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Network Troubleshooting: Fix Connection Problems

Networks break. Connections fail. You need to fix them.

Here's the thing: Network troubleshooting is systematic. Check one thing at a time. Find the problem.

Basic Checks​

Check Interface Status​

ip link show
ip addr show

My take: Check if interface is up. Check if it has an IP. Start here.

Test Connectivity​

ping 8.8.8.8                      # Test internet
ping google.com                   # Test DNS
ping 192.168.1.1                  # Test gateway

My take: ping tests connectivity. Use it to isolate problems.

Troubleshooting Steps​

1. Check Interface​

ip link show
# Is it UP?

My take: Interface must be UP. If DOWN, bring it up.

2. Check IP Address​

ip addr show
# Does it have an IP?

My take: Interface needs an IP. If missing, configure it.

3. Check Gateway​

ip route show
# Is default route set?

My take: Gateway is needed for external access. Check it.

4. Test Local Network​

ping 192.168.1.1                  # Gateway
ping 192.168.1.100               # Another host

My take: Test local network first. Then test internet.

5. Test DNS​

nslookup google.com
dig google.com

My take: DNS must work for names. Test it.

Common Tools​

ping: Test Connectivity​

ping -c 4 8.8.8.8                # 4 packets
ping -c 4 google.com              # Test DNS too

My take: ping is basic. Use it first.

traceroute: Trace Path​

traceroute google.com
tracepath google.com              # Alternative

My take: traceroute shows the path. Useful for finding where it breaks.

netstat/ss: View Connections​

ss -tlnp                          # Listening ports
ss -tn                            # Active connections
netstat -tlnp                     # Legacy

My take: ss is modern. netstat is legacy. Use ss.

tcpdump: Packet Capture​

sudo tcpdump -i eth0
sudo tcpdump -i eth0 port 80

My take: tcpdump captures packets. Advanced. Use when needed.

Common Problems​

No IP Address​

# Check DHCP
sudo dhclient eth0

# Or set static
sudo ip addr add 192.168.1.100/24 dev eth0

My take: No IP = no network. Get an IP.

Can't Reach Gateway​

# Check gateway
ip route show

# Add gateway
sudo ip route add default via 192.168.1.1

My take: Gateway is needed. Set it.

DNS Not Working​

# Check DNS
cat /etc/resolv.conf

# Test DNS
nslookup google.com

My take: DNS must work. Check configuration.

Common Mistakes (I've Made These)​

  1. Not checking interface status: Interface must be UP. Check first.

  2. Skipping steps: Troubleshoot systematically. Don't skip steps.

  3. Not testing locally first: Test local network before testing internet.

  4. Ignoring DNS: DNS problems look like connectivity problems. Check DNS.

  5. Not checking firewall: Firewalls can block. Check them.

Real-World Examples​

Debug Network Issue​

# 1. Check interface
ip link show

# 2. Check IP
ip addr show

# 3. Test gateway
ping 192.168.1.1

# 4. Test internet
ping 8.8.8.8

# 5. Test DNS
ping google.com

Find Network Problems​

# Check routing
ip route show

# Check DNS
cat /etc/resolv.conf

# Test connectivity
ping -c 4 8.8.8.8
traceroute google.com

What's Next?​

Now that you can troubleshoot networks, let's talk about Firewall iptables to secure networks.

Personal note: Network problems used to frustrate me. Then I learned systematic troubleshooting. Now I check one thing at a time. Find the problem. Fix it. It's methodical.