4b7eca8a46
All checks were successful
Reindex Knowledge Base / reindex (push) Successful in 3s
Adds title, description, type, domain, and tags frontmatter to every doc for improved KB semantic search. The description field is prepended to every search chunk, and domain/type/tags enable filtered queries. Type values: context, guide, runbook, reference, troubleshooting Domain values match directory structure (networking, docker, etc.) Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
191 lines
3.4 KiB
Markdown
191 lines
3.4 KiB
Markdown
---
|
|
title: "Network Troubleshooting Commands"
|
|
description: "Quick-reference diagnostic commands for connectivity, port debugging, firewall rules, SSL certificates, DNS resolution, and bandwidth testing."
|
|
type: reference
|
|
domain: networking
|
|
tags: [troubleshooting, diagnostics, commands, firewall, ssl, dns, bandwidth]
|
|
---
|
|
|
|
# Network Troubleshooting Reference
|
|
|
|
## Connectivity Issues
|
|
|
|
### Basic connectivity tests
|
|
```bash
|
|
# Test connectivity
|
|
ping <target_ip>
|
|
ping -c 4 <target_ip> # Linux (4 packets)
|
|
|
|
# Test specific port
|
|
telnet <target_ip> <port>
|
|
nc -zv <target_ip> <port> # netcat
|
|
nmap -p <port> <target_ip>
|
|
|
|
# DNS resolution
|
|
nslookup <hostname>
|
|
dig <hostname>
|
|
host <hostname>
|
|
```
|
|
|
|
### Network interface debugging
|
|
```bash
|
|
# Show interfaces
|
|
ip addr show
|
|
ifconfig
|
|
|
|
# Show routing table
|
|
ip route
|
|
route -n
|
|
|
|
# Show network statistics
|
|
netstat -i
|
|
ss -i
|
|
```
|
|
|
|
## Service Issues
|
|
|
|
### Port and process debugging
|
|
```bash
|
|
# Show listening ports
|
|
netstat -tulpn
|
|
ss -tulpn
|
|
lsof -i :<port>
|
|
|
|
# Find process using port
|
|
fuser <port>/tcp
|
|
lsof -i tcp:<port>
|
|
|
|
# Show all connections
|
|
netstat -an
|
|
ss -an
|
|
```
|
|
|
|
### Service status
|
|
```bash
|
|
# Systemd services
|
|
systemctl status <service>
|
|
systemctl is-active <service>
|
|
systemctl is-enabled <service>
|
|
|
|
# Service logs
|
|
journalctl -u <service> -f
|
|
journalctl -u <service> --since "1 hour ago"
|
|
```
|
|
|
|
## Firewall Debugging
|
|
|
|
### iptables
|
|
```bash
|
|
# List rules
|
|
iptables -L -n -v
|
|
iptables -t nat -L -n -v
|
|
|
|
# Check if rule exists
|
|
iptables -C INPUT -p tcp --dport 80 -j ACCEPT
|
|
|
|
# Temporarily disable firewall
|
|
systemctl stop iptables # CentOS/RHEL
|
|
ufw disable # Ubuntu
|
|
```
|
|
|
|
### UFW (Ubuntu)
|
|
```bash
|
|
# Check status
|
|
ufw status verbose
|
|
|
|
# Show rules by number
|
|
ufw status numbered
|
|
|
|
# Check logs
|
|
tail -f /var/log/ufw.log
|
|
```
|
|
|
|
## SSL/TLS Issues
|
|
|
|
### Certificate debugging
|
|
```bash
|
|
# Check certificate details
|
|
openssl x509 -in certificate.crt -text -noout
|
|
openssl s_client -connect <hostname>:443 -servername <hostname>
|
|
|
|
# Check certificate chain
|
|
openssl s_client -connect <hostname>:443 -showcerts
|
|
|
|
# Test SSL connection
|
|
curl -vI https://<hostname>
|
|
wget --no-check-certificate -O /dev/null https://<hostname>
|
|
```
|
|
|
|
### Certificate expiration
|
|
```bash
|
|
# Check expiration date
|
|
openssl x509 -in certificate.crt -enddate -noout
|
|
|
|
# Check remote certificate expiration
|
|
echo | openssl s_client -servername <hostname> -connect <hostname>:443 2>/dev/null | openssl x509 -noout -dates
|
|
```
|
|
|
|
## DNS Issues
|
|
|
|
### DNS server testing
|
|
```bash
|
|
# Test specific DNS server
|
|
nslookup <hostname> <dns_server>
|
|
dig @<dns_server> <hostname>
|
|
|
|
# Flush DNS cache
|
|
# Linux (systemd-resolved)
|
|
systemctl restart systemd-resolved
|
|
# Linux (nscd)
|
|
systemctl restart nscd
|
|
```
|
|
|
|
### DNS configuration
|
|
```bash
|
|
# Check DNS settings
|
|
cat /etc/resolv.conf
|
|
cat /etc/systemd/resolved.conf
|
|
|
|
# Test DNS resolution order
|
|
getent hosts <hostname>
|
|
```
|
|
|
|
## Performance Issues
|
|
|
|
### Bandwidth testing
|
|
```bash
|
|
# iperf3 testing
|
|
# Server: iperf3 -s
|
|
# Client: iperf3 -c <server_ip>
|
|
|
|
# wget speed test
|
|
wget -O /dev/null http://speedtest.tele2.net/100MB.zip
|
|
```
|
|
|
|
### Network latency
|
|
```bash
|
|
# Continuous ping with timestamps
|
|
ping -D <target>
|
|
|
|
# MTR (better than traceroute)
|
|
mtr <target>
|
|
mtr --report <target>
|
|
```
|
|
|
|
## Quick Diagnostics
|
|
|
|
### One-liner network check
|
|
```bash
|
|
# Basic network health
|
|
ping -c 1 8.8.8.8 && echo "Internet OK" || echo "No Internet"
|
|
|
|
# Service accessibility
|
|
nc -zv localhost 80 && echo "HTTP service running" || echo "HTTP service down"
|
|
```
|
|
|
|
### Network interface stats
|
|
```bash
|
|
# Interface statistics
|
|
cat /proc/net/dev
|
|
watch -n 1 cat /proc/net/dev
|
|
``` |