Difference in du and df results
We normally see issues where the “du” and “df” outputs different disk space usage.
A common scenario is filling up of /var partition due to log files.
We usually clear the logs and the “du” outputs the expected result whereas “df” still shows the earlier reading.
This is due to the fact that, some user processes keeps the deleted files open. “du” doesn’t show that since the file (if it is removed), or the file contents (if it is cleared) is not there in the current working directory.
“df” will continue to display the previous disk space, until the space is de-allocated. To de-allocate this space either the file descriptor of the file should close or the process should exit.
Therefore you can try restarting the daemons used by these files or reboot the server.
How can I check to see if DNS is working?
Dont use “ping” to test DNS. Use tools called “nslookup” and “dig”.
Check to see whether or not your registrar is pointing your domain to the correct nameservers. The “whois” information is often wrong, and should not be used. Go to a command prompt/console and enter the command:
nslookup -type=NS yourdomainname.com
If the response does not contain all of the correct name servers, then you should contact your registrar and have them fix it.
Check to see whether a particular server is responding, you can add the server name:
nslookup www.yourdomainname.com ns1.yournameserver.com
If the response has a bunch of ‘root-servers’ that means the server does not know about the domain name and is referring you elsewhere.
To look for a certain record type, like the “MX” record or the “SOA” record, you can use the parameter “-type=MX” or “-type=SOA”.
nslookup -type=MX yourdomainname.com
In order to see all TCP sockets currently open, run (as root).
If all of the sockets in question on your webserver port (port 80), you should be able to list only those with
lsof is another tool for locating open files. lsof (no options) will list all files opened by any processes currently running. To restrict this to processes owned by username, use
Booting into Emergency Mode
In emergency mode, you are booted into the most minimal environment possible. The root file system is mounted read-only and almost nothing is set up. The main advantage of emergency mode over single-user mode is that the init files are not loaded. If init is corrupted or not working, you can still mount file systems to recover data that could be lost during a re-installation.
To boot into emergency mode, use the same method for single-user mode,
“Booting into Single-User Mode” with one exception, replace the keyword single with the keyword emergency.
Reinstalling the Boot Loader
In many cases, the GRUB boot loader can mistakenly be deleted, corrupted, or replaced by other operating systems.
The following steps detail the process on how GRUB is reinstalled on the master boot record:
Boot the system from an installation boot medium.
Type linux rescue at the installation boot prompt to enter the rescue environment.
Type chroot /mnt/sysimage to mount the root partition.
Type /sbin/grub-install /dev/hda to reinstall the GRUB boot loader, where /dev/hda is the boot partition.
Review the /boot/grub/grub.conf file, as additional entries may be needed for GRUB to control additional operating systems.
Reboot the system.
