -exec chmod 644 {} \;

How to recursively chmod only PHP files (only for PHP files with the extension .php):

find . -type f -name '*.php' -exec chmod 644 {} \;

You can change the “.php” in the line above to whatever file extension that you want.

To determine what daemon or program is listening on a port in Linux or OS X you can use the lsof command. You need to run the command while logged in as root or if your operating system supports sudo like OS X, you can use that.

Command to run in Linux:

lsof -i -nP

Command to run in OS X:

sudo lsof -i -nP

This tip is really a generic *Nix command so it should not only work in OS X but also in other flavors of UNIX such as Linux.

In OS X a lot of the log files are auto-archived into compressed GZIP files in Tiger (Mac OS X 10.4) and BZ2 files in Leopard (Mac OS X 10.5). If you need to search for an occurrence of a word or other string of text in a compressed GZIP file, you can use the following terminal command:

zgrep someString theLogFileName.log.gz

As an example, say you want to search for the word “error” in a compressed system log in Tiger. To do this you would launch the Terminal application and run the command:

zgrep error /var/log/system.log.0.gz

Alternatively you can search all compressed system log files in a single command by using an asterisk (*) as a “wild card” character:

zgrep error /var/log/system.log*

Leopard uses the BZ2 compression scheme for most of its log files and you can similarly search those files by using the bzgrep command. For example, say you wanted to search all the application firewall logs for instances of the word “connection”. To do this you would run the following command in the Terminal application:

bzgrep connection /var/log/appfirewall.log*

If you need to search for a string that consists of more than one word separated by spaces, you would need to surround the search string in quotation marks:

bzgrep "connection attempt" /var/log/appfirewall.log*