Network Troubleshooting

Problem-Solving Model

Work from bottom to top:

1. link and interface
2. IP and subnet
3. routing
4. DNS
5. transport port
6. firewall or policy
7. application

Quick Commands

ip -br a
ip r
ping -c 2 gateway
ping -c 2 1.1.1.1
dig example.com +short
nc -vz host 443
curl -vk https://example.com
ss -tulpn
tcpdump -nn -i any host 10.0.0.10

Common Cases

IP Works but the Domain Does Not

This usually points to:

• DNS
• TLS or SNI
• reverse proxy or virtual host config

Only One Subnet Is Broken

Check:

• routing
• ACLs or security groups
• VLAN
• DHCP options

TCP Timeout

This usually means:

• firewall drops the packet
• asymmetric routing
• the backend is not reachable

Connection Refused

This usually means:

• the process is not listening
• it listens on 127.0.0.1 when an external interface was expected

Practice

• timeout and refused are different classes of problems.
• curl -vk often tells you more than ping.
• Use tcpdump when higher-level commands still do not explain where packets disappear.