Ping sweep with nmap

Need to find a free IP in your address range? Use NMAP to find out which addresses are currently used:

nmap -sP

ntpdate equivalent command when running NTP service

If your Linux clock is more than a minute or two off, you might be tempted to use the “ntpdate” program to update the clock one time. However, the ntpdate program cannot run while the ntp daemon is running (not to mention the fact the ntpdate function is set to be retired).

You can mimic the functionality of the ntpdate command by issuing the “ntpd -q -g” command. This updates the clock, telling the ntp daemon to ignore the ‘sanity limit’ of 1000 seconds, and exit after updating the clock.

Oracle version numbers explained

The significance of the version numbers are (lets use as an example):

9 is the version number
0 is the new features release number
1 is the maintenance release number
1 is the generic patch set number
2 is the platform-specific patch set number


I found this excellent listing of Cache-Control definitions on

Cache-Control: no-store
This object may not be stored in any cache, even the requestor’s browser cache.

Cache-Control: no-cache
This object may be held in any cache but it must be revalidated every time it is requested.

Cache-Control: private
This object can be stored in the requesting browser´s cache but not in a shared cache …

Cache-Control: must-revalidate
Tells caches that they must obey any freshness information you give them about an object. The HTTP allows caches to take liberties with the freshness of objects; by specifying this header, you’re telling the cache that you want it to strictly follow your rules.

Cache-Control: proxy-revalidate
Similar to must-revalidate, except that it only applies to proxy caches.