Just a quick Stub for the simple debugging stuff. More details will follow as I find or make time.
- iftop ( Interface statistics. )
- htop ( Prettier version of top. )
- nmon ( Does most of the above. )
- strace ( CLI execution of code or connect to a running process )
- tcpdump ( To verify connections are going where you think they are. )
- nmap ( To see if ports are open externally when netstat or ss says they are internally. )
- telnet ( More versatile than you would think. Test connections to services manually. )