Source Code

DNS & Networking

Debug DNS resolution, network connectivity, and HTTP issues. Covers dig/nslookup, port testing, firewall rules, curl diagnostics, /etc/hosts, proxy configuration, and certificate troubleshooting.

When to Use

DNS name not resolving or resolving to wrong IP
Connection refused / connection timed out errors
Diagnosing firewall or security group rules
HTTP requests failing for unclear reasons
Proxy configuration issues
SSL/TLS certificate errors
Testing connectivity between services

DNS Debugging

Query DNS records

# A record (IP address)
dig example.com
dig +short example.com

# Specific record types
dig example.com MX        # Mail servers
dig example.com CNAME     # Aliases
dig example.com TXT       # Text records (SPF, DKIM, etc.)
dig example.com NS        # Name servers
dig example.com AAAA      # IPv6 address
dig example.com SOA       # Start of Authority

# Query a specific DNS server
dig @8.8.8.8 example.com
dig @1.1.1.1 example.com

# Trace the full resolution path
dig +trace example.com

# Reverse lookup (IP → hostname)
dig -x 93.184.216.34

# nslookup (simpler, works everywhere)
nslookup example.com
nslookup example.com 8.8.8.8    # Query specific server
nslookup -type=MX example.com

# host (simplest)
host example.com
host -t MX example.com

Check DNS propagation

# Query multiple public DNS servers
for dns in 8.8.8.8 1.1.1.1 9.9.9.9 208.67.222.222; do
    echo -n "$dns: "
    dig +short @"$dns" example.com
done

# Check TTL (time to live)
dig example.com | grep -E '^\S+\s+\d+\s+IN\s+A'
# The number is TTL in seconds

Local DNS issues

# Check /etc/resolv.conf (which DNS server the system uses)
cat /etc/resolv.conf

# Check /etc/hosts (local overrides)
cat /etc/hosts

# Flush DNS cache
# macOS:
sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder
# Linux (systemd-resolved):
sudo systemd-resolve --flush-caches
# Windows:
ipconfig /flushdns

# Check if systemd-resolved is running (Linux)
resolvectl status

/etc/hosts patterns

# /etc/hosts — local DNS overrides (no TTL, instant)

# Point a domain to localhost (for development)
127.0.0.1    myapp.local
127.0.0.1    api.myapp.local

# Block a domain
0.0.0.0      ads.example.com

# Test a migration (point domain to new server before DNS change)
203.0.113.50    example.com
203.0.113.50    www.example.com

# Multiple names for one IP
192.168.1.100   db.local redis.local cache.local

Port and Connectivity Testing

Test if a port is open

# nc (netcat) — most reliable
nc -zv example.com 443
nc -zv -w 5 example.com 80    # 5 second timeout

# Test multiple ports
for port in 22 80 443 5432 6379; do
    nc -zv -w 2 example.com $port 2>&1
done

# /dev/tcp (bash built-in, no extra tools needed)
timeout 3 bash -c 'echo > /dev/tcp/example.com/443' && echo "Open" || echo "Closed"

# curl (also tests HTTP)
curl -sI -o /dev/null -w "%{http_code}" https://example.com

# Test from inside a Docker container
docker exec my-container nc -zv db 5432

Network path diagnostics

# traceroute (show network hops)
traceroute example.com

# mtr (continuous traceroute with stats — best for finding packet loss)
mtr example.com
mtr -r -c 20 example.com   # Report mode, 20 packets

# ping
ping -c 5 example.com

# Show local network interfaces
ip addr show          # Linux
ifconfig              # macOS / older Linux

# Show routing table
ip route show         # Linux
netstat -rn           # macOS
route -n              # Linux (older)

Check listening ports

# What's listening on which port (Linux)
ss -tlnp
ss -tlnp | grep :8080

# macOS
lsof -i -P -n | grep LISTEN
lsof -i :8080

# Older Linux
netstat -tlnp
netstat -tlnp | grep :8080

# Which process is using a port
lsof -i :3000
fuser 3000/tcp   # Linux

curl Diagnostics

Verbose request inspection

# Full verbose output (headers, TLS handshake, timing)
curl -v https://api.example.com/endpoint

# Show timing breakdown
curl -o /dev/null -s -w "
    DNS:        %{time_namelookup}s
    Connect:    %{time_connect}s
    TLS:        %{time_appconnect}s
    TTFB:       %{time_starttransfer}s
    Total:      %{time_total}s
    Status:     %{http_code}
    Size:       %{size_download} bytes
" https://api.example.com/endpoint

# Show response headers only
curl -sI https://api.example.com/endpoint

# Follow redirects and show each hop
curl -sIL https://example.com

# Resolve a domain to a specific IP (bypass DNS)
curl --resolve example.com:443:203.0.113.50 https://example.com

# Use a specific network interface
curl --interface eth1 https://example.com

Debug common HTTP issues

# Test with different HTTP versions
curl --http1.1 https://example.com
curl --http2 https://example.com

# Test with specific TLS version
curl --tlsv1.2 https://example.com
curl --tlsv1.3 https://example.com

# Ignore certificate errors (debugging only)
curl -k https://self-signed.example.com

# Send request with custom Host header (virtual hosts)
curl -H "Host: example.com" https://203.0.113.50/

# Test CORS preflight
curl -X OPTIONS -H "Origin: http://localhost:3000" \
     -H "Access-Control-Request-Method: POST" \
     -v https://api.example.com/endpoint

Firewall Basics

iptables (Linux)

