Network debugging

On target box:

• Check my IP from outside
• curl -s checkip.dyndns.org | sed -e 's/.*Current IP Address: //' -e 's/<.*$//'
• Check what interfaces exist on your box:
• ip addr
• Run a tcpdump 
• Also run tcpdump on any gateway between your boxes that you have access to 

On own box:

• Fire up netcat try to connect
• nc target_ip target_port

On all boxes:

• Run traceroute and follow the IPs. Do we go to a gateway that we shouldn't?
  • mtr  8.8.8.8
  • mtr
• See traffic routing rules for this box: 
• ip route show 
• ip addr
• netstat -ra
• Arping is like ping but lower down, corresponding to layer 2:
•  arping 192.168.1.1

Understanding tcpdump:

• Does the first part of the handshake get thru?
• Do we respond to the above handshake?
• Does it go thru the gateway (both ways)?

Other stuff to check on target box:

• Is iptables blocking it? 
• sudo iptables -L -n | grep 21 
• Is the process listening on that port?
• sudo netstat -tulpn | grep