Archives for May 2018

The login to specific users can be allowed or denied using the file /etc/pam.d/remote. Other network services uses the files /etc/pam.d/sshd or /etc/pam.d/password-auth for allowing or denying the access. Whereas telnet uses the file /etc/pam.d/remote. Denying telnet login to specific users 1. Add the users you want to deny the telnet login to, in the file /etc/user.deny. # vi /etc/user.deny user01 user02 user03 2. Add the following line to the /etc/pam.d/remote file to deny telnet … [Read more...] about How to allow or deny telnet login to specific users only in CentOS/RHEL

The device node name of a disk (/dev/sda, /dev/hda, /dev/vda, etc.) may change in some situations. For example, after switching cables around or upgrading certain packages, sda & sdc could swap places. This causes problems when /etc/fstab references filesystems by the disk names. Instead, we can use filesystem labels to refer them in /etc/fstab file. This allows us for identifying a filesystem without resorting to ephemeral block device names. Note: The UUIDs and labels are not required if a … [Read more...] about How to Change the filesystem Labels in CentOS/RHEL

The cachedir specifies the directory where yum stores downloaded packages, although with keepcache set to 0, yum does not store these packages after installing them. Consider a case where the /var filesystem where yum cache is located is full and there is no diskspace for yum to download the system updates. Also, you do not have set the keepcache variable to 0. The "yum install" command would end up with the error like: insufficient space in download directory … [Read more...] about How to change the default location (/var/cache/yum) of yum cache

Red Hat Enterprise Linux natively supports storage that has multiple paths presented to the OS. Configuring CentOS/RHEL to use the multipath functionality is a straightforward process: Installation If not already installed, install the device-mapper-multipath package: # yum install device-mapper-multipath Location of multipath devices When devices are brought under the control of device-mapper-multipath the new multipathed devices can be seen in three different places under the /dev … [Read more...] about How to Install and Configure Device Mapper Multipath in CentOS/RHEL 6,7

The System Security Services Daemon (SSSD) provides access to remote identity and authentication providers. Providers are configured as back ends with SSSD acting as an intermediary between local clients and any configured back-end provider. The local clients connect to SSSD and then SSSD contacts the providers. Benefits of SSSD include: Reduced load: Clients do not have to contact the identification/authentication servers directly; they need to contact only SSSD. Offline authentication: … [Read more...] about Understanding System Security Services Daemon (SSSD)

Once a multipath device is down, I/O to this device is blocked. That can cause hang when collecting a sosreport or scanning/refreshing a disk. An example of a Dead Multipath device is shown below: 360060e80166bac0000016bac000000da dm-49 HITACHI,OPEN-V size=200G features='0' hwhandler='0' wp=rw '-+- policy='round-robin 0' prio=0 status=enabled |- 8:0:0:34 sdiq 135:160 failed faulty running |- 8:0:1:34 sdix 8:272 failed faulty running |- 9:0:1:34 sdjl 8:496 failed faulty running |- … [Read more...] about How to Remove a Dead Mulitpath Device without Reboot in CentOS/RHEL

The Problem When trying to re-add a device to a raid, you get the following message: # mdadm /dev/md2 --add /dev/sda1 mdadm: Cannot open /dev/sda1: Device or resource busy Whereas the mdstat command indicates that the device is inactive. # cat /proc/mdstat Personalities : md2 : inactive sda1[0](S) 292928154 blocks super 1.2 unused devices: [none] mdadm fail to create software raid device with Device or resource busy error message. The solution Pre-checks Before we … [Read more...] about mdadm: Cannot open /dev/sda1: Device or resource busy

The iostat command when run would produce an output like the following, with names such as dm-0 instead of device-mapper paths like /dev/mapper/mpath0: # iostat Linux 2.6.18-164.11.1.el5 (hostname) 03/19/2010 avg-cpu: %user %nice %system %iowait %steal %idle 4.15 0.40 5.60 7.25 0.00 82.60 Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn hda 102.14 2327.29 533.51 1422881 326182 hda1 0.25 … [Read more...] about How to map devices in the iostat command output to their corresponding device mapper path