Network Debugging Guide

Initial Issue

  • User reported ping pool.ntp.org freezing the console.

  • Subsequent tests (ping 8.8.8.8, curl, mtr) showed either freezes or incomplete data.

Diagnostic Steps

1. Basic Connectivity Checks

# Check interface status
ip link show
ip a

# Test raw connectivity
nc -zv 8.8.8.8 80
arping -I eth0 192.168.1.1

2. Routing and DNS

# Verify routing table
ip route

# Test DNS resolution
nslookup google.com
dig google.com

3. Firewall Inspection

# Check iptables
sudo iptables -L

# Check nftables
sudo nft list ruleset

4. Advanced Diagnostics

# Kernel network stats
sudo netstat -i
sudo ethtool -S eth0

# Strace analysis
strace ping -c 1 8.8.8.8

Key Findings

  1. Network Stack Behavior:

    • sendto() calls succeeded (outbound packets sent)

    • poll() timed out (no replies received)

    • ARP working (gateway reachable at L2)

  2. Identified Issues:

    • Outbound packets not being routed back

    • Possible causes: * Gateway misconfiguration * ISP blocking * NAT/firewall interference

Solutions Applied

  1. Gateway Testing:

    • Recommended testing router’s WAN connectivity

    • Suggested bypassing gateway via hotspot

  2. ISP Verification:

    • Recommended VPN test to bypass potential ISP blocking

  3. Kernel Parameters:

    • Checked ICMP echo settings:

      sudo sysctl net.ipv4.icmp_echo_ignore_all

  4. Alternative Networks:

    • Suggested testing with different network interfaces

Final Recommendations

  1. Conclusive Tests:

    • Use mobile hotspot to isolate gateway issues

    • Test with VPN to check for ISP blocking

  2. Administrative Actions:

    • Contact ISP if gateway cannot ping 8.8.8.8

    • Check router logs for dropped packets

  3. Persistent Configuration:

    • For DNS issues:

      echo "nameserver 8.8.8.8" | sudo tee /etc/resolv.conf

    • For routing issues:

      sudo ip route add default via <gateway_ip> dev <interface>