If you check the 'ifconfig -a' after a fresh install of RHEL/CentOS 6,7 system, you would find the interfaces virbr0 name. Here is an example from freshly installed CentOS 7 system. # ifconfig -a .... virbr0: flags=4099[UP,BROADCAST,MULTICAST] mtu 1500 inet 192.168.122.1 netmask 255.255.255.0 broadcast 192.168.122.255 ether 52:54:00:d5:f2:0c txqueuelen 1000 (Ethernet) RX packets 0 bytes 0 (0.0 B) RX errors 0 dropped 0 overruns 0 frame 0 TX … [Read more...] about CentOS / RHEL 6,7 : How to disable or delete virbr0 interface
CentOS / RHEL 7 : How to install kernel from rescue mode
A situation where the system has no kernel to boot from maybe because all the kernel are removed from the system or due to a corruption, Kernel installation from rescue mode is required. You can still install a new kernel from rescue mode and make the system up and be running again. Boting into rescue mode Boot the system using installation DVD or ISO and enter into the rescue mode. Follow the steps below for a detailed instruction on how to boot into rescue mode. 1. Attach the ISO … [Read more...] about CentOS / RHEL 7 : How to install kernel from rescue mode
CentOS / RHEL 7 : How to boot into rescue mode from installation DVD/ISO
When there is an issue with booting up RHEL / CentOS 7 system you can boot it into rescue mode using installation DVD or ISO to troubleshoot the issue. You can have the access to the disk where the OS is installed, to have a look at the problem or do a recovery from a corrupt kernel etc. Booting into rescue mode using DVD/ISO Follow the steps below to boot the system into rescue mode using installation media such as DVD or ISO. 1. Attach the ISO image You can use an actual installation … [Read more...] about CentOS / RHEL 7 : How to boot into rescue mode from installation DVD/ISO
CentOS / RHEL 7 : How to remove rescue image using grubby
The ask is to remove the default rescue image that gets installed when you install a CentOS/RHEL 7 system. To list out the rescue image under /boot directory : [root@geeklab ~]# ls -lrt /boot total 135164 -rw-------. 1 root root 3113253 Nov 22 2016 System.map-3.10.0-514.el7.x86_64 -rw-r--r--. 1 root root 137696 Nov 22 2016 config-3.10.0-514.el7.x86_64 -rwxr-xr-x. 1 root root 5392080 Nov 22 2016 vmlinuz-3.10.0-514.el7.x86_64 -rw-r--r--. 1 root root 277953 Nov 22 2016 … [Read more...] about CentOS / RHEL 7 : How to remove rescue image using grubby
Linux interview questions – Special permissions (SUID, SGID and sticky bit)
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. Below are few of the most commonly asked Linux interview questions on the special permissions like SUID, SGID and sticky bit. What is Set User ID (setuid)? SUID is a … [Read more...] about Linux interview questions – Special permissions (SUID, SGID and sticky bit)
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 volume open, follow the steps below. 1. Find the major,minor number corresponding to the Logical Volume: # dmsetup info -c | grep [lvname] For Example: # dmsetup … [Read more...] about lvremove Command Fails With Error “LVM – Can’t remove open logical volume”
Yum Command Fails with “Another app is currently holding the yum lock” in CentOS/ RHEL 7
While issuing any yum command, you would get an error shown below: # yum list Loaded plugins: fastestmirror, langpacks Existing lock /var/run/yum.pid: another copy is running as pid 13023. Another app is currently holding the yum lock; waiting for it to exit... The other application is: PackageKit Memory : 26 M RSS (430 MB VSZ) Started: Sat Sep 30 18:31:21 2017 - 00:33 ago State : Sleeping, pid: 13023 The error indicates that another instance is running for the yum … [Read more...] about Yum Command Fails with “Another app is currently holding the yum lock” in CentOS/ RHEL 7
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 firewall configuration for the physical network. If xen environment is not used, these rules are not needed at all. In a … [Read more...] about What causes iptables to load every time after a reboot even when it’s completely turned off
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 after a reboot you have set and saved iptables firewall rules and they are still not loaded after a reboot. After a reboot, the iptables rules are not loaded, and … [Read more...] about CentOS / RHEL : iptables troubleshooting guide
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 boot-up and the quiet option eliminates most of the boot messages. what loglevels are available Removing rhgb quiet attributes, increases the … [Read more...] about CentOS / RHEL 6 : How to change the verbosity of debug logs during booting