Network Debugging

Summarizing some of my thought process, first determine what layer the issue is happening in. Probably layer 7 (HTTP, DNS, or other protocols), layer 4 (transport layer TCP UDP), or layer 3 (network layer IPv4, ICMP). Always describe the problem in a sentence.

• HTTP and DNS layer 7 issues
  • nslookup and ping can be useful for debugging layer 7 DNS issues, traceroute lets you trace the DNS lookup
  • /etc/resolv.conf contents generally dictate what nameservers will be used and with what domain suffixes during DNS lookup
  • /etc/hosts can locally alias certain address to records
  • curl -v is useful for checking HTTP response headers and body
• tcp udp layer 4 issues
  • wireshark can be very useful for catching issues here, like which requests are being sent and where
  • netstat -tulpn can be useful for whitebox testing to see if a machine is listening on the right port
• routing layer 3 issues
  • ICMP is a protocol, doesn't have port abstraction
    • protocol used by layer 3 debugging tools
  • ifconfig is used for information about network interfaces
  • ping of course is an easy way to see if a hostname or address is discoverable
  • route will display the routing table
  • bridge link show can be used to display