Troubleshooting
IP Configuration
Use ifconfig or ip to see if the interface is up, and if so, if it is configured.
Network Driver
If the interface can't be brought up, maybe the correct device driver for the network card(s) is not loaded. Check with lsmod if the network driver is loaded as a kernel module, or by examining relevant pseudo-files in /proc and /sys, such as /proc/interrupts or /sys/class/net.
Connectivity
Use ping to see if the network is visible, checking for response time and packet loss. traceroute can follow packets through the network, while mtr can do this in a continuous fashion. Use of these utilities can tell you if the problem is local or on the Internet.
Default Gateway and Routing Configuration
Run route -n and see if the routing table makes sense.
Hostname Resolution
Run dig or host on a URL and see if DNS is working properly.
Send an HTTP request with curl
exec 3<>/dev/tcp/localhost/8000
printf 'GET /health HTTP/1.1\r\nHost: localhost\r\nConnection: close\r\n\r\n' >&3
cat <&3
HTTP/1.1 200 OK
Health Check Ok