There are two special permissions that can be set on executable files: Set User ID (setuid) and Set Group ID (sgid). These permissions allow the file being executed to be executed with the privileges of the owner or the group. Similarly, there are two special permissions for directories: the sticky bit and the setgid bit. […]
Archives for September 2017
lvremove Command Fails With Error “LVM – Can’t remove open logical volume”
You want to delete a logical volume but, when running the command “lvremove”, you get the following output: # umount /data # lvremove /dev/vg1/lv1 Can’t remove open logical volume “lv1” Solution The above error usually means that the volume is still in use by some process. To find the process that is keeping the logical […]
What causes iptables to load every time after a reboot even when it’s completely turned off
Question : Even though iptables is turned OFF using ‘chkconfig –level 345 iptables off’, ‘service iptables status’ still displays some iptables rules after every reboot. Answer The Libvirtd process will add iptables rules into iptables when starting libvirtd. iptables will run when starting libvirtd, even if iptables was disabled before. These rules will not impact […]
CentOS / RHEL : iptables troubleshooting guide
The iptables utility controls the network packet filtering code in the Linux kernel. The iptables feature is used to set up, maintain, and inspect the tables of IP packet filter rules in the Linux kernel. The post discusses the most commonly encountered issues with iptables and how to resolve them. iptables rules do not load […]
CentOS / RHEL 6 : How to change the verbosity of debug logs during booting
Removing the rhgb and the quiet kernel command-line parameters, either by editing the /boot/grub/grub.conf file or by using the grub editing mode during boot. Replace these parameters with loglevel parameter. This change is generally sufficient to provide as much data as is needed to debug a boot failure. The option rhgb is used for graphical […]
CentOS / RHEL 5, 6 : how to disable NetworkManager
What is NetworkManager? – NetworkManager is a dynamic network control and configuration system that attempts to keep network devices and connections up and active when they are available. – NetworkManager consists of a core daemon, a GNOME Notification Area applet that provides network status information, and graphical configuration tools that can create, edit and remove […]
Linux OS Service ‘microcode_ctl’
Service Name microcode_ctl Description microcode_ctl updates a microcode in IA32 CPU. The microcode is prepared as a file /etc/firmware/microcode.dat. As this service can fix microcode bugs by updating CPU microcode this should be available.m however only for Intel IA32. Basically, running “service microcode_ctl start” doesn’t start up a daemon, it first checks if the processor […]
CentOS / RHEL 6 : How to boot into rescue mode
Rescue mode is the same as a single-user mode. This mode can be used when a condition exists that prevents your system from completing the regular boot process. The system attempts to mount local file systems and start some system services. But rescue mode does not start the network service and does not allow other […]
CentOS / RHEL 6 : Lock User Account After N Number of Incorrect Login Attempts
Often a requirement in a secure environment is to lockdown users after they enter a wrong password for a specified number of times. This makes the system protect againt attacks likes password dictionary attacks. The post describes how to lock an account after N incorrect login attempts using pam.d files. Lock user after N incorrect […]
CentOS / RHEL 6 : how to start the services interactively during boot (to disable/abort some services)
There is an option with RHEL / CentOS 6 to start the services interactively during system boot up. We can use it to troubleshoot any booting issues related to services. By default, this feature is not available and you need to enable it using either of the 2 methods described below. Method 1 – Using […]