Setting Password for Single User Mode in Linux adds security to Linux boxes. If someone accesses your server physically & reboots the server and then tries to change the root password from single-user mode. In that case, if the password is set on single-user mode, one has to first enter the root password, then only […]
CentOS/RHEL 5
“Server refused to allocate pty” – Unable to login to CentOS/RHEL
The Problem On an CentOS/RHEL 6 server, all logins started to fail to the system with the following errors: Server refused to allocate pty /etc/profile: line 48: /dev/null: Permission denied /etc/profile: line 76: /dev/n ull: Permission denied /etc/profile: line 76: /dev/null: Permission denied /etc/pr ofile: line 76: /dev/null: Permission denied /etc/profile: line 76: /dev/null: Permission […]
“Connection reset by peer” – error while ssh into a CentOS/RHEL system with a specific user only
The Problem When trying to open an ssh connection to a system with a specific account, it failed with ‘Connection reset by peer’. Other users can successfully connect with ssh to this system. Below is an example showing the failed login to node [NODE2] with the account ‘oracle’. [oracle@NODE1]$ ssh oracle@[NODE2] oracle@[NODE2]’s password: Read from […]
How to Configure NFS Services (server-side) to Run on Static Ports as an Alternative CentOS/RHEL 5 and 6
By default, some NFS services (server_side) use well-known ports but some normally just use random ports. The use of random ports does not work so well with strict firewall rules blocking random yet unknown ports. So, sometimes it is best to manually alter or specify static ports for NFS services (server_side) to use. This post […]
FATAL: Error inserting rds_rdma
The Problem RDS module is not loading after rebooting the nodes of the cluster. Therefore, the CRS cannot run on any of the nodes. When trying to load the module the following errors are displayed: # modprobe rds_rdma FATAL: Error inserting rds_rdma /lib/modules/2.6.18-274.18.1.0.1.el5/updates/net/rds/rds_rdma.ko): Unknown symbol in module, or unknown parameter (see dmesg) dmesg output shows […]
Failed to start LSB: Bring up/down networking – On restarting network service CentOS/RHEL (DHCP client)
The Problem Cannot reliably get networking to start on a server when using DHCP: # sudo /etc/init.d/network restart Restarting network (via systemctl): Job for network.service failed because the control process exited with error code. See “systemctl status network.service” and “journalctl -xe” for details. [FAILED] # journalctl -xe Nov 29 18:57:42 hostaname.domain.com dhclient[37653]: exiting. Nov 29 […]
How to Capture More Logs in /var/log/dmesg for CentOS/RHEL
Sometimes not all log messages in /var/log/dmesg can be captured after Linux OS/kernel boots up because of too many block devices attached to the server. This post illustrates another way to record all bootup log messages besides configuring the serial console. Basically kernel parameter “log_buf_len” can be configured to increase kernel log buffer size, Name: […]
How to enable additional scsi logging in CentOS/RHEL
Question: How to turn on scsi extended debug messages? What are the field/flag definitions of scsi_logging_level? Additional scsi logging messages can be enabled by writing to /proc/sys/dev/scsi/logging_level either via use of the echo or preferably the sysctl command. The kernel parameter consists of ten packed fields, each 3 bits in length. Each field can have […]
How to Disable rpc.quotad Service in CentOS/RHEL 6 and 7
This post explains how to disable rpc.quotad service on CentOS/RHEL 6 or 7 machines. The protocol used by rpc.rquotad is an unsecured and obsolete protocol and should be disabled. Note: This procedure can disrupt NFS clients relying on disk quotas. To avoid issues, schedule a maintenance outage and stop these NFS clients. For CentOS/RHEL 6 […]
How to configure Linux Resource Groups (cgroups) for MySQL
The following provides the procedure for configuring and using resource groups under Linux: 1. To use cgroups, you must install the “libcgroup” package on your system. # yum install libcgroup 2. Create a config file for cgroups (/etc/cgconfig.conf) and add below configuration. # vi /etc/cgconfig.conf mount { cpu = /cgroup/cpumem; cpuset = /cgroup/cpumem; memory = […]