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Archives for September 2018

How to Configure TCP Keepalive option in Solaris

by admin

Typically, idle TCP connections are maintained indefinitely once established, even if no communication occurs between host systems. This is quite normal in TCP. In some cases, keeping the connection open may inappropriately consume host and/or application resources (normally TCP port ranges), if for example: the remote host crashes or otherwise undergoes some type of non-orderly […]

Filed Under: Solaris, Solaris 11

How to Test Port [TCP/UDP] Connectivity from a Linux Server

by admin

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 […]

Filed Under: Linux

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

by admin

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 […]

Filed Under: CentOS/RHEL 6, CentOS/RHEL 7, Linux

Interview Questions : Linux Package Manager (RPM)

by admin

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 […]

Filed Under: Linux

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

by admin

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 […]

Filed Under: CentOS/RHEL 7, Linux

SSH Connection Refused by TCP Wrapper

by admin

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: […]

Filed Under: Linux

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

by admin

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 […]

Filed Under: Linux

What is the refid in ntpq -p output?

by admin

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 […]

Filed Under: Linux

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

by admin

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 […]

Filed Under: CentOS/RHEL 6, CentOS/RHEL 7, Linux

What is HBA Queue Depth and How to Check the Current Queue Depth Value and how to Change it

by admin

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 […]

Filed Under: CentOS/RHEL 6, CentOS/RHEL 7, Linux

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