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Diagnose Network Connectivity Issues

Troubleshoot packet loss, high latency, and connectivity failures between servers.

intermediate

8 min read

LightYear Team

Updated April 24, 2026

networkingtroubleshootinglatencypacket-loss

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Step 1 — Basic Connectivity Test

>_BASH

$# Test internet connectivity
$ping -c 10 8.8.8.8
$
$# Test DNS resolution
$nslookup google.com
$
$# Test a specific port
$nc -zv google.com 443

Step 2 — Trace the Network Path

>_BASH

$# Traceroute (shows each hop)
$traceroute 8.8.8.8
$
$# MTR (combines ping + traceroute)
$apt-get install -y mtr
$mtr --report 8.8.8.8

Sample MTR output:

HOST: my-server              Loss%   Snt   Last   Avg  Best  Wrst StDev
  1. 10.0.0.1                 0.0%    10    0.5   0.5   0.4   0.6   0.1
  2. 203.0.113.1              0.0%    10    1.2   1.3   1.1   1.5   0.1
  3. ???                     100.0%    10    0.0   0.0   0.0   0.0   0.0

A hop showing 100% loss but subsequent hops working is normal (ICMP filtering).

Step 3 — Check Network Interface

>_BASH

$# Interface statistics
$ip -s link show eth0
$
$# Check for errors
$ethtool -S eth0 | grep -i error
$
$# Check routing table
$ip route show

Step 4 — Bandwidth Testing

>_BASH

$# Install iperf3
$apt-get install -y iperf3
$
$# On server B (receiver)
$iperf3 -s
$
$# On server A (sender)
$iperf3 -c <SERVER_B_IP> -t 30

Expected results:

Same region, private network: 10+ Gbps
Same region, public network: 1–5 Gbps
Cross-region: 100–500 Mbps

Step 5 — Check Firewall Rules

>_BASH

$# List iptables rules
$iptables -L -n -v
$
$# Check if traffic is being dropped
$iptables -L -n -v | grep DROP

Step 6 — DNS Issues

>_BASH

$# Check DNS servers
$cat /etc/resolv.conf
$
$# Test DNS resolution speed
$time nslookup google.com
$
$# Use a different DNS server
$nslookup google.com 1.1.1.1

Common Issues and Fixes

Issue	Symptom	Fix
Firewall blocking	Connection refused	Add firewall rule
MTU mismatch	Large packets dropped	Set MTU to 1450
DNS failure	Cannot resolve hostnames	Fix /etc/resolv.conf
Routing issue	Partial connectivity	Check ip route

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