1. nslookup <url>
check what's your url is known by the dns server.
2. ping <IP>
Is the IP alive? If ping get 100% loss, it could mean two things, either the host is down or it is blocking the ICMP traffic.
3. curl <url>
check if the web application you are trying to access is alive.
4. netstat -nultp
ssh into the server and check what's going on there. run the command on the server machine to list all the listening ports of your application, for example, your java program's listening ports are:
netstat -nultp | grep java
5. nc <IP> <port>
Your web application might be listening on a port such as 8799, which you don't know how to communicate with. Use netcat, the general purpose net client to connect to the port to tell if the port is open.
6. openssl s_client -connect <IP:port>
Whenever your web application get authentication issue during network connection, get the public key from the destination server, then check whatever your web client was looking for are there.
7. tcpdump -i eth0 -s0 -v port 80
Still have issue? Try to get the pcap from the source host, load balancer, proxy server, destination host, then look into them, looking for issues such as connection reset, socket reuse, traffic drop, wait timed out, concurrency bug, etc.
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