The Problem The GRUB2 files went missing under the /boot directory. Missing '/boot', /boot/grub2 folder and /boot/grub2/grub.cfg file which are essential to boot the system. How to recover these missing boot files? The Solution Follow the steps outlined below to recover the missing files under /boot directory. 1. Verify if the Boot directory contents are really missing or the /boot is not mounted itself: # ls /boot/grub2/ # mount | grep boot 2. Install the GRUB2 again using the … [Read more...] about How to Recover Missing GRUB2 Files from /boot
Linux
How to Stop SSH Session From Getting Timed Out
The Linux ssh server never disconnects ssh session from server side by default, check switch/router/firewall configuration to avoid the issue without changing the configuration of ssh servers/clients. If the ssh session is getting disconnected due to inactivity and one wants to stop this then please follow the below steps either on client or server side as described below: Client Side Settings To enable the keep alive system-wide (root access required), edit the configuration file … [Read more...] about How to Stop SSH Session From Getting Timed Out
How to Setup SSH keys for “passwordless” SSH Login on CentOS/RHEL
The post outlines the steps to configure a passwordless ssh between 2 CentOS/RHEL hosts. The steps although remains almost the same with slight changes across all the Linux distributions. 1. Log in as the user that you want to set up the ssh keys, in this case, we are using user "geek". 2. Create a private and public key : $ ssh-keygen Generating public/private rsa key pair. Enter file in which to save the key (/home/geek/.ssh/id_rsa): Created directory '/home/geek/.ssh'. Enter … [Read more...] about How to Setup SSH keys for “passwordless” SSH Login on CentOS/RHEL
How to test port connectivity using nc utility in CentOS/RHEL
The nc utility, which is also called netcat, comes in very handy when you want to test TCP/IP servers and clients. netcat is a computer networking utility for reading from and writing to network connections using TCP or UDP. netcat can be installed using the below command, if not already installed. # yum -y install nmap-ncat Open a port using nc A port can be opened on CentOS/RHEL by using netcat for testing purpose. From the man page of nc command. # man nc -l, --listen (Listen … [Read more...] about How to test port connectivity using nc utility in CentOS/RHEL
Why Does “netstat” Output Show Many Connections in CLOSE_WAIT Status?
Applications that handle a huge number of TCP connections, either as a server or as a client, will often see a large number of sockets in TIME_WAIT state. The sockets in TIME_WAIT can be seen with the “netstat -epn –tcp ” command. If the number of TIME_WAIT sockets gets too large, your address space will be exhausted, causing a disruption of TCP traffic. The possible state values for TCP sockets are as follows: Connection StateMeaning BOUND Bound, ready to connect or listen. … [Read more...] about Why Does “netstat” Output Show Many Connections in CLOSE_WAIT Status?
How to Test Port [TCP/UDP] Connectivity from a Linux Server
Here is a short post to check port [TCP/UDP] connectivity from a Linux server. A TCP/IP network connection may be either blocked, dropped, open, or filtered. These actions are generally controlled by the IPtables firewall the system uses and is independent of any process or program that may be listening on a network port. Telnet and nc are common tools used to test port connectivity from Linux server. Telnet can be used to test tcp port connections, where as nc can be used to test both … [Read more...] about How to Test Port [TCP/UDP] Connectivity from a Linux Server
Kernel Logs Warning Messages “kernel: Possible SYN flooding on port X. Sending cookies” is logged”
The Problem Sometimes Linux kernel logs warning messages as followings: Mar 7 09:17:14 hostname kernel: TCP: Possible SYN flooding on port 26450. Sending cookies. or Mar 7 09:17:14 hostname kernel: TCP: Possible SYN flooding on port 26450. Dropping request. The Solution This is a warning message, which indicates that the server is frequently attempted to connect to the specific port, and the kernel warns that this might possibly be an SYN flood attack(=DoS(Denial of Service) … [Read more...] about Kernel Logs Warning Messages “kernel: Possible SYN flooding on port X. Sending cookies” is logged”
Interview Questions : Linux Package Manager (RPM)
This post will cover the most frequently reported how-to questions of Enterprise Linux Common Package Manager (RPM). Although not often used, rpm is a must know when you are facing a Linux interview. What is RPM? RPM stands for Red Hat Package Manager which provides for installing, upgrading and removing packages on Linux distributions. It is a free software tool. Originally developed by Red Hat for Red Hat Linux, RPM is now used by many Linux distributions. It has also been ported to some … [Read more...] about Interview Questions : Linux Package Manager (RPM)
Troubleshooting “connection refused” From Remote Servers in CentOS/RHEL 7 (Either Firewalld or iptables service issue)
The Problem An application is getting "connection refused" from other servers. The application is accessible from localhost and also it listens to the expected port. The Solution This may be a case of Firewall on the local server dropping inbound connection attempts from other servers. By default, CentOS/RHEL 7 uses the FIREWALLD service to manage the IPTABLES rules. The older IPTABLES subsystem is still available and may be used directly if the FIREWALLD service is disabled. Determine … [Read more...] about Troubleshooting “connection refused” From Remote Servers in CentOS/RHEL 7 (Either Firewalld or iptables service issue)
SSH Connection Refused by TCP Wrapper
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