The Geek Diary

Archives for September 2018

Kernel Logs Warning Messages “kernel: Possible SYN flooding on port X. Sending cookies” is logged”

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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 […]

Interview Questions : Linux Package Manager (RPM)

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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 […]

Troubleshooting “connection refused” From Remote Servers in CentOS/RHEL 7 (Either Firewalld or iptables service issue)

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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 […]

SSH Connection Refused by TCP Wrapper

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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: […]

“ntpq -pn” command returns with error “Name or service not known”

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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 […]

What is the refid in ntpq -p output?

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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 […]

Why Does “/var/log/messages” Report Martian Packets

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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: 08:00:00:00:45:00:00:6c:00:00:40:00:40:11:9f:aa:c0:a8:0c:c6:c0:a8:0c:c0 […]

How to Configure Network Teaming in CentOS/RHEL 7

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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 […]