# List all rules
sudo iptables -L -n -v

# Allow incoming on port 80
sudo iptables -A INPUT -p tcp --dport 80 -j ACCEPT

# Allow incoming from specific IP
sudo iptables -A INPUT -s 203.0.113.0/24 -p tcp --dport 22 -j ACCEPT

# Block incoming on a port
sudo iptables -A INPUT -p tcp --dport 3306 -j DROP

# Save rules (persist across reboot)
sudo iptables-save > /etc/iptables/rules.v4

ufw (simpler, Ubuntu/Debian)

# Enable
sudo ufw enable

# Allow/deny
sudo ufw allow 80/tcp
sudo ufw allow 443/tcp
sudo ufw allow from 203.0.113.0/24 to any port 22
sudo ufw deny 3306

# Check status
sudo ufw status verbose

# Reset all rules
sudo ufw reset

macOS firewall

# Check status
sudo /usr/libexec/ApplicationFirewall/socketfilterfw --getglobalstate

# Enable
sudo /usr/libexec/ApplicationFirewall/socketfilterfw --setglobalstate on

# Allow an application
sudo /usr/libexec/ApplicationFirewall/socketfilterfw --add /usr/local/bin/myapp

Proxy Configuration

Environment variables

# Set proxy for most CLI tools
export HTTP_PROXY=http://proxy.example.com:8080
export HTTPS_PROXY=http://proxy.example.com:8080
export NO_PROXY=localhost,127.0.0.1,.internal.example.com

# For curl specifically
export http_proxy=http://proxy.example.com:8080  # lowercase also works

# With authentication
export HTTPS_PROXY=http://user:password@proxy.example.com:8080

Test through proxy

# curl with explicit proxy
curl -x http://proxy.example.com:8080 https://httpbin.org/ip

# SOCKS proxy
curl --socks5 localhost:1080 https://httpbin.org/ip

# Verify your external IP through proxy
curl -x http://proxy:8080 https://httpbin.org/ip
curl https://httpbin.org/ip  # Compare with direct

# Test proxy connectivity
curl -v -x http://proxy:8080 https://example.com 2>&1 | grep -i "proxy\|connect"

Common proxy issues

# Node.js fetch/undici does NOT respect HTTP_PROXY
# Use undici ProxyAgent or node-fetch with http-proxy-agent

# Git through proxy
git config --global http.proxy http://proxy:8080
git config --global https.proxy http://proxy:8080
# Remove:
git config --global --unset http.proxy

# npm through proxy
npm config set proxy http://proxy:8080
npm config set https-proxy http://proxy:8080

# pip through proxy
pip install --proxy http://proxy:8080 package-name

Certificate Troubleshooting

# Check certificate from a server
echo | openssl s_client -connect example.com:443 -servername example.com 2>/dev/null | \
  openssl x509 -noout -subject -issuer -dates

# Check expiry
echo | openssl s_client -connect example.com:443 2>/dev/null | \
  openssl x509 -noout -enddate

# Download certificate chain
openssl s_client -showcerts -connect example.com:443 < /dev/null 2>/dev/null | \
  awk '/BEGIN CERT/,/END CERT/' > chain.pem

# Verify a certificate against CA bundle
openssl verify -CAfile /etc/ssl/certs/ca-certificates.crt server.pem

# Check certificate for a specific hostname (SNI)
openssl s_client -connect cdn.example.com:443 -servername cdn.example.com

# Common error: "certificate has expired"
# Check the date on the server:
date
# If the system clock is wrong, certs will appear invalid

Quick Diagnostics Script

#!/bin/bash
# net-check.sh — Quick network diagnostics
TARGET="${1:?Usage: net-check.sh <hostname> [port]}"
PORT="${2:-443}"

echo "=== Network Check: $TARGET:$PORT ==="

echo -n "DNS resolution: "
IP=$(dig +short "$TARGET" | head -1)
[[ -n "$IP" ]] && echo "$IP" || echo "FAILED"

echo -n "Ping: "
ping -c 1 -W 3 "$TARGET" > /dev/null 2>&1 && echo "OK" || echo "FAILED (may be blocked)"

echo -n "Port $PORT: "
nc -zv -w 5 "$TARGET" "$PORT" 2>&1 | grep -q "succeeded\|open" && echo "OPEN" || echo "CLOSED/FILTERED"

if [[ "$PORT" == "443" || "$PORT" == "8443" ]]; then
    echo -n "TLS: "
    echo | openssl s_client -connect "$TARGET:$PORT" -servername "$TARGET" 2>/dev/null | \
      grep -q "Verify return code: 0" && echo "VALID" || echo "INVALID/ERROR"

    echo -n "Certificate expiry: "
    echo | openssl s_client -connect "$TARGET:$PORT" 2>/dev/null | \
      openssl x509 -noout -enddate 2>/dev/null | sed 's/notAfter=//'
fi

echo "=== Done ==="

Tips

dig +short is the fastest way to check DNS from the command line. Use @8.8.8.8 to bypass local caching.
nc -zv is the simplest port connectivity test. If nc isn't available, use bash's /dev/tcp.
curl's -w format string with timing variables is the fastest way to diagnose slow HTTP requests: DNS, connect, TLS, and TTFB are all visible.
DNS changes propagate based on TTL. Check the current TTL with dig before expecting a DNS change to take effect.
/etc/hosts changes take effect immediately (no TTL, no propagation delay). Use it to test domain migrations before changing DNS.
When debugging "connection refused": first verify the port is open with nc, then check the service is actually listening with ss -tlnp or lsof -i.
mtr is better than traceroute for diagnosing packet loss — it runs continuously and shows per-hop loss percentages.
Node.js, Python requests, and many libraries do NOT automatically use HTTP_PROXY environment variables. Check each tool's proxy documentation.

dns-networking