The Problem Server failed to login through ssh with below errors. From ssh client: $ ssh -vvv root@10.131.12.10 OpenSSH_7.6p1, LibreSSL 2.6.2 debug1: Reading configuration data /etc/ssh/ssh_config debug1: /etc/ssh/ssh_config line 48: Applying options for * debug2: ssh_connect_direct: needpriv 0 debug1: Connecting to 10.131.12.10 port 22. debug1: Connection established. debug1: key_load_public: No such file or directory debug1: identity file /Users/yaozhenqiang/.ssh/id_rsa type … [Read more...] about SSH Connection Refused by TCP Wrapper
“ntpq -pn” command returns with error “Name or service not known”
The Problem Executing the command "ntpq -pn" returns with the error: # ntpq -p Name or service not known In some cases, you may also encounter an error shown below: # ntpq -pn Servname not supported for ai_socktype The Solution The "Name or service not known" error will be seen if the system cannot resolve "localhost" for the ntpq command. Make sure we have pointer to "127.0.0.1 localhost" in /etc/hosts file. 1. Fix the /etc/hosts file to map 127.0.0.1 to localhost, i.e., there … [Read more...] about “ntpq -pn” command returns with error “Name or service not known”
What is the refid in ntpq -p output?
Question When executing an ntpq -p command, what is the refid? $ ntpq -p remote refid st t when poll reach delay offset jitter ======================================================================================== *myntp.example.com 192.168.102.252 2 u 161 1024 377 0.841 -0.442 1.365 Answer A stratum-0 is an actual time source. It is connected to a server which is a stratum-1 server. Systems that use a stratum-1 system as their time … [Read more...] about What is the refid in ntpq -p output?
Why Does “/var/log/messages” Report Martian Packets
There are entries in the /var/log/messages file as shown below: # tailf /var/log/messages Aug 22 11:08:21 server kernel: martian source 192.168.12.197 from 192.168.12.198, on dev eth0 Aug 22 11:08:21 server kernel: ll header: 08:00:00:00:45:00:01:00:00:00:40:00:40:11:9f:11:c0:a8:0c:c6:c0:a8:0c:c5 Aug 22 11:08:22 server kernel: martian source 192.168.12.192 from 192.168.12.198, on dev eth0 Aug 22 11:08:22 server kernel: ll header: … [Read more...] about Why Does “/var/log/messages” Report Martian Packets
What is HBA Queue Depth and How to Check the Current Queue Depth Value and how to Change it
What is Fiber Channel HBA Queue Depth The queue depth indicates the number of I/O requests that are "in flight", that have been requested but not yet been acknowledged to have been completed when communicating with a SAN storage. These requests can be configured per single Logical Unit Number (LUN) that is accessed or based on the HBA. The maximum queue depth describes the value of requests which is allowed to be in flight at maximum. The maximum queue depth setting can significantly influence … [Read more...] about What is HBA Queue Depth and How to Check the Current Queue Depth Value and how to Change it
How to disable ping responses from CentOS/RHEL system
The Problem As a part of the hardening process user wants to tune the system to not respond to an ICMP (ping) request. How this can be enabled on a CentOS/RHEL system? The Solution To disable ping or ICMP responses from a CentOS/RHEL system, we can use kernel parameter icmp_echo_ignore_all in /etc/sysctl.conf. 1. On the server where you need to disable ping responses, review the current setting related to responses to ping messages with the following command: # cat … [Read more...] about How to disable ping responses from CentOS/RHEL system
How to Configure Network Teaming in CentOS/RHEL 7
On network servers, it is an important requirement to ensure that the network interface is available at all times. On Red Hat Enterprise Linux 7, you can create aggregated network interfaces. Use network teaming or network bonding to accomplish this goal. In earlier versions of RHEL, network bonding was the default method for creating aggregated network interfaces. In RHEL 7, network teaming has been added as a solution. The main difference between these two is that network bonding happened … [Read more...] about How to Configure Network Teaming in CentOS/RHEL 7
lvcreate/lvremove Failed with Error “Can’t remove merging snapshot logical volume”
The Problem Unable create or remove snapshot volume as error below: # lvcreate -s -L 3G -n snapopt /dev/vgOS/opt Logical volume "snapopt" already exists in volume group "vgOS" Whereas the logical volume is not present in any Volume Group. # lvs LV VG Attr LSize Pool Origin Data% Meta% Move Log Cpy%Sync Convert cplex vgOPT -wi-ao---- 3.00g oracle vgOPT -wi-ao---- 20.00g home vgOS -wi-ao---- 2.00g opt vgOS Owi-aos--- 6.45g root vgOS -wi-ao---- 6.00g swap vgOS -wi-ao---- 2.00g tmp … [Read more...] about lvcreate/lvremove Failed with Error “Can’t remove merging snapshot logical volume”
How to revert an accidental chown or chmod on a directory in CentOS/RHEL
The Problem The user has recursively changed the permissions or ownerships of a directory unintentionally. For example: # chmod -R 777 /etc He may have changed the ownership of the files/directories as well. How can we revert the permissions and ownership to the default value without reinstalling the system? The Solution The following example uses /etc for demonstrative purposes. You may have any directory in place of /etc for recovery. The process remains the same. The rpm command has … [Read more...] about How to revert an accidental chown or chmod on a directory in CentOS/RHEL
How to restore the permission/ownership to default in case of accidental change of permission/ownership
The problem Accidentally the user executed the following command. # chmod -R 777 / Is there any way to revert back to the original permissions? The Solution Several options exist for resetting permissions on files and directories across an installed system. 1. Restore from the last-known-good backup This option, while usually least desired, offers the best chance of restoring all file permissions back to their correct values. (This does require the values to have been correct at … [Read more...] about How to restore the permission/ownership to default in case of accidental change of permission/ownership