Openssl error: SSL routines:SSL23_GET_SERVER_HELLO:unknown protocol


If you see similar errors as below when using openssl to test a https site, it is most likely caused by the SNI that is configured in the reverse proxy or server, like Nginx. openssl s_client -connect bla.bla.com:443 CONNECTED(00000003) 51089:error:140770FC:SSL routines:SSL23_GET_SERVER_HELLO:unknown protocol:/BuildRoot/Library/Caches/com.apple.xbs/Sources/OpenSSL098/OpenSSL098-59.60.1/src/ssl/s23_clnt.c:618: To make it work, you need to specify the hostname in the command, … Continue reading Openssl error: SSL routines:SSL23_GET_SERVER_HELLO:unknown protocol

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Find open ports from Linux kernel


Have you ever tried to find out the open ports in a Linux box without using the common tools like netstat, nmap, lsof, ss ... ? In some cases,  the tools are just not available. e.g In a cutdown version Linux server or container, and you don't have the permission to install any software. I … Continue reading Find open ports from Linux kernel

Troubleshoot high CPU usage java process


This is a real troubleshooting example that I just did yesterday for a high CPU usage java application. The application uses tomcat and runs on AWS EC2. Login into the box, and change to root user so you can see all users' process. sudo su - Install htop if you have not installed it before, the run it. … Continue reading Troubleshoot high CPU usage java